the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 06 Mar 2024
Technical note: RA138 Calcite U-Pb LA-ICP-MS primary reference material
Abstract. A promising primary reference material for U-Pb LA-ICP-MS carbonate dating is analysed and reported here. The new RM is a botryoidal cement (C1) from sample RA138. The sample was collected in outcrop strata of mid-Carboniferous (Uppermost Mississippian, upper Serpukhovian) in northern Spain near La Robla, and multiple aliquots have been meticulously prepared for distribution. The RA138 is characterised by variable U/Pb ratios (from ~1 to ~19) and a relatively high and homogeneous U content (~4 ppm). This material exhibits a low age uncertainty (0.2 %, 2s; unanchored, ID-TIMS), allowing for the establishment of a well-defined isochron, particularly when anchored to the initial Pb ratio using LA-ICP-MS. ID-TIMS analyses of micro drilled C1 cement (17 sub-samples) produce a lower intercept age of 321.99 ± 0.65 Ma, an initial 207Pb/206Pb ratio of 0.8495 ± 0.0065, and a Mean Square of Weighted Deviations (MSWD) of 5.1. The systematic uncertainty of 1.5 % observed in repeated LA-ICP-MS analyses challenges previous estimations of 2–2.5 % based on repeated analyses of ASH-15D and JT using WC-1 as primary reference material, underscoring the precision and reliability of RA138 for U-Pb dating applications.
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RC1: 'Comment on gchron-2024-7', Greg Ludvigson, 01 Apr 2024
This paper by Guillong et al. advances an important goal of characterizing a primary reference standard for the calcite U-Pb isotopes. It is well written and well documented, but I do have a few minor criticisms for the authors to consider:
- In line 31, please consider inserting Gulbranson et al., 2022 ( https://doi.org/10.3390/geosciences12090346 ) in this list of appropriate citations. Also include it at the end of the list of citations in line 35)
- In line 48, please clarify the status of Samankassou et al. (2024). Is this also referred to as "submitted" as in line 63?
- As presently shown the cathodoluminescence image in Figure 1b is too dark to see the difference in luminescence colors. I do understand the viewpoint that original images should be shown as collected, but given the reality of dim luminescence in many geological samples, I recommend increasing the brightness and contrast of the image so that it conveys appropriate information. There are many other things that operators can do to achieve this end in original native photomicrographs, such as increasing accelerating voltage, beam current, or streaming helium into the chamber. Given all of these possible variables, I suggest simply doing some digital processing of the image to clearly show the features of interest.
- Great job on an important contribution!
Citation: https://doi.org/10.5194/gchron-2024-7-RC1 -
AC2: 'Reply on RC1', Marcel Guillong, 02 May 2024
We would like to thank Greg Ludvigson for the comments.
- We will add the requested literature Gulbranson et al., 2022 to the revised manuscript.
- Yes, this is the same manuscript we reference and we will update the status on submission of the revised manuscript.
- We agree that the CL image in Figure 1b is too dark and will enhance the image quality for a revised version of the manuscript so that more information becomes visible. As this is a stitched image of about 30 individual images, I suspect that the stitching software decreased the brightness to make it balanced, and I as user did not correct it. An image enhancement is no problem.
- –
Citation: https://doi.org/10.5194/gchron-2024-7-AC2
-
CC1: 'Comment on gchron-2024-7', Sota Niki, 07 Apr 2024
Publisher’s note: this comment is a copy of RC2 and its content was therefore removed.
Citation: https://doi.org/10.5194/gchron-2024-7-CC1 -
RC2: 'Comment on gchron-2024-7', Niki Sota, 09 Apr 2024
General comments
Carbonate U–Pb geochronology is increasingly important for various research fields in geosciences, and lack of high-quality reference materials (RMs) are critical issue for acquiring reliable age data. Owing to the limited numbers and the poor quality of RMs, applications of carbonate U–Pb geochronology can be retarded. The RM138 presented in the manuscript is well-characterised and demonstrating the better homogeneity in terms of the U–Pb age compared to previously reported carbonate RMs. Although I believe that this manuscript should be of interest to the audience of GChron, and be suitable for publication, there remains several questions and points should be addressed.
Specific comments
L13 As for terminology, “U–Pb“ (en dash) rather than “U-Pb“ (hyphen) is recommended for describing the relationship between U and Pb as parent and descendent isotopes.
