REACTION MECHANISM IN SILICEOUS DOLOMITE CONTACT AUREOLES: TABULAR OLIVINE GROWTH

Tabular olivines in siliceous dolomites of the Ubehebe Peak contact aureole were first described by Roselle and Baumgartner (1997). They attributed the olivine morphology to a surface kinetic growth mechanism. A re-investigation of these olivine textures led to the realization that diffusion was an important factor for texture development. Textures were studied with optical microscopy, the electron microprobe, and micro X-ray tomography (µCT) with a Skyscan 1072 system. Calcite haloes surround large individual forsterite laths (5-20mm), while calcite is conspicuously absent in the matrix of most samples. The µCT-images reveal a complex intergrowth of olivine with each other, forming a self-supported network. Individual crystals of olivine are skeletal, with embayments filled by calcite. Stable isotopes of O and C of dolomite and calcite, as well as O-isotopes of forsterite indicate grain scale equilibrium of calcite and forsterite, but disequilibrium with dolomite for both product phases. Calcites are up to 3‰ depleted in ¹⁸O, indicating limited infiltration of ¹⁸O depleated fluid during growth. Isotopic composition of dolomite appears to little affected by the infiltration. Simple diffusional texture models using the program SEG (Foster, 1993) successfully produce a calcite rim around the growing olivine crystal. Once precipitated, calcite is difficult to remove from the olivine grain boundary, since chemical potential gradients are fixed by calcite. This forces the olivine to overgrow its calcite rim and its dentritic growth. Growth models for the reaction do+tr -> fo+cc result in significant precipitation of dolomite in the matrix. Calcite produced by the tremolite-forming reaction in the matrix is removed by the tremolite consuming reaction producing dolomite in the matrix. Olivine growth by the metastable reaction do+qz -> fo+cc results in minimal dolomite precipitation in the matrix. Taken together, with the isotopic and textural evidence, these models indicate production of olivine from the metastable assemblage do+qz.