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 -
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
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