the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Potential impacts of chemical weathering on feldspar luminescence dating properties
- 1Institute of Earth Surface Dynamics, University of Lausanne, Lausanne, 1015, Switzerland
- 2Department of Civil and Environmental Engineering, University of California, 760 Davis Hall, Berkeley, USA
- 1Institute of Earth Surface Dynamics, University of Lausanne, Lausanne, 1015, Switzerland
- 2Department of Civil and Environmental Engineering, University of California, 760 Davis Hall, Berkeley, USA
Abstract. Chemical weathering alters the chemical composition of mineral grains. As a result, trapped-charge dating signals of primary silicates may be progressively modified. In this study, we artificially weathered three feldspar specimens to understand the effect of proton- and ligand-promoted dissolution on their luminescence properties. We conducted kinetic experiments over 720 h using two solutions: (1) oxalic acid (pH 3, 20 °C), an organic acid with chelating abilities, and (2) aqua regia (pH < 1, 40 °C), a mixture of strong acids creating aggressive acid hydrolysis conditions. These two solutions were chosen to approach over laboratory timescales some of the changes that may occur over geological time scales as minerals weather in nature.
The effect of the extracting solutions on mineral dissolution was investigated by following the concentration of elements accumulating in solution, while changes in feldspar surface morphology was assessed by scanning electron microscopy (SEM). Subsequent changes in feldspar luminescence in the near-UV (~340 nm) and blue (~410 nm) thermoluminescence (TL) and infrared stimulated luminescence (IRSL) emission bands were assessed at the multi- and/or single-grain levels to gain insight into the emission spectra, dose response, saturation, and anomalous fading characteristics of the feldspars. In all experiments, only minor feldspar dissolution was observed after 720 h with < 5 % of total Al, Si, Na, and Ca appearing in the aqueous phase, while 5–8 % of the total Mn and Fe were extracted. In general, aqua regia, the more chemically-aggressive solution, had a larger effect on feldspar dissolution compared to that of oxalic acid. Additionally, our results showed that although the TL and IRSL intensities changed slightly with increasing artificial weathering time, the feldspar luminescence properties were otherwise unmodified. This suggests that chemical alteration of feldspar surfaces may not affect luminescence dating signals obtained from natural samples.
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Melanie Bartz et al.
Interactive discussion
Status: closed
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RC1: 'Comment on gchron-2022-3', Paulramasamy Morthekai, 09 Apr 2022
General comments:
The authors have artificially weathered three feldspar mineral specimens (2 albite and 1 microcline) using oxalic acid and aqua regia for a month time. Luminescence characteristics of unweathered and weathered (at different time steps like 4, 10 and 30 days) grains were characterized. TL spectra, TL glow curve, IRSL decay curve, IRSL based dose response curve and fading rate were measured to check the change in the luminescence characteristics of these 3 feldspars. Very little change was observed in all the luminescence characteristics. The authors performed this study (artificial weathering) before they embark on the naturally weathered materials.
Although it is difficult to mimic the natural weathering process in laboratory, as is reflected in an unchanging luminescence characteristics, it is an important work. It is structured and presented in a way it can be understood and reproduced. However, I feel the sample selection is not proper because essentially 2 feldspar specimens were considered (albite and microcline) which under-represent the feldspar types. They could have considered 1) geochemical end members (orthoclase and anorthoclase in addition to albite), and 2) order-disorder representing feldspar (like sanidine in addition to microcline). I have some more comments and suggestions before it is accepted for publication.
Specific comments:
- It is said that XRD measurements were done but the results are not shown. As XRD measurements have already been done, the order-disorder parameter could have been calculated and considered as another luminescence parameter. This parameter becomes relevant as the blue emission is hypothesized to be associated with an oxygen ion trapped in between 2 Al ions (Al-O1- -Al). Hence monitoring this order-disorder parameter using XRD measurements with the time points will give better understanding.
- In line with the earlier suggestion, change in the TL intensity and IRSL intensity before and after the artificial weathering i.e., the effect of weathering in controlling the residence time of trapped electron/hole is a direct and important luminescence dating characteristic. In simple terms whether weathering make a sediment younger than actual. This measurement demand that all the experiments should happen in dark environment. So, the reduction (or no change) in laboratory induced luminescence intensity before and after weathering will suffice (only 2 time points). For measurements shown in Figures 4 and 5, as I understood, laboratory irradiation was given after each time point.
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AC1: 'Comment on gchron-2022-3', Melanie Bartz, 12 Jul 2022
We thank Paulramasamy Morthekai for reviewing our manuscript and the constructive comments, which will improve the manuscript. In the following we will reply to the comments in detail:
"However, I feel the sample selection is not proper because essentially 2 feldspar specimens were considered (albite and microcline) which under-represent the feldspar types. They could have considered 1) geochemical end members (orthoclase and anorthoclase in addition to albite), and 2) order-disorder representing feldspar (like sanidine in addition to microcline)."
