the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 18 Apr 2024
Short communication: New analytical approach on (U-Th)/He dating of Fe-hydroxide with an example using goethite from the Amerasian Basin, Arctic Ocean
Abstract. We propose a new analytical approach for (U-Th)/He dating of Fe-hydroxides, that includes sealing samples in quartz ampoules and demonstrates its suitability as a reliable tool for the investigation of geological processes. The (U-Th)/He ages of goethite clasts and vein from Fe- and Mn-oxide mineralization rocks recovered from the slope of Chukchi Borderland in the Amerasia Basin demonstrate remarkable reproducibility, yielding a weighted mean age of 8.6 ± 0.3 Ma (n=4) and 4.8 ± 0.4 Ma (n=2), respectively, providing insights into the Neogene mineralization history of the region. This study also focuses on the sample preparation technique, that might influence the (U-Th)/He ages. Our data indicate that a significant fraction of U can be leached from the goethite during sonication by distilled water which might result in over-dispersed (U-Th)/He ages.
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RC1: 'Comment on egusphere-2024-992', Xiao-Dong Deng, 12 May 2024
Review of: Short communication: New analytical approach on (U-Th)/He dating of Fe-hydroxide with an example using goethite from the Amerasian Basin, Arctic Ocean
By: Olga Valentinovna Yakubovich et al.
I’m glad to see work on improving (U-Th)/He dating protocol for Fe-hydroxides. This manuscript provides a new sample loading method and the sample preparation technique for Fe-hydroxides (U-Th)/He dating. The sealing samples in quartz ampoules method is useful to solve the loss of U and Th during the sample helium extraction by heating processes. For sample preparation, authors proposed that U may be leached from goethite during the sonication by distilled water and crushing processes. This finding is helpful and may be beneficial for (U-Th)/He dating sample preparation. I think it would fit in this journal. Below I list my comments in more detail and hope it will help the authors improve this paper because I think they provide interesting protocol to contribute (U-Th)/He dating for Fe-hydroxides.
The application of the sealing samples in quartz ampoules
This sample loading method significantly solve the U and Th loss problem, but also bring two disadvantages: one is the relatively high blank of the quartz ampoule, the other is the release of helium gas controlled by helium diffusion through the quartz ampoules. My concern is authors proposed that the characters of helium released from quartz ampoules at different heating stages can reflect the helium retentivity in goethite samples. Although helium easily diffuses through the quartz, the helium release patterns in Fig.5 show all samples (except DG 1015) have similar patterns and release helium at high temperatures (350-1250 °C). This is obviously different with the traditional goethite helium release pattern showing the peak of helium release commonly occur during phase transformation at ca.300 °C. In my opinion, these helium release patterns may be controlled by the helium diffusion parameters of quartz. In this case, I suggest authors may fill helium gas in quartz ampoules and test the helium diffusion characters in quartz. This may help to determine whether the helium release patterns reflect the gas release characters of goethite sample or quartz ampoule, because it is useful to understand helium loss, recrystallisation, and fluid and mineral inclusions in goethite samples. For DG 1015 sample, the helium release pattern is obviously different with others in figure 5, and shows the peak of helium release occur at ~350°C, this is consistent with the goethite commonly release the helium during the phase transformation at ~300°C. In this case, this quartz ampoules may be broken during the heating or improperly sealed quartz ampoule. Finally, I expect authors add all the helium release patterns in figure 5, especially DG969 that show the oldest age in the same goethite clast, then we can identify the reason why some grains yield the dispersed ages.
Goethite U loss during the sonication by distilled water
Investigating goethite U loss during the sonication by distilled water is a very welcomed contribution. Authors design two sonication washing stages during the sample preparation and the results show up to 7.8% U in one vein grain sample can be leached during sonication in the distilled water. As we known, the minerals in this sample are possibly stable in water, because it collected from dredge haul DR7 at 3400 m water depth. In this case, I wonder whether the U is the dissolved ion or occur as suspended colloidal particles in the distilled water after the sonication. In the first case, the U loss may significantly affect the dating results, and this suggests (U-Th)/He isotopic system may be open in water. The second case may not hinder us to obtain the reliable ages. Numerous studies reveal that sonication is commonly useful to isolate smaller size aggregates, but this physical disintegration produce lots of fine particles in water, even after centrifugation and filtration, it also incomplete removal of fine suspended colloidal particles from the supernatant prior to element analysis. The results of leaching experiments in Table 3 show that less than 0.3% U loss occur at first sonication stage, while most of U (up to 7.8%) loss at second stage involved to the crushed and sonicated grain sample. Is it possible that the sonication of the crushed sample increase the suspended colloidal particles in water? In addition, why the U loss of crushed samples with two (first and second) steps is much higher than those with only second step? Authors also find the leachates contain amount of Mn and Fe, why do not list all these results? If possible, the relationship between U-Th and Mn-Fe may provide some information on whether U and Th are from dissolved ions or fine suspended particles.
General comments
80: Table 1 show that minerals in DR7 are composed of goethite, birnessite, and other minerals, this means the sample used to (U-Th)/He dating are not pure goethite, how to evaluate the (U-Th)/He dating results from these mineral mixtures.
90: Figure 2 show multiple generation of Fe-Mn-hydroxides or oxides and small monazite may occur in this sample, these mineral associations could affect the dating results.
115: All the samples used to (U-Th)/He dating were not washed. Why do not carry out some washed samples to test they may yield systematic older ages?
120: How do you seal the quartz ampoule? If use oxyhydrogen flame, is it possible to heat the ampoule and cause the loss of helium?
170, 205-210: The U and Th is not only derived from mineral surface adsorption but also from some suspended particles. Authors may list the concentration of other elements, such as Mn, Fe, Al, P….
216, 218: “Qu ampoule” -quartz ampoule
234-239: How about the genesis of dating sample, are they precipitated from the sea water or hydrothermal fluid?
Fig.5 Why only plot six goethite fragments patterns? How about the patterns of ID 966 and 969?
249-260: Sample DG969 has the lowest U, but sample DG 1031 has the highest U, the two samples yield erroneously old and reproducible ages. In this case, it cannot be simply explained by U-loss of these samples.
Citation: https://doi.org/10.5194/egusphere-2024-992-RC1 -
AC1: 'Reply on RC1', Olga Yakubovich, 28 Jun 2024
Dear Xiao-Dong Deng,
Thank you very much for the valuable comments and suggestions on how to improve the manuscript.
There are two main concerns that were highlighted in your comment: (1) the effect of the quartz ampoule on the He release pattern, and (2) goethite U loss during the sonication by the distilled water.
Quartz ampoule
We agree that helium release pattern is controlled by helium diffusion parameters of the quartz ampoule. Experiments with the empty quartz ampoules – which still have some residual atmospheric helium (the residual pressure is ~ 10-3 torr, which is roughly 10-11 cm3 STP of He) – revealed that helium is completely released at temperatures of 350 °C from the ampoules after 30 minutes of heating. Thus, complex He release patterns of the goethite samples cannot be easily explained by the ampoule effect. As an additional argument that these patterns are not the methodological “artefacts” we can refer to our experience in (U-Th)/He dating of pyrite and native gold (Yakubovich et al., 2020; 2014) and Pt-He dating of isoferroplatinum by the same technique (Yakubovich, 2013) – for each of these minerals He release patterns via quartz ampoule differs from what we have seen in goethite.
It is very interesting question, indeed, why during the phase transformation He is not completely released from the goethite. But the close type effect we have observed for isoferroplatinum. Under strong mechanical deformations Pt-alloy change its crystal structure from Pm-3m to Fm-3m, but no signs of He-loss were observed (Mochalov et al., 2019). It might be somehow related to the velocity of the transformation process, but it is only a hypothesis.
Goethite U loss during the sonication
It is very hard to prove that U, that we have measured in the solutions, is the dissolved U. We cannot exclude the presence of the tiny floating goethite particles in the solutions.
Our main arguments are:
(1) shift of the Th/U ratio in the solution relative to the Th/U ratio of the residual goethite (from 0.06 to 3; Table 3). Tiny floating particles of goethite are likely to have the same Th/U ratio as a goethite itself;
(2) leaching experiments of the goethite from the same DR-7-001 sample revealed that 2–3% of U is leached by 1M acetic acid/Na acetate buffer (pH 5), while Th is not (see the Figure attached). The same patterns were observed in Fe- and Mn-oxides by Konstantinova et al., 2018; Koschinsky and Hein, 2003.
However, the presence of Fe and Mn in the analyzed solutions and the condition of the nebulizer of the ICP-MS after these measurements are in a favor that at least some of the U and Th is related to floating goethite particles. We will add it to the discussion in the text of the manuscript.
Answer on General comments
80: Table 1 show that minerals in DR7 are composed of goethite, birnessite, and other minerals, this means the sample used to (U-Th)/He dating are not pure goethite, how to evaluate the (U-Th)/He dating results from these mineral mixtures.
For (U-Th)/He dating we used cement of the sample DR7-001, which according to XRD data is mainly ( >95%) goethite. For dating we picked the grains without visible under binocular microscope inclusions of other minerals. So likely, the purity of the analyzed samples was higher than 95%. The presence of other phases, of course, could affect the (U-Th)/He ages, but we assume that their impact is insignificant.
90: Figure 2 show multiple generation of Fe-Mn-hydroxides or oxides and small monazite may occur in this sample, these mineral associations could affect the dating results.
Agree, we will add this to the discussion
115: All the samples used to (U-Th)/He dating were not washed. Why do not carry out some washed samples to test they may yield systematic older ages?
Agree, but we are not able to do these measurements with the same material at the moment, but we will do it with other samples in the future.
120: How do you seal the quartz ampoule? If use oxyhydrogen flame, is it possible to heat the ampoule and cause the loss of helium?
The flame is narrow, quartz ampoule is relatively long, thermal conductivity of quartz is low, sealing is fast. We have done the (U-Th)/He dating of the sealed Durango Apatite earlier. The (U-Th)/He age was correct.
170, 205-210: The U and Th is not only derived from mineral surface adsorption but also from some suspended particles. Authors may list the concentration of other elements, such as Mn, Fe, Al, P….
We haven’t calibrated the ICP-MS on all these elements prior the measurements (only on U and Th). Thus, these values are only semiquantitive.
216, 218: “Qu ampoule” -quartz ampoule
Corrected
234-239: How about the genesis of dating sample, are they precipitated from the sea water or hydrothermal fluid?
These samples a likely of hydrothermal origin. Based on their morphology, textures and chemistry they are distinct from hydrogenic Fe-Mn crust. The origin of these Fe-oxides will be discussed in the detail in the article by Hein et al (under preparation).
Fig.5 Why only plot six goethite fragments patterns? How about the patterns of ID 966 and 969?
The reason why we excluded these two samples is that their heating protocol was different. But we will add them to the figure with this notion.
249-260: Sample DG969 has the lowest U, but sample DG 1031 has the highest U, the two samples yield erroneously old and reproducible ages. In this case, it cannot be simply explained by U-loss of these samples.
Agree, will add this to the discussion.
