Isotopic compositions of asteroidal liquid water trapped in fluid inclusions of chondrites

Abstract

Copyright © 2014 by The Geochemical Society of Japan. primitive materials in the solar system and chondrite-like materials formed the terrestrial planets, isotopic compositions of chondritic liquid water provide a direct evidence to reveal the origin of water of terrestrial planets. The fluid inclusions were found in halite (NaCl) and sylvite (KCl) (hereafter collectively called “halite”) from two ordinary chondrite regolith breccias (Monahans (1998), hereafter simply “Monahans” (H5), and Zag (H3-6)) (Rubin et al., 2002; Zolensky et al., 1999, 2000). Both meteorites contain millimeter to centimeter-sized aggregates of blue to purple halite containing aqueous fluid inclusions in the matrix. The halite grains were dated by K–Ar, Rb–Sr and I–Xe systematics to be ~4.5 Ga (Bogard et al., 2001; Whitby et al., 2000; Zolensky et al., 1999), and thus the trapped aqueous fluids are at least as ancient. These halite grains are blue to purple owing to the gradual accumulation of trapped electrons in cation vacancies (Nassau, 1983), resulting, probably, from the beta decay of 40K. Since recently recrystallized halite would be colorless, fluid within the colored halite grains must be pre-terrestrial in origin. Heating/freezing studies of the fluid inclusions in Monahans halite grains demonstrated that they were trapped at approximately 25°C (Zolensky et al., 1999), and their presence in the halite requires their Isotopic compositions of asteroidal liquid water trapped in fluid inclusions of chondrites