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| 14 Jun 2024
Development of an integrated analytical platform of clay minerals separation, characterization and 40K/40Ar dating
Abstract. Isotopic dating is a valuable method to constrain the timing of lithospheric processes: geodynamic episodes, ore deposition and geothermal regimes. The K-Ar dating technique has the main advantage of being applied to ubiquitous K-bearing minerals that crystallize in various temperatures, from magmatic to low temperatures. Clays are of significant interest among all K-bearing minerals, as they crystallize during various hydro-thermo-dynamic processes. Nonetheless, the dating of illites by the K-Ar method is not straightforward. K-Ar dates on illite usually rely on a mixed isotopic signal referring to various illitic populations that might have experienced isotopic resetting or re-crystallization processes. Therefore, reliable K-Ar dates on illite depend on (1) the grain size separation of large amounts of clay fractions, (2) the study of the morphology, mineralogy and crystallography, (3) the determination of precise K-Ar dates on each clay size fraction and (4) the meaningful interpretation of ages using either end-member ages or the Illite-Age-Analysis (IAA) method. This paper describes the instrumentation and methods recently developed at the GeoRessources laboratory of the University of Lorraine to obtain valuable ages on illite mixtures.
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RC1: 'Comment on egusphere-2024-1150', Jesús Solé, 19 Jul 2024
General comments
I have read with interest the article “Development of an integrated analytical platform of clay minerals separation, characterization and 40K/40Ar dating”. The K-Ar dating of clays, particularly illite, has been used for sixty years but has had a renaissance in the last two decades. Its application to diagenesis and structural processes (faults, tectonic uplift, low-grade deformation, water circulation, and others) has promoted its use. The K-Ar method is particularly well-suited to the dating of clays, as the 40Ar-39Ar method, although more powerful, is analytically and interpretatively difficult in this case. It should be added that the closure of several Ar-Ar laboratories worldwide makes establishing a new K-Ar laboratory good news.
After a few detailed readings, I have not found any major problems in this contribution, which seems well-detailed and structured. However, some minor points need to be adressed, so I suggest it be published with minor revisions.
Since there are a few points to discuss, I have listed them below by line number, regardless of whether they are minor typographical errors or points that need better discussion. I am not a native English speaker, but I believe the manuscript is well-written. There are some minor language details that I have marked and perhaps some others that I have not detected.
Specific comments
Title. I propose three minor changes: “Development of an integrated analytical platform for clay mineral separation, characterization, and K-Ar dating.” The first two changes are grammatical, and the last is a suggestion to standardize the title with the main text where the term K-Ar is always used.
Line 27. Change to: “... detailed by Schaeffer and Zähringer (1966) and by ...”
Line 31. Point (1) is not strictly true. There is evidence that electron capture is affected by pressure, but this does not affect any "real" dating by the K-Ar method.
Line 32. Point (2) is erroneous. The 40K/K ratio decreases over time; this is the basis of the K-Ar geochronometer. The authors mean that it is constant for all samples at present, which is also strictly false, but within the margins of error currently achievable, it can be considered true.
Line 32. Point (3) needs to be worded a little differently, as stating that all 40Ar comes from the decay of 40K does not exclude 40Ar trapped during the rock's formation and originating from the decay of 40K before the system closed. The later point (5) also does not eliminate this case. Saying that 40Ar comes from the decay of 40K within the mineral will solve the problem.
Line 33. Point (4) we know is false, but we must assume it for atmospheric correction. In any case, most of the air in the extraction lines has a modern atmospheric composition. A different case occurs when measuring very old samples with trapped air bubbles (fluid inclusions) that could give values different from modern air (and that have been studied for this reason).
Line 34. As mentioned, point (5) does not exclude radiogenic argon trapped during rock formation.
Line 35. “The latter might be hypothetical ...” I would say it is typically false in these cases, which is why K-Ar (Ar-Ar) can be used to obtain data on the thermal history, as detailed already in the manuscript.
Line 39. “This age is then related to ...” I would modify this sentence as follows: “This age is related to the crystallization event in the case of fast cooling (e.g., unaltered volcanic rocks), the closure time for slow cooling rocks (e.g., plutonic, metamorphic) or ...”
Line 53. “One of the main concerns ...”
Line 61. The word “desorption” introduced here and used later raises some doubts. The Merriam-Webster dictionary defines it as “to remove (a sorbed substance) by the reverse of adsorption or absorption.” But this is not what happened with argon; argon was not absorbed. It would be better to name it ‘the extraction line,’ but I will leave it to the authors' discretion.
Line 68. “... (40Ar*, radiogenic daughter) ...”
Line 72. “... depends on the age and the argon content ...” It would be better to say “... depends on the argon content, i.e., potassium concentration and age ...”
Line 76. “... of argon ...”
Table 1. The lambda-beta value of Steiger and Jäger is 4.962. Where do the errors come from? They are not specified in the Steiger and Jäger publication.
Line 85. From here to the end of the article, including the tables, it is unclear whether the errors are expressed as one or two standard deviations. If the deviations differ, they should be stated now or noted on each occasion.
Line 102. “... released during sample melting ...”
Line 102. Add carbon dioxide to the list; it is quite common.
Lines 114-116. “To compare one analysis to another ... and a charcoal trap.” Is there a lot of volume change in the complete line? I assume the problem is the furnace.
Line 117. According to the published graphs by physicists, the highest cross-section of electronic impact ionization for argon is 80-100 eV, so why is the source regulated at 60 eV? To diminish the background?
Line 125. The sensitivity of the spectrometer also depends on the transmission, typically controlled by the slit widths and the quality of the focus given by the ion source, the magnet, and additional electrical filters, if any.
Line 139. Figure 4 and equation (2). The Y-axis value of Figure 2 is given in pA, but equation (2) seems to provide the values in fA. This should be stated in the text or Figure 2 modified accordingly. What is the relationship between the 40Ar intensity of -644 fA when DE=0 (i.e., no gas) deduced from equation (2) and the values of the analytical blank in Table 2, discussed below? The correlation in Figure 2 is very good, but I would expect the intersection to be positive, implying a positive analytical blank (atmospheric or otherwise). Is there any correction somewhere in the calculation?
