^1,2Alan E. Rubin
Meteoritics & Planetary Science (in Press) Open Access Link to Article [https://doi.org/10.1111/maps.70080]
¹Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, California, USA
²Maine Mineral & Gem Museum, Bethel, Maine, USA
Published by arrangement with John Wiley & Sons

The O-, N-, Mo-, Ru-, Os-, Cr-, Ti-, Ni-, Fe-, Nd-, Ca-, Zn-, Sr-, and Mg-isotopic compositions of enstatite chondrites are essentially identical to those of the Earth and Moon. These correspondences suggest enstatite chondrites formed at ≈1 AU as the only known chondrite groups that accreted in the vicinity of a major planet. Bulk Earth has a higher Mg/Si weight ratio (1.09) than enstatite chondrites (0.63–0.76) and aubrites (0.84). Earth could have accreted from a mixture of these materials along with forsterite (Mg/Si = 1.73) and niningerite [(Mg,Fe)S] from the lower mantles of aubritic parent asteroids whose crusts and upper mantles were stripped off by hit-and-run collisions. The highly reducing conditions in which enstatite chondrites formed resulted from the dehydration of the inner regions of the nebula caused by outward diffusion of water vapor; this lowered the H2O/H2 ratio of the gas. The minor fraction of oxidized material in enstatite chondrites formed earlier—when the H2O/H2 ratio was briefly enhanced by inward-migrating ice particles. Enstatite chondrites are the most shocked chondrite groups, exhibiting a large variety of shock features—for example, deformed silicate lattices; petrofabrics; brecciation; shock veins; metal globules; coesite; impact-melt textures; impact-produced phases (keilite, sinoite, graphite and F-rich minerals); and fractionated bulk REE patterns. The Ar-Ar, Rb-Sr and I-Xe ages of enstatite chondrites indicate many of these rocks were shocked early in Solar System history, 4520–4563 Ma ago. This interval stretches back to the period of giant-planet migration, when the 1 AU region became dynamically excited.