演題概要

2P-20

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炭素質コンドライトにおけるハロゲン濃度の定量化とI-Xe年代測定

(東大先端研)
^o日比谷由紀・ 角野浩史

Understanding water-rock interactions and thermal metamorphism in carbonaceous chondrites, which are considered potential sources of Earth's water, is of critical importance. Halogens are significant tracers of these processes due to their high volatility and incompatibility. Therefore, it is important to quantify the halogen abundances in carbonaceous chondrites. Analytical methods, such as Radiochemical Neutron Activation Analysis (RNAA) and Neutron Irradiation Noble Gas Mass Spectrometry (NI-NGMS), have shown discrepancies in determining halogen abundances in meteorites. Possible causes of these discrepancies include methodological differences, variations in sample preparation, and sample heterogeneity. Here, we analyzed halogen concentrations in chips and powder samples from the Allende meteorite. The results showed consistency regardless of the sample form. However, one of the chip samples showed significantly higher concentrations for all halogens. Extraction of this sample's high-temperature fraction to distinguish meteorite-intrinsic halogens from terrestrial contamination resulted in Br/Cl and I/Cl ratios that deviated even further from the literature values, indicating that the discrepancies were due to sample-derived heterogeneity. The I-Xe dating suggests thermal conditions around 4558 Ma in Allende, consistent with thermochronological modeling. The age of the Jbilet Winselwan meteorite (4517 ± 15 Ma) indicates a post-aqueous alteration thermal disturbance around this age.