Origin of mass‐independent oxygen isotope variation among ureilites: Clues from chondrites and primitive achondrites

Abstract

Ureilite meteorites are abundant, carbon‐rich, primitive achondrites made of coarse‐grained, equilibrated olivine and pyroxene (usually pigeonite). They probably sample the baked, heterogeneous, melt‐depleted mantle of a large, once‐chondritic parent body that was broken up catastrophically while still young and hot. Heterogeneity in the parent body is inferred from a considerable “slope‐1” variation from one meteorite to another in oxygen isotopes (−2.5‰ < Δ17O < −0.2‰), which correlates with both molar FeO/MgO (range 0.03–0.35) and molar FeO/MnO (range 3–57), i.e., Δ17O correlates with the redox state. No consensus has yet emerged on the cause of these correlated trends. One view favors their inheritance via silicates from hot nebular (preaccretion) processes. Another invokes smelting (reduction of FeO by C in the hot parent body). Here, guided mainly by similar trends among equilibrated ordinary and R chondrites, studies of their unequilibrated counterparts, and work on other primitive achondrites, we propose a new model for ureilites in which the parent body accreted nebular ice with high‐∆17O, Mg‐rich silicates with low ∆17O, and varying amounts of metallic iron. Water from the thawing ice then oxidized the metal yielding secondary FeO‐bearing minerals with high ∆17O that, with metamorphism, became incorporated into the ureilite silicates. FeO/MgO, FeO/MnO, and ∆17O correlate because they rose in unison by amounts that varied spatially, depending on the local amount of metal that was oxidized. We suggest that the parent body was so large (radius ≫ 100 km) that smelting was inhibited and that carbon played a passive role in ureilite evolution. Although ureilites are regarded as complicated meteorites, we believe our analysis explains their mass‐independent oxygen isotope trend and related FeO variation through well‐understood processes and enlightens our understanding of the evolution of early planetesimals from cold, wet bodies to hot, dry ones.

Cosmic ray exposure ages for ureilites—New data and a literature study

“…; Sanders et al. ). The heterogeneity is indicated by the wide variation in bulk composition, Fo content of olivine, and oxygen isotopes among the various ureilite meteorites (e.g., Clayton and Mayeda ; Downes et al.…”

“…Considering the final steps of the UPB history, Goodrich et al (2004) suggested that the UPB broke up early in the solar system, i.e., about 5 Ma after CAI formation, and reassembled as one or more daughter bodies (ureilite daughter body, UDB). The known ureilites are derived most likely from one of the heterogeneous brecciated UDBs (Downes et al 2008;Herrin et al 2010a;Warren 2012;Goodrich et al 2015;Sanders et al 2017). The heterogeneity is indicated by the wide variation in bulk composition, Fo content of olivine, and oxygen isotopes among the various ureilite meteorites (e.g., Clayton and Mayeda 1988;Downes et al 2008;Warren 2012).…”

Metamorphism of four desert ureilites and luminescence spectroscopy of defects in ureilitic diamonds

“…This might have occurred within the ureilite parent body, which initially could be rich in volatile components (Sanders et al. ).…”

“…Experiments have shown that various H 2 -CO-CO 2 -H 2 O gas mixtures are active reduction agents and are able to reduce iron from oxides to a metal (Kemppainen et al 2012) at temperatures assumed for ureilites' petrogenetic processes. This might have occurred within the ureilite parent body, which initially could be rich in volatile components (Sanders et al 2017).…”

“…Main group ureilites are coarse-grained ultramafic rocks (Goodrich 1992;Mittlefehldt et al 1998). Ureilites are generally considered as primitive achondrites probably formed due to igneous processes in an isotopically heterogeneous asteroidal body (Warren 2011;Sanders et al 2017). It is widely accepted that ureilites are mostly restites of partial melting or, rarely, cumulates of unknown primitive materials, but detailed scenarios of this process are controversial; in particular, the role of C-CO equilibrium in ureilite petrogenesis is debated (Warren 2012;Goodrich et al 2013;Barrat et al 2016).…”