Zircon As Magma Monitor: Robust, Temperature-Dependent Partition Coefficients From Glass And Zircon Surface And Rim Measurements From Natural Systems

Analysis of natural mineral/host glass pairs provides robust zircon-melt partition coefficients applicable to natural systems. We analyzed zircon rims (outer similar to 15 mu m of grain interiors) or surfaces (1-2 mu m deep pits on crystal faces) and glasses in tholeiitic, calc-alkaline, and alkaline dacites and rhyolites from diverse settings (continental extension, AZ-NV, USA; hot spot/spreading center, Iceland; continental arc, Mount St. Helens (MSH), WA, USA). MSH Kds are based on eruption-age surfaces with adhering glass, which should closely approach crystal-melt equilibrium. We parameterize trivalent rare earth element (REE) Kds by X*[Ti]y for Sm to Lu, Nb, Th, and U, where X = 2.5-3600 and y = -0.73 to -1.3 for Sm to Lu. Kds for all elements span more than an order of magnitude but are highly coherent. REE Kds fit lattice strain model parabolas well, and all Kds show strong negative correlations with T indicators. Useful Kds for zircon can be estimated from Ti-zircon - Kd(element) correlations. MSH Kds based on surface analyses are consistent with those from conventional rim analysis. When paired with zircon ages, modeled compositions of MSH melts corroborate and strengthen previous conclusions regarding history and evolution of the MSH magmatic system through time.