We appreciate the comment on the sample selection. Our samples were carefully chosen for our purposes, because most of the luminescence dating studies focus on K-rich and Na-rich feldspars. This is why we have decided to work on a microcline and albites. Regarding one of the plagioclase samples, we have chosen a Na-feldspar with a higher Ca-content in order to cover the more weatherable feldspar end-members and to discover whether this sample would show more changes with weathering time in its luminescence properties. As presented in the manuscript, this is not the case. We agree that an anorthite would likely reflect more changes due to its higher Ca content in comparison to K- and Na-feldspar end-members, but we think that an anorthite is not representative for most of the luminescence dating approaches. Likewise, sanidine, which forms in felsic volcanic rocks, is very rare in soils and was therefore not chosen in our experiments.
About the specific comments, we agree on both points and I expand below:
"1. It is said that XRD measurements were done but the results are not shown. As XRD measurements have already been done, the order-disorder parameter could have been calculated and considered as another luminescence parameter. This parameter becomes relevant as the blue emission is hypothesized to be associated with an oxygen ion trapped in between 2 Al ions (Al-O1- -Al). Hence monitoring this order-disorder parameter using XRD measurements with the time points will give better understanding."
We agree that order-disorder structure of feldspars is an important characteristic of luminescence emissions, which becomes especially true for the Al-O—Al (i.e., blue emission). However, we think that a detailed discussion about the XRD data will not give much new value to the manuscript. Additional XRD experiments on the treated albite samples after 240 h did not show any differences except for intensity.
"2. In line with the earlier suggestion, change in the TL intensity and IRSL intensity before and after the artificial weathering i.e., the effect of weathering in controlling the residence time of trapped electron/hole is a direct and important luminescence dating characteristic. In simple terms whether weathering make a sediment younger than actual. This measurement demand that all the experiments should happen in dark environment. So, the reduction (or no change) in laboratory induced luminescence intensity before and after weathering will suffice (only 2 time points). For measurements shown in Figures 4 and 5, as I understood, laboratory irradiation was given after each time point."
We agree that the experimental approach the reviewer recommend (i.e. irradiation before chemical treatments) would be very interesting. We considered using this approach but decided that the experiment would be too complicated. Starting the experiments by irradiating the samples prior to chemical treatments would add additional uncertainty in terms of natural dose distribution and the effect of irradiation without preheating on the distribution of anomalous fading, especially as we chose to carry out the chemical treatments at higher temperatures (40 °C for aqua regia), which could further modify the dose distribution of highly unstable charge. It would be interesting to try this experiment in future work.
We are happy to follow up a further discussion,
Melanie Bartz (on behalf of the co-authors)
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RC2: 'Comment on gchron-2022-3', Anonymous Referee #2, 14 Jun 2022
Comments on ‘Potential impacts of chemical weathering on feldspar luminescence dating properties’ by Bartz et al.
A data rich manuscript presenting straightforward experiments to mimic chemical weathering and monitor associated changes in luminescence properties. The work is new and well within the remit of GChron. Chemical weathering has long been suspected as an important issue in luminescence dating but very few studies have so far been attempted to clarify its role. The manuscript under consideration adds to our rather limited understanding of feldspar luminescence alteration due to mineral surface dissolution. It is the nature of such an initial study that many more parameters remain to be explored but Bartz et al manage to establish a first set of results that will form a base for further investigations.
The manuscript is mainly well produced, needs some streamlining and more carful wording (comments below) and should be publishable after minor revisions.
General comments:
Please reflect on the terminology used: I found the term ‘weathering’ confusing where used as sample label. You are working on museums specimens that will probably not have been exposed to weathering in nature. So may be use ‘treated’, ‘etched’, or other terms that better describe the state of a sample. Please use similar terms throughout the manuscript (and also in figure captions).
A related issue: In the discussion, please highlight that your results were obtained on museum specimens. Mineral grains extracted from sediments may respond significantly different to etching due to very variable pre-depositional mechanical and chemical alteration.
Parts of the results and discussion section need more precise wording, loosing superfluous words and jargon. A job for the senior author?
Specific comments:
Abstract:
Line 16: change ‘was accessed’ to ‘were accessed’
Lines 24 and 25: A rather strong wording given the limited number of specimens and acid treatments used. I would like to suggest to weaken this statement.
Main text:
Line 67: Wording - what is ‘challenging’ about putting an adequate optical filter in the detection pathway?
Line 73: ‘…well as changes’ change to ‘…well as to changes’
Line 104: ‘(unweathered)’? would ‘(untreated)’ be more adequate? Please print units with all digits: 0 h, 4 h, 96 h, 240 h, and 720 h.
Lines 109 & 110: wording - change: ‘hydrolysis conditions, which allow it to efficiently leach transition metals and trace elements from the surface of minerals without destructuring the silicate’ to ‘hydrolysis conditions and efficiently leaches transition metals and trace elements from the mineral surface without destroying the silicate’
Line 105 versus line 115 – please clarify: was shaking applied in both experiments over the full period?
Line 120: wording – ‘weathering time point’ change to ‘experiment duration’ or similar
Line 123: Unclear – ‘Sc was used as an internal standard’. Have values been normalised to Sc concentration?
Line 139: Change ‘were used’ to ‘was used’
Line 143: Change ‘were weighted’ to ‘was weighted’
Line 149: Unclear: ‘background noise’ – given your measurement setup I expect that the majority of background signal was due to black body radiation and not noise. Did you check the reproducibility of the setup?
