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the Creative Commons Attribution 4.0 License.
| 13 Mar 2024
Production rate calibration for cosmogenic 10Be in pyroxene by applying a rapid fusion method to 10Be-saturated samples from the Transantarctic Mountains, Antarctica
Abstract. Measurements of multiple cosmogenic nuclides in a single sample are valuable for various applications of cosmogenic nuclide exposure dating and allow for correcting exposure ages for surface weathering and erosion and establishing exposure-burial history. Here we provide advances in the measurement of cosmogenic 10Be in pyroxene and constraints on the production rate which provide new opportunities for measurements of multi-nuclide systems, such as 10Be/3He, in pyroxene-bearing samples. We extracted and measured cosmogenic 10Be in pyroxene from two sets of Ferrar Dolerite samples collected from the Transantarctic Mountains in Antarctica. One set of samples has 10Be concentrations close to saturation which allows for the production rate calibration of 10Be in pyroxene by assuming production-erosion equilibrium. The other set of samples, which has a more recent exposure history, is used to determine if a rapid fusion method can be successfully applied to samples with Holocene to Last-Glacial-Maximum exposure ages. From measured 10Be concentrations in the near-saturation sample set we find the production rate of 10Be in pyroxene to be 3.74 +/- 0.10 atoms g-1 yr-1 and is consistent with 10Be/3He paired nuclide ratios from samples assumed to have simple exposure. Given the high 10Be concentration measured in this sample set, a sample mass of ~0.5 g of pyroxene is sufficient for the extraction of cosmogenic 10Be from pyroxene using a rapid fusion method. However, for the set of samples having low 10Be concentrations, measured concentrations were higher than expected. We attribute spuriously high 10Be concentration to potential failure in removing all meteoric 10Be and/or a highly variable and poorly quantified measurement background.
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RC1: 'Comment on egusphere-2024-702', Anonymous Referee #1, 30 Mar 2024
This manuscript describes a method for extracting 10Be from pyroxene using a rapid fusion technique and presents a new calibration of the 10Be production rate in pyroxene. The fusion technique is employed on two sets of pyroxene samples from Antarctica, one that is near saturation with respect to in situ 10Be and another with late-glacial exposure ages. The near-saturated samples, in which the in situ-produced inventory greatly exceeds any potential meteoric 10Be contamination, are then used to calibrate the production rate of 10Be. However, the measured 10Be concentrations in the late-glacial samples are higher than predicted by this production rate, which the authors attribute to either incomplete removal of meteoric 10Be or blank issues.
I believe that this contribution to Geochronology will be useful for those seeking a 10Be target mineral suitable for quartz-poor lithologies. I found the manuscript to be well-organized, the presentation concise and straightforward, and the figures appropriate and illustrative. My principal concern is that the authors do not consider the compositional variability of pyroxene and the impact of this variability on production rates. I think that providing some constraints on the composition of the analyzed pyroxenes would be useful for generalizing these results to pyroxenes of different compositions. In addition, I have some questions that I hope will clarify several points in the manuscript.
General Comments:
1.) Dependence of the production rate on target chemistry
Is the production rate of 10Be in pyroxene dependent on the composition of the pyroxene? Unlike quartz, which is characterized by a tightly constrained target chemistry, pyroxene is not a single mineral, but rather a group of minerals of diverse compositions. I would argue that the production rate of 10Be in pyroxene is likely to vary with pyroxene composition.
To illustrate this with a “back-of-the-envelope” calculation, assume that, to a first order, all production of 10Be in pyroxene occurs through spallation on oxygen. In enstatite (with an endmember composition of Mg2Si2O6) oxygen accounts for 47.8% of the molar mass. Now, consider that in another common pyroxene, hypersthene (En50Fs50) (MgFeSi2O6), oxygen accounts for 41.3% of the molar mass. This implies that the production rate in enstatite is ca. 16% higher than in hypersthene. The actual difference in production rate is likely to be even larger when considering that 10Be production from Fe is lower than from Mg. Similar calculations can be made for other pyroxenes, for example the difference between enstatite and hedenbergite (CaFeSi2O6) should be greater than 24%.