Thanks again for the comments,
Regards, Olga
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AC1: 'Reply on RC1', Olga Yakubovich, 28 Jun 2024
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RC2: 'Comment on egusphere-2024-992', Florian Hofmann, 15 May 2024
This manuscript suggests a new analytical approach to (U-Th)/He dating of goethite samples using the single-aliquot method. Since U can be volatilized during laser heating, the authors propose to encapsulate goethite samples in quartz ampoules which will let helium diffuse through but will retain any volatilized U. The U is then recovered by dissolving the glass ampoule together with its contents. This method is applied to submarine iron oxide nodules.
I appreciate seeing a new approach for dealing with U volatilization in iron oxides. This is a promising technique that could provide a way to deal with U volatilization without having to make changes to the extraction line, which makes the application easier and more accessible.
While the application of (U-Th)/He dating to submarine nodules is exciting, I am wondering whether this was the best choice of sample for an exploratory study into new analytical techniques. In my opinion, samples with more independent age constraints would have been a better test of this method since the efficacy of this method could be verified. However, these samples are sufficiently documented to be a good first demonstration of the technique.
My major analytical concern about this approach is whether the samples can be completely degassed and whether the resulting helium can be completely extracted from the ampoule. Incomplete extraction of helium would lead to an underestimation of the (U-Th)/He age. Have any of the samples been heated to temperatures higher than 1150 °C? Some of the samples seem to still outgas a significant amount of helium in the 1000-1150 °C range (Fig. 5). Even though the amount of helium extracted declines in the 1150 °C step, there could still be helium in more retentive domains. Bimodal release patterns are not unusual and are dependent on the distribution of diffusional domain sizes in the samples.
Samples could be tested for complete extraction by heating previously heated samples to even higher temperatures or melting them with flux to determine whether more helium is released. If the material is homogeneous in age and eU concentration, a separate set of samples could be weighed and completely degassed using flux and/or high temperatures. The 4He concentrations obtained from samples heated in the quartz ampoules could be compared against the concentrations from the reference samples to check for complete extraction.
Another issue is the complete recovery of the volatilized U, which could equally affect the calculated ages. I suggest dissolving a number of unheated fragments of each structure and determining U and Th concentrations. The obtained U and Th concentrations of the heated samples can then be compared against these reference values. This would also demonstrate how homogeneous the sample is with regard to parent nuclides.
I think that the efficacy of this method is somewhat overstated. At several points throughout the manuscript, the authors claim that their samples show “no signs of overdispersion”, yet they report an MSWD of 3.4, which is very clearly overdispersed. This should be addressed. Adding additional replicate measurements for the same material and re-evaluating the MSWDs of these larger datasets might increase the significance of the findings.
I think that this manuscript is a good exploration of the quartz ampoule technique for goethite. However, several aspects of the technique, such as complete extraction of helium and full recovery of any volatilized U, need to be demonstrated with a larger number of replicate analyses of samples with more independent age constraints before the method can be claimed to be robust. Such additional analyses could be incorporated into this manuscript to make the conclusions more significant, but if this is deemed to be beyond the scope of this study, I would recommend scaling down the claims as to the robustness and reproducibility of the method.
Overall, I think this manuscript is a good fit for a GChron short communication. This manuscript is a great contribution towards establishing the quartz ampoule degassing method and will be interesting to the (U-Th)/He community at large. However, the issues brought up here and by other reviewers should be addressed before publication. I, therefore, recommend that the manuscript be accepted subject to minor revisions.
Detailed comments:
Line 17: I think “remarkable reproducibility” is overstating the reproducibility of n=4 with MSDW=3.4 and n=2 with MSDW=1.4.
Line 19: Insert “a” between “that” and “significant”.
Line 30: Change “mineral” to “minerals”.
Line 32: What is “sufficient” retentivity? Published studies generally indicate retention in the range of 80-98% for comparable material at Earth-surface conditions. I am wondering whether these samples are comparable to the supergene samples for which diffusion experiments have been performed so far. These samples could benefit from 4He/3He diffusion experiments to determine the retention of radiogenic helium.
Lines 34-36: Since these are not exhaustive lists, I suggest adding “e.g.” to these references.
Line 36: Change “successful” to “successfully”.
Line 44: Add citations to support that these methods are “typically” applied.
Lines 74-75: What is the observation that dark-colored goethite has better crystallinity based on? XRD/SEM/both?
Line 108: 5% HNO3 alone might not be enough to stabilize Th in the solution. Typically, a small amount of HF is added to solutions to stabilize Th during the measurement process. Th fractionation can occur in the ICP-MS tubing if it isn’t sufficiently stabilized.
Line 126: Was the sensitivity based solely on comparison with a mineral standard? Was there any internal standardization with an air or other gaseous standard from a tank?
Line 127: What is the repeatability of the 10 measurements of the mineral standard? The manuscript never seems to come back to this, and I don’t see these measurements in the results tables. Please report these results.
Line 131: I’m not sure what “in the camera of the mass spectrometer” refers to. Please clarify.
Line 214, Line 285: The results have MSDWs of 3.4 and 1.4, respectively. These values are both >1, which counts as overdispersed. A value of 1.4 is reasonably close to a univariate normal distribution to claim that the results aren’t overdispersed, but that is for n=2. However, an MSWD value of 3.4 is definitely overdispersed at n=4. (U-Th)/He ages in a variety of minerals are typically overdispersed, so that’s not unusual. It is most likely an effect of zonation and our incomplete understanding of the (U-Th)/He system rather than the measurement process, as long as U volatilization during heating is avoided.
Line 216: The abbreviation “Qu ampoule” isn’t defined. I suggest just using “quartz ampoule” instead. There are several more uses in the same paragraph that should be changed as well.
Line 223: I wouldn’t characterize the O2 tank as “dangerously explosive”. It is a thick-walled vessel filled significantly below its rated pressure. Since it is filled with pure O2 and no combustible fuel is present in the tank, it does not present an explosion hazard. Mishandling could lead to damage to the mass spectrometer and extraction line, the risk of which can be mitigated by implementing redundancy and fail-safe measures. Overall, the amounts of oxygen are small, comparable to the natural amount of oxygen present in the air inside a vented extraction line. Pressurized oxygen tanks filled to much higher pressures are common in labs and other settings and can be safely used if handled and stored correctly.
Figure 5: Not all of the performed analyses are shown here. Please add the missing patterns. Additionally, the use of colors to distinguish between sets of samples might present a barrier to accessibility. I suggest using a different symbol for each sample/set of analyses and an accessible color scheme to help visual interpretation. Some of the values given in this figure use dots as decimal symbols while others use commas. This should be consistently applied to the rest of the manuscript as well.
Figure 6: While age-eU plots are a good tool to diagnose radiation damage effects and U loss, the small number of samples and the small range of eU values (2-3 ppm) make the interpretation of this plot difficult. A larger dataset might reveal patterns. Also, use the same symbols and color scheme as suggested for Fig. 5.
Line 259: The presence of zircon and monazite in the sample might affect the helium release pattern. These phases are likely to be more retentive than goethite and would lead to some of the helium being released at higher temperatures. Having the helium release patterns of these samples in Fig. 5 would be helpful. A possible strategy to prevent inference from mineral inclusions would be to pre-screen samples with microCT and only pick inclusion-free grains for analysis.
Line 271: How are you distinguishing between recrystallization of existing material and newly formed material from a second period of goethite crystallization?
Citation: https://doi.org/10.5194/egusphere-2024-992-RC2 -
AC2: 'Reply on RC2', Olga Yakubovich, 28 Jun 2024
Dear Florian Hofmann,
Thank you very much for the valuable comments and suggestions on how to improve the manuscript.
There were several major concerns that you focus in your comment: (1) the choice of the object for testing the technique; (2) the completeness of He release from the ampoule; (3) complete recovery of U from the samples; (4) robustness of the technique.
Object
We agree that the samples with independent age constrains might be better to analyze in order to verify the reliability of the technique. But at the same time, the deep underwater goethite samples have some obvious advantage – they are surrounded by steady and low temperature (around 0°C) environment. It excludes any continuous thermal loss of He by the samples during their geological history, one of the factors that affected (U-Th)/He ages.
Helium release
Firstly, I would like to note, that diffusion of He through the quartz ampoule is fast at 1150 °C. So, the main question is if 1150 °C is enough for complete He release from the goethite grain?
During the samples heating, we are continuously measuring the He concentration in the mass-spectrometer chamber. Thus, the valve that connects furnace and analyzer is open during the experiment. Therefore, we are able to measure the total amount of He released at the present temperature, as well as the way He was released (see Fig 1 and 2 in attachments). The case that you refer in the comment is usually observed as the steady growth of the He concentration in the chamber of mass-spectrometer. We have not observed it for these samples – there were no further growth of He signal. Given that I would suggest that 1150°C in our case was enough, but for future experiments I would definitely increase the upper limit for at least for 1250°C.
Uranium recovery
We have some bulk data from the ICP-MS measurements of the HLY0805 samples. Uranium content is in range from 1.5 – 2.5 ppm (n=5). Uranium concentrations in the heated in the quartz ampoule goethite grains are from 1.4 to 2.8 ppm (Table 2). We do not have enough material to perform the type of analyses that you have recommended, but we do not see any signs of U-loss, with the exception of sample ID 969, which had shown the lowest U content (Table 2).
Robustness of the technique
Agree, we do not have enough data to discuss overdispersion, and comparison of the technique with other methodological approaches. We will follow your suggestion in the discussion section of the manuscript.
Answer to the detailed comments:
Line 17: I think “remarkable reproducibility” is overstating the reproducibility of n=4 with MSDW=3.4 and n=2 with MSDW=1.4.
agree
Line 19: Insert “a” between “that” and “significant”.
corrected
Line 30: Change “mineral” to “minerals”.
corrected
Line 32: What is “sufficient” retentivity? Published studies generally indicate retention in the range of 80-98% for comparable material at Earth-surface conditions. I am wondering whether these samples are comparable to the supergene samples for which diffusion experiments have been performed so far. These samples could benefit from 4He/3He diffusion experiments to determine the retention of radiogenic helium.
They are comparable based on their chemistry and crystallography. But they still can benefit from 4He/3Heexperiments as the expected to have lower impact of thermal He loss.
Lines 34-36: Since these are not exhaustive lists, I suggest adding “e.g.” to these references.
agree
Line 36: Change “successful” to “successfully”.
corrected
Line 44: Add citations to support that these methods are “typically” applied.
ok
Lines 74-75: What is the observation that dark-colored goethite has better crystallinity based on? XRD/SEM/both?
Yes, based on XRD data mainly. Dark one has better crystallinity.
Line 108: 5% HNO3 alone might not be enough to stabilize Th in the solution. Typically, a small amount of HF is added to solutions to stabilize Th during the measurement process. Th fractionation can occur in the ICP-MS tubing if it isn’t sufficiently stabilized.
We have not added HF to the solution. How strong is this effect? We measure the standard Th and U calibration solutions in 5% HNO3 without additional HF as well. The Th/U ratio is quite stable during the measurement set. Thus, I don’t expect it to dramatically change the results of the leaching experiments.
Line 126: Was the sensitivity based solely on comparison with a mineral standard? Was there any internal standardization with an air or other gaseous standard from a tank?