Line 160: Please print units with all digits.
Line 168: Change ‘were used’ to ‘was used’
Line 171: ‘feldspar sample compared to the pure’ change to ‘feldspar sample and given as percentage of the pure’
Line 173: ‘higher Ca content’ needs a comparator - compared to?
Line 198: ‘The same’ better ‘Similar’
Line 204: Please print units with all digits.
Line 215: wording – While the shape of the TL emissions is unaltered even after …. the intensities of the TL emissions increased….
Lines 219 - 223: wording – is it ‘no significant change can be detected due to inter-aliquot variability?
Chapter 4.2.2 – please reword. Shorten and focus using more precise terminology and descriptions. Some suggestions here:
Similar lines 230 - 233 – is the scatter significant? If not, refrain from speculating.
Line 235: ‘up to saturation’ - be more precise: The highest applied dose was 2250 Gy – the dose response (S&) is still growing…
Line 236: unclear – delete: ‘Normalised to the initial D0 values,’
Line 241: change ‘could be’ to ‘were’
Discussion lines 280-290. Is this a good analogue for chemical weathering in, for example, a soil?
Line 315: unclear – ‘when using the blue filter pack.’ Is it ‘the 410 nm emission.’?
Figure 1:
I find the semi-logarithmic graph design not very clear. May I suggest plotting the data as percentage change on a linear scale? You can show all results in the same graph by normalizing the concentrations to the initial vale. This should reveal potential trends more easily. In addition to the different symbols, you may also want to use a colour scheme.
Figure 5, Caption: Unclear – ‘All curves’? is it ‘All symbols’?
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AC2: 'Reply on RC2', Melanie Bartz, 12 Jul 2022
We highly appreciate the time of referee#2 in reviewing our manuscript. The comments and suggestions of modification of the manuscript are very constructive and will improve our manuscript.
"Please reflect on the terminology used: I found the term ‘weathering’ confusing where used as sample label. You are working on museums specimens that will probably not have been exposed to weathering in nature. So may be use ‘treated’, ‘etched’, or other terms that better describe the state of a sample. Please use similar terms throughout the manuscript (and also in figure captions).
A related issue: In the discussion, please highlight that your results were obtained on museum specimens. Mineral grains extracted from sediments may respond significantly different to etching due to very variable pre-depositional mechanical and chemical alteration.
Parts of the results and discussion section need more precise wording, loosing superfluous words and jargon. A job for the senior author?"
We agree that «weathering» is not a proper term for laboratory treatments and we decided on using «treated» throughout the manuscript. We will also make clear in the discussion that we deal with museum specimens rather than minerals extracted from natural environments to highlight potential differences from soils/sediments (i.e. mineral mixtures etc.). In general we will work on the wording throughout the manuscript.
Regarding the specific comments we will do the following changes:
"Line 67: Wording - what is ‘challenging’ about putting an adequate optical filter in the detection pathway?"
We mentioned the challenge of measuring a pure luminescence emission as the blue filter pack still transmits a small part of the UV wavelength region. Small changes in luminescence properties with chemical treatment in the UV emission might also have an effect in the blue luminescence emission usually used in luminescence dating applications. We will add information to the manuscript.
"Line 73: ‘…well as changes’ change to ‘…well as to changes’"
We will work on the grammar/wording throughout the manuscript.
"Line 104: ‘(unweathered)’? would ‘(untreated)’ be more adequate? Please print units with all digits: 0 h, 4 h, 96 h, 240 h, and 720 h."
We will use the terms «treated» or «untreated» in the revised manuscript.
"Lines 109 & 110: wording - change: ‘hydrolysis conditions, which allow it to efficiently leach transition metals and trace elements from the surface of minerals without destructuring the silicate’ to ‘hydrolysis conditions and efficiently leaches transition metals and trace elements from the mineral surface without destroying the silicate’"
We will work on the grammar/wording throughout the manuscript.
"Line 105 versus line 115 – please clarify: was shaking applied in both experiments over the full period?"
Yes, shaking was applied in both experiments. We will make the experimental conditions clear in the methods section of the manuscript.
"Line 120: wording – ‘weathering time point’ change to ‘experiment duration’ or similar"
We agree that «weathering time point» might not be the most appropriate term and we will change it to «treatment duration».
"Line 123: Unclear – ‘Sc was used as an internal standard’. Have values been normalised to Sc concentration?"
The Sc standard was used for investigating the stability of the ICP-OES measurements. A normalisation approach was not applied as the Sc readings were constant, which means that the analysis was stable and that normalisation would not change the results. We will briefly explain that in the methods section.
"Line 139: Change ‘were used’ to ‘was used’, Line 143: Change ‘were weighted’ to ‘was weighted’, Line 168: Change ‘were used’ to ‘was used’"
We will work on the grammar throughout the manuscript.
"Line 149: Unclear: ‘background noise’ – given your measurement setup I expect that the majority of background signal was due to black body radiation and not noise. Did you check the reproducibility of the setup?"
