Stable-isotopic anomalies and the accretionary assemblage of the Earth and Mars: A subordinate role for carbonaceous chondrites

Abstract

Plots such as ε 54Cr vs. ε 50Ti and ε 54Cr vs. Δ 17O reveal a fundamental dichotomy among planetary materials. The “carbonaceous” chondrites, by virtue of high ε 50Ti and high ε 62Ni, as well as, especially for any given Δ 17O, high ε 54Cr, are separated by a wide margin from all other materials. The significance of the bimodality is further manifested by several types of meteorites with petrological-geochemical characteristics that suggest membership in the opposite category from the true pedigree as revealed by the stable isotopes. Ureilites, for example, despite having diversely low Δ 17O and about the same average carbon content as the most C-rich carbonaceous chondrite, have clear stable-isotopic signatures of noncarbonaceous pedigree. The striking bimodality on the ε 54Cr vs. ε 50Ti and ε 54Cr vs. Δ 17O diagrams suggests that the highest taxonomic division in meteorite/planetary classification should be between carbonaceous and noncarbonaceous materials. The bimodality 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). In any event, both the Earth and Mars plot squarely within the noncarbonaceous composition-space. Applying the lever rule to putative mixing lines on the ε 50Ti vs. ε 54Cr and Δ 17O vs. ε 54Cr diagrams, the carbonaceous/(carbonaceous + noncarbonaceous) mixing ratio C/( C + NC) is most likely close to (very roughly) 24% for Earth and 9% for Mars. Estimated upper limits for C/( C + NC) are 32% for Earth and 18% for Mars. However, the uncertainties are such that isotopic data do not require or even significantly suggest that Earth has higher C/( C + NC) than Mars. Among known chondrite groups, EH yields a relatively close fit to the stable-isotopic composition of Earth.