We calibrate the mass-spectrometer only by mineral standards. We use two of them: Knyaghinya meteorite and platinum from the Santiago River. We also constantly measure (U-Th)/He age of the Durango apatite.
Line 127: What is the repeatability of the 10 measurements of the mineral standard? The manuscript never seems to come back to this, and I don’t see these measurements in the results tables. Please report these results.
I mean that the MS is calibrated by the mineral standard. Ten measurements mean that we obtained the average value of He content based on 10 scans (as far as I know some labs do 20 scans). I have rewritten this part to make it clear. I don’t think that these results should be presented in the manuscript, but I can provide them in a xls file if necessary. The original file from the MS looks like in Fig. 2 (attached). Or it is the question regarding the general performance of the MS? I guess that the best impression of the reproducibility of the MS can be find here: https://onlinelibrary.wiley.com/doi/abs/10.1111/ggr.12502
Line 131: I’m not sure what “in the camera of the mass spectrometer” refers to. Please clarify.
Chamber
Line 214, Line 285: The results have MSDWs of 3.4 and 1.4, respectively. These values are both >1, which counts as overdispersed. A value of 1.4 is reasonably close to a univariate normal distribution to claim that the results aren’t overdispersed, but that is for n=2. However, an MSWD value of 3.4 is definitely overdispersed at n=4. (U-Th)/He ages in a variety of minerals are typically overdispersed, so that’s not unusual. It is most likely an effect of zonation and our incomplete understanding of the (U-Th)/He system rather than the measurement process, as long as U volatilization during heating is avoided.
Agree, corrected
Line 216: The abbreviation “Qu ampoule” isn’t defined. I suggest just using “quartz ampoule” instead. There are several more uses in the same paragraph that should be changed as well.
Ok
Line 223: I wouldn’t characterize the O2 tank as “dangerously explosive”. It is a thick-walled vessel filled significantly below its rated pressure. Since it is filled with pure O2 and no combustible fuel is present in the tank, it does not present an explosion hazard. Mishandling could lead to damage to the mass spectrometer and extraction line, the risk of which can be mitigated by implementing redundancy and fail-safe measures. Overall, the amounts of oxygen are small, comparable to the natural amount of oxygen present in the air inside a vented extraction line. Pressurized oxygen tanks filled to much higher pressures are common in labs and other settings and can be safely used if handled and stored correctly.
Thank you! I had a wrong impression about the oxygen tank! Corrected in the text.
Figure 5: Not all of the performed analyses are shown here. Please add the missing patterns. Additionally, the use of colors to distinguish between sets of samples might present a barrier to accessibility. I suggest using a different symbol for each sample/set of analyses and an accessible color scheme to help visual interpretation. Some of the values given in this figure use dots as decimal symbols while others use commas. This should be consistently applied to the rest of the manuscript as well.
Ok, agree, it would be better
Figure 6: While age-eU plots are a good tool to diagnose radiation damage effects and U loss, the small number of samples and the small range of eU values (2-3 ppm) make the interpretation of this plot difficult. A larger dataset might reveal patterns. Also, use the same symbols and color scheme as suggested for Fig. 5.
Agree
Line 259: The presence of zircon and monazite in the sample might affect the helium release pattern. These phases are likely to be more retentive than goethite and would lead to some of the helium being released at higher temperatures. Having the helium release patterns of these samples in Fig. 5 would be helpful. A possible strategy to prevent inference from mineral inclusions would be to pre-screen samples with microCT and only pick inclusion-free grains for analysis.
Agree, but the microCT is a next methodological step. We have added all samples to the Fig.5.
Line 271: How are you distinguishing between recrystallization of existing material and newly formed material from a second period of goethite crystallization?
We do not. Both options are possible. Made it clearer in the text.
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AC2: 'Reply on RC2', Olga Yakubovich, 28 Jun 2024
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RC3: 'Comment on egusphere-2024-992', Hevelyn S. Monteiro, 22 May 2024
General comments
Yakubovich et al. propose an alternative analytical approach for dating goethite by the (U-Th)/He method. To avoid U-loss during heating for He extraction, multiple grains (1-7 mg) are encapsulated in vacuum-sealed quartz ampoules. The quartz ampoules containing the grains are then dissolved in a solution of aqua regia + HF + HClO4, and U and Th isotopes are analyzed via isotope dilution method.
Two samples of Fe-Mn-crusts collected from the Egiazarov Trough, Amerasian Basin, Artic Ocean, were used to test this alternative methodology. The Fe-Mn-crusts are a mixture of goethite and Mn-oxides and other phases (e.g., lepidocrocite, feroxyhyte, quartz, feldspars, clays, clinochlore, etc.). Goethites from Fe-Mn-crusts are porous and poorly crystallized. Yakubovich et al. propose that U and Th are leached from the samples during ultrasonication and interpret the variable Th/Uleachate and Th/Ugrain values as evidence for preferential U remobilization.
It is encouraging to see researchers interested in further refining the (U-Th)/He method applied to the study of supergene phases, as obtaining meaningful ages for these complex materials can be challenging and time consuming. Therefore, Yakubovich et al.’s proposal of an alternative approach to measure the ages of goethites is worth pursuing. However, before the manuscript can be accepted for publication, some important aspects of the work need to be assessed:
- suitability of chosen samples for (U-Th)/He geochronology;
- interpretation of leaching experiments results;
- interpretation of the (U-Th)/He results.
The successful application of a geochronological method depends on sample selection. Since 1999, ~ 2600 goethite (U-Th)/He ages have been produced by four laboratories. The most prolific laboratory, the noble gas facility at Caltech, is responsible for ~2000 of those results. The most important criterion to ensure reliable results is to select pure well crystallized goethite. Powdery goethite and poorly crystalline goethite intergrown with other minerals should be carefully avoided. Early in our studies, to test whether poorly crystalline goethites produce reliable results, we carried out analyses of co-existing crystalline (generally black or brown dense goethite samples) and poorly crystalline (generally yellow, porous and powdery goethite) samples. The combined (U-Th)/He-4He/3He experiments show that the latter are unsuitable for geochronology and should be avoided. Yakubovich et al. chose to work with impure, poorly crystalline goethites, the very same type of goethite that should be avoided in (U-Th)/He geochronology. Thus, their results, as interesting as they are, are useful to confirm that impure poorly crystalline goethites are not suitable for (U-Th)/He geochronology. Their findings cannot be generalized to all goethites, particularly to the types of goethites carefully selected for geochronology by most laboratories and analysts.
As demonstrated in this study, impure poorly crystalline goethite suffers from several limitations. One of these limitations is that U and Th may occur both within the goethite crystalline structure but also adsorbed or as a surface precipitate on the high surface area fine grained porous phases (porous goethites, manganese oxides, clays, silica minerals). Most He produced by U and Th adsorbed or precipitated on mineral surfaces will be lost, and the analytical results will provide no information on the sample’s age. In addition, U and Th on those surface sites should be readily desorbed and removed by aqueous solutions, both during their geological history and during sample preparation and cleaning procedures, further challenging the interpretation of results obtained from this type of material. Yakubovich et al.’s results clearly show why these types of samples should be avoided if the major objective is to date a goethite occurrence.
In the interpretation of their (U-Th)/He results, Yakubovich et al. raise the possibility that the Fe-Mn-crusts were formed deep in the Earth’s crust and then acquired their ages after cooling below a certain closure temperature. There is plenty of evidence suggesting the hydrogenetic origin of the Artic Ocean Fe-Mn-crusts (Hein et al., 2017). To avoid misinterpretation of the results, the authors should focus their interpretation of the results in the context of changing environmental conditions at the contact between detrital sediments and sea water. More importantly, the fact the samples are made of a mixture of minerals, not pure goethite, precludes a robust interpretation on the precipitation ages of goethites. Finally, based on our current knowledge about the helium retentivity in powdery, poorly crystalline goethites, the samples analyzed in this work probably lost >70% of their helium, a challenge in interpreting results obtained from poorly crystalline mineral assemblages. These limitations should be incorporated in the discussion.
It would be beneficial if the authors were to compare the results obtained in poorly crystalline and impure samples with an additional set of results obtained from pure crystalline goethite using the exact same analytical approach. This comparative analysis would strengthen the credibility of the approach implemented in this study and provide a more comprehensive understanding of its applicability. Finally, the complex nature of the Fe-Mn-crusts investigated by Yakubovich et al. demands a more detailed characterization of the paragenetic relationships between the different minerals in the samples to determine exactly which phase(s) was(were) dated.
By addressing these points, the authors can enhance the overall quality and impact of their research, making a significant contribution to the field of geochronology.
Specific comments
Lines 37-39: How do Artic Ocean Fe-Mn-crusts (especially DR7-001) differ from Pacific Ocean Fe-Mn-crusts studied by Basu et al. (2006)?
Lines 64-65: What is the percentage of goethite and Mn-oxides in these samples? The indicated values of > 25%, 5-25%, <5% are not sufficient information. The authors should be able to quantify the percentages of the main mineral phases in the samples analyzed by XRD. The reader would benefit from a figure illustrating the X-ray diffraction patterns obtained for each type of material.
Line 115-118: What was the sampling strategy adopted by the researchers? The dark-brown clasts were selected from sample DR7-002 and the yellow-brown vein grains come from sample DR7-001? If so, what is the relationship between the two types of grains analyzed?
Line 116: If (U-Th)/He results were obtained only for sample DR7-001, is this sample also a breccia?
Line 127: Please, tabulate 4He amounts measured for calibration standard.
Lines 130-131: 4He amounts are retrieved from multiple grains (as shown in Figure 4), not an individual goethite grain.
Lines 182-183: Would it be possible that heating of the grain during sealing of the quartz ampoule caused some U-volatilization as well as He loss?
Lines 195-211: Yakubovich et al. claim that U-loss during sample ultrasonication results in ages that are overly dispersed. In most environments, sorption of U and Th ions/complexes will be less favorable compared to other abundant dissolved metal ions/complexes, suggesting only a minor contribution to the U-Th-He system. U and Th incorporated in the goethite structure during crystal growth would dominate the production of He in a goethite grain. However, the samples used in this study are not pure goethites. Therefore, U and Th can be adsorbed to surfaces exhibiting very distinct characteristics, presenting different adsorption-desorption reaction rates. Additionally, microporosity increases both the surface area of minerals available for sorption reactions and the number of pores to which ions migrate, the sites where they are trapped, and the ease of desorption during ultrasonication. These phenomena may be particularly important in the multi-mineral grains analyzed by Yakubovich et al., but not as significant for well-crystallized, dense goethite masses commonly selected for (U-Th)/He dating. Cleaning mm- to µm-sized goethite grains in an ultrasonic bath at room temperature for 10-15 minutes aims to facilitate the removal of loosely attached fine particles of goethite and other phases (e.g., adsorbed clays or dust) from the surface of the grains to minimize interference on He, U, and Th measurements of the target phase.
Line 214: The calculated MSWDs for the analyses of dark and vein “goethites” show otherwise.
