REE geochemistry of oldhamite-dominated clasts from the Norton County aubrite: Igneous origin of oldhamite

Abstract

Oldhamite-dominated lithic clasts represent a new igneous lithology of the aubrite parent body. They contain single crystals of oldhamite up to 2 cm in size, with inclusions of ferromagnesian alabandite, troilite, daubreelite, caswellsilverite, and Fe,Ni metal; they are usually in intimate contact with a silicate portion consisting of enstatite, forsterite, and/or plagioclase. Textural evidence for igneous origin includes apparent primary igneous grain boundaries between oldhamite and forsterite, coarse grain size, and the presence of round, droplet-like Mn-Fe-Mg-Cr-Na sulfide inclusions within oldhamite which appear to represent an immiscible sulfide liquid. We propose that the oldhamite-dominated lithology formed during the melting and fractionation of enstatite chondrite-like precursor material and represents a locally CaS-rich facies. During melting, two mutually immiscible sulfide liquids—a Ca sulfide and a Mg-Fe-Mn-Cr-Na sulfide—formed in the silicate magma. Upon cooling, the immiscible Sulfides crystallized, forming large oldhamite crystals containing inclusions of Mn-Fe-Mg-Cr-Na-bearing sulfides; forsterite, enstatite, and plagioclase crystallized from the surrounding silicate melt. At subsolidus temperatures, tiny ferromagnesian alabandite crystals exsolved from oldhamite. REE abundances in oldhamite are high (about 200 × CI), but REE patterns are nearly identical within single crystals and from clast to clast, indicating equilibrium conditions. High REE abundances have been cited as evidence that oldhamite grains in aubrites are nebular relics. However, we find it difficult to imagine that the rather homogeneous REE patterns of oldhamite in the oldhamite-dominated lithology of Norton County are not the result of equilibration of the REEs with a silicate melt during formation of the igneous aubrites through parent body melting, differentiation, fractionation, and cooling, where peak temperatures of around 1450–1500°C must have been reached. We conclude that oldhamite in the oldhamite-dominated lithology of Norton County is of igneous origin and that its REE abundances were established by equilibration with the aubrite silicate melt.