L43 I would like to recommend using either LA-ICP-MS or LA-ICPMS as a consistent abbreviation for laser ablation ICP mass spectrometry through the manuscript.
L59 Where in the manuscript is the google satellite image shown?
L77 Although the authors describe the correction scheme for U–Pb isotopic data obtained by LA-ICP-MS in detail, the actual values for key correction parameters, such as the relative sensitivity ratio of U and Pb, mass bias factors, and down-hole fractionation, are not stated. In objectively assessing the data quality, I would like to suggest that these values are shown in the manuscript.
L112 The notation for isotopes should be changed from Mg24 to 24Mg.
L137 In the manuscript, the 235U/238U value of the sample is assumed to be 1/137.818 for the calculation of the mass bias factor as a representative value for the natural uranium isotopic ratio. The value of 1/137.818 was previously determined from zircon and apatite reported by Hiess and co-authors, and the value may not necessarily apply to carbonates. In fact, the 235U/238U value of marine carbonates deviates from the value for zircon and apatite, and some carbonates can demonstrate fractionated 235U/238U potentially depending on redox conditions. For carbonates, the degree of potential isotopic fractionation for 235U/238U is within 0.1%, but this can be a cause of significant systematic error for high-precision U–Pb isotopic analysis based on ID-TIMS. Although quantitative evaluation for the systematic error may be difficult without measuring the actual 235U/238U isotope ratios, I would like to recommend mentioning the potential systematic error arising from the assumption of the natural U isotopic ratio in the manuscript.
L204 In Fig. 3, the treatment of the ID-TIMS data points for the D16 domain in the C1 cement with high non-radiogenic Pb contents can appear arbitrary. The authors indicate that there is a contribution from non-C1 phases, but, for a clear rationale, showing some evidence for containing non-C1 components within the D16 domain is preferable. For instance, some elements enriched in non-C1 phases (e.g., Mg, Mn, and Fe) should be also high for the aliquots of the D16 domain. In addition, if there are any magnified photographs of the D16 domain before isotope analysis, I would like to recommend including them in the manuscript.
Fig 2. B For easy understanding from readers, I would like to recommend demonstrating the intercept point of the regression line and the Concordia curve on the Tera-Wasserburg diagram.
Citation: https://doi.org/10.5194/gchron-2024-7-RC2 -
AC1: 'Reply on RC2', Marcel Guillong, 02 May 2024
We Thank Niki Sota for the detailed review and the valuable comments that will help to improve the manuscript.
L13: We agree that the U–Pb terminology should be consistent throughout the manuscript, and we will make sure the revised manuscript is, and as suggested we will use the dash (–) instead of the minus (-).
L43: We agree that the abbreviation of Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) should be consistent throughout the manuscript and we will adjust it.
L59: There is no Google earth image and we do not intend to add a google map / earth image as everyone can enter the coordinates and look at the location directly in google earth. Additionally, the google satellite image is not perfectly accurate to the reel coordinates and might change in future. That is also why there are two set of coordinates, the “real” ones in the (EPSG:4258 (ETRS89) and the ones for google earth/maps image that shows the location on the satellite image. To prevent any confusion, we will change the revised manuscript and just report the real coordinates.
L77: The correction factors for the 23 sessions can be added but we think that this information is of limited help. Specifically:
- The mass bias for the 207/206 correction is found to be very small and stable for different sessions (usually less than 1% variation).
- The correction factor for 238U/206Pb is highly variable due to variations in ionisation efficiency and strongly depends on the daily tuning of the ICP-MS instrument. In Wu S, et al. 2022, as an example the correction factors are given and vary between 1.063 and 1.189 for 25 sessions. We observed similar values, but other instruments might have different values and different ranges.
- The correct assessment of downhole fractionation using calcite is difficult due to variable initial Pb content, possible surface Pb contamination, and the generally small amount of change in the ratio due to the low drill rate and large craters. Again, an example is given in Wu S, et al. for WC-1, we observe similar results.
The present manuscript focuses on the characterisation of RA-138 with ID-TIMS and LA-ICP-MS including the repeatability and does not specifically deal with the data reduction method and matrix effects. This was already described in Guillong et. al 2020 and Wu, S et al 2022 and we do not think repeating this content would add value to this manuscript.
L112: We agree and will change in the revised version of the manuscript.
