EXPERIMENTAL RARE-EARTH-ELEMENT PARTITIONING IN OLDHAMITE - IMPLICATIONS FOR THE IGNEOUS ORIGIN OF AUBRITIC OLDHAMITE

Aubritic oldhamite (CaS) has been the subject of intense study recentl y because it is the major rare-earth-element (REE) carrier in aubrites , has a variety of REE patterns comparable to those in unequilibrated enstatite chondrites and has an extraordinarily high melting point as a pure substance (2525 degrees C). These latter two facts have caused some authors to assert that much of the aubritic oldhamite is an unmel ted nebular relict, rather than of igneous origin. We have conducted R EE partitioning experiments between oldhamite and silicate melt using an aubritic bulk composition at 1200 degrees C and 1300 degrees C and subsolidus annealing experiments. All experiments produced crystalline oldhamite, with a range of compositions, glass and Fe metal, as well as enstatite, SiO2, diopside and troilite in some charges. Rare-earth- element partitioning is strongly dependent on oldhamite composition an d temperature. Subsolidus annealing results in larger partition coeffi cients for some oldhamite grains, particularly those in contact with t roilite. All experimental oldhamite/silicate melt partition coefficien ts are <20 and the vast majority are <5, which is similar to those rep orted in the literature and is two orders of magnitude less than those inferred for natural aubritic oldhamite. These partition coefficients preclude a simple igneous model, since REE abundances in aubritic old hamite are greater than would be predicted on the basis of the experim ental partition coefficients. Our experimental partition coefficients are consistent with a relict nebular origin for aubritic oldhamite, al though experimental evidence that suggests melting of oldhamite at tem peratures lower than that reached on the aubrite parent body are clear ly inconsistent with the nebular model. Our experiments are consistent also with a complex igneous history. Oldhamite REE patterns may refle ct a complex process of partial melting, melt removal, fractional crys tallization and subsolidus annealing and exsolution. These mechanisms (primarily fractional crystallization and subsolidus annealing) can pr oduce a wide range of REE patterns in aubritic oldhamite, as well as e levated (100-1000 x CI) REE abundances observed in aubritic oldhamite.