Characterization of the End-Stages of Fractional Crystallization

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MeurerEAR-0229702The Stillwater complex is a large body of layered mafic igneous rock formed by fractional crystallization of a basaltic magma--the most common type of magma generated in the Earth. As minerals grow and separate from a crystallizing basaltic magma, its composition becomes increasingly enriched in elements not included in the minerals, termed incompatible elements. Incompatible elements are used in the study of igneous rocks to document tectonic settings, extents of melting of a source region, and extents of fractional crystallization. This study will document the behavior of incompatible elements during the final stages of the crystallization of the Stillwater complex by examining the distribution, abundance, and composition of the last rocks to have crystallized. The Stillwater complex is of both economic and academic interest, as it hosts the bulk of the platinum reserves of the US and is a natural laboratory for understanding basaltic liquid crystallization. An understanding of the end stages of fractional crystallization in this intrusion will provide a more reliable basis for interpreting incompatible elements concentrations in igneous systems in general. It will also help in understanding how platinum was concentrated to more than 1000 times normal in the mineralized parts of this intrusion.