¹B. J. Tkalcec,²P. Tack,²E. De Pauw,²B. Vekemans,³T. Nakamura,⁴J. Garrevoet,⁴G. Falkenberg,²L. Vincze,¹F. E. Brenker
Meteoritics & Planetary Society (in Press) Link to Article [https://doi.org/10.1111/maps.13797]
¹Department of Geosciences, Goethe University Frankfurt, Altenhoeferallee 1, Frankfurt am Main, 60438 Germany
²Department of Chemistry, XMI Research Group, Ghent University, Krijgslaan 281 S12, Ghent, 9000 Belgium
³Department of Earth Science, Tohoku University, Sendai, Miyagi, 980-8578 Japan
⁴Deutsches Elektronen-Synchrotron DESY, Notkestr. 85, Hamburg, 22607 Germany
Published by arrangement with John Wiley & Sons

Reliable identification of chondrules, calcium-aluminum-rich inclusions (CAIs), carbonate grains, and Ca-phosphate grains at depth within untouched, unprepared chondritic samples by a nondestructive analytical method, such as synchrotron X-ray fluorescence (SXRF) computed tomography (CT), is an essential first step before intrusive analytical and sample preparation methods are performed. The detection of a local Ca-enrichment could indicate the presence of such a component, all of which contain Ca as major element and/or Ca-bearing minerals, allowing it to be precisely located at depth within a sample. However, the depth limitation from which Ca-K fluorescence can travel through a chondrite sample (e.g., ∼115 µm through material of 1.5 g cm⁻³) to XRF detectors leaves many Ca-bearing components undetected at deeper depths. In comparison, Sr-K lines travel much greater distances (∼1700 µm) through the same sample density and are, thus, detected from much greater depths. Here, we demonstrate a clear, positive, and preferential correlation between Ca and Sr and conclude that Sr-detection can be used as proxy for the presence of Ca (and, thus, Ca-bearing components) throughout mm-sized samples of carbonaceous chondritic material. This has valuable implications, especially for sample return missions from carbonaceous C-type asteroids, such as Ryugu or Bennu. Reliable localization, identification, and targeted analysis by SXRF of Ca-bearing chondrules, CAIs, and carbonates at depth within untouched, unprepared samples in the initial stages of a multianalysis investigation insures the valuable information they hold of pre- and post-accretion processes in the early solar system is neither corrupted nor destroyed in subsequent processing and analyses.

¹J.N.Connelly,²A.A.Nemchin,³R.E.Merle,⁴J.F.Snape,³M.J.Whitehouse,¹M.Bizzarro
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2022.02.026]
¹Centre for Star and Planet Formation, GLOBE Institute, University of Copenhagen, Øster Voldgade, 5-7, DK-1350, Copenhagen, Denmark
²School of Earth and Planetary Sciences (EPS), Curtin University, GPO Box U1987, Perth, WA 6845, Australia
³Department of Earth Sciences, Natural Resources and Sustainable Development, Uppsala University, Villavägen 16, 75236 Uppsala, Sweden
⁴Faculty of Earth and Life Sciences, VU Amsterdam, De Boelelaan 1085,1081 HV Amsterdam, the Netherlands
Copyright Elsevier

Previous isotope studies of lunar samples have demonstrated that volatile loss was an important part of the early history of the Moon. The radiogenic U-Pb system, where Pb has a significantly lower T_50% condensation temperature than U, has the capacity to both recognize and calibrate the extent of volatile loss but this approach has been hindered by terrestrial Pb contamination of samples. We employ a novel method that integrates analyses of individual samples by Ion Microprobe and Thermal Ionization mass spectrometry to correct for ubiquitous common Pb contamination, a method that results in significantly higher estimates for µ-values (²³⁸U/²⁰⁴Pb) than previously reported. Using this method, six of seven samples of low-Ti basaltic meteorites return µ-values between 1900 and 9600, values that are consistent with a re-evaluation of published results that return µ-values of 510-2900 for both low- and high-Ti basalts. While some degree of fractionation during partial melting may increase µ-values, we infer that the source region(s) for the basalts must also have had elevated µ-values by the time the lunar magma ocean solidified. Models to account for the available initial Pb isotopic compositions of lunar basalts in light of timing constraints from thermal modelling imply that their source regions had a µ-value of at least 280, consistent with the elevated µ-values of lunar basalts and that inferred for their sources. Such high µ-values are attributed to the preferential loss of more than 99% of the Pb over U relative to a precursor with a Mars-like composition in the aftermath of the giant impact. Such an extensive loss of Pb is consistent with previously reported losses of other elements with comparable volatility, namely Zn, Rb, Ga and CrO₂. Finally, our modelling of highly-radiogenic lunar Pb isotopes assuming crystallization of the lunar magma ocean over 10’s of millions of years shows that the elevated µ-values allows for, but does not require, a young Moon formation age.