Lines 215-217: Laser heating of encapsulated goethite grain at 950 °C for 6 min ensures complete 4He extraction and no U-volatilization from samples suitable for (U-Th)/He geochronology. This is routinely done in laboratories around the world. Now, the quartz ampoule method potentially offers the opportunity to perform helium diffusion studies if it can help to avoid phase breakdown during step-heat experiments.
Lines 238-293: What are the complications in using the quartz ampoule method to derive He diffusion parameters?
Lines 239-240: I do not understand this statement.
No correlation exists between He release pattern and (U-Th)/He ages, reflecting an insignificant thermal loss of 4He.
Line 266: Goethite is the most thermodynamically stable iron-oxyhydroxide in the near surface environment. It partially dissolves during interaction with acidic solutions, but its thermodynamic stability does not change. For instance, goethites as old as 70 Ma have been found near the surface of massive sulfide deposits.
Technical corrections
Title: The word goethite in the title is misleading. The samples analyzed by Yakubovich et al. are not pure goethite. In addition, this method can be applied to other phases as well. I suggest changing the title to “A new analytical approach on (U-Th)/He dating by sample encapsulation in quartz ampoules under vacuum: application to Fe-Mn-crusts, Amerasian Basin, Arctic Ocean”.
Figure 1: Enlarge sample photos, indicate on the photo the different types of material (dendrites, botryoidal, glassy, cellular-like, etc.), and show the areas sampled for geochronology and X-ray diffraction analysis.
Figure 2: (B) replace massive with film or cement.
Figure 5: HD+vs Temperature and He vs Temperature: What do the patterns of HD+ and He release with temperature teach us about sample behavior under vacuum?
Figure 5: These helium release patterns do not say anything about goethite behavior because the grains are not pure goethite.
Line 14: replace Fe-hydroxides with Fe-(oxyhydr)oxides.
Line 19-20: Here are some potentially contributors for overdispersion of (U-Th)/He ages: multiple generations of Fe-(oxyhydr)oxides, He diffusion from poorly crystalline material, alpha recoil, mineral inclusions, and the presence of fine particles attached to the grain surface. The sentence needs to be rewritten.
Line 30-31: Suggestion: “Goethite is one of the most common Fe-(oxy)hydroxide minerals formed during the hydrolyzation of rocks, making it a desirable mineral for dating various surface and subsurface geological processes.”
Lines 34, 36: replace successful with successfully.
Line 45: Shuster and Farley (2005) discuss diffusion experiment results obtained for quartz. Is there an equivalent citation pertinent to goethite?
Line 75: What do replacements mean?
Line 131: camera == chamber?
Line 131: How much HD+ was detected during the step-heat experiment? Does the amount of HD+ differ between the types of samples?
Lines 215, 218, 220: Qu == quartz?
Citation: https://doi.org/10.5194/egusphere-2024-992-RC3 -
AC3: 'Reply on RC3', Olga Yakubovich, 28 Jun 2024
Dear Hevelyn S. Monteiro,
Thank you very much for the valuable comments and suggestions on how to improve the manuscript.
The major concern that was highlighted in your comment is the suitability of the chosen samples for (U-Th)/He geochronology. The quality of the studied material has a direct impact on the interpretation of the leaching experiments and (U-Th)/He ages.
We agree, that hydrogenetic Fe-Mn crusts are not suitable for (U-Th)/He geochronology due to the significant content of extraterrestrial He-rich dust, their high porosity and specific surface area, as well as poorly crystalline main minerals: vernadite (also called d-MnO2), and X-ray amorphous iron oxyhydroxide (Hein et al., 2000).
The samples that we have analyzed are hydrothermal underwater Fe-hydroxides, not the fragments of the hydrogenetic Fe-Mn crust described by Hein et al., 2017. We are not discussing in this manuscript the origin of these samples as it is a subject of the article which is under preparation by Hein with coauthors.
The samples mainly consist of pure crystalline goethite (>95%). Based on XRD data the darker grains have higher crystallinity than the yellowish ones. Therefore, they might be considered as a standard material which is used for (U-Th)/He dating. The sequential leaching experiments reflect likely presence of some sorbted U on the surface of the goethite grains, and the interpretation of the (U-Th)/He can be done based on the data available for hypergenic crystalline goethite.
Answer to Specific comments
Lines 37-39: How do Artic Ocean Fe-Mn-crusts (especially DR7-001) differ from Pacific Ocean Fe-Mn-crusts studied by Basu et al. (2006)?
The sample DR7-001 is not described as Fe-Mn crust. Based on its morphology, texture, predominantly Fe content with low rare elements concentrations as well as higher crystallinity goethite, studied samples likely has a hydrothermal origin (Hein, J.R. et al 2000. Handbook of Marine Mineral Deposits. p. 239-279).
Lines 64-65: What is the percentage of goethite and Mn-oxides in these samples? The indicated values of > 25%, 5-25%, <5% are not sufficient information. The authors should be able to quantify the percentages of the main mineral phases in the samples analyzed by XRD. The reader would benefit from a figure illustrating the X-ray diffraction patterns obtained for each type of material.
We will add the XRD images and provide additional information
Line 115-118: What was the sampling strategy adopted by the researchers? The dark-brown clasts were selected from sample DR7-002 and the yellow-brown vein grains come from sample DR7-001? If so, what is the relationship between the two types of grains analyzed?
All fragments of goethite mineralization were manually extracted only from DR7-001 sample: two dark-brown clasts of the breccia and yellow-brown vein. Subsamples from the yellow-brown vein material and from dark-brown gains were treated as separate samples. We suppose two different types of goethite is associated to different crystallinity.
Line 116: If (U-Th)/He results were obtained only for sample DR7-001, is this sample also a breccia?
Yes, it is
Line 127: Please, tabulate 4He amounts measured for calibration standard.
Ok
Lines 130-131: 4He amounts are retrieved from multiple grains (as shown in Figure 4), not an individual goethite grain.
Corrected
Lines 182-183: Would it be possible that heating of the grain during sealing of the quartz ampoule caused some U-volatilization as well as He loss?
The flame is narrow, quartz ampoule is relatively long, thermal conductivity of quartz is low, sealing is fast. We have done the (U-Th)/He dating of the sealed Durango Apatite earlier. The (U-Th)/He age was correct.
Lines 195-211: Yakubovich et al. claim that U-loss during sample ultrasonication results in ages that are overly dispersed. In most environments, sorption of U and Th ions/complexes will be less favorable compared to other abundant dissolved metal ions/complexes, suggesting only a minor contribution to the U-Th-He system. U and Th incorporated in the goethite structure during crystal growth would dominate the production of He in a goethite grain. However, the samples used in this study are not pure goethites. Therefore, U and Th can be adsorbed to surfaces exhibiting very distinct characteristics, presenting different adsorption-desorption reaction rates. Additionally, microporosity increases both the surface area of minerals available for sorption reactions and the number of pores to which ions migrate, the sites where they are trapped, and the ease of desorption during ultrasonication. These phenomena may be particularly important in the multi-mineral grains analyzed by Yakubovich et al., but not as significant for well-crystallized, dense goethite masses commonly selected for (U-Th)/He dating. Cleaning mm- to µm-sized goethite grains in an ultrasonic bath at room temperature for 10-15 minutes aims to facilitate the removal of loosely attached fine particles of goethite and other phases (e.g., adsorbed clays or dust) from the surface of the grains to minimize interference on He, U, and Th measurements of the target phase.
Agree, U-loss might be significant only for multi-mineral grains. Large grains are unlikely to lose significant amount of U, as their surface to volume ratio is low.
Line 214: The calculated MSWDs for the analyses of dark and vein “goethites” show otherwise.
Corrected
Lines 215-217: Laser heating of encapsulated goethite grain at 950 °C for 6 min ensures complete 4He extraction and no U-volatilization from samples suitable for (U-Th)/He geochronology. This is routinely done in laboratories around the world. Now, the quartz ampoule method potentially offers the opportunity to perform helium diffusion studies if it can help to avoid phase breakdown during step-heat experiments.
From our point of view the ampoule method do not help in establishing the diffusion parameters, as it cannot avoid the phase breakdown. But it solves the problem of U-loss. Though the most laboratories release He from the grains at temperature <1000 °C, there are some papers that suggest that in some cases more intense heating is required (e.g. Wernicke and Lippolt, 1993; Farley and Flowers, 2012; Farley and McKeon, 2015; Hofmann et al., 2020).
Lines 238-293: What are the complications in using the quartz ampoule method to derive He diffusion parameters?
Ampoules are not uniform. They have different surface volume and thickness of the walls. Thus, in order to measure He diffusion parameters of the sample it is necessary firstly to solve this equation for the ampoule. The proper design of the experiment will allow to measure He diffusion parameters of the sample. But will add uncertainties.
Lines 239-240: I do not understand this statement.
No correlation exists between He release pattern and (U-Th)/He ages, reflecting an insignificant thermal loss of 4He.
Rewritten
Line 266: Goethite is the most thermodynamically stable iron-oxyhydroxide in the near surface environment. It partially dissolves during interaction with acidic solutions, but its thermodynamic stability does not change. For instance, goethites as old as 70 Ma have been found near the surface of massive sulfide deposits.
Agree
Technical corrections
Title: The word goethite in the title is misleading. The samples analyzed by Yakubovich et al. are not pure goethite. In addition, this method can be applied to other phases as well. I suggest changing the title to “A new analytical approach on (U-Th)/He dating by sample encapsulation in quartz ampoules under vacuum: application to Fe-Mn-crusts, Amerasian Basin, Arctic Ocean”.
Agree, changed to “A new analytical approach on (U-Th)/He dating by sample encapsulation in quartz ampoules under vacuum: application to goethite, Amerasian Basin, Arctic Ocean”.
Figure 1: Enlarge sample photos, indicate on the photo the different types of material (dendrites, botryoidal, glassy, cellular-like, etc.), and show the areas sampled for geochronology and X-ray diffraction analysis.
Corrected
Figure 2: (B) replace massive with film or cement.
Corrected
Figure 5: HD+vs Temperature and He vs Temperature: What do the patterns of HD+ and He release with temperature teach us about sample behavior under vacuum?
Figure 5: These helium release patterns do not say anything about goethite behavior because the grains are not pure goethite.
The sample is > 95% goethite. We have added this to figure caption.
Line 14: replace Fe-hydroxides with Fe-(oxyhydr)oxides.
Corrected
Line 19-20: Here are some potentially contributors for overdispersion of (U-Th)/He ages: multiple generations of Fe-(oxyhydr)oxides, He diffusion from poorly crystalline material, alpha recoil, mineral inclusions, and the presence of fine particles attached to the grain surface. The sentence needs to be rewritten.
Corrected
Line 30-31: Suggestion: “Goethite is one of the most common Fe-(oxy)hydroxide minerals formed during the hydrolyzation of rocks, making it a desirable mineral for dating various surface and subsurface geological processes.”
Thanks, corrected
Lines 34, 36: replace successful with successfully.
Corrected
Line 45: Shuster and Farley (2005) discuss diffusion experiment results obtained for quartz. Is there an equivalent citation pertinent to goethite?
Corrected
Line 75: What do replacements mean?
Newly formed minerals
Line 131: camera == chamber?