Although comparisons between endmember compositions give a maximum range of variation, it seems likely that the variation in production rate due to compositional differences would very often be much greater than the ca. 2% analytical uncertainties on 10Be measurements. I wonder if such variation might be the source of the scatter between estimates of 10Be production rates in pyroxene from different studies (as shown in Figure 3). The Balter-Kennedy data, which you note agrees well with yours, was also obtained from pyroxenes from the Ferrar dolerite, which may have a similar composition to those you analyzed.
Although this critique does not in any way invalidate the measurements presented in this study, I think that it would be useful to know to which pyroxene composition this production rate applies. I encourage the authors to provide some constraints on the composition of the pyroxenes that they have analyzed. At a minimum, these constraints could be literature values for pyroxene from the Ferrar dolerite, although direct measurements of the composition of the analyzed pyroxenes would be even more useful. Likewise, if the authors believe that compositional variation is not important, then I think that this argument should be made in the text.
2.) Clarification of analytical methods
I think that in several places some of the methodological descriptions should be clarified, given that presentation and evaluation of the fusion approach for preparing pyroxene samples is a principal point of this study. In specific, I found the latter part of the fusion procedure description to be hard to follow (please see specific comments from lines 140-146).
Likewise, I think that the discussion of the 3He measurements and data analysis is pretty spartan. I understand that the 3He data may not be the focus of the manuscript, but the authors might consider developing this further for the benefit both of readers who are not familiar with 3He as well as for those readers interested in the details of the data analysis. For example, you might consider including some brief explanation of the nuances of 3He, such as capture of cosmogenic and radiogenic thermal neutrons by Li, recoil losses, mantle He etc., which are alluded to later in the manuscript but are never explained.
Specific Comments:
Line 17: Do you mean secular equilibrium here? To me, “production-erosion equilibrium” sounds like steady-state erosion, but in Eqn. 1 you do not consider erosion and at line 76 you give a statement of secular equilibrium. In a few places in the manuscript, you say “production-decay saturation,” which may also be a better way of phrasing this. I recommend changing “production-erosion equilibrium” to “production-decay saturation” or “secular equilibrium” throughout.
Line 79: Isn’t the production rate weighted towards the younger portion of that time interval by radioactive decay? What is the actual averaging time?
Line 97: If the pyroxenes in the Ferrar dolerite have a range of compositions, does using a magnetic separator to separate the pyroxene mean that you end-up selecting the more Fe-rich pyroxene fraction?
Line 102: Why do you begin this sentence by stating that a fine grain size reduces the amount of meteoric 10Be stored in the grain fractures, but then say that you do not need to powder the samples? Wouldn’t the first clause suggest that by powdering the samples you would decrease meteoric contamination?
Line 105: What volume of acid was used vs. mass of sample? In other words, is the fraction dissolved limited by the acid volume (because the reaction goes to equilibrium) or by the reaction time?
Line 111: Does powdering after mineral separation and HF-leaching introduce the possibility for contamination? (Either from residual dust from previous samples processed in the shatterbox or from fragments of the material that the mill is made from?). It seems like this might be problematic if the shatterbox is routinely used to process soils or other high 10Be/9Be materials.
Line 123: At what temperature were these samples fused?
Line 136: Maybe it would be worthwhile to state here that BeF2 is soluble and most other fluorides are not? This is necessary to know to understand this method.
Line 138: Do you remove the KClO4 by centrifugation?
Line 140-146: How you get from the KClO4 precipitate and a supernatant with the dissolved Be to a cathode that is ready to measure is not clear to me. Do you remove the KClO4 precipitate by centrifugation, evaporate the excess HClO4, and then redissolve the residues in 12 mL of HNO3? What concentration do you mean by “dilute”?
When do you precipitate the Be and at what pH?
How do you determine Be yields? By ICP-OES measurements of aliquots? If so, when were these taken? Or do you determine the yield gravimetrically from the mass of the BeO product?