We agree that the background signal was very minor compared to that of the black body radiation in the high temperature region (>350 °C). However, the signal-to-noise ratio was lower in the low temperature region, which corresponds to the main luminescence emissions for our samples and correction/subtraction of the background signal was necessary. We checked the reproducibility of the spectra between aliquots (n=3) and explored only minor differences in intensity. We will give additional information in the manuscript.
"Line 160: Please print units with all digits., Line 204: Please print units with all digits."
We will change that.
"Line 171: ‘feldspar sample compared to the pure’ change to ‘feldspar sample and given as percentage of the pure’"
We will change that sentence accordingly.
"Line 173: ‘higher Ca content’ needs a comparator - compared to?"
Sample ALB1 was compared to sample ALB2 as both samples are albites. We will state that in the manuscript.
"Line 198: ‘The same’ better ‘Similar’"
We will work on the wording throughout the manuscript.
"Line 215: wording – While the shape of the TL emissions is unaltered even after …. the intensities of the TL emissions increased….
Lines 219 - 223: wording – is it ‘no significant change can be detected due to inter-aliquot variability?"
This is correct. Inter-aliquot variability is more evident rather than «real» changes in TL intensities after chemical treatment. We will present these results more clearly in the revised manuscript.
"Chapter 4.2.2 – please reword. Shorten and focus using more precise terminology and descriptions. Some suggestions here:
Similar lines 230 - 233 – is the scatter significant? If not, refrain from speculating.
Line 235: ‘up to saturation’ - be more precise: The highest applied dose was 2250 Gy – the dose response (S&) is still growing…
Line 236: unclear – delete: ‘Normalised to the initial D0 values,’
Line 241: change ‘could be’ to ‘were’"
Thank you for the suggestions of modification, we will work on the wording of the whole manuscript to be more precise and to avoid speculative wording.
"Discussion lines 280-290. Is this a good analogue for chemical weathering in, for example, a soil?"
It is comparable to weathering processes of specific minerals. Whether those processes can be good analogues in natural environments depends on the complex interplay between intrinsic (mineral properties) and extrinsic factors (environmental properties).
"Line 315: unclear – ‘when using the blue filter pack.’ Is it ‘the 410 nm emission.’?"
Yes, it is the 410 nm emission, we will make it clear in the manuscript.
" Figure 1:
I find the semi-logarithmic graph design not very clear. May I suggest plotting the data as percentage change on a linear scale? You can show all results in the same graph by normalizing the concentrations to the initial vale. This should reveal potential trends more easily. In addition to the different symbols, you may also want to use a colour scheme."
We will modify figure 1. In the new version, we will plot the data as percentages and normalise the data to their initial values. However, plotting all data in one graph would likely overload the figure and make it difficult to follow due to the amount of data points and associated symbols/colours. Therefore, we would like to keep the number of figures.
"Figure 5, Caption: Unclear – ‘All curves’? is it ‘All symbols’?"
Yes, this should be «all symbols», we will revise the caption.
We are happy to follow up a further discussion,
Melanie Bartz (on behalf of the co-authors)
-
AC2: 'Reply on RC2', Melanie Bartz, 12 Jul 2022
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AC1: 'Comment on gchron-2022-3', Melanie Bartz, 12 Jul 2022
We thank Paulramasamy Morthekai for reviewing our manuscript and the constructive comments, which will improve the manuscript. In the following we will reply to the comments in detail:
"However, I feel the sample selection is not proper because essentially 2 feldspar specimens were considered (albite and microcline) which under-represent the feldspar types. They could have considered 1) geochemical end members (orthoclase and anorthoclase in addition to albite), and 2) order-disorder representing feldspar (like sanidine in addition to microcline)."
We appreciate the comment on the sample selection. Our samples were carefully chosen for our purposes, because most of the luminescence dating studies focus on K-rich and Na-rich feldspars. This is why we have decided to work on a microcline and albites. Regarding one of the plagioclase samples, we have chosen a Na-feldspar with a higher Ca-content in order to cover the more weatherable feldspar end-members and to discover whether this sample would show more changes with weathering time in its luminescence properties. As presented in the manuscript, this is not the case. We agree that an anorthite would likely reflect more changes due to its higher Ca content in comparison to K- and Na-feldspar end-members, but we think that an anorthite is not representative for most of the luminescence dating approaches. Likewise, sanidine, which forms in felsic volcanic rocks, is very rare in soils and was therefore not chosen in our experiments.
About the specific comments, we agree on both points and I expand below:
"1. It is said that XRD measurements were done but the results are not shown. As XRD measurements have already been done, the order-disorder parameter could have been calculated and considered as another luminescence parameter. This parameter becomes relevant as the blue emission is hypothesized to be associated with an oxygen ion trapped in between 2 Al ions (Al-O1- -Al). Hence monitoring this order-disorder parameter using XRD measurements with the time points will give better understanding."
We agree that order-disorder structure of feldspars is an important characteristic of luminescence emissions, which becomes especially true for the Al-O—Al (i.e., blue emission). However, we think that a detailed discussion about the XRD data will not give much new value to the manuscript. Additional XRD experiments on the treated albite samples after 240 h did not show any differences except for intensity.