Line 150, Figure 5. It seems to me there is some error in the equations under the figure. Perhaps it needs to be explained better how the equations are derived, particularly (3); see the comment below.
Line 153. “The number of 40Ar* atoms released per gram of sample is then:”
Line 153, equation (3). It should be better explained what DEs and DEp are, and it should also be stated that m is the mass in grams.
Lines 163-164. Is the oxygen peak sufficient to know that there is not still too much water, hydrogen, carbon dioxide, or hydrocarbons in the line? If the spectrometer is used to scan, it means the sample has already been introduced; does this not delay the first argon reading too much from time zero? Wouldn't it be better to have a "pipette" with a gas quadrupole in some section of the line to sample a small (but constant) volume of the gas before introduction? The information these instruments provide about the vacuum quality is very significant. Once you have tried one, you cannot work without it.
Lines 167. “... the absence of radiogenic argon in the system ...” But there could also be hydrocarbons, or HCl at mass 36, that modify the 36Ar reading, not just radiogenic argon.
Lines 170-172 and Table 2. In my opinion, the composition of the electronic blank can be problematic. The values are certainly much lower than those of the furnace blank in the case of 40Ar, but in the case of 36Ar, the negative signal value is very close to the 36Ar of the furnace blank. Since it is unclear how the 4.7 fA of the furnace blank has been measured, I cannot confirm. However, suppose the value of 4.7 fA is directly the reading of the multimeter (or the equivalent conversion of voltage to current). In that case, the "base" value of -1.0 fA should be subtracted, giving 5.7 fA, which is not an atmospheric blank (40Ar/36Ar = 1358.8/5.7 = 238.4). The furnace blank may have some atomic or molecular species at 36 Da that is not 36Ar. If the same calculation applies to the "1 DE" values, the 40/36 ratio would be 298.6, which is exactly the value of air. I think the AX amplifier should be adjusted; it surely has a circuit to adjust the zero (offset) with a potentiometer to leave it positive, not negative, to avoid possible confusion. These comments might seem exaggerated, but we must consider that the latest-generation spectrometers have very stable electronics and provide high-precision data, so these details are relevant. All this should be clarified in the manuscript.
Table 2. The furnace blank is quite high, approximately 0.1 V for 40Ar. In modern laser systems, the blank is much lower by two or three orders of magnitude. According to the text, I understand that this is compensated by the weight of the samples analyzed, which makes the signal, on average, about ten times larger than the analytical blank. Even so, reducing the furnace blank by an order of magnitude would be a good goal.
Line 177. “Three micas (or related) ...” It is better to say “Three phyllosilicates (muscovite and glauconite) ...”
Line 191. “... various illite polytypes ...”
Line 237. “... using a Bruker D2 ...”
Line 243. “... (Kübler, 1966) ...”
Lines 337-338. Another explanation is that, at atmospheric pressure, heating oxidizes the Fe+2 that may be present, and the weight increases slightly, causing the weight loss due to water loss not to be so great. This phenomenon often occurs when determining the loss on ignition (LOI) of rock samples.
Line 348-349, equation (5). What is the value of the sensitivity S? With the data provided in the article, I inverted the equation and obtained S=1.5e-11 pA/at. Is this correct?
References
The Charbit et al. citations in the bibliography and the text are 1984a and 1984b, but they have been repeated in the bibliography, and both references are the same.
In the main text, line 375, Eremin (2005) is cited, which is not in the bibliography.
The Kübler, B (1966) citation is misspelled (“Klübler”).
In line 238, Moore & Reynolds (1997) is cited but does not appear in the bibliography.
In line 334, Zimmermann & Odin (1979) are cited but do not appear in the bibliography.
I hope these comments are helpful to the authors and that I have not misunderstood anything.
Sincerely,
Jesús Solé
Does the paper address relevant scientific questions within the scope of GI?
- Does the paper present novel concepts, ideas, tools, or data? YES
- Are substantial conclusions reached? YES
- Are the scientific methods and assumptions valid and clearly outlined? YES
- Are the results sufficient to support the interpretations and conclusions? YES
- Is the description of experiments and calculations sufficiently complete and precise to allow their reproduction by fellow scientists (traceability of results)? YES, with some comments.
- Do the authors give proper credit to related work and clearly indicate their own new/original contribution? YES
- Does the title clearly reflect the contents of the paper? YES
- Does the abstract provide a concise and complete summary? YES
- Is the overall presentation well-structured and clear? YES
- Is the language fluent and precise? YES
- Are mathematical formulae, symbols, abbreviations, and units correctly defined and used? YES, with some comments
- Should any parts of the paper (text, formulae, figures, tables) be clarified, reduced, combined, or eliminated? NO
- Are the number and quality of references appropriate? YES
- Is the amount and quality of supplementary material appropriate? -
Citation: https://doi.org/10.5194/egusphere-2024-1150-RC1 -
AC1: 'Reply on RC1', Marie Gerardin, 25 Jul 2024
Dear Dr. Jesus Solé,
Thank you very much for carefully reviewing our manuscript, providing helpful and valuable comments. The minor comments on english writing or cited reference have been directly corrected in the text. An answer is addressed below each point that needed further discussion.
Regards,
Marie GérardinLine 31. Point (1) is not strictly true. There is evidence that electron capture is affected by pressure, but this does not affect any "real" dating by the K-Ar method.
Line 32. Point (2) is erroneous. The 40K/K ratio decreases over time; this is the basis of the K-Ar geochronometer. The authors mean that it is constant for all samples at present, which is also strictly false, but within the margins of error currently achievable, it can be considered true.
