REE GEOCHEMISTRY OF OLDHAMITE-DOMINATED CLASTS FROM THE NORTON COUNTYAUBRITE - IGNEOUS ORIGIN OF OLDHAMITE

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, troili te, daubreelite, caswellsilverite, and Fe,Ni metal; they are usually i n intimate contact with a silicate portion consisting of enstatite, fo rsterite, and/or plagioclase. Textural evidence for igneous origin inc ludes 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 re present an immiscible sulfide liquid. We propose that the oldhamite-do minated lithology formed during the melting and fractionation of ensta tite chondrite-like precursor material and represents a locally CaS-ri ch facies. During melting, two mutually immiscible sulfide liquids-a C a sulfide and a Mg-Fe-Mn-Cr-Na sulfide-formed in the silicate magma. U pon cooling, the immiscible sulfides crystallized, forming large oldha mite crystals containing inclusions of Mn-Fe-Mg-Cr-Na-bearing sulfides ; forsterite, enstatite, and plagioclase crystallized from the surroun ding silicate melt. At subsolidus temperatures, tiny ferromagnesian al abandite crystals exsolved from oldhamite. REE abundances in oldhamite are high (about 200 X CI), but REE patterns are nearly identical with in single crystals and from clast to clast, indicating equilibrium con ditions. High REE abundances have been cited as evidence that oldhamit e grains in aubrites are nebular relics. However, we find it difficult to imagine that the rather homogeneous REE patterns of oldhamite in t he oldhamite-dominated lithology of Norton County are not the result o f equilibration of the REEs with a silicate melt during formation of t he igneous aubrites through parent body melting, differentiation, frac tionation, and cooling, where peak temperatures of around 1450-1500-de grees-C must have been reached. We conclude that oldhamite in the oldh amite-dominated lithology of Norton County is of igneous origin and th at its REE abundances were established by equilibration with the aubri te silicate melt.