Computer Science
Scientific paper
Jul 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994metic..29q.546w&link_type=abstract
Meteoritics (ISSN 0026-1114), vol. 29, no. 4, p. 546
Computer Science
Meteoritic Composition, Temperature Effects, Trace Elements, Ureilites, Depletion, Geochemistry, Mineralogy, Volatility
Scientific paper
While genetic processes of many meteorite groups are debated, ureilites are arguably the most enigmatic of all: even the question of an igneous vs. a nebular origin is unsettled. We do know that the ureilites or their parent material represent at least two and perhaps as many as six batches of nebular material. The ureilite enigma reflects the small number of specimens for which data are known. The Antarctic populations provide a disproportionately large number of ureilites and we report RNAA data for 15 trace elements in 18 samples representing 15 separate falls. While the contents of these elements in the ureilite suite is highly variable, the ureilite data reveal several trends: strong depletion of Rb, Cs, and Tl, averaging 0.003-0.007 x Cl; depletion of highly labile Cd, Bi, and In, averaging 0.01 x Cl; lesser, but still severe, depletion of moderately labile Ag, Se, and Te, averaging 0.05 x Cl; surprisingly strong depletions of refractory U and of siderophilic Au, Co, Sb, and Ga, to averages of 0.1-0.2 x Cl; and Zn levels averaging 0.6 x Cl. The compositions of ureilites seem to exclude carbonaceous chondrites as an immediate precursor. Ureilite compositions do not reflect loss of vapor but, rather, geochemical, condensed-phase fractionations involving the presence/absence of components. The clearest of these fractionations, loss of feldspar from the ureilite precursor, is indicated by data for the alkalis and Tl. In most meteorites, it is the high thermal lability of Tl that is important in establishing its content in ureilites; it is condensed-state behavior that is responsible for its covariation with alkalis. While high-intensity shock left its imprint in the minerals of nearly all ureilites, unlike the situation in L chondrites it failed to alter preshock contents of even the most liable of trace elements.
Lipschutz Michael E.
Wang Shao-Min
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