Line 32. Point (3) needs to be worded a little differently, as stating that all 40Ar comes from the decay of 40K does not exclude 40Ar trapped during the rock's formation and originating from the decay of 40K before the system closed. The later point (5) also does not eliminate this case. Saying that 40Ar comes from the decay of 40K within the mineral will solve the problem.
Line 33. Point (4) we know is false, but we must assume it for atmospheric correction. In any case, most of the air in the extraction lines has a modern atmospheric composition. A different case occurs when measuring very old samples with trapped air bubbles (fluid inclusions) that could give values different from modern air (and that have been studied for this reason).
Line 34. As mentioned, point (5) does not exclude radiogenic argon trapped during rock formation.
Reply: Thank you for sharing clarifications about the 5 points that must be addressed to apply K-Ar method. As written in the text, those 5 points are assumptions that we need to consider to apply the method but we know some of them are strictly false, as you rigorously mentioned. I made some modifications in the text following your suggestion and added “It is considered that…” to avoid confusion.
Line 61. The word “desorption” introduced here and used later raises some doubts. The Merriam-Webster dictionary defines it as “to remove (a sorbed substance) by the reverse of adsorption or absorption.” But this is not what happened with argon; argon was not absorbed. It would be better to name it ‘the extraction line,’ but I will leave it to the authors' discretion.
Reply: I agree with this comment, Ar is not physio-sorbed on the surface of micas but trapped in the interlayer. I will replace the word “desorption” by “extraction”, more adapted to radiogenic argon.
Table 1. The lambda-beta value of Steiger and Jäger is 4.962. Where do the errors come from? They are not specified in the Steiger and Jäger publication.
Reply: Thank you for pointing out this mistake. At the beginning of the writing, I evaluated the contribution of an error on the decay constants on the age. I probably took the error mentioned in Guillou et al. (2021). I found that this contribution is negligible compared to the error on the Ar measurement and especially on %K2O. The errors have been removed.
Line 85. From here to the end of the article, including the tables, it is unclear whether the errors are expressed as one or two standard deviations. If the deviations differ, they should be stated now or noted on each occasion.
Reply: One sentence has been added in section 2.1: “If not specified, all errors reported in this paper are expressed as one standard deviation.”
Lines 114-116. “To compare one analysis to another ... and a charcoal trap.” Is there a lot of volume change in the complete line? I assume the problem is the furnace.
Reply: I’m afraid I don’t understand this comment and especially what stands for “volume change”. The gas in the line travels through multiple volumes from the furnace to the MS and is sorbed on a charcoal trap (either to purify the gas or to attract the gas near the MS). In order to compare one MS measurement to another, we need to perform the analysis in an equivalent volume. I choose this volume to be the one of the MS + one “introduction” volume before entering MS.
Line 117. According to the published graphs by physicists, the highest cross-section of electronic impact ionization for argon is 80-100 eV, so why is the source regulated at 60 eV? To diminish the background?
Reply: Thank you for raising this question. I was not aware of these studies about the optimal energy to ionize argon. The source is regulated at 60 eV to reduce the proportion of double ionizations (Ar++, i.e. m/z = 20). I will run some tests in the energy range of 80-100 eV.
Line 125. The sensitivity of the spectrometer also depends on the transmission, typically controlled by the slit widths and the quality of the focus given by the ion source, the magnet, and additional electrical filters, if any.
Reply: Thank you for those details that are definitely included in the “black box” that I called in the text: “the ionization capacities of the source”. I did not risk to make an exhaustive list of all parameters influencing the ionization capacities because it could be interpreted as if we can control all of them.
Line 139. Figure 4 and equation (2). The Y-axis value of Figure 2 is given in pA, but equation (2) seems to provide the values in fA. This should be stated in the text or Figure 2 modified accordingly. What is the relationship between the 40Ar intensity of -644 fA when DE=0 (i.e., no gas) deduced from equation (2) and the values of the analytical blank in Table 2, discussed below? The correlation in Figure 2 is very good, but I would expect the intersection to be positive, implying a positive analytical blank (atmospheric or otherwise). Is there any correction somewhere in the calculation?
Reply: All intensities have been changed in pA in eq. 2. Eq. 2 is an empirical equation based on the signal obtained at nDE (n≥1). This equation is meant to correct the signal from pressure effect in the source. Then it has no meaning when there is no gas in the mass spectrometer. The signal at 0 DE (blank) is then not included in the data to build the equation. The following sentence is added in the text: “Note that this equation is valid when I(40Ar) > 0.01 pA corresponding to the lowest signal measurable in the mass spectrometer (see the blank measurements at section 2.6).” This condition concerns the signal intensity because this is what we measure. The DE is calculated from I.
Line 150, Figure 5. It seems to me there is some error in the equations under the figure. Perhaps it needs to be explained better how the equations are derived, particularly (3); see the comment below.
Reply: The paragraph above Fig.5 has been changed. Hopefully, this will help the reader to better understand the calculation performed after each step of the analysis.
Lines 163-164. Is the oxygen peak sufficient to know that there is not still too much water, hydrogen, carbon dioxide, or hydrocarbons in the line? If the spectrometer is used to scan, it means the sample has already been introduced; does this not delay the first argon reading too much from time zero? Wouldn't it be better to have a "pipette" with a gas quadrupole in some section of the line to sample a small (but constant) volume of the gas before introduction? The information these instruments provide about the vacuum quality is very significant. Once you have tried one, you cannot work without it.
Reply: When the gas enters the MS, it is not in the scan mode. Argon 40, 38 and 36 are measured simultaneously during 200 s (one point every 2s). Then, we use the scan mode of the MS to measure the oxygen pics. The former analysis of argon can be disregarded if the oxygen pics are too high. Indeed it would be very interesting to work with a small quadrupole like a RGA to measure the quality of the purification before analysis into the MS.
Lines 167. “... the absence of radiogenic argon in the system ...” But there could also be hydrocarbons, or HCl at mass 36, that modify the 36Ar reading, not just radiogenic argon.
