NOBLE GASES IN THE GRAIN-SIZE FRACTIONS OF PRESOLAR DIAMOND FROM THE BORISKINO CM2 METEORITE

Abstract

Presolar diamond recovered from the Boriskino CM2 meteorite was separated into size fractions by sedimentation in an ultracentrifuge. The average grain sizes of the fractions ranged from ~1.4 to ~5.3 nm. Noble gases were measured in each diamond fraction and in the bulk sample in the course of stepped pyrolysis up to 700 and subsequent oxidation between 600 and 900. The following main results were obtained: (a) fine diamond fractions are strongly different from coarse ones in the elemental composition of the P3 component of noble gases, which is most likely related to element fractionation during implantation into diamond grains; (b)in presolar diamond from a single meteorite, a wide range of the Xe-P3/Xe-HL ratio was detected for the first time, from 0.130.06 for the fine fraction to 3.95 0.37 for the coarse fraction, which is suggestive of different energies of ions of the P3 and HL noble gas components during their implantation into diamond grains; (c) the calculated upper limit of 136Xe/132Xe for isotopically anomalous xenon (0.71 0.04) in the fine fraction is identical to a value of 0.7 determined by Hass and Lewis (1994) for Xe-LH in diamond from ther- mally metamorphosed meteorites. Such a coincidence suggests that Xe-HL was formed as an individual com- ponent before its implantation into the grains of presolar diamond as a result of mixing of the products of the r- and p-processes of nucleosynthesis during a supernova explosion with xenon of the isotopically normal com- ponent of noble gases. The elemental composition of the latter component is significantly different from that of the P3 component, at least with respect to 36Ar/132Xe.