Corrected
Line 131: How much HD+ was detected during the step-heat experiment? Does the amount of HD+ differ between the types of samples?
Yes. Most of the hydrogen was observed at the temperatures of 350 C, which presence in the chamber of MS is likely related to transformation of goethite to hematite. Nevertheless, the amount of HD for yellowish grains was lower, than for dark ones (we expected the otherwise pattern)
Lines 215, 218, 220: Qu == quartz?
Corrected
Citation: https://doi.org/10.5194/egusphere-2024-992-AC3
Status: closed
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RC1: 'Comment on egusphere-2024-992', Xiao-Dong Deng, 12 May 2024
Review of: Short communication: New analytical approach on (U-Th)/He dating of Fe-hydroxide with an example using goethite from the Amerasian Basin, Arctic Ocean
By: Olga Valentinovna Yakubovich et al.
I’m glad to see work on improving (U-Th)/He dating protocol for Fe-hydroxides. This manuscript provides a new sample loading method and the sample preparation technique for Fe-hydroxides (U-Th)/He dating. The sealing samples in quartz ampoules method is useful to solve the loss of U and Th during the sample helium extraction by heating processes. For sample preparation, authors proposed that U may be leached from goethite during the sonication by distilled water and crushing processes. This finding is helpful and may be beneficial for (U-Th)/He dating sample preparation. I think it would fit in this journal. Below I list my comments in more detail and hope it will help the authors improve this paper because I think they provide interesting protocol to contribute (U-Th)/He dating for Fe-hydroxides.
The application of the sealing samples in quartz ampoules
This sample loading method significantly solve the U and Th loss problem, but also bring two disadvantages: one is the relatively high blank of the quartz ampoule, the other is the release of helium gas controlled by helium diffusion through the quartz ampoules. My concern is authors proposed that the characters of helium released from quartz ampoules at different heating stages can reflect the helium retentivity in goethite samples. Although helium easily diffuses through the quartz, the helium release patterns in Fig.5 show all samples (except DG 1015) have similar patterns and release helium at high temperatures (350-1250 °C). This is obviously different with the traditional goethite helium release pattern showing the peak of helium release commonly occur during phase transformation at ca.300 °C. In my opinion, these helium release patterns may be controlled by the helium diffusion parameters of quartz. In this case, I suggest authors may fill helium gas in quartz ampoules and test the helium diffusion characters in quartz. This may help to determine whether the helium release patterns reflect the gas release characters of goethite sample or quartz ampoule, because it is useful to understand helium loss, recrystallisation, and fluid and mineral inclusions in goethite samples. For DG 1015 sample, the helium release pattern is obviously different with others in figure 5, and shows the peak of helium release occur at ~350°C, this is consistent with the goethite commonly release the helium during the phase transformation at ~300°C. In this case, this quartz ampoules may be broken during the heating or improperly sealed quartz ampoule. Finally, I expect authors add all the helium release patterns in figure 5, especially DG969 that show the oldest age in the same goethite clast, then we can identify the reason why some grains yield the dispersed ages.
Goethite U loss during the sonication by distilled water
Investigating goethite U loss during the sonication by distilled water is a very welcomed contribution. Authors design two sonication washing stages during the sample preparation and the results show up to 7.8% U in one vein grain sample can be leached during sonication in the distilled water. As we known, the minerals in this sample are possibly stable in water, because it collected from dredge haul DR7 at 3400 m water depth. In this case, I wonder whether the U is the dissolved ion or occur as suspended colloidal particles in the distilled water after the sonication. In the first case, the U loss may significantly affect the dating results, and this suggests (U-Th)/He isotopic system may be open in water. The second case may not hinder us to obtain the reliable ages. Numerous studies reveal that sonication is commonly useful to isolate smaller size aggregates, but this physical disintegration produce lots of fine particles in water, even after centrifugation and filtration, it also incomplete removal of fine suspended colloidal particles from the supernatant prior to element analysis. The results of leaching experiments in Table 3 show that less than 0.3% U loss occur at first sonication stage, while most of U (up to 7.8%) loss at second stage involved to the crushed and sonicated grain sample. Is it possible that the sonication of the crushed sample increase the suspended colloidal particles in water? In addition, why the U loss of crushed samples with two (first and second) steps is much higher than those with only second step? Authors also find the leachates contain amount of Mn and Fe, why do not list all these results? If possible, the relationship between U-Th and Mn-Fe may provide some information on whether U and Th are from dissolved ions or fine suspended particles.
General comments
80: Table 1 show that minerals in DR7 are composed of goethite, birnessite, and other minerals, this means the sample used to (U-Th)/He dating are not pure goethite, how to evaluate the (U-Th)/He dating results from these mineral mixtures.
90: Figure 2 show multiple generation of Fe-Mn-hydroxides or oxides and small monazite may occur in this sample, these mineral associations could affect the dating results.
115: All the samples used to (U-Th)/He dating were not washed. Why do not carry out some washed samples to test they may yield systematic older ages?
120: How do you seal the quartz ampoule? If use oxyhydrogen flame, is it possible to heat the ampoule and cause the loss of helium?
170, 205-210: The U and Th is not only derived from mineral surface adsorption but also from some suspended particles. Authors may list the concentration of other elements, such as Mn, Fe, Al, P….
216, 218: “Qu ampoule” -quartz ampoule
234-239: How about the genesis of dating sample, are they precipitated from the sea water or hydrothermal fluid?
Fig.5 Why only plot six goethite fragments patterns? How about the patterns of ID 966 and 969?
249-260: Sample DG969 has the lowest U, but sample DG 1031 has the highest U, the two samples yield erroneously old and reproducible ages. In this case, it cannot be simply explained by U-loss of these samples.
Citation: https://doi.org/10.5194/egusphere-2024-992-RC1 -
AC1: 'Reply on RC1', Olga Yakubovich, 28 Jun 2024
Dear Xiao-Dong Deng,
Thank you very much for the valuable comments and suggestions on how to improve the manuscript.
There are two main concerns that were highlighted in your comment: (1) the effect of the quartz ampoule on the He release pattern, and (2) goethite U loss during the sonication by the distilled water.
Quartz ampoule
We agree that helium release pattern is controlled by helium diffusion parameters of the quartz ampoule. Experiments with the empty quartz ampoules – which still have some residual atmospheric helium (the residual pressure is ~ 10-3 torr, which is roughly 10-11 cm3 STP of He) – revealed that helium is completely released at temperatures of 350 °C from the ampoules after 30 minutes of heating. Thus, complex He release patterns of the goethite samples cannot be easily explained by the ampoule effect. As an additional argument that these patterns are not the methodological “artefacts” we can refer to our experience in (U-Th)/He dating of pyrite and native gold (Yakubovich et al., 2020; 2014) and Pt-He dating of isoferroplatinum by the same technique (Yakubovich, 2013) – for each of these minerals He release patterns via quartz ampoule differs from what we have seen in goethite.
It is very interesting question, indeed, why during the phase transformation He is not completely released from the goethite. But the close type effect we have observed for isoferroplatinum. Under strong mechanical deformations Pt-alloy change its crystal structure from Pm-3m to Fm-3m, but no signs of He-loss were observed (Mochalov et al., 2019). It might be somehow related to the velocity of the transformation process, but it is only a hypothesis.
Goethite U loss during the sonication
It is very hard to prove that U, that we have measured in the solutions, is the dissolved U. We cannot exclude the presence of the tiny floating goethite particles in the solutions.
Our main arguments are:
(1) shift of the Th/U ratio in the solution relative to the Th/U ratio of the residual goethite (from 0.06 to 3; Table 3). Tiny floating particles of goethite are likely to have the same Th/U ratio as a goethite itself;
(2) leaching experiments of the goethite from the same DR-7-001 sample revealed that 2–3% of U is leached by 1M acetic acid/Na acetate buffer (pH 5), while Th is not (see the Figure attached). The same patterns were observed in Fe- and Mn-oxides by Konstantinova et al., 2018; Koschinsky and Hein, 2003.
However, the presence of Fe and Mn in the analyzed solutions and the condition of the nebulizer of the ICP-MS after these measurements are in a favor that at least some of the U and Th is related to floating goethite particles. We will add it to the discussion in the text of the manuscript.
Answer on General comments
80: Table 1 show that minerals in DR7 are composed of goethite, birnessite, and other minerals, this means the sample used to (U-Th)/He dating are not pure goethite, how to evaluate the (U-Th)/He dating results from these mineral mixtures.
For (U-Th)/He dating we used cement of the sample DR7-001, which according to XRD data is mainly ( >95%) goethite. For dating we picked the grains without visible under binocular microscope inclusions of other minerals. So likely, the purity of the analyzed samples was higher than 95%. The presence of other phases, of course, could affect the (U-Th)/He ages, but we assume that their impact is insignificant.
90: Figure 2 show multiple generation of Fe-Mn-hydroxides or oxides and small monazite may occur in this sample, these mineral associations could affect the dating results.
Agree, we will add this to the discussion
115: All the samples used to (U-Th)/He dating were not washed. Why do not carry out some washed samples to test they may yield systematic older ages?
Agree, but we are not able to do these measurements with the same material at the moment, but we will do it with other samples in the future.
120: How do you seal the quartz ampoule? If use oxyhydrogen flame, is it possible to heat the ampoule and cause the loss of helium?
The flame is narrow, quartz ampoule is relatively long, thermal conductivity of quartz is low, sealing is fast. We have done the (U-Th)/He dating of the sealed Durango Apatite earlier. The (U-Th)/He age was correct.
170, 205-210: The U and Th is not only derived from mineral surface adsorption but also from some suspended particles. Authors may list the concentration of other elements, such as Mn, Fe, Al, P….
We haven’t calibrated the ICP-MS on all these elements prior the measurements (only on U and Th). Thus, these values are only semiquantitive.
216, 218: “Qu ampoule” -quartz ampoule
Corrected
234-239: How about the genesis of dating sample, are they precipitated from the sea water or hydrothermal fluid?
These samples a likely of hydrothermal origin. Based on their morphology, textures and chemistry they are distinct from hydrogenic Fe-Mn crust. The origin of these Fe-oxides will be discussed in the detail in the article by Hein et al (under preparation).
Fig.5 Why only plot six goethite fragments patterns? How about the patterns of ID 966 and 969?
The reason why we excluded these two samples is that their heating protocol was different. But we will add them to the figure with this notion.
249-260: Sample DG969 has the lowest U, but sample DG 1031 has the highest U, the two samples yield erroneously old and reproducible ages. In this case, it cannot be simply explained by U-loss of these samples.
Agree, will add this to the discussion.
Thanks again for the comments,
Regards, Olga
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AC1: 'Reply on RC1', Olga Yakubovich, 28 Jun 2024
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RC2: 'Comment on egusphere-2024-992', Florian Hofmann, 15 May 2024
This manuscript suggests a new analytical approach to (U-Th)/He dating of goethite samples using the single-aliquot method. Since U can be volatilized during laser heating, the authors propose to encapsulate goethite samples in quartz ampoules which will let helium diffuse through but will retain any volatilized U. The U is then recovered by dissolving the glass ampoule together with its contents. This method is applied to submarine iron oxide nodules.