Reply: This is true that the mass 36 could be contaminated by hydrocarbons or HCl if the purification of the sample didn’t operate properly. I complete the sentence by “or any hydrocarbons or HCl remaining in the system”.
Lines 170-172 and Table 2. In my opinion, the composition of the electronic blank can be problematic. The values are certainly much lower than those of the furnace blank in the case of 40Ar, but in the case of 36Ar, the negative signal value is very close to the 36Ar of the furnace blank. Since it is unclear how the 4.7 fA of the furnace blank has been measured, I cannot confirm. However, suppose the value of 4.7 fA is directly the reading of the multimeter (or the equivalent conversion of voltage to current). In that case, the "base" value of -1.0 fA should be subtracted, giving 5.7 fA, which is not an atmospheric blank (40Ar/36Ar = 1358.8/5.7 = 238.4). The furnace blank may have some atomic or molecular species at 36 Da that is not 36Ar. If the same calculation applies to the "1 DE" values, the 40/36 ratio would be 298.6, which is exactly the value of air. I think the AX amplifier should be adjusted; it surely has a circuit to adjust the zero (offset) with a potentiometer to leave it positive, not negative, to avoid possible confusion. These comments might seem exaggerated, but we must consider that the latest-generation spectrometers have very stable electronics and provide high-precision data, so these details are relevant. All this should be clarified in the manuscript.
Reply: A sentence has been added under Table2: “* signals corrected from the electronic blank”. Indeed, all signals presented in this paper are corrected from the electronic blank. It means that the intensity of the 40Ar in the furnace blank is measured at 1366.1 fA and the 36Ar is measured at 3.7 fA. In the future, I will adjust the signal to zero to avoid confusion.
Table 2. The furnace blank is quite high, approximately 0.1 V for 40Ar. In modern laser systems, the blank is much lower by two or three orders of magnitude. According to the text, I understand that this is compensated by the weight of the samples analyzed, which makes the signal, on average, about ten times larger than the analytical blank. Even so, reducing the furnace blank by an order of magnitude would be a good goal.
Reply: Indeed, the blank is low enough to allow the measurement of radiogenic argon from the samples but is still quite high. This will be reduced with further improvements on the heating device. A new furnace is expected in 2025 with higher heating performance. Notably, the furnace will reach high temperatures in less time limiting the gas release of the furnace surfaces.
Lines 337-338. Another explanation is that, at atmospheric pressure, heating oxidizes the Fe+2 that may be present, and the weight increases slightly, causing the weight loss due to water loss not to be so great. This phenomenon often occurs when determining the loss on ignition (LOI) of rock samples.
Reply: Thank you for this comment. I was not aware of the weight increase by Fe2+ oxidation. I added this hypothesis to explain my result in the Appendix.
Line 348-349, equation (5). What is the value of the sensitivity S? With the data provided in the article, I inverted the equation and obtained S=1.5e-11 pA/at. Is this correct?
Reply: Yes, this value is around 1.6e-11 pA/at. The sensitivity can also be calculated using the equation 4 and considering that is 50.68 pA.
Citation: https://doi.org/10.5194/egusphere-2024-1150-AC1 -
RC2: 'Reply on AC1', Jesús Solé, 01 Aug 2024
I appreciate the comments that clarify my questions, and I agree with your explanations. I don't have any more questions.
Regarding the electron energy for the ionization of the noble gases, it is true that some older instruments used about 70eV, as it was a compromise between sensitivity and double ionization. Some people (myself included) have lowered the energy to about 40eV to reduce the interferences of double ionization of argon 40 and CO2 44 during the measurement of neon at masses 20 and 22. This reduction in energy leads to a decrease in sensitivity (precision) but increases accuracy. Other instruments use 100eV or even 110eV. In modern high-resolution instruments, it is no longer necessary or desirable to modify the value recommended by the manufacturer to avoid altering the conditions of the ion source (sensitivity, linearity, etc).
The probably best publication on the electron energy to ionize noble gases is:
Rejoub, B. G. Lindsay, and R. F. Stebbings (2002) Determination of the absolute partial and total cross sections for electron-impact ionization of the rare gases. Physical Review A, 65, 042713, 8 pp.
I am confident that your new laboratory will be a great success.
Sincerely,
Jesús Solé
Citation: https://doi.org/10.5194/egusphere-2024-1150-RC2
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RC2: 'Reply on AC1', Jesús Solé, 01 Aug 2024
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RC3: 'Comment on egusphere-2024-1150', Marek Tulej, 12 Aug 2024
General comments
The manuscript by Gerardin et al. contributes to development of the K-Ar dating technique. This technique is known already for decades and numerous studies on various mineralogical phases have proved its feasibility for conducting the material formation ages in time range from thousands to billions years. This method is also promising for in situ chronology applications and is of considerable interest for future landing planetary missions (e.g. on Mars). Though, the instrumentation used to analyze the abundances of K and Ar differs to the current method, the performance studies showed its applicability for in situ application. Among other interest clay material considered here is important mineral phases in searches for extinct and extant life on the other planets. Thus, the current studies can be helpful for the currently important topic in planetary sciences.
The manuscript described the studies conducted on illite which is isolated from clay phase by authors by using isolation system specifically developed for this application. The abundance analysis and dating results are convincing and encouraging proving feasibility of the current approach for delivering accurate and precise analysis of even uneasy samples. The manuscript is well-structured and the experimental part sufficiently detailed so could be reproduced with an ease by others if necessary.
Overall this is well-written manuscript, well justified with respect to analyses of clays, important group of mineral phases and the results of analysis are of high quality. At this point, I do not see also any minor corrections to be added to the last version of manuscript. Thanks to first reviewer my job here is easier and again, followed the first reviewer I recommend publishing the manuscript after the last changes.