"2. In line with the earlier suggestion, change in the TL intensity and IRSL intensity before and after the artificial weathering i.e., the effect of weathering in controlling the residence time of trapped electron/hole is a direct and important luminescence dating characteristic. In simple terms whether weathering make a sediment younger than actual. This measurement demand that all the experiments should happen in dark environment. So, the reduction (or no change) in laboratory induced luminescence intensity before and after weathering will suffice (only 2 time points). For measurements shown in Figures 4 and 5, as I understood, laboratory irradiation was given after each time point."
We agree that the experimental approach the reviewer recommend (i.e. irradiation before chemical treatments) would be very interesting. We considered using this approach but decided that the experiment would be too complicated. Starting the experiments by irradiating the samples prior to chemical treatments would add additional uncertainty in terms of natural dose distribution and the effect of irradiation without preheating on the distribution of anomalous fading, especially as we chose to carry out the chemical treatments at higher temperatures (40 °C for aqua regia), which could further modify the dose distribution of highly unstable charge. It would be interesting to try this experiment in future work.
We are happy to follow up a further discussion,
Melanie Bartz (on behalf of the co-authors)
Peer review completion
Interactive discussion
Status: closed
-
RC1: 'Comment on gchron-2022-3', Paulramasamy Morthekai, 09 Apr 2022
General comments:
The authors have artificially weathered three feldspar mineral specimens (2 albite and 1 microcline) using oxalic acid and aqua regia for a month time. Luminescence characteristics of unweathered and weathered (at different time steps like 4, 10 and 30 days) grains were characterized. TL spectra, TL glow curve, IRSL decay curve, IRSL based dose response curve and fading rate were measured to check the change in the luminescence characteristics of these 3 feldspars. Very little change was observed in all the luminescence characteristics. The authors performed this study (artificial weathering) before they embark on the naturally weathered materials.
Although it is difficult to mimic the natural weathering process in laboratory, as is reflected in an unchanging luminescence characteristics, it is an important work. It is structured and presented in a way it can be understood and reproduced. However, I feel the sample selection is not proper because essentially 2 feldspar specimens were considered (albite and microcline) which under-represent the feldspar types. They could have considered 1) geochemical end members (orthoclase and anorthoclase in addition to albite), and 2) order-disorder representing feldspar (like sanidine in addition to microcline). I have some more comments and suggestions before it is accepted for publication.
Specific comments:
- It is said that XRD measurements were done but the results are not shown. As XRD measurements have already been done, the order-disorder parameter could have been calculated and considered as another luminescence parameter. This parameter becomes relevant as the blue emission is hypothesized to be associated with an oxygen ion trapped in between 2 Al ions (Al-O1- -Al). Hence monitoring this order-disorder parameter using XRD measurements with the time points will give better understanding.
- In line with the earlier suggestion, change in the TL intensity and IRSL intensity before and after the artificial weathering i.e., the effect of weathering in controlling the residence time of trapped electron/hole is a direct and important luminescence dating characteristic. In simple terms whether weathering make a sediment younger than actual. This measurement demand that all the experiments should happen in dark environment. So, the reduction (or no change) in laboratory induced luminescence intensity before and after weathering will suffice (only 2 time points). For measurements shown in Figures 4 and 5, as I understood, laboratory irradiation was given after each time point.
-
AC1: 'Comment on gchron-2022-3', Melanie Bartz, 12 Jul 2022
We thank Paulramasamy Morthekai for reviewing our manuscript and the constructive comments, which will improve the manuscript. In the following we will reply to the comments in detail:
"However, I feel the sample selection is not proper because essentially 2 feldspar specimens were considered (albite and microcline) which under-represent the feldspar types. They could have considered 1) geochemical end members (orthoclase and anorthoclase in addition to albite), and 2) order-disorder representing feldspar (like sanidine in addition to microcline)."
We appreciate the comment on the sample selection. Our samples were carefully chosen for our purposes, because most of the luminescence dating studies focus on K-rich and Na-rich feldspars. This is why we have decided to work on a microcline and albites. Regarding one of the plagioclase samples, we have chosen a Na-feldspar with a higher Ca-content in order to cover the more weatherable feldspar end-members and to discover whether this sample would show more changes with weathering time in its luminescence properties. As presented in the manuscript, this is not the case. We agree that an anorthite would likely reflect more changes due to its higher Ca content in comparison to K- and Na-feldspar end-members, but we think that an anorthite is not representative for most of the luminescence dating approaches. Likewise, sanidine, which forms in felsic volcanic rocks, is very rare in soils and was therefore not chosen in our experiments.
About the specific comments, we agree on both points and I expand below:
"1. It is said that XRD measurements were done but the results are not shown. As XRD measurements have already been done, the order-disorder parameter could have been calculated and considered as another luminescence parameter. This parameter becomes relevant as the blue emission is hypothesized to be associated with an oxygen ion trapped in between 2 Al ions (Al-O1- -Al). Hence monitoring this order-disorder parameter using XRD measurements with the time points will give better understanding."
