Experimental REE partitioning in oldhamite: Implications for the igneous origin of aubritic oldhamite

Details Experimental REE partitioning in oldhamite: Implications for the igneous origin of aubritic oldhamite Experimental REE partitioning in oldhamite: Implications for the igneous origin of aubritic oldhamite

May 1997

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997m%26ps...32..395d&link_type=abstract

Meteoritics, vol. 32, pages 395-412

Computer Science

4

Scientific paper

Aubritic oldhamite (CaS) has been the subject of intense study recently, because it is the major 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 iC). These latter two facts have caused some authors to assert that much of the aubritic oldhamite is an unmelted nebular relict, rather than of igneous origin. We have conducted REE partitioning experiments between oldhamite and silicate melt using an aubritic bulk composition at 1200 and 1300 iC 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. REE partitioning is strongly dependent on oldhamite composition and temperature. Subsolidus annealing results in higher partition coefficients for some oldhamite grains, particularly those in contact with troilite. All experimental oldhamite/silicate melt partition coefficients are <20 and the vast majority are <5, similar to those reported in the literature and are 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 oldhamite are greater than would be predicted on the basis of the experimental partition coefficients. Our experimental partition coefficients are consistent with a relict nebular origin for aubritic oldhamite, although experimental evidence which suggests melting of oldhamite at temperatures lower than that reached on the aubrite parent body are clearly inconsistent with the nebular model. Our experiments are also consistent with a complex igneous history. Oldhamite REE patterns may reflect a complex process of partial melting, melt removal, fractional crystallization and subsolidus annealing and exsolution. These mechanisms (primarily fractional crystallization and subsolidus annealing) can produce a wide range of REE patterns in aubritic oldhamite, as well as elevated (100-1000xCI) REE abundances observed in aubritic oldhamite.

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