Hydrogen-isotope Constraints on the Origin and Evolution of the Carbonaceous Chondrites

Introduction: The carbonaceous chondrites are among the most primitive solid objects in the solar system and plausibly represent building blocks of the terrestrial planets, moons and asteroids [1,2]. Although they are generally less differentiated and metamorphosed than other meteorite groups, the car-bonaceous chondrites are compositionally and textur-ally diverse, in part reflecting processes acting on them after their accretion [3,4]. One of the more important of these processes was aqueous alteration, either by water formed from melting co-accreted ice or by infiltration of externally-derived water produced elsewhere on their meteorite parent bodies [5]. Several recent models of the protoliths, sources of water, and conditions and mechanisms of this water-rock reaction have been proposed based on evidence from oxygen isotope geochemistry [e.g., 6,7]. In this study, we reexamine this issue considering the added constraint of hydrogen isotope compositions of CV, CO, CM and CI carbona-ceous chondrites. We particularly focus on the CM chondrites, which preserve the most diverse record of aqueous alteration. Samples and methods: We present new measurements of hydrogen isotope compositions of mechanically micro-sampled portions of nine carbona-ceous chondrites, including five CM chondrites (LEW 85311, Murchison, Mighei, Murray, Cold Bokkeveld), two CV chondrites (Allende and Mokoia) and two CI chondrites (Ivuna and Orgueil). We will also show data for Tagish Lake, although these are preliminary at present and not discussed in the following paragraphs. Three classes of materials were prepared from these samples: fragments of matrix, free of visible coarse crystalline material, veins or heterogeneities in color; central portions of large, aqueously altered chondrules; and aqueously altered loose fragments of coarse sili-cates (chondrules and/or CAI's). All samples were analysed using a recently-developed method of on-line stepped combustion, reduction of evolved water, and continuous flow mass spectrometry of product H2 [8]. This method reduces the sample size for hydrogen isotope analysis by ca. three to four orders of magnitude compared to conventional techniques, such that detailed stepped heating experiments can be conducted on tens to hundreds of micrograms of water-rich materials. Briefly, samples are loaded in a 1/8 " O.D. alumina tube and dried for 12 hours at 50˚C under a He flow. A movable resistance heater is then slid over the portion of the alumina tube