We agree that order-disorder structure of feldspars is an important characteristic of luminescence emissions, which becomes especially true for the Al-O—Al (i.e., blue emission). However, we think that a detailed discussion about the XRD data will not give much new value to the manuscript. Additional XRD experiments on the treated albite samples after 240 h did not show any differences except for intensity.
"2. In line with the earlier suggestion, change in the TL intensity and IRSL intensity before and after the artificial weathering i.e., the effect of weathering in controlling the residence time of trapped electron/hole is a direct and important luminescence dating characteristic. In simple terms whether weathering make a sediment younger than actual. This measurement demand that all the experiments should happen in dark environment. So, the reduction (or no change) in laboratory induced luminescence intensity before and after weathering will suffice (only 2 time points). For measurements shown in Figures 4 and 5, as I understood, laboratory irradiation was given after each time point."
We agree that the experimental approach the reviewer recommend (i.e. irradiation before chemical treatments) would be very interesting. We considered using this approach but decided that the experiment would be too complicated. Starting the experiments by irradiating the samples prior to chemical treatments would add additional uncertainty in terms of natural dose distribution and the effect of irradiation without preheating on the distribution of anomalous fading, especially as we chose to carry out the chemical treatments at higher temperatures (40 °C for aqua regia), which could further modify the dose distribution of highly unstable charge. It would be interesting to try this experiment in future work.
We are happy to follow up a further discussion,
Melanie Bartz (on behalf of the co-authors)
-
RC2: 'Comment on gchron-2022-3', Anonymous Referee #2, 14 Jun 2022
Comments on ‘Potential impacts of chemical weathering on feldspar luminescence dating properties’ by Bartz et al.
A data rich manuscript presenting straightforward experiments to mimic chemical weathering and monitor associated changes in luminescence properties. The work is new and well within the remit of GChron. Chemical weathering has long been suspected as an important issue in luminescence dating but very few studies have so far been attempted to clarify its role. The manuscript under consideration adds to our rather limited understanding of feldspar luminescence alteration due to mineral surface dissolution. It is the nature of such an initial study that many more parameters remain to be explored but Bartz et al manage to establish a first set of results that will form a base for further investigations.
The manuscript is mainly well produced, needs some streamlining and more carful wording (comments below) and should be publishable after minor revisions.
General comments:
Please reflect on the terminology used: I found the term ‘weathering’ confusing where used as sample label. You are working on museums specimens that will probably not have been exposed to weathering in nature. So may be use ‘treated’, ‘etched’, or other terms that better describe the state of a sample. Please use similar terms throughout the manuscript (and also in figure captions).
A related issue: In the discussion, please highlight that your results were obtained on museum specimens. Mineral grains extracted from sediments may respond significantly different to etching due to very variable pre-depositional mechanical and chemical alteration.
Parts of the results and discussion section need more precise wording, loosing superfluous words and jargon. A job for the senior author?
Specific comments:
Abstract:
Line 16: change ‘was accessed’ to ‘were accessed’
Lines 24 and 25: A rather strong wording given the limited number of specimens and acid treatments used. I would like to suggest to weaken this statement.
Main text:
Line 67: Wording - what is ‘challenging’ about putting an adequate optical filter in the detection pathway?
Line 73: ‘…well as changes’ change to ‘…well as to changes’
Line 104: ‘(unweathered)’? would ‘(untreated)’ be more adequate? Please print units with all digits: 0 h, 4 h, 96 h, 240 h, and 720 h.
Lines 109 & 110: wording - change: ‘hydrolysis conditions, which allow it to efficiently leach transition metals and trace elements from the surface of minerals without destructuring the silicate’ to ‘hydrolysis conditions and efficiently leaches transition metals and trace elements from the mineral surface without destroying the silicate’
Line 105 versus line 115 – please clarify: was shaking applied in both experiments over the full period?
Line 120: wording – ‘weathering time point’ change to ‘experiment duration’ or similar
Line 123: Unclear – ‘Sc was used as an internal standard’. Have values been normalised to Sc concentration?
Line 139: Change ‘were used’ to ‘was used’
Line 143: Change ‘were weighted’ to ‘was weighted’
Line 149: Unclear: ‘background noise’ – given your measurement setup I expect that the majority of background signal was due to black body radiation and not noise. Did you check the reproducibility of the setup?
Line 160: Please print units with all digits.
Line 168: Change ‘were used’ to ‘was used’
Line 171: ‘feldspar sample compared to the pure’ change to ‘feldspar sample and given as percentage of the pure’
Line 173: ‘higher Ca content’ needs a comparator - compared to?
Line 198: ‘The same’ better ‘Similar’
Line 204: Please print units with all digits.
Line 215: wording – While the shape of the TL emissions is unaltered even after …. the intensities of the TL emissions increased….
Lines 219 - 223: wording – is it ‘no significant change can be detected due to inter-aliquot variability?
Chapter 4.2.2 – please reword. Shorten and focus using more precise terminology and descriptions. Some suggestions here:
Similar lines 230 - 233 – is the scatter significant? If not, refrain from speculating.
Line 235: ‘up to saturation’ - be more precise: The highest applied dose was 2250 Gy – the dose response (S&) is still growing…
Line 236: unclear – delete: ‘Normalised to the initial D0 values,’
Line 241: change ‘could be’ to ‘were’
Discussion lines 280-290. Is this a good analogue for chemical weathering in, for example, a soil?
