Compositions of enstatite (EH3, EH4,5 and EL6) chondrites: Implications regarding their formation

Abstract

We report concentrations of 30 elements in 34 replicate samples of 17 enstatite chondrites, including 8 EH3 chondrites. Unweathered EH3 and EH4,5 chondrites appear to be compositionally indistinguishable, and, therefore, observed differences in phase compositions reflect metamorphism of EH4,5 chondrites. The system EH3 kamacite and perryite is essentially equivalent to EH4,5 kamacite, suggesting that perryite 1. (1) condensed on metal at high nebular temperatures, 2. (2) dissolved into the kamacite during the metamorphism of the EH4,5 chondrites. Our data revealed some unexpected fractionations between closely related elements in EL chondrites: the La Sm ratio is resolvably lower and the Co Ni ratio resolvably higher than in EH chondrites. The EL chondrites have a fractionated “refractory lithophile” abundance pattern relative to CI, unlike other chondrite groups. Ni-normalized refractory siderophile abundances are higher and volatile siderophile abundances lower in EL relative to EH. The Au Ni ratio is ~1.4× the CI ratio in both EL and EH. Antarctic EH chondrites show similar patterns of element loss during weathering, independent of petrologic type. Factor analysis suggests that appreciable La condensed together with sulfides that nucleated on the metal; Na and Br are also associated with this component. The remaining rare earths are in an oldhamite-rich component. Vanadium and Cr are strongly correlated. The highly reduced state of the enstatite chondrites as well as their unique EL fractionation patterns seem most consistent with formation in the innermost part of the solar nebula.