Apatite Partitioning in Intermediate Melts

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Apatite is an important accessory phase in igneous and metamorphic rocks and may act as an important reservoir for rare earth elements, phosphorus, and other elements in the lower crust. This study presents new trace element partition coefficients between apatite and intermediate melts at 1 GPa. Our results show that the rare earth elements, Y, and Sr partition strongly into the crystal structure of apatite (ap/meltD values 12.61, 1.32, 18.49 for Y, Sr and Gd, respectively), while large ion lithophile elements and high field strength elements are strongly incompatible (ap/meltD values between 0.03 and 0.48). These results have important implications for geochemical models which rely on the interpretation of rare earth element ratios in melts, such as the amphibole sponge model, as the high partition coefficients between apatite and melts can complicate the signature often attributed to amphibole fractionation. The fractionation of small (<5% modally) amounts of apatite significantly alter the Dy/Yb and Dy/Dy* trajectory of a differentiating melt, with fractionation of 10% apatite causing depletion of Dy/Yb in a melt after 80% crystallization.

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