Line 315: unclear – ‘when using the blue filter pack.’ Is it ‘the 410 nm emission.’?
Figure 1:
I find the semi-logarithmic graph design not very clear. May I suggest plotting the data as percentage change on a linear scale? You can show all results in the same graph by normalizing the concentrations to the initial vale. This should reveal potential trends more easily. In addition to the different symbols, you may also want to use a colour scheme.
Figure 5, Caption: Unclear – ‘All curves’? is it ‘All symbols’?
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AC2: 'Reply on RC2', Melanie Bartz, 12 Jul 2022
We highly appreciate the time of referee#2 in reviewing our manuscript. The comments and suggestions of modification of the manuscript are very constructive and will improve our manuscript.
"Please reflect on the terminology used: I found the term ‘weathering’ confusing where used as sample label. You are working on museums specimens that will probably not have been exposed to weathering in nature. So may be use ‘treated’, ‘etched’, or other terms that better describe the state of a sample. Please use similar terms throughout the manuscript (and also in figure captions).
A related issue: In the discussion, please highlight that your results were obtained on museum specimens. Mineral grains extracted from sediments may respond significantly different to etching due to very variable pre-depositional mechanical and chemical alteration.
Parts of the results and discussion section need more precise wording, loosing superfluous words and jargon. A job for the senior author?"
We agree that «weathering» is not a proper term for laboratory treatments and we decided on using «treated» throughout the manuscript. We will also make clear in the discussion that we deal with museum specimens rather than minerals extracted from natural environments to highlight potential differences from soils/sediments (i.e. mineral mixtures etc.). In general we will work on the wording throughout the manuscript.
Regarding the specific comments we will do the following changes:
"Line 67: Wording - what is ‘challenging’ about putting an adequate optical filter in the detection pathway?"
We mentioned the challenge of measuring a pure luminescence emission as the blue filter pack still transmits a small part of the UV wavelength region. Small changes in luminescence properties with chemical treatment in the UV emission might also have an effect in the blue luminescence emission usually used in luminescence dating applications. We will add information to the manuscript.
"Line 73: ‘…well as changes’ change to ‘…well as to changes’"
We will work on the grammar/wording throughout the manuscript.
"Line 104: ‘(unweathered)’? would ‘(untreated)’ be more adequate? Please print units with all digits: 0 h, 4 h, 96 h, 240 h, and 720 h."
We will use the terms «treated» or «untreated» in the revised manuscript.
"Lines 109 & 110: wording - change: ‘hydrolysis conditions, which allow it to efficiently leach transition metals and trace elements from the surface of minerals without destructuring the silicate’ to ‘hydrolysis conditions and efficiently leaches transition metals and trace elements from the mineral surface without destroying the silicate’"
We will work on the grammar/wording throughout the manuscript.
"Line 105 versus line 115 – please clarify: was shaking applied in both experiments over the full period?"
Yes, shaking was applied in both experiments. We will make the experimental conditions clear in the methods section of the manuscript.
"Line 120: wording – ‘weathering time point’ change to ‘experiment duration’ or similar"
We agree that «weathering time point» might not be the most appropriate term and we will change it to «treatment duration».
"Line 123: Unclear – ‘Sc was used as an internal standard’. Have values been normalised to Sc concentration?"
The Sc standard was used for investigating the stability of the ICP-OES measurements. A normalisation approach was not applied as the Sc readings were constant, which means that the analysis was stable and that normalisation would not change the results. We will briefly explain that in the methods section.
"Line 139: Change ‘were used’ to ‘was used’, Line 143: Change ‘were weighted’ to ‘was weighted’, Line 168: Change ‘were used’ to ‘was used’"
We will work on the grammar throughout the manuscript.
"Line 149: Unclear: ‘background noise’ – given your measurement setup I expect that the majority of background signal was due to black body radiation and not noise. Did you check the reproducibility of the setup?"
We agree that the background signal was very minor compared to that of the black body radiation in the high temperature region (>350 °C). However, the signal-to-noise ratio was lower in the low temperature region, which corresponds to the main luminescence emissions for our samples and correction/subtraction of the background signal was necessary. We checked the reproducibility of the spectra between aliquots (n=3) and explored only minor differences in intensity. We will give additional information in the manuscript.
"Line 160: Please print units with all digits., Line 204: Please print units with all digits."
We will change that.
"Line 171: ‘feldspar sample compared to the pure’ change to ‘feldspar sample and given as percentage of the pure’"
We will change that sentence accordingly.
"Line 173: ‘higher Ca content’ needs a comparator - compared to?"
Sample ALB1 was compared to sample ALB2 as both samples are albites. We will state that in the manuscript.
"Line 198: ‘The same’ better ‘Similar’"
We will work on the wording throughout the manuscript.
"Line 215: wording – While the shape of the TL emissions is unaltered even after …. the intensities of the TL emissions increased….
Lines 219 - 223: wording – is it ‘no significant change can be detected due to inter-aliquot variability?"
