Mm. Wheelock et al., REE GEOCHEMISTRY OF OLDHAMITE-DOMINATED CLASTS FROM THE NORTON COUNTYAUBRITE - IGNEOUS ORIGIN OF OLDHAMITE, Geochimica et cosmochimica acta, 58(1), 1994, pp. 449-458
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.