Citation: https://doi.org/10.5194/egusphere-2024-1150-RC3 -
AC2: 'Reply on RC3', Marie Gerardin, 19 Aug 2024
Dear Pr. Marek Tulej,
Thank you very much for your encouraging comments based on the application of the K-Ar dating technique. As you mentionned, this technique can be useful in planetary sciences, especially when the system is miniaturized for in situ measurements during missions. This topic is studied by the GEOPS team in Orsay-Paris, where the K content is determined by LIBS. This promising technique allows for combining the K and the Ar measurements and is also studied by Dr Jesus Solé in the Institute of Geology in Mexico.
Regards
Marie GerardinCitation: https://doi.org/10.5194/egusphere-2024-1150-AC2
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AC2: 'Reply on RC3', Marie Gerardin, 19 Aug 2024
Interactive discussion
Status: closed
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RC1: 'Comment on egusphere-2024-1150', Jesús Solé, 19 Jul 2024
General comments
I have read with interest the article “Development of an integrated analytical platform of clay minerals separation, characterization and 40K/40Ar dating”. The K-Ar dating of clays, particularly illite, has been used for sixty years but has had a renaissance in the last two decades. Its application to diagenesis and structural processes (faults, tectonic uplift, low-grade deformation, water circulation, and others) has promoted its use. The K-Ar method is particularly well-suited to the dating of clays, as the 40Ar-39Ar method, although more powerful, is analytically and interpretatively difficult in this case. It should be added that the closure of several Ar-Ar laboratories worldwide makes establishing a new K-Ar laboratory good news.
After a few detailed readings, I have not found any major problems in this contribution, which seems well-detailed and structured. However, some minor points need to be adressed, so I suggest it be published with minor revisions.
Since there are a few points to discuss, I have listed them below by line number, regardless of whether they are minor typographical errors or points that need better discussion. I am not a native English speaker, but I believe the manuscript is well-written. There are some minor language details that I have marked and perhaps some others that I have not detected.
Specific comments
Title. I propose three minor changes: “Development of an integrated analytical platform for clay mineral separation, characterization, and K-Ar dating.” The first two changes are grammatical, and the last is a suggestion to standardize the title with the main text where the term K-Ar is always used.
Line 27. Change to: “... detailed by Schaeffer and Zähringer (1966) and by ...”
Line 31. Point (1) is not strictly true. There is evidence that electron capture is affected by pressure, but this does not affect any "real" dating by the K-Ar method.
Line 32. Point (2) is erroneous. The 40K/K ratio decreases over time; this is the basis of the K-Ar geochronometer. The authors mean that it is constant for all samples at present, which is also strictly false, but within the margins of error currently achievable, it can be considered true.
Line 32. Point (3) needs to be worded a little differently, as stating that all 40Ar comes from the decay of 40K does not exclude 40Ar trapped during the rock's formation and originating from the decay of 40K before the system closed. The later point (5) also does not eliminate this case. Saying that 40Ar comes from the decay of 40K within the mineral will solve the problem.
Line 33. Point (4) we know is false, but we must assume it for atmospheric correction. In any case, most of the air in the extraction lines has a modern atmospheric composition. A different case occurs when measuring very old samples with trapped air bubbles (fluid inclusions) that could give values different from modern air (and that have been studied for this reason).
Line 34. As mentioned, point (5) does not exclude radiogenic argon trapped during rock formation.
Line 35. “The latter might be hypothetical ...” I would say it is typically false in these cases, which is why K-Ar (Ar-Ar) can be used to obtain data on the thermal history, as detailed already in the manuscript.
Line 39. “This age is then related to ...” I would modify this sentence as follows: “This age is related to the crystallization event in the case of fast cooling (e.g., unaltered volcanic rocks), the closure time for slow cooling rocks (e.g., plutonic, metamorphic) or ...”
Line 53. “One of the main concerns ...”
Line 61. The word “desorption” introduced here and used later raises some doubts. The Merriam-Webster dictionary defines it as “to remove (a sorbed substance) by the reverse of adsorption or absorption.” But this is not what happened with argon; argon was not absorbed. It would be better to name it ‘the extraction line,’ but I will leave it to the authors' discretion.
Line 68. “... (40Ar*, radiogenic daughter) ...”
Line 72. “... depends on the age and the argon content ...” It would be better to say “... depends on the argon content, i.e., potassium concentration and age ...”
Line 76. “... of argon ...”
Table 1. The lambda-beta value of Steiger and Jäger is 4.962. Where do the errors come from? They are not specified in the Steiger and Jäger publication.
Line 85. From here to the end of the article, including the tables, it is unclear whether the errors are expressed as one or two standard deviations. If the deviations differ, they should be stated now or noted on each occasion.
Line 102. “... released during sample melting ...”
Line 102. Add carbon dioxide to the list; it is quite common.
Lines 114-116. “To compare one analysis to another ... and a charcoal trap.” Is there a lot of volume change in the complete line? I assume the problem is the furnace.
Line 117. According to the published graphs by physicists, the highest cross-section of electronic impact ionization for argon is 80-100 eV, so why is the source regulated at 60 eV? To diminish the background?
Line 125. The sensitivity of the spectrometer also depends on the transmission, typically controlled by the slit widths and the quality of the focus given by the ion source, the magnet, and additional electrical filters, if any.
Line 139. Figure 4 and equation (2). The Y-axis value of Figure 2 is given in pA, but equation (2) seems to provide the values in fA. This should be stated in the text or Figure 2 modified accordingly. What is the relationship between the 40Ar intensity of -644 fA when DE=0 (i.e., no gas) deduced from equation (2) and the values of the analytical blank in Table 2, discussed below? The correlation in Figure 2 is very good, but I would expect the intersection to be positive, implying a positive analytical blank (atmospheric or otherwise). Is there any correction somewhere in the calculation?
Line 150, Figure 5. It seems to me there is some error in the equations under the figure. Perhaps it needs to be explained better how the equations are derived, particularly (3); see the comment below.
Line 153. “The number of 40Ar* atoms released per gram of sample is then:”
Line 153, equation (3). It should be better explained what DEs and DEp are, and it should also be stated that m is the mass in grams.