This is correct. Inter-aliquot variability is more evident rather than «real» changes in TL intensities after chemical treatment. We will present these results more clearly in the revised manuscript.
"Chapter 4.2.2 – please reword. Shorten and focus using more precise terminology and descriptions. Some suggestions here:
Similar lines 230 - 233 – is the scatter significant? If not, refrain from speculating.
Line 235: ‘up to saturation’ - be more precise: The highest applied dose was 2250 Gy – the dose response (S&) is still growing…
Line 236: unclear – delete: ‘Normalised to the initial D0 values,’
Line 241: change ‘could be’ to ‘were’"
Thank you for the suggestions of modification, we will work on the wording of the whole manuscript to be more precise and to avoid speculative wording.
"Discussion lines 280-290. Is this a good analogue for chemical weathering in, for example, a soil?"
It is comparable to weathering processes of specific minerals. Whether those processes can be good analogues in natural environments depends on the complex interplay between intrinsic (mineral properties) and extrinsic factors (environmental properties).
"Line 315: unclear – ‘when using the blue filter pack.’ Is it ‘the 410 nm emission.’?"
Yes, it is the 410 nm emission, we will make it clear in the manuscript.
" Figure 1:
I find the semi-logarithmic graph design not very clear. May I suggest plotting the data as percentage change on a linear scale? You can show all results in the same graph by normalizing the concentrations to the initial vale. This should reveal potential trends more easily. In addition to the different symbols, you may also want to use a colour scheme."
We will modify figure 1. In the new version, we will plot the data as percentages and normalise the data to their initial values. However, plotting all data in one graph would likely overload the figure and make it difficult to follow due to the amount of data points and associated symbols/colours. Therefore, we would like to keep the number of figures.
"Figure 5, Caption: Unclear – ‘All curves’? is it ‘All symbols’?"
Yes, this should be «all symbols», we will revise the caption.
We are happy to follow up a further discussion,
Melanie Bartz (on behalf of the co-authors)
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AC2: 'Reply on RC2', Melanie Bartz, 12 Jul 2022
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AC1: 'Comment on gchron-2022-3', Melanie Bartz, 12 Jul 2022
We thank Paulramasamy Morthekai for reviewing our manuscript and the constructive comments, which will improve the manuscript. In the following we will reply to the comments in detail:
"However, I feel the sample selection is not proper because essentially 2 feldspar specimens were considered (albite and microcline) which under-represent the feldspar types. They could have considered 1) geochemical end members (orthoclase and anorthoclase in addition to albite), and 2) order-disorder representing feldspar (like sanidine in addition to microcline)."
We appreciate the comment on the sample selection. Our samples were carefully chosen for our purposes, because most of the luminescence dating studies focus on K-rich and Na-rich feldspars. This is why we have decided to work on a microcline and albites. Regarding one of the plagioclase samples, we have chosen a Na-feldspar with a higher Ca-content in order to cover the more weatherable feldspar end-members and to discover whether this sample would show more changes with weathering time in its luminescence properties. As presented in the manuscript, this is not the case. We agree that an anorthite would likely reflect more changes due to its higher Ca content in comparison to K- and Na-feldspar end-members, but we think that an anorthite is not representative for most of the luminescence dating approaches. Likewise, sanidine, which forms in felsic volcanic rocks, is very rare in soils and was therefore not chosen in our experiments.
About the specific comments, we agree on both points and I expand below:
"1. It is said that XRD measurements were done but the results are not shown. As XRD measurements have already been done, the order-disorder parameter could have been calculated and considered as another luminescence parameter. This parameter becomes relevant as the blue emission is hypothesized to be associated with an oxygen ion trapped in between 2 Al ions (Al-O1- -Al). Hence monitoring this order-disorder parameter using XRD measurements with the time points will give better understanding."
We agree that order-disorder structure of feldspars is an important characteristic of luminescence emissions, which becomes especially true for the Al-O—Al (i.e., blue emission). However, we think that a detailed discussion about the XRD data will not give much new value to the manuscript. Additional XRD experiments on the treated albite samples after 240 h did not show any differences except for intensity.
"2. In line with the earlier suggestion, change in the TL intensity and IRSL intensity before and after the artificial weathering i.e., the effect of weathering in controlling the residence time of trapped electron/hole is a direct and important luminescence dating characteristic. In simple terms whether weathering make a sediment younger than actual. This measurement demand that all the experiments should happen in dark environment. So, the reduction (or no change) in laboratory induced luminescence intensity before and after weathering will suffice (only 2 time points). For measurements shown in Figures 4 and 5, as I understood, laboratory irradiation was given after each time point."
We agree that the experimental approach the reviewer recommend (i.e. irradiation before chemical treatments) would be very interesting. We considered using this approach but decided that the experiment would be too complicated. Starting the experiments by irradiating the samples prior to chemical treatments would add additional uncertainty in terms of natural dose distribution and the effect of irradiation without preheating on the distribution of anomalous fading, especially as we chose to carry out the chemical treatments at higher temperatures (40 °C for aqua regia), which could further modify the dose distribution of highly unstable charge. It would be interesting to try this experiment in future work.
We are happy to follow up a further discussion,
Melanie Bartz (on behalf of the co-authors)
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