When you say “redissolving the precipitated sample” at line 141 to which precipitated sample do you refer (at the end of the last paragraph it sounded like your beryllium was dissolved in 12 mL HNO3) and in which acid do you redissolve in?
When you say “these samples” are you referring to all your samples, or only to a subset? Earlier you mentioned that you changed your flux/Na2SO4 ratio at some point. Are you referring to something to do with this change?
Is the idea here that some K was present in the product, which led to a dilution effect in the ion source, reducing the beam currents? If you measured the beryllium yield by ICP-OES, then did you also look for other elements, such as K? Why would K co-precipitate with the Be? Isn’t K highly soluble at the mildly alkaline pH that you typically precipitate Be at?
Table 2: Why do you think that a higher Be yield is not correlated with a higher beam current?
Table 3: Please add the errors on the 10Be SLHL production rates. Also, I think that it might be worthwhile to include the Stone scaling factors in this table. This would make your production rates easier to recalculate.
Figure 1: Is it necessary to present both the “measured” and “corrected” 10Be production rates on the figure? Aren’t the “corrected” rates the actual rates after applying the thickness and shielding factors? Are the rates presented in Table 3 and that you state are calculated using Eqn. 1 the “corrected” or “measured” rates? If they are “corrected”, then I would recommend adding the thickness and shielding factors to Eqn. 1.
With respect to the outliers, are there two samples here, or is it like the others, where you present a “measured” and a “corrected” value? They are not distinguished by color.
More generally, I would recommend sticking to presenting one set of rates, including the thickness and shielding factors (i.e., the “corrected” rates), in all tables and figures. I think this would avoid confusion between the two, and keep attention focused on the best estimate of the production rate.
Line 198: How large is this subtraction?
Line 203-205: It seems to me that this outlier rejection could be justified more rigorously, perhaps using a statistical test, such as Chauvenet’s criterion. Moreover, why do you think it is an outlier? Could it be related to variations in the target chemistry between samples?
Line 205: Why quote the standard deviation here rather than the standard error of the mean?
Line 209-212: Please explain this more clearly. Was this production rate cross-calibrated against 3He? Is the reason limiting assumptions are required that 10Be decays significantly over the relevant exposure timescales, but stable 3He does not?
Figure 2: What 3He production rate was used to calculate this figure and did you consider muon-production of 3He and 10Be? Which scaling model was used for the site-specific normalization? Did you use your production rate for 10Be or the average of your production rate and the Balter-Kennedy et al. data? Why are the error ellipses tilted?
Lines 224-229: What production rates do these studies give?
Lines 230-240: The LSDn scaling model may be time-dependent, but doesn’t that not really matter at high latitudes where the cutoff rigidity is 0? Isn’t it that LSDn takes into consideration the softening of the energy spectrum with decreasing altitude and the dependence of each production reaction on different cosmic ray energies?
How are you calculating the LSDn scaling factors? LSDn is reaction-specific in the sense that it takes into consideration the excitation functions for each production reaction (e.g., neutron and proton spallation on Si and O for quartz). Do you just use the LSDn factors for quartz, or do you also consider the other elements that are present in pyroxene (e.g., Ca, Mg, Fe) and the different relative proportions of Si and O?
Also, to what extent is the modern elevation and air pressure of these samples’ representative of the average air pressure that they have experienced over the multi-million-year integration time of the cosmogenic nuclide signal, given glacio-isostatic fluctuations and glacial-interglacial climate change (which might affect local air pressure)?
Line 236: Can you quantify this agreement? Perhaps using a goodness of fit metric, such as the MSWD?
Figure 3: Is the number in the bottom left corner of panels b.) St and Lm the mean production rate considering all the data in the plot? What is the error on this value?
What is the meaning of P in the LSDn panels? What do you mean by a non-dimensional correction factor in this context? How was the correction factor determined? What are you correcting? Is it that you are correcting the production rate inferred from a non-reaction specific scaling model?
I would recommend distinguishing the data from Blard and Eaves from those of your study using color or symbology. Right now, each scaling model is colored differently, but this is probably not necessary because each has its own plot. Instead, you could give each sample set (i.e., Blard, Eaves, yours) a color that is consistent across all three plots.