Lines 163-164. Is the oxygen peak sufficient to know that there is not still too much water, hydrogen, carbon dioxide, or hydrocarbons in the line? If the spectrometer is used to scan, it means the sample has already been introduced; does this not delay the first argon reading too much from time zero? Wouldn't it be better to have a "pipette" with a gas quadrupole in some section of the line to sample a small (but constant) volume of the gas before introduction? The information these instruments provide about the vacuum quality is very significant. Once you have tried one, you cannot work without it.
Lines 167. “... the absence of radiogenic argon in the system ...” But there could also be hydrocarbons, or HCl at mass 36, that modify the 36Ar reading, not just radiogenic argon.
Lines 170-172 and Table 2. In my opinion, the composition of the electronic blank can be problematic. The values are certainly much lower than those of the furnace blank in the case of 40Ar, but in the case of 36Ar, the negative signal value is very close to the 36Ar of the furnace blank. Since it is unclear how the 4.7 fA of the furnace blank has been measured, I cannot confirm. However, suppose the value of 4.7 fA is directly the reading of the multimeter (or the equivalent conversion of voltage to current). In that case, the "base" value of -1.0 fA should be subtracted, giving 5.7 fA, which is not an atmospheric blank (40Ar/36Ar = 1358.8/5.7 = 238.4). The furnace blank may have some atomic or molecular species at 36 Da that is not 36Ar. If the same calculation applies to the "1 DE" values, the 40/36 ratio would be 298.6, which is exactly the value of air. I think the AX amplifier should be adjusted; it surely has a circuit to adjust the zero (offset) with a potentiometer to leave it positive, not negative, to avoid possible confusion. These comments might seem exaggerated, but we must consider that the latest-generation spectrometers have very stable electronics and provide high-precision data, so these details are relevant. All this should be clarified in the manuscript.
Table 2. The furnace blank is quite high, approximately 0.1 V for 40Ar. In modern laser systems, the blank is much lower by two or three orders of magnitude. According to the text, I understand that this is compensated by the weight of the samples analyzed, which makes the signal, on average, about ten times larger than the analytical blank. Even so, reducing the furnace blank by an order of magnitude would be a good goal.
Line 177. “Three micas (or related) ...” It is better to say “Three phyllosilicates (muscovite and glauconite) ...”
Line 191. “... various illite polytypes ...”
Line 237. “... using a Bruker D2 ...”
Line 243. “... (Kübler, 1966) ...”
Lines 337-338. Another explanation is that, at atmospheric pressure, heating oxidizes the Fe+2 that may be present, and the weight increases slightly, causing the weight loss due to water loss not to be so great. This phenomenon often occurs when determining the loss on ignition (LOI) of rock samples.
Line 348-349, equation (5). What is the value of the sensitivity S? With the data provided in the article, I inverted the equation and obtained S=1.5e-11 pA/at. Is this correct?
References
The Charbit et al. citations in the bibliography and the text are 1984a and 1984b, but they have been repeated in the bibliography, and both references are the same.
In the main text, line 375, Eremin (2005) is cited, which is not in the bibliography.
The Kübler, B (1966) citation is misspelled (“Klübler”).
In line 238, Moore & Reynolds (1997) is cited but does not appear in the bibliography.
In line 334, Zimmermann & Odin (1979) are cited but do not appear in the bibliography.
I hope these comments are helpful to the authors and that I have not misunderstood anything.
Sincerely,
Jesús Solé
Does the paper address relevant scientific questions within the scope of GI?
- Does the paper present novel concepts, ideas, tools, or data? YES
- Are substantial conclusions reached? YES
- Are the scientific methods and assumptions valid and clearly outlined? YES
- Are the results sufficient to support the interpretations and conclusions? YES
- Is the description of experiments and calculations sufficiently complete and precise to allow their reproduction by fellow scientists (traceability of results)? YES, with some comments.
- Do the authors give proper credit to related work and clearly indicate their own new/original contribution? YES
- Does the title clearly reflect the contents of the paper? YES
- Does the abstract provide a concise and complete summary? YES
- Is the overall presentation well-structured and clear? YES
- Is the language fluent and precise? YES
- Are mathematical formulae, symbols, abbreviations, and units correctly defined and used? YES, with some comments
- Should any parts of the paper (text, formulae, figures, tables) be clarified, reduced, combined, or eliminated? NO
- Are the number and quality of references appropriate? YES
- Is the amount and quality of supplementary material appropriate? -
Citation: https://doi.org/10.5194/egusphere-2024-1150-RC1 -
AC1: 'Reply on RC1', Marie Gerardin, 25 Jul 2024
Dear Dr. Jesus Solé,
Thank you very much for carefully reviewing our manuscript, providing helpful and valuable comments. The minor comments on english writing or cited reference have been directly corrected in the text. An answer is addressed below each point that needed further discussion.
Regards,
Marie GérardinLine 31. Point (1) is not strictly true. There is evidence that electron capture is affected by pressure, but this does not affect any "real" dating by the K-Ar method.
Line 32. Point (2) is erroneous. The 40K/K ratio decreases over time; this is the basis of the K-Ar geochronometer. The authors mean that it is constant for all samples at present, which is also strictly false, but within the margins of error currently achievable, it can be considered true.
Line 32. Point (3) needs to be worded a little differently, as stating that all 40Ar comes from the decay of 40K does not exclude 40Ar trapped during the rock's formation and originating from the decay of 40K before the system closed. The later point (5) also does not eliminate this case. Saying that 40Ar comes from the decay of 40K within the mineral will solve the problem.
Line 33. Point (4) we know is false, but we must assume it for atmospheric correction. In any case, most of the air in the extraction lines has a modern atmospheric composition. A different case occurs when measuring very old samples with trapped air bubbles (fluid inclusions) that could give values different from modern air (and that have been studied for this reason).
Line 34. As mentioned, point (5) does not exclude radiogenic argon trapped during rock formation.
