Trace element study of formation processes of carbonaceous chondrites and regolith processes on Fayetteville parent body

Xiaoyue Xiao, Purdue University

Abstract

Labile trace elements are used to establish thermal histories and evolutionary episodes of meteorites. Fourteen trace elements--Ag, Au, Bi, Cd, Co, Cs, Ga, In, Rb, Sb, Se, Te, Tl, and Zn--covering most labile trace elements, were determined by radiochemical neutron activation analysis, in samples of C1-6 and unique carbonaceous (C) chondrites, mainly from Antarctica and a few non-Antarctic specimens, and in samples of dark matrix, and dark and light inclusions of the Fayetteville H chondrite regolith breccia. C chondrites: C1-normalized mean concentrations for the 9 most labile elements Ag $\to$ In are quite constant in most carbonaceous chondrites. Trace element contents in Antarctic and non-Antarctic carbonaceous chondrites are similar: they are essentially unaltered by weathering in Antarctica and by secondary metamorphism at $\geq$500$\sp\circ$C in an open-system. Concentrations of highly labile trace elements reveal a continuous distribution from 1.0 $\times$ C1 to 0.1 $\times$ C1 for C2-6 chondrites and unique carbonaceous chondrites, and there is considerable overlap between different petrographic types. Siderophile ratios (Ga/Co and Au/Co) exhibit trends similar to those of highly labile elements. The entire suite of carbonaceous chondrites do not seem to sample a few, compositionally-distinct parent materials but, rather, a compositional continuum in which parent materials forming under more oxidizing conditions incorporated lesser complements of volatiles, essentially unfractionated from cosmic composition. This may well reflect a continuum in formation conditions (temperature, duration, water/rock ratios) represented by oxygen isotope variations during preterrestrial genetic processes of parent materials. Fayetteville regolith breccia: Most dark inclusions had similar mean trace element concentrations as matrix but were compositionally more heterogeneous. Two dark inclusions were unique, one having apparently formed from impact-melted soil, the other being a mixture of 90% matrix--10% C2M chondrite. The light inclusion proved compositionally similar to H4-6 chondrite falls except for its higher contents of labile Bi, Cd, In, and Tl. Dark portions of such breccias apparently represent compacted regolith soil, produced by isochemical comminution of "normal" dark and light inclusions, plus inclusions of local and exotic provenance.

Degree

Ph.D.

Advisors

Lipschutz, Purdue University.

Subject Area

Chemistry|Geochemistry|Astronomy|Astrophysics

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