I appreciate seeing a new approach for dealing with U volatilization in iron oxides. This is a promising technique that could provide a way to deal with U volatilization without having to make changes to the extraction line, which makes the application easier and more accessible.
While the application of (U-Th)/He dating to submarine nodules is exciting, I am wondering whether this was the best choice of sample for an exploratory study into new analytical techniques. In my opinion, samples with more independent age constraints would have been a better test of this method since the efficacy of this method could be verified. However, these samples are sufficiently documented to be a good first demonstration of the technique.
My major analytical concern about this approach is whether the samples can be completely degassed and whether the resulting helium can be completely extracted from the ampoule. Incomplete extraction of helium would lead to an underestimation of the (U-Th)/He age. Have any of the samples been heated to temperatures higher than 1150 °C? Some of the samples seem to still outgas a significant amount of helium in the 1000-1150 °C range (Fig. 5). Even though the amount of helium extracted declines in the 1150 °C step, there could still be helium in more retentive domains. Bimodal release patterns are not unusual and are dependent on the distribution of diffusional domain sizes in the samples.
Samples could be tested for complete extraction by heating previously heated samples to even higher temperatures or melting them with flux to determine whether more helium is released. If the material is homogeneous in age and eU concentration, a separate set of samples could be weighed and completely degassed using flux and/or high temperatures. The 4He concentrations obtained from samples heated in the quartz ampoules could be compared against the concentrations from the reference samples to check for complete extraction.
Another issue is the complete recovery of the volatilized U, which could equally affect the calculated ages. I suggest dissolving a number of unheated fragments of each structure and determining U and Th concentrations. The obtained U and Th concentrations of the heated samples can then be compared against these reference values. This would also demonstrate how homogeneous the sample is with regard to parent nuclides.
I think that the efficacy of this method is somewhat overstated. At several points throughout the manuscript, the authors claim that their samples show “no signs of overdispersion”, yet they report an MSWD of 3.4, which is very clearly overdispersed. This should be addressed. Adding additional replicate measurements for the same material and re-evaluating the MSWDs of these larger datasets might increase the significance of the findings.
I think that this manuscript is a good exploration of the quartz ampoule technique for goethite. However, several aspects of the technique, such as complete extraction of helium and full recovery of any volatilized U, need to be demonstrated with a larger number of replicate analyses of samples with more independent age constraints before the method can be claimed to be robust. Such additional analyses could be incorporated into this manuscript to make the conclusions more significant, but if this is deemed to be beyond the scope of this study, I would recommend scaling down the claims as to the robustness and reproducibility of the method.
Overall, I think this manuscript is a good fit for a GChron short communication. This manuscript is a great contribution towards establishing the quartz ampoule degassing method and will be interesting to the (U-Th)/He community at large. However, the issues brought up here and by other reviewers should be addressed before publication. I, therefore, recommend that the manuscript be accepted subject to minor revisions.
Detailed comments:
Line 17: I think “remarkable reproducibility” is overstating the reproducibility of n=4 with MSDW=3.4 and n=2 with MSDW=1.4.
Line 19: Insert “a” between “that” and “significant”.
Line 30: Change “mineral” to “minerals”.
Line 32: What is “sufficient” retentivity? Published studies generally indicate retention in the range of 80-98% for comparable material at Earth-surface conditions. I am wondering whether these samples are comparable to the supergene samples for which diffusion experiments have been performed so far. These samples could benefit from 4He/3He diffusion experiments to determine the retention of radiogenic helium.
Lines 34-36: Since these are not exhaustive lists, I suggest adding “e.g.” to these references.
Line 36: Change “successful” to “successfully”.
Line 44: Add citations to support that these methods are “typically” applied.
Lines 74-75: What is the observation that dark-colored goethite has better crystallinity based on? XRD/SEM/both?
Line 108: 5% HNO3 alone might not be enough to stabilize Th in the solution. Typically, a small amount of HF is added to solutions to stabilize Th during the measurement process. Th fractionation can occur in the ICP-MS tubing if it isn’t sufficiently stabilized.
Line 126: Was the sensitivity based solely on comparison with a mineral standard? Was there any internal standardization with an air or other gaseous standard from a tank?
Line 127: What is the repeatability of the 10 measurements of the mineral standard? The manuscript never seems to come back to this, and I don’t see these measurements in the results tables. Please report these results.
Line 131: I’m not sure what “in the camera of the mass spectrometer” refers to. Please clarify.
Line 214, Line 285: The results have MSDWs of 3.4 and 1.4, respectively. These values are both >1, which counts as overdispersed. A value of 1.4 is reasonably close to a univariate normal distribution to claim that the results aren’t overdispersed, but that is for n=2. However, an MSWD value of 3.4 is definitely overdispersed at n=4. (U-Th)/He ages in a variety of minerals are typically overdispersed, so that’s not unusual. It is most likely an effect of zonation and our incomplete understanding of the (U-Th)/He system rather than the measurement process, as long as U volatilization during heating is avoided.
Line 216: The abbreviation “Qu ampoule” isn’t defined. I suggest just using “quartz ampoule” instead. There are several more uses in the same paragraph that should be changed as well.
Line 223: I wouldn’t characterize the O2 tank as “dangerously explosive”. It is a thick-walled vessel filled significantly below its rated pressure. Since it is filled with pure O2 and no combustible fuel is present in the tank, it does not present an explosion hazard. Mishandling could lead to damage to the mass spectrometer and extraction line, the risk of which can be mitigated by implementing redundancy and fail-safe measures. Overall, the amounts of oxygen are small, comparable to the natural amount of oxygen present in the air inside a vented extraction line. Pressurized oxygen tanks filled to much higher pressures are common in labs and other settings and can be safely used if handled and stored correctly.
Figure 5: Not all of the performed analyses are shown here. Please add the missing patterns. Additionally, the use of colors to distinguish between sets of samples might present a barrier to accessibility. I suggest using a different symbol for each sample/set of analyses and an accessible color scheme to help visual interpretation. Some of the values given in this figure use dots as decimal symbols while others use commas. This should be consistently applied to the rest of the manuscript as well.
Figure 6: While age-eU plots are a good tool to diagnose radiation damage effects and U loss, the small number of samples and the small range of eU values (2-3 ppm) make the interpretation of this plot difficult. A larger dataset might reveal patterns. Also, use the same symbols and color scheme as suggested for Fig. 5.
Line 259: The presence of zircon and monazite in the sample might affect the helium release pattern. These phases are likely to be more retentive than goethite and would lead to some of the helium being released at higher temperatures. Having the helium release patterns of these samples in Fig. 5 would be helpful. A possible strategy to prevent inference from mineral inclusions would be to pre-screen samples with microCT and only pick inclusion-free grains for analysis.
Line 271: How are you distinguishing between recrystallization of existing material and newly formed material from a second period of goethite crystallization?
Citation: https://doi.org/10.5194/egusphere-2024-992-RC2 -
AC2: 'Reply on RC2', Olga Yakubovich, 28 Jun 2024
Dear Florian Hofmann,
Thank you very much for the valuable comments and suggestions on how to improve the manuscript.
There were several major concerns that you focus in your comment: (1) the choice of the object for testing the technique; (2) the completeness of He release from the ampoule; (3) complete recovery of U from the samples; (4) robustness of the technique.
Object
We agree that the samples with independent age constrains might be better to analyze in order to verify the reliability of the technique. But at the same time, the deep underwater goethite samples have some obvious advantage – they are surrounded by steady and low temperature (around 0°C) environment. It excludes any continuous thermal loss of He by the samples during their geological history, one of the factors that affected (U-Th)/He ages.
Helium release
Firstly, I would like to note, that diffusion of He through the quartz ampoule is fast at 1150 °C. So, the main question is if 1150 °C is enough for complete He release from the goethite grain?
During the samples heating, we are continuously measuring the He concentration in the mass-spectrometer chamber. Thus, the valve that connects furnace and analyzer is open during the experiment. Therefore, we are able to measure the total amount of He released at the present temperature, as well as the way He was released (see Fig 1 and 2 in attachments). The case that you refer in the comment is usually observed as the steady growth of the He concentration in the chamber of mass-spectrometer. We have not observed it for these samples – there were no further growth of He signal. Given that I would suggest that 1150°C in our case was enough, but for future experiments I would definitely increase the upper limit for at least for 1250°C.
Uranium recovery
We have some bulk data from the ICP-MS measurements of the HLY0805 samples. Uranium content is in range from 1.5 – 2.5 ppm (n=5). Uranium concentrations in the heated in the quartz ampoule goethite grains are from 1.4 to 2.8 ppm (Table 2). We do not have enough material to perform the type of analyses that you have recommended, but we do not see any signs of U-loss, with the exception of sample ID 969, which had shown the lowest U content (Table 2).
Robustness of the technique
Agree, we do not have enough data to discuss overdispersion, and comparison of the technique with other methodological approaches. We will follow your suggestion in the discussion section of the manuscript.
Answer to the detailed comments:
Line 17: I think “remarkable reproducibility” is overstating the reproducibility of n=4 with MSDW=3.4 and n=2 with MSDW=1.4.
agree
Line 19: Insert “a” between “that” and “significant”.
corrected
Line 30: Change “mineral” to “minerals”.
corrected
Line 32: What is “sufficient” retentivity? Published studies generally indicate retention in the range of 80-98% for comparable material at Earth-surface conditions. I am wondering whether these samples are comparable to the supergene samples for which diffusion experiments have been performed so far. These samples could benefit from 4He/3He diffusion experiments to determine the retention of radiogenic helium.
They are comparable based on their chemistry and crystallography. But they still can benefit from 4He/3Heexperiments as the expected to have lower impact of thermal He loss.
Lines 34-36: Since these are not exhaustive lists, I suggest adding “e.g.” to these references.
agree
Line 36: Change “successful” to “successfully”.
corrected
Line 44: Add citations to support that these methods are “typically” applied.
ok
Lines 74-75: What is the observation that dark-colored goethite has better crystallinity based on? XRD/SEM/both?
Yes, based on XRD data mainly. Dark one has better crystallinity.
Line 108: 5% HNO3 alone might not be enough to stabilize Th in the solution. Typically, a small amount of HF is added to solutions to stabilize Th during the measurement process. Th fractionation can occur in the ICP-MS tubing if it isn’t sufficiently stabilized.
We have not added HF to the solution. How strong is this effect? We measure the standard Th and U calibration solutions in 5% HNO3 without additional HF as well. The Th/U ratio is quite stable during the measurement set. Thus, I don’t expect it to dramatically change the results of the leaching experiments.
Line 126: Was the sensitivity based solely on comparison with a mineral standard? Was there any internal standardization with an air or other gaseous standard from a tank?
We calibrate the mass-spectrometer only by mineral standards. We use two of them: Knyaghinya meteorite and platinum from the Santiago River. We also constantly measure (U-Th)/He age of the Durango apatite.
Line 127: What is the repeatability of the 10 measurements of the mineral standard? The manuscript never seems to come back to this, and I don’t see these measurements in the results tables. Please report these results.
