Modeling enstatite chondrites: Reduced rocks with a pinch of oxidized material (affected by varying <scp> H ₂ O </scp> (g)) derived from planetesimals shocked during the epoch of giant–planet migration

Abstract 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 H 2 O/H 2 ratio of the gas. The minor fraction of oxidized material in enstatite chondrites formed earlier—when the H 2 O/H 2 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.