Reply: Thank you for sharing clarifications about the 5 points that must be addressed to apply K-Ar method. As written in the text, those 5 points are assumptions that we need to consider to apply the method but we know some of them are strictly false, as you rigorously mentioned. I made some modifications in the text following your suggestion and added “It is considered that…” to avoid confusion.
Line 61. The word “desorption” introduced here and used later raises some doubts. The Merriam-Webster dictionary defines it as “to remove (a sorbed substance) by the reverse of adsorption or absorption.” But this is not what happened with argon; argon was not absorbed. It would be better to name it ‘the extraction line,’ but I will leave it to the authors' discretion.
Reply: I agree with this comment, Ar is not physio-sorbed on the surface of micas but trapped in the interlayer. I will replace the word “desorption” by “extraction”, more adapted to radiogenic argon.
Table 1. The lambda-beta value of Steiger and Jäger is 4.962. Where do the errors come from? They are not specified in the Steiger and Jäger publication.
Reply: Thank you for pointing out this mistake. At the beginning of the writing, I evaluated the contribution of an error on the decay constants on the age. I probably took the error mentioned in Guillou et al. (2021). I found that this contribution is negligible compared to the error on the Ar measurement and especially on %K2O. The errors have been removed.
Line 85. From here to the end of the article, including the tables, it is unclear whether the errors are expressed as one or two standard deviations. If the deviations differ, they should be stated now or noted on each occasion.
Reply: One sentence has been added in section 2.1: “If not specified, all errors reported in this paper are expressed as one standard deviation.”
Lines 114-116. “To compare one analysis to another ... and a charcoal trap.” Is there a lot of volume change in the complete line? I assume the problem is the furnace.
Reply: I’m afraid I don’t understand this comment and especially what stands for “volume change”. The gas in the line travels through multiple volumes from the furnace to the MS and is sorbed on a charcoal trap (either to purify the gas or to attract the gas near the MS). In order to compare one MS measurement to another, we need to perform the analysis in an equivalent volume. I choose this volume to be the one of the MS + one “introduction” volume before entering MS.
Line 117. According to the published graphs by physicists, the highest cross-section of electronic impact ionization for argon is 80-100 eV, so why is the source regulated at 60 eV? To diminish the background?
Reply: Thank you for raising this question. I was not aware of these studies about the optimal energy to ionize argon. The source is regulated at 60 eV to reduce the proportion of double ionizations (Ar++, i.e. m/z = 20). I will run some tests in the energy range of 80-100 eV.
Line 125. The sensitivity of the spectrometer also depends on the transmission, typically controlled by the slit widths and the quality of the focus given by the ion source, the magnet, and additional electrical filters, if any.
Reply: Thank you for those details that are definitely included in the “black box” that I called in the text: “the ionization capacities of the source”. I did not risk to make an exhaustive list of all parameters influencing the ionization capacities because it could be interpreted as if we can control all of them.
Line 139. Figure 4 and equation (2). The Y-axis value of Figure 2 is given in pA, but equation (2) seems to provide the values in fA. This should be stated in the text or Figure 2 modified accordingly. What is the relationship between the 40Ar intensity of -644 fA when DE=0 (i.e., no gas) deduced from equation (2) and the values of the analytical blank in Table 2, discussed below? The correlation in Figure 2 is very good, but I would expect the intersection to be positive, implying a positive analytical blank (atmospheric or otherwise). Is there any correction somewhere in the calculation?
Reply: All intensities have been changed in pA in eq. 2. Eq. 2 is an empirical equation based on the signal obtained at nDE (n≥1). This equation is meant to correct the signal from pressure effect in the source. Then it has no meaning when there is no gas in the mass spectrometer. The signal at 0 DE (blank) is then not included in the data to build the equation. The following sentence is added in the text: “Note that this equation is valid when I(40Ar) > 0.01 pA corresponding to the lowest signal measurable in the mass spectrometer (see the blank measurements at section 2.6).” This condition concerns the signal intensity because this is what we measure. The DE is calculated from I.
Line 150, Figure 5. It seems to me there is some error in the equations under the figure. Perhaps it needs to be explained better how the equations are derived, particularly (3); see the comment below.
Reply: The paragraph above Fig.5 has been changed. Hopefully, this will help the reader to better understand the calculation performed after each step of the analysis.
Lines 163-164. Is the oxygen peak sufficient to know that there is not still too much water, hydrogen, carbon dioxide, or hydrocarbons in the line? If the spectrometer is used to scan, it means the sample has already been introduced; does this not delay the first argon reading too much from time zero? Wouldn't it be better to have a "pipette" with a gas quadrupole in some section of the line to sample a small (but constant) volume of the gas before introduction? The information these instruments provide about the vacuum quality is very significant. Once you have tried one, you cannot work without it.
Reply: When the gas enters the MS, it is not in the scan mode. Argon 40, 38 and 36 are measured simultaneously during 200 s (one point every 2s). Then, we use the scan mode of the MS to measure the oxygen pics. The former analysis of argon can be disregarded if the oxygen pics are too high. Indeed it would be very interesting to work with a small quadrupole like a RGA to measure the quality of the purification before analysis into the MS.
Lines 167. “... the absence of radiogenic argon in the system ...” But there could also be hydrocarbons, or HCl at mass 36, that modify the 36Ar reading, not just radiogenic argon.
Reply: This is true that the mass 36 could be contaminated by hydrocarbons or HCl if the purification of the sample didn’t operate properly. I complete the sentence by “or any hydrocarbons or HCl remaining in the system”.