L137: We acknowledge that the choice of 238U/235U measured in accessory minerals by Hiess et al. (2012) is not ideal. This value corresponds to δ238U = -0.19 +/- 0.3 ‰ (2 sigma, relative to CRM 112a) and comes with a large uncertainty which is propagated into every date. We did not measure 238U/235U in our aliquots and have no independent knowledge of the appropriate value; however, compilations of δ238U in modern and ancient marine carbonates show significant variability in natural systems. For example, Chen et al. (2021) compiled data where Phanerozoic carbonates are on average δ238U = -0.37 ‰, with a significant spread of ca. 0.6 ‰ (2 SD). We do not know whether this could be random (i.e. compositions vary for each of our aliquots) but suspect that U isotopic composition should be coherent at least within the same cement generation. As such, a deviation of δ238U from the Hiess value should result in a systematic error, as suggested by the reviewer.
The magnitude of this error can be estimated. Assuming that RA138 has δ238U = -0.37 of average Phanerozoic carbonate instead of -0.19 results in a negligible shift of the isochron age of 10 ka (30 ppm relative) towards younger values and has no effect on the value of the common Pb intercept. As such, in this particular case and at the currently achievable level of precision in ID-TIMS, the exact 238U/235U of the carbonate can be neglected.
We note, however, that future studies employing high-precision U-Pb geochronology of (particularly old) carbonates should consider directly analysing 238U/235U in the unknowns to establish this value. In younger samples, estimates of initial 234U/238U disequilibria are likely to be the dominant limitation to accuracy; however, in developing RA138 as a reference material we are interested in the raw date before any disequilibrium corrections rather than its true age.
Chen, F. L. H. Tissot, M. F. Jansen, A. Bekker, C. X. Liu, N. X. Nie, G. P. Halverson, J. Veizer, N. Dauphas, The uranium isotopic record of shales and carbonates through geologic time. Geochimica et Cosmochimica Acta 300, 164-191 (2021).
L204: We agree that for the ID-TIMS results showing some clear evidence for containing non-C1 components within the D16 domain would be preferable, however this is difficult to achieve as all the solutions have either been used or were disposed so a direct analysis for elevated Mg, Mn and Fe is not possible. A high-resolution image of the sample prior to the micro drill sampling does not exist and would not help as we selected the sampling location based on the CL image (high resolution) presented in Figure 3 and we (obviously) target the reliable domains. However, the CL image is from the surface only, and the micro drill sampling removes several 100 to 1000s of micrometres into the sample. On the CL image there is a partial cut from the sawing between sampling locations 4 and 6 revealing a finite depth of the C1 phase. Based on this observation we think it is valid to suggest that the micro drilling might have touched a different phase. Another CL image would reveal this, unfortunately the sample was used for other experiments and extensive micro drilling.
Fig 2. B: We will extend the x scale to include the intercept of the regression with the Concordia curve for the revised version of the manuscript
Citation: https://doi.org/10.5194/gchron-2024-7-AC1
-
AC1: 'Reply on RC2', Marcel Guillong, 02 May 2024
Status: closed
-
RC1: 'Comment on gchron-2024-7', Greg Ludvigson, 01 Apr 2024
This paper by Guillong et al. advances an important goal of characterizing a primary reference standard for the calcite U-Pb isotopes. It is well written and well documented, but I do have a few minor criticisms for the authors to consider:
- In line 31, please consider inserting Gulbranson et al., 2022 ( https://doi.org/10.3390/geosciences12090346 ) in this list of appropriate citations. Also include it at the end of the list of citations in line 35)
- In line 48, please clarify the status of Samankassou et al. (2024). Is this also referred to as "submitted" as in line 63?
- As presently shown the cathodoluminescence image in Figure 1b is too dark to see the difference in luminescence colors. I do understand the viewpoint that original images should be shown as collected, but given the reality of dim luminescence in many geological samples, I recommend increasing the brightness and contrast of the image so that it conveys appropriate information. There are many other things that operators can do to achieve this end in original native photomicrographs, such as increasing accelerating voltage, beam current, or streaming helium into the chamber. Given all of these possible variables, I suggest simply doing some digital processing of the image to clearly show the features of interest.
- Great job on an important contribution!
Citation: https://doi.org/10.5194/gchron-2024-7-RC1 -
AC2: 'Reply on RC1', Marcel Guillong, 02 May 2024
We would like to thank Greg Ludvigson for the comments.
- We will add the requested literature Gulbranson et al., 2022 to the revised manuscript.
- Yes, this is the same manuscript we reference and we will update the status on submission of the revised manuscript.