I mean that the MS is calibrated by the mineral standard. Ten measurements mean that we obtained the average value of He content based on 10 scans (as far as I know some labs do 20 scans). I have rewritten this part to make it clear. I don’t think that these results should be presented in the manuscript, but I can provide them in a xls file if necessary. The original file from the MS looks like in Fig. 2 (attached). Or it is the question regarding the general performance of the MS? I guess that the best impression of the reproducibility of the MS can be find here: https://onlinelibrary.wiley.com/doi/abs/10.1111/ggr.12502
Line 131: I’m not sure what “in the camera of the mass spectrometer” refers to. Please clarify.
Chamber
Line 214, Line 285: The results have MSDWs of 3.4 and 1.4, respectively. These values are both >1, which counts as overdispersed. A value of 1.4 is reasonably close to a univariate normal distribution to claim that the results aren’t overdispersed, but that is for n=2. However, an MSWD value of 3.4 is definitely overdispersed at n=4. (U-Th)/He ages in a variety of minerals are typically overdispersed, so that’s not unusual. It is most likely an effect of zonation and our incomplete understanding of the (U-Th)/He system rather than the measurement process, as long as U volatilization during heating is avoided.
Agree, corrected
Line 216: The abbreviation “Qu ampoule” isn’t defined. I suggest just using “quartz ampoule” instead. There are several more uses in the same paragraph that should be changed as well.
Ok
Line 223: I wouldn’t characterize the O2 tank as “dangerously explosive”. It is a thick-walled vessel filled significantly below its rated pressure. Since it is filled with pure O2 and no combustible fuel is present in the tank, it does not present an explosion hazard. Mishandling could lead to damage to the mass spectrometer and extraction line, the risk of which can be mitigated by implementing redundancy and fail-safe measures. Overall, the amounts of oxygen are small, comparable to the natural amount of oxygen present in the air inside a vented extraction line. Pressurized oxygen tanks filled to much higher pressures are common in labs and other settings and can be safely used if handled and stored correctly.
Thank you! I had a wrong impression about the oxygen tank! Corrected in the text.
Figure 5: Not all of the performed analyses are shown here. Please add the missing patterns. Additionally, the use of colors to distinguish between sets of samples might present a barrier to accessibility. I suggest using a different symbol for each sample/set of analyses and an accessible color scheme to help visual interpretation. Some of the values given in this figure use dots as decimal symbols while others use commas. This should be consistently applied to the rest of the manuscript as well.
Ok, agree, it would be better
Figure 6: While age-eU plots are a good tool to diagnose radiation damage effects and U loss, the small number of samples and the small range of eU values (2-3 ppm) make the interpretation of this plot difficult. A larger dataset might reveal patterns. Also, use the same symbols and color scheme as suggested for Fig. 5.
Agree
Line 259: The presence of zircon and monazite in the sample might affect the helium release pattern. These phases are likely to be more retentive than goethite and would lead to some of the helium being released at higher temperatures. Having the helium release patterns of these samples in Fig. 5 would be helpful. A possible strategy to prevent inference from mineral inclusions would be to pre-screen samples with microCT and only pick inclusion-free grains for analysis.
Agree, but the microCT is a next methodological step. We have added all samples to the Fig.5.
Line 271: How are you distinguishing between recrystallization of existing material and newly formed material from a second period of goethite crystallization?
We do not. Both options are possible. Made it clearer in the text.
-
AC2: 'Reply on RC2', Olga Yakubovich, 28 Jun 2024
-
RC3: 'Comment on egusphere-2024-992', Hevelyn S. Monteiro, 22 May 2024
General comments
Yakubovich et al. propose an alternative analytical approach for dating goethite by the (U-Th)/He method. To avoid U-loss during heating for He extraction, multiple grains (1-7 mg) are encapsulated in vacuum-sealed quartz ampoules. The quartz ampoules containing the grains are then dissolved in a solution of aqua regia + HF + HClO4, and U and Th isotopes are analyzed via isotope dilution method.
Two samples of Fe-Mn-crusts collected from the Egiazarov Trough, Amerasian Basin, Artic Ocean, were used to test this alternative methodology. The Fe-Mn-crusts are a mixture of goethite and Mn-oxides and other phases (e.g., lepidocrocite, feroxyhyte, quartz, feldspars, clays, clinochlore, etc.). Goethites from Fe-Mn-crusts are porous and poorly crystallized. Yakubovich et al. propose that U and Th are leached from the samples during ultrasonication and interpret the variable Th/Uleachate and Th/Ugrain values as evidence for preferential U remobilization.
It is encouraging to see researchers interested in further refining the (U-Th)/He method applied to the study of supergene phases, as obtaining meaningful ages for these complex materials can be challenging and time consuming. Therefore, Yakubovich et al.’s proposal of an alternative approach to measure the ages of goethites is worth pursuing. However, before the manuscript can be accepted for publication, some important aspects of the work need to be assessed:
- suitability of chosen samples for (U-Th)/He geochronology;
- interpretation of leaching experiments results;
- interpretation of the (U-Th)/He results.
The successful application of a geochronological method depends on sample selection. Since 1999, ~ 2600 goethite (U-Th)/He ages have been produced by four laboratories. The most prolific laboratory, the noble gas facility at Caltech, is responsible for ~2000 of those results. The most important criterion to ensure reliable results is to select pure well crystallized goethite. Powdery goethite and poorly crystalline goethite intergrown with other minerals should be carefully avoided. Early in our studies, to test whether poorly crystalline goethites produce reliable results, we carried out analyses of co-existing crystalline (generally black or brown dense goethite samples) and poorly crystalline (generally yellow, porous and powdery goethite) samples. The combined (U-Th)/He-4He/3He experiments show that the latter are unsuitable for geochronology and should be avoided. Yakubovich et al. chose to work with impure, poorly crystalline goethites, the very same type of goethite that should be avoided in (U-Th)/He geochronology. Thus, their results, as interesting as they are, are useful to confirm that impure poorly crystalline goethites are not suitable for (U-Th)/He geochronology. Their findings cannot be generalized to all goethites, particularly to the types of goethites carefully selected for geochronology by most laboratories and analysts.
As demonstrated in this study, impure poorly crystalline goethite suffers from several limitations. One of these limitations is that U and Th may occur both within the goethite crystalline structure but also adsorbed or as a surface precipitate on the high surface area fine grained porous phases (porous goethites, manganese oxides, clays, silica minerals). Most He produced by U and Th adsorbed or precipitated on mineral surfaces will be lost, and the analytical results will provide no information on the sample’s age. In addition, U and Th on those surface sites should be readily desorbed and removed by aqueous solutions, both during their geological history and during sample preparation and cleaning procedures, further challenging the interpretation of results obtained from this type of material. Yakubovich et al.’s results clearly show why these types of samples should be avoided if the major objective is to date a goethite occurrence.
In the interpretation of their (U-Th)/He results, Yakubovich et al. raise the possibility that the Fe-Mn-crusts were formed deep in the Earth’s crust and then acquired their ages after cooling below a certain closure temperature. There is plenty of evidence suggesting the hydrogenetic origin of the Artic Ocean Fe-Mn-crusts (Hein et al., 2017). To avoid misinterpretation of the results, the authors should focus their interpretation of the results in the context of changing environmental conditions at the contact between detrital sediments and sea water. More importantly, the fact the samples are made of a mixture of minerals, not pure goethite, precludes a robust interpretation on the precipitation ages of goethites. Finally, based on our current knowledge about the helium retentivity in powdery, poorly crystalline goethites, the samples analyzed in this work probably lost >70% of their helium, a challenge in interpreting results obtained from poorly crystalline mineral assemblages. These limitations should be incorporated in the discussion.
It would be beneficial if the authors were to compare the results obtained in poorly crystalline and impure samples with an additional set of results obtained from pure crystalline goethite using the exact same analytical approach. This comparative analysis would strengthen the credibility of the approach implemented in this study and provide a more comprehensive understanding of its applicability. Finally, the complex nature of the Fe-Mn-crusts investigated by Yakubovich et al. demands a more detailed characterization of the paragenetic relationships between the different minerals in the samples to determine exactly which phase(s) was(were) dated.
By addressing these points, the authors can enhance the overall quality and impact of their research, making a significant contribution to the field of geochronology.
Specific comments
Lines 37-39: How do Artic Ocean Fe-Mn-crusts (especially DR7-001) differ from Pacific Ocean Fe-Mn-crusts studied by Basu et al. (2006)?
Lines 64-65: What is the percentage of goethite and Mn-oxides in these samples? The indicated values of > 25%, 5-25%, <5% are not sufficient information. The authors should be able to quantify the percentages of the main mineral phases in the samples analyzed by XRD. The reader would benefit from a figure illustrating the X-ray diffraction patterns obtained for each type of material.
Line 115-118: What was the sampling strategy adopted by the researchers? The dark-brown clasts were selected from sample DR7-002 and the yellow-brown vein grains come from sample DR7-001? If so, what is the relationship between the two types of grains analyzed?
Line 116: If (U-Th)/He results were obtained only for sample DR7-001, is this sample also a breccia?
Line 127: Please, tabulate 4He amounts measured for calibration standard.
Lines 130-131: 4He amounts are retrieved from multiple grains (as shown in Figure 4), not an individual goethite grain.
Lines 182-183: Would it be possible that heating of the grain during sealing of the quartz ampoule caused some U-volatilization as well as He loss?
Lines 195-211: Yakubovich et al. claim that U-loss during sample ultrasonication results in ages that are overly dispersed. In most environments, sorption of U and Th ions/complexes will be less favorable compared to other abundant dissolved metal ions/complexes, suggesting only a minor contribution to the U-Th-He system. U and Th incorporated in the goethite structure during crystal growth would dominate the production of He in a goethite grain. However, the samples used in this study are not pure goethites. Therefore, U and Th can be adsorbed to surfaces exhibiting very distinct characteristics, presenting different adsorption-desorption reaction rates. Additionally, microporosity increases both the surface area of minerals available for sorption reactions and the number of pores to which ions migrate, the sites where they are trapped, and the ease of desorption during ultrasonication. These phenomena may be particularly important in the multi-mineral grains analyzed by Yakubovich et al., but not as significant for well-crystallized, dense goethite masses commonly selected for (U-Th)/He dating. Cleaning mm- to µm-sized goethite grains in an ultrasonic bath at room temperature for 10-15 minutes aims to facilitate the removal of loosely attached fine particles of goethite and other phases (e.g., adsorbed clays or dust) from the surface of the grains to minimize interference on He, U, and Th measurements of the target phase.
Line 214: The calculated MSWDs for the analyses of dark and vein “goethites” show otherwise.
Lines 215-217: Laser heating of encapsulated goethite grain at 950 °C for 6 min ensures complete 4He extraction and no U-volatilization from samples suitable for (U-Th)/He geochronology. This is routinely done in laboratories around the world. Now, the quartz ampoule method potentially offers the opportunity to perform helium diffusion studies if it can help to avoid phase breakdown during step-heat experiments.
Lines 238-293: What are the complications in using the quartz ampoule method to derive He diffusion parameters?
Lines 239-240: I do not understand this statement.
No correlation exists between He release pattern and (U-Th)/He ages, reflecting an insignificant thermal loss of 4He.