Lines 170-172 and Table 2. In my opinion, the composition of the electronic blank can be problematic. The values are certainly much lower than those of the furnace blank in the case of 40Ar, but in the case of 36Ar, the negative signal value is very close to the 36Ar of the furnace blank. Since it is unclear how the 4.7 fA of the furnace blank has been measured, I cannot confirm. However, suppose the value of 4.7 fA is directly the reading of the multimeter (or the equivalent conversion of voltage to current). In that case, the "base" value of -1.0 fA should be subtracted, giving 5.7 fA, which is not an atmospheric blank (40Ar/36Ar = 1358.8/5.7 = 238.4). The furnace blank may have some atomic or molecular species at 36 Da that is not 36Ar. If the same calculation applies to the "1 DE" values, the 40/36 ratio would be 298.6, which is exactly the value of air. I think the AX amplifier should be adjusted; it surely has a circuit to adjust the zero (offset) with a potentiometer to leave it positive, not negative, to avoid possible confusion. These comments might seem exaggerated, but we must consider that the latest-generation spectrometers have very stable electronics and provide high-precision data, so these details are relevant. All this should be clarified in the manuscript.
Reply: A sentence has been added under Table2: “* signals corrected from the electronic blank”. Indeed, all signals presented in this paper are corrected from the electronic blank. It means that the intensity of the 40Ar in the furnace blank is measured at 1366.1 fA and the 36Ar is measured at 3.7 fA. In the future, I will adjust the signal to zero to avoid confusion.
Table 2. The furnace blank is quite high, approximately 0.1 V for 40Ar. In modern laser systems, the blank is much lower by two or three orders of magnitude. According to the text, I understand that this is compensated by the weight of the samples analyzed, which makes the signal, on average, about ten times larger than the analytical blank. Even so, reducing the furnace blank by an order of magnitude would be a good goal.
Reply: Indeed, the blank is low enough to allow the measurement of radiogenic argon from the samples but is still quite high. This will be reduced with further improvements on the heating device. A new furnace is expected in 2025 with higher heating performance. Notably, the furnace will reach high temperatures in less time limiting the gas release of the furnace surfaces.
Lines 337-338. Another explanation is that, at atmospheric pressure, heating oxidizes the Fe+2 that may be present, and the weight increases slightly, causing the weight loss due to water loss not to be so great. This phenomenon often occurs when determining the loss on ignition (LOI) of rock samples.
Reply: Thank you for this comment. I was not aware of the weight increase by Fe2+ oxidation. I added this hypothesis to explain my result in the Appendix.
Line 348-349, equation (5). What is the value of the sensitivity S? With the data provided in the article, I inverted the equation and obtained S=1.5e-11 pA/at. Is this correct?
Reply: Yes, this value is around 1.6e-11 pA/at. The sensitivity can also be calculated using the equation 4 and considering that is 50.68 pA.
Citation: https://doi.org/10.5194/egusphere-2024-1150-AC1 -
RC2: 'Reply on AC1', Jesús Solé, 01 Aug 2024
I appreciate the comments that clarify my questions, and I agree with your explanations. I don't have any more questions.
Regarding the electron energy for the ionization of the noble gases, it is true that some older instruments used about 70eV, as it was a compromise between sensitivity and double ionization. Some people (myself included) have lowered the energy to about 40eV to reduce the interferences of double ionization of argon 40 and CO2 44 during the measurement of neon at masses 20 and 22. This reduction in energy leads to a decrease in sensitivity (precision) but increases accuracy. Other instruments use 100eV or even 110eV. In modern high-resolution instruments, it is no longer necessary or desirable to modify the value recommended by the manufacturer to avoid altering the conditions of the ion source (sensitivity, linearity, etc).
The probably best publication on the electron energy to ionize noble gases is:
Rejoub, B. G. Lindsay, and R. F. Stebbings (2002) Determination of the absolute partial and total cross sections for electron-impact ionization of the rare gases. Physical Review A, 65, 042713, 8 pp.
I am confident that your new laboratory will be a great success.
Sincerely,
Jesús Solé
Citation: https://doi.org/10.5194/egusphere-2024-1150-RC2
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RC2: 'Reply on AC1', Jesús Solé, 01 Aug 2024
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RC3: 'Comment on egusphere-2024-1150', Marek Tulej, 12 Aug 2024
General comments
The manuscript by Gerardin et al. contributes to development of the K-Ar dating technique. This technique is known already for decades and numerous studies on various mineralogical phases have proved its feasibility for conducting the material formation ages in time range from thousands to billions years. This method is also promising for in situ chronology applications and is of considerable interest for future landing planetary missions (e.g. on Mars). Though, the instrumentation used to analyze the abundances of K and Ar differs to the current method, the performance studies showed its applicability for in situ application. Among other interest clay material considered here is important mineral phases in searches for extinct and extant life on the other planets. Thus, the current studies can be helpful for the currently important topic in planetary sciences.
The manuscript described the studies conducted on illite which is isolated from clay phase by authors by using isolation system specifically developed for this application. The abundance analysis and dating results are convincing and encouraging proving feasibility of the current approach for delivering accurate and precise analysis of even uneasy samples. The manuscript is well-structured and the experimental part sufficiently detailed so could be reproduced with an ease by others if necessary.
Overall this is well-written manuscript, well justified with respect to analyses of clays, important group of mineral phases and the results of analysis are of high quality. At this point, I do not see also any minor corrections to be added to the last version of manuscript. Thanks to first reviewer my job here is easier and again, followed the first reviewer I recommend publishing the manuscript after the last changes.
Citation: https://doi.org/10.5194/egusphere-2024-1150-RC3 -
AC2: 'Reply on RC3', Marie Gerardin, 19 Aug 2024
Dear Pr. Marek Tulej,
Thank you very much for your encouraging comments based on the application of the K-Ar dating technique. As you mentionned, this technique can be useful in planetary sciences, especially when the system is miniaturized for in situ measurements during missions. This topic is studied by the GEOPS team in Orsay-Paris, where the K content is determined by LIBS. This promising technique allows for combining the K and the Ar measurements and is also studied by Dr Jesus Solé in the Institute of Geology in Mexico.
Regards
Marie GerardinCitation: https://doi.org/10.5194/egusphere-2024-1150-AC2
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AC2: 'Reply on RC3', Marie Gerardin, 19 Aug 2024
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