- We agree that the CL image in Figure 1b is too dark and will enhance the image quality for a revised version of the manuscript so that more information becomes visible. As this is a stitched image of about 30 individual images, I suspect that the stitching software decreased the brightness to make it balanced, and I as user did not correct it. An image enhancement is no problem.
- –
Citation: https://doi.org/10.5194/gchron-2024-7-AC2
-
CC1: 'Comment on gchron-2024-7', Sota Niki, 07 Apr 2024
Publisher’s note: this comment is a copy of RC2 and its content was therefore removed.
Citation: https://doi.org/10.5194/gchron-2024-7-CC1 -
RC2: 'Comment on gchron-2024-7', Niki Sota, 09 Apr 2024
General comments
Carbonate U–Pb geochronology is increasingly important for various research fields in geosciences, and lack of high-quality reference materials (RMs) are critical issue for acquiring reliable age data. Owing to the limited numbers and the poor quality of RMs, applications of carbonate U–Pb geochronology can be retarded. The RM138 presented in the manuscript is well-characterised and demonstrating the better homogeneity in terms of the U–Pb age compared to previously reported carbonate RMs. Although I believe that this manuscript should be of interest to the audience of GChron, and be suitable for publication, there remains several questions and points should be addressed.
Specific comments
L13 As for terminology, “U–Pb“ (en dash) rather than “U-Pb“ (hyphen) is recommended for describing the relationship between U and Pb as parent and descendent isotopes.
L43 I would like to recommend using either LA-ICP-MS or LA-ICPMS as a consistent abbreviation for laser ablation ICP mass spectrometry through the manuscript.
L59 Where in the manuscript is the google satellite image shown?
L77 Although the authors describe the correction scheme for U–Pb isotopic data obtained by LA-ICP-MS in detail, the actual values for key correction parameters, such as the relative sensitivity ratio of U and Pb, mass bias factors, and down-hole fractionation, are not stated. In objectively assessing the data quality, I would like to suggest that these values are shown in the manuscript.
L112 The notation for isotopes should be changed from Mg24 to 24Mg.
L137 In the manuscript, the 235U/238U value of the sample is assumed to be 1/137.818 for the calculation of the mass bias factor as a representative value for the natural uranium isotopic ratio. The value of 1/137.818 was previously determined from zircon and apatite reported by Hiess and co-authors, and the value may not necessarily apply to carbonates. In fact, the 235U/238U value of marine carbonates deviates from the value for zircon and apatite, and some carbonates can demonstrate fractionated 235U/238U potentially depending on redox conditions. For carbonates, the degree of potential isotopic fractionation for 235U/238U is within 0.1%, but this can be a cause of significant systematic error for high-precision U–Pb isotopic analysis based on ID-TIMS. Although quantitative evaluation for the systematic error may be difficult without measuring the actual 235U/238U isotope ratios, I would like to recommend mentioning the potential systematic error arising from the assumption of the natural U isotopic ratio in the manuscript.
L204 In Fig. 3, the treatment of the ID-TIMS data points for the D16 domain in the C1 cement with high non-radiogenic Pb contents can appear arbitrary. The authors indicate that there is a contribution from non-C1 phases, but, for a clear rationale, showing some evidence for containing non-C1 components within the D16 domain is preferable. For instance, some elements enriched in non-C1 phases (e.g., Mg, Mn, and Fe) should be also high for the aliquots of the D16 domain. In addition, if there are any magnified photographs of the D16 domain before isotope analysis, I would like to recommend including them in the manuscript.
Fig 2. B For easy understanding from readers, I would like to recommend demonstrating the intercept point of the regression line and the Concordia curve on the Tera-Wasserburg diagram.
Citation: https://doi.org/10.5194/gchron-2024-7-RC2 -
AC1: 'Reply on RC2', Marcel Guillong, 02 May 2024
We Thank Niki Sota for the detailed review and the valuable comments that will help to improve the manuscript.
L13: We agree that the U–Pb terminology should be consistent throughout the manuscript, and we will make sure the revised manuscript is, and as suggested we will use the dash (–) instead of the minus (-).
L43: We agree that the abbreviation of Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) should be consistent throughout the manuscript and we will adjust it.