Line 266: Goethite is the most thermodynamically stable iron-oxyhydroxide in the near surface environment. It partially dissolves during interaction with acidic solutions, but its thermodynamic stability does not change. For instance, goethites as old as 70 Ma have been found near the surface of massive sulfide deposits.
Technical corrections
Title: The word goethite in the title is misleading. The samples analyzed by Yakubovich et al. are not pure goethite. In addition, this method can be applied to other phases as well. I suggest changing the title to “A new analytical approach on (U-Th)/He dating by sample encapsulation in quartz ampoules under vacuum: application to Fe-Mn-crusts, Amerasian Basin, Arctic Ocean”.
Figure 1: Enlarge sample photos, indicate on the photo the different types of material (dendrites, botryoidal, glassy, cellular-like, etc.), and show the areas sampled for geochronology and X-ray diffraction analysis.
Figure 2: (B) replace massive with film or cement.
Figure 5: HD+vs Temperature and He vs Temperature: What do the patterns of HD+ and He release with temperature teach us about sample behavior under vacuum?
Figure 5: These helium release patterns do not say anything about goethite behavior because the grains are not pure goethite.
Line 14: replace Fe-hydroxides with Fe-(oxyhydr)oxides.
Line 19-20: Here are some potentially contributors for overdispersion of (U-Th)/He ages: multiple generations of Fe-(oxyhydr)oxides, He diffusion from poorly crystalline material, alpha recoil, mineral inclusions, and the presence of fine particles attached to the grain surface. The sentence needs to be rewritten.
Line 30-31: Suggestion: “Goethite is one of the most common Fe-(oxy)hydroxide minerals formed during the hydrolyzation of rocks, making it a desirable mineral for dating various surface and subsurface geological processes.”
Lines 34, 36: replace successful with successfully.
Line 45: Shuster and Farley (2005) discuss diffusion experiment results obtained for quartz. Is there an equivalent citation pertinent to goethite?
Line 75: What do replacements mean?
Line 131: camera == chamber?
Line 131: How much HD+ was detected during the step-heat experiment? Does the amount of HD+ differ between the types of samples?
Lines 215, 218, 220: Qu == quartz?
Citation: https://doi.org/10.5194/egusphere-2024-992-RC3 -
AC3: 'Reply on RC3', Olga Yakubovich, 28 Jun 2024
Dear Hevelyn S. Monteiro,
Thank you very much for the valuable comments and suggestions on how to improve the manuscript.
The major concern that was highlighted in your comment is the suitability of the chosen samples for (U-Th)/He geochronology. The quality of the studied material has a direct impact on the interpretation of the leaching experiments and (U-Th)/He ages.
We agree, that hydrogenetic Fe-Mn crusts are not suitable for (U-Th)/He geochronology due to the significant content of extraterrestrial He-rich dust, their high porosity and specific surface area, as well as poorly crystalline main minerals: vernadite (also called d-MnO2), and X-ray amorphous iron oxyhydroxide (Hein et al., 2000).
The samples that we have analyzed are hydrothermal underwater Fe-hydroxides, not the fragments of the hydrogenetic Fe-Mn crust described by Hein et al., 2017. We are not discussing in this manuscript the origin of these samples as it is a subject of the article which is under preparation by Hein with coauthors.
The samples mainly consist of pure crystalline goethite (>95%). Based on XRD data the darker grains have higher crystallinity than the yellowish ones. Therefore, they might be considered as a standard material which is used for (U-Th)/He dating. The sequential leaching experiments reflect likely presence of some sorbted U on the surface of the goethite grains, and the interpretation of the (U-Th)/He can be done based on the data available for hypergenic crystalline goethite.
Answer to Specific comments
Lines 37-39: How do Artic Ocean Fe-Mn-crusts (especially DR7-001) differ from Pacific Ocean Fe-Mn-crusts studied by Basu et al. (2006)?
The sample DR7-001 is not described as Fe-Mn crust. Based on its morphology, texture, predominantly Fe content with low rare elements concentrations as well as higher crystallinity goethite, studied samples likely has a hydrothermal origin (Hein, J.R. et al 2000. Handbook of Marine Mineral Deposits. p. 239-279).
Lines 64-65: What is the percentage of goethite and Mn-oxides in these samples? The indicated values of > 25%, 5-25%, <5% are not sufficient information. The authors should be able to quantify the percentages of the main mineral phases in the samples analyzed by XRD. The reader would benefit from a figure illustrating the X-ray diffraction patterns obtained for each type of material.
We will add the XRD images and provide additional information
Line 115-118: What was the sampling strategy adopted by the researchers? The dark-brown clasts were selected from sample DR7-002 and the yellow-brown vein grains come from sample DR7-001? If so, what is the relationship between the two types of grains analyzed?
All fragments of goethite mineralization were manually extracted only from DR7-001 sample: two dark-brown clasts of the breccia and yellow-brown vein. Subsamples from the yellow-brown vein material and from dark-brown gains were treated as separate samples. We suppose two different types of goethite is associated to different crystallinity.
Line 116: If (U-Th)/He results were obtained only for sample DR7-001, is this sample also a breccia?
Yes, it is
Line 127: Please, tabulate 4He amounts measured for calibration standard.
Ok
Lines 130-131: 4He amounts are retrieved from multiple grains (as shown in Figure 4), not an individual goethite grain.
Corrected
Lines 182-183: Would it be possible that heating of the grain during sealing of the quartz ampoule caused some U-volatilization as well as He loss?
The flame is narrow, quartz ampoule is relatively long, thermal conductivity of quartz is low, sealing is fast. We have done the (U-Th)/He dating of the sealed Durango Apatite earlier. The (U-Th)/He age was correct.
Lines 195-211: Yakubovich et al. claim that U-loss during sample ultrasonication results in ages that are overly dispersed. In most environments, sorption of U and Th ions/complexes will be less favorable compared to other abundant dissolved metal ions/complexes, suggesting only a minor contribution to the U-Th-He system. U and Th incorporated in the goethite structure during crystal growth would dominate the production of He in a goethite grain. However, the samples used in this study are not pure goethites. Therefore, U and Th can be adsorbed to surfaces exhibiting very distinct characteristics, presenting different adsorption-desorption reaction rates. Additionally, microporosity increases both the surface area of minerals available for sorption reactions and the number of pores to which ions migrate, the sites where they are trapped, and the ease of desorption during ultrasonication. These phenomena may be particularly important in the multi-mineral grains analyzed by Yakubovich et al., but not as significant for well-crystallized, dense goethite masses commonly selected for (U-Th)/He dating. Cleaning mm- to µm-sized goethite grains in an ultrasonic bath at room temperature for 10-15 minutes aims to facilitate the removal of loosely attached fine particles of goethite and other phases (e.g., adsorbed clays or dust) from the surface of the grains to minimize interference on He, U, and Th measurements of the target phase.
Agree, U-loss might be significant only for multi-mineral grains. Large grains are unlikely to lose significant amount of U, as their surface to volume ratio is low.
Line 214: The calculated MSWDs for the analyses of dark and vein “goethites” show otherwise.
Corrected
Lines 215-217: Laser heating of encapsulated goethite grain at 950 °C for 6 min ensures complete 4He extraction and no U-volatilization from samples suitable for (U-Th)/He geochronology. This is routinely done in laboratories around the world. Now, the quartz ampoule method potentially offers the opportunity to perform helium diffusion studies if it can help to avoid phase breakdown during step-heat experiments.
From our point of view the ampoule method do not help in establishing the diffusion parameters, as it cannot avoid the phase breakdown. But it solves the problem of U-loss. Though the most laboratories release He from the grains at temperature <1000 °C, there are some papers that suggest that in some cases more intense heating is required (e.g. Wernicke and Lippolt, 1993; Farley and Flowers, 2012; Farley and McKeon, 2015; Hofmann et al., 2020).
Lines 238-293: What are the complications in using the quartz ampoule method to derive He diffusion parameters?
Ampoules are not uniform. They have different surface volume and thickness of the walls. Thus, in order to measure He diffusion parameters of the sample it is necessary firstly to solve this equation for the ampoule. The proper design of the experiment will allow to measure He diffusion parameters of the sample. But will add uncertainties.
Lines 239-240: I do not understand this statement.
No correlation exists between He release pattern and (U-Th)/He ages, reflecting an insignificant thermal loss of 4He.
Rewritten
Line 266: Goethite is the most thermodynamically stable iron-oxyhydroxide in the near surface environment. It partially dissolves during interaction with acidic solutions, but its thermodynamic stability does not change. For instance, goethites as old as 70 Ma have been found near the surface of massive sulfide deposits.
Agree
Technical corrections
Title: The word goethite in the title is misleading. The samples analyzed by Yakubovich et al. are not pure goethite. In addition, this method can be applied to other phases as well. I suggest changing the title to “A new analytical approach on (U-Th)/He dating by sample encapsulation in quartz ampoules under vacuum: application to Fe-Mn-crusts, Amerasian Basin, Arctic Ocean”.
Agree, changed to “A new analytical approach on (U-Th)/He dating by sample encapsulation in quartz ampoules under vacuum: application to goethite, Amerasian Basin, Arctic Ocean”.
Figure 1: Enlarge sample photos, indicate on the photo the different types of material (dendrites, botryoidal, glassy, cellular-like, etc.), and show the areas sampled for geochronology and X-ray diffraction analysis.
Corrected
Figure 2: (B) replace massive with film or cement.
Corrected
Figure 5: HD+vs Temperature and He vs Temperature: What do the patterns of HD+ and He release with temperature teach us about sample behavior under vacuum?
Figure 5: These helium release patterns do not say anything about goethite behavior because the grains are not pure goethite.
The sample is > 95% goethite. We have added this to figure caption.
Line 14: replace Fe-hydroxides with Fe-(oxyhydr)oxides.
Corrected
Line 19-20: Here are some potentially contributors for overdispersion of (U-Th)/He ages: multiple generations of Fe-(oxyhydr)oxides, He diffusion from poorly crystalline material, alpha recoil, mineral inclusions, and the presence of fine particles attached to the grain surface. The sentence needs to be rewritten.
Corrected
Line 30-31: Suggestion: “Goethite is one of the most common Fe-(oxy)hydroxide minerals formed during the hydrolyzation of rocks, making it a desirable mineral for dating various surface and subsurface geological processes.”
Thanks, corrected
Lines 34, 36: replace successful with successfully.
Corrected
Line 45: Shuster and Farley (2005) discuss diffusion experiment results obtained for quartz. Is there an equivalent citation pertinent to goethite?
Corrected
Line 75: What do replacements mean?
Newly formed minerals
Line 131: camera == chamber?
Corrected
Line 131: How much HD+ was detected during the step-heat experiment? Does the amount of HD+ differ between the types of samples?
Yes. Most of the hydrogen was observed at the temperatures of 350 C, which presence in the chamber of MS is likely related to transformation of goethite to hematite. Nevertheless, the amount of HD for yellowish grains was lower, than for dark ones (we expected the otherwise pattern)
Lines 215, 218, 220: Qu == quartz?
Corrected
Citation: https://doi.org/10.5194/egusphere-2024-992-AC3
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