Finally, I think that it would be worthwhile to remind the reader in the caption where the samples from Blard and Eaves were collected.
Line 246: This one sentence paragraph seems “tacked-on” to me and doesn’t really follow from your previous two paragraphs or the adjacent figure. I would consider deleting it. It doesn’t seem to further your argument and I think that it disrupts the flow of the text. Alternatively, you could develop this sentence into a full paragraph summarizing this section and perhaps bring in some comparison with the Blard and Eaves studies.
Line 248: Is this the production rate from Balter-Kennedy? I find this confusing because you were just discussing Blard and Eaves, and this value seems to differ from those in your figure. Would it be worthwhile to add the data from Balter-Kennedy to Figure 3?
Line 259: I recommend developing this assumption a bit. Thus far, I do not think that you have discussed non-cosmogenic sources of 3He in your samples.
Line 266: Which regression technique do you use to fit this slope?
Line 294: It is unclear to me here what you are normalizing by, please explain. Why is the range in normalized residual for the high-concentration samples (ca. -2 to 6) almost as great as for the low-concentration samples (ca. -2 to 8)?
Line 308-312: I think that your experiment may be fundamentally different from a leaching series experiment performed on an individual sample. I think that you might not see a decline in meteoric 10Be with increasing loss fraction if each sample is weathered to a different degree and contains a different fraction of secondary minerals, grain size distribution, fracture density, etc. that may give each sample a different initial amount of meteoric 10Be.
Lines 314-321: Alright, but what if you were to take the alternative approach and subtract the blank associated with the batch rather than the average of all blanks? Might this be more representative of the contamination associated with that batch? Would this change your results?
Moreover, would a different blank subtraction be sufficient to fully explain your results if the samples are higher by 394,000 and 840,000 atoms and your max blank is only 288,000 atoms? Isn’t it true that you could not account for this much 10Be unless there was more contamination than indicated by your blanks? If so, this would seem to support the meteoric 10Be hypothesis.
Line 366: The final paragraph seems underdeveloped to me. I recommend adding some discussion of what these “new opportunities” are and reiterating that analyzing 10Be in pyroxene opens quartz-poor landscapes.
Technical Corrections:
Line 17: Please add a comma before “which”
Line 19-21: Please modify this sentence to read more smoothly, perhaps by replacing “and” with a comma and “which”
Line 24-25: Please revise to read more smoothly. Perhaps by making “concentration” plural, deleting “potential” and clarifying what you mean by “measurement background,” which sounds like you are referring to something to do with the AMS measurements.
Line 40: I think that it is vague what you mean by “multiple nuclides” – I recommend stating “paired 10Be/3He”.
Line 70: What do you mean by “upper” here? Is it “higher” in the sense of greater elevation? Or is it a geographic term (like Upper Midwest)?
Line 119: I would recommend sticking to saying either “CCF” as in the preceding paragraph, or the University of Vermont, but not switching between the two.
Line 130: Please remind the reader that the 1st set of samples are the high concentration samples.
Line 143-144: I do not think that the parenthetical clause makes this sentence any clearer. You could probably just leave this out.
Figure 1: In the last sentence of the caption please reword to read more smoothly.
Table 2: Caption – Please change “measure” to “measured”
Is there a typographical error in the units on the 10Be conc.? Should these be 107 atoms g-1? Likewise, the 3He conc. units are missing a -1 superscript on the g. Also, the 10 on “10Be” in the caption should be in superscript.
Line 202: Please change “saturations” to “saturation.”
Line 291: I would recommend stating "in situ” here, either before or in lieu of “cosmogenic”
Line 318-320: I think that these two sentences are self-evident and can be deleted.
Line 357: Please insert “a” before “previously”
Line 357-358: Please add citations to the previously published production rate and paired nuclide ratios (from Balter-Kennedy?), to make explicit that you are referring to this study and not Blard or Eaves.
Citation: https://doi.org/10.5194/egusphere-2024-702-RC1 - RC2: 'Comment on egusphere-2024-702', Samuel Niedermann, 09 Apr 2024
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