L59: There is no Google earth image and we do not intend to add a google map / earth image as everyone can enter the coordinates and look at the location directly in google earth. Additionally, the google satellite image is not perfectly accurate to the reel coordinates and might change in future. That is also why there are two set of coordinates, the “real” ones in the (EPSG:4258 (ETRS89) and the ones for google earth/maps image that shows the location on the satellite image. To prevent any confusion, we will change the revised manuscript and just report the real coordinates.
L77: The correction factors for the 23 sessions can be added but we think that this information is of limited help. Specifically:
- The mass bias for the 207/206 correction is found to be very small and stable for different sessions (usually less than 1% variation).
- The correction factor for 238U/206Pb is highly variable due to variations in ionisation efficiency and strongly depends on the daily tuning of the ICP-MS instrument. In Wu S, et al. 2022, as an example the correction factors are given and vary between 1.063 and 1.189 for 25 sessions. We observed similar values, but other instruments might have different values and different ranges.
- The correct assessment of downhole fractionation using calcite is difficult due to variable initial Pb content, possible surface Pb contamination, and the generally small amount of change in the ratio due to the low drill rate and large craters. Again, an example is given in Wu S, et al. for WC-1, we observe similar results.
The present manuscript focuses on the characterisation of RA-138 with ID-TIMS and LA-ICP-MS including the repeatability and does not specifically deal with the data reduction method and matrix effects. This was already described in Guillong et. al 2020 and Wu, S et al 2022 and we do not think repeating this content would add value to this manuscript.
L112: We agree and will change in the revised version of the manuscript.
L137: We acknowledge that the choice of 238U/235U measured in accessory minerals by Hiess et al. (2012) is not ideal. This value corresponds to δ238U = -0.19 +/- 0.3 ‰ (2 sigma, relative to CRM 112a) and comes with a large uncertainty which is propagated into every date. We did not measure 238U/235U in our aliquots and have no independent knowledge of the appropriate value; however, compilations of δ238U in modern and ancient marine carbonates show significant variability in natural systems. For example, Chen et al. (2021) compiled data where Phanerozoic carbonates are on average δ238U = -0.37 ‰, with a significant spread of ca. 0.6 ‰ (2 SD). We do not know whether this could be random (i.e. compositions vary for each of our aliquots) but suspect that U isotopic composition should be coherent at least within the same cement generation. As such, a deviation of δ238U from the Hiess value should result in a systematic error, as suggested by the reviewer.
The magnitude of this error can be estimated. Assuming that RA138 has δ238U = -0.37 of average Phanerozoic carbonate instead of -0.19 results in a negligible shift of the isochron age of 10 ka (30 ppm relative) towards younger values and has no effect on the value of the common Pb intercept. As such, in this particular case and at the currently achievable level of precision in ID-TIMS, the exact 238U/235U of the carbonate can be neglected.
We note, however, that future studies employing high-precision U-Pb geochronology of (particularly old) carbonates should consider directly analysing 238U/235U in the unknowns to establish this value. In younger samples, estimates of initial 234U/238U disequilibria are likely to be the dominant limitation to accuracy; however, in developing RA138 as a reference material we are interested in the raw date before any disequilibrium corrections rather than its true age.
Chen, F. L. H. Tissot, M. F. Jansen, A. Bekker, C. X. Liu, N. X. Nie, G. P. Halverson, J. Veizer, N. Dauphas, The uranium isotopic record of shales and carbonates through geologic time. Geochimica et Cosmochimica Acta 300, 164-191 (2021).
L204: We agree that for the ID-TIMS results showing some clear evidence for containing non-C1 components within the D16 domain would be preferable, however this is difficult to achieve as all the solutions have either been used or were disposed so a direct analysis for elevated Mg, Mn and Fe is not possible. A high-resolution image of the sample prior to the micro drill sampling does not exist and would not help as we selected the sampling location based on the CL image (high resolution) presented in Figure 3 and we (obviously) target the reliable domains. However, the CL image is from the surface only, and the micro drill sampling removes several 100 to 1000s of micrometres into the sample. On the CL image there is a partial cut from the sawing between sampling locations 4 and 6 revealing a finite depth of the C1 phase. Based on this observation we think it is valid to suggest that the micro drilling might have touched a different phase. Another CL image would reveal this, unfortunately the sample was used for other experiments and extensive micro drilling.
Fig 2. B: We will extend the x scale to include the intercept of the regression with the Concordia curve for the revised version of the manuscript
Citation: https://doi.org/10.5194/gchron-2024-7-AC1
-
AC1: 'Reply on RC2', Marcel Guillong, 02 May 2024
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