Stable isotopes and the noncarbonaceous derivation of ureilites, in common with nearly all differentiated planetary materials

The abundant, diverse ureilite meteorites are peridotitic asteroidal mantle restites that have remarkably high bulk carbon contents (average 3 wt%) and have long been linked to the so-called carbonaceous chondrites (although this term is potentially misleading, because the high petrologic type "carbonaceous" chondrites are, if anything, C-poor compared to ordinary chondrites). Ureilite oxygen isotopic compositions, i.e., diversely negative (CCAM-like) Delta O-17, viewed in isolation, have long been viewed as confirming the carbonaceous-chondritic derivation hypothesis. However, a very different picture emerges through analysis of a compilation of recently published high-precision isotopic data for chromium, titanium and nickel for ureilites and various other planetary materials. Ureilites have lower epsilon Ni-62 and far lower epsilon Ti-50 and epsilon Cr-54 than any known variety of carbonaceous chondrite. On a plot of epsilon Ti-50 vs. epsilon Cr-54, and similarly Delta O-17 vs. epsilon Cr-54, ureilite compositions cluster far from and in a direction approximately orthogonal to a trend internal to the carbonaceous chondrites, and the carbonaceous chondrites are separated by a wide margin from all other planetary materials. I conclude that notwithstanding the impressive resemblance to carbonaceous chondrites in terms of diversely negative Delta O-17, the ureilite precursors accreted from preponderantly noncarbonaceous (sensu stricto) materials. Despite total depletion of basaltic matter, the ureilites retain moderate pyroxene/olivine ratios; which is an expected outcome from simple partial melting of moderate-SiO2/(FeO + MgO) noncarbonaceous chondritic material, but would imply an additional process of major reduction of FeO if the precursor material were carbonaceous-chondritic. The striking bimodality of planetary materials on the epsilon Ti-50 vs. epsilon Cr-54 and Delta O-17 vs. epsilon Cr-54 diagrams may be an extreme manifestation of the effects of episodic accretion of early solids in the protoplanetary nebula. However, an alternative, admittedly speculative, explanation is that the bimodality corresponds to a division between materials that originally accreted in the outer solar system (carbonaceous) and materials that accreted in the inner solar system (noncarbonaceous, including the ureilites). (C) 2011 Elsevier Ltd. All rights reserved.