2015 GSA Annual Meeting in Baltimore, Maryland, USA (1-4 November 2015)

Paper No. 145-1
Presentation Time: 9:00 AM-6:30 PM


BOHRSON, Wendy A., Department of Geological Sciences, Central Washington University, 400 E. University Way, Ellensburg, WA 98926, SPERA, Frank J., Earth Science, University of California, Santa Barbara, Santa Barbara, CA 93106 and SCHMIDT, Jason S., Department of Earth Science, University of California, Santa Barbara, CA 93106, bohrson@geology.cwu.edu

Most terrestrial magmas form and evolve as a consequence of processes such as partial melting of heterogeneous mantle, degassing, magma recharge/mixing (R), assimilation (A), and crystallization (FC). Current debate focuses on defining reliable ways to quantify mantle vs. crustal contributions from observed compositions. Two mass- and energy-constrained thermodynamic models provide insight into compositional variations introduced by crustal processes. A "toy" model quantifies RFC in a binary eutectic system, and the Magma Chamber Simulator models RAFC in a multicomponent-multiphase system. Both require limited thermodynamic, compositional, and mass input, and produce phase equilibria, major/trace element, and radiogenic isotope results. Mixing models that involve hotter olivine-dominated magma and cooler cpx+plagioclase magma reveal constraints that can be applied to the ongoing Pu'u O'o eruption, Kilauea (e.g., Thornber and colleagues, 2003, 2015). Near-continuous recharge and self-mixing of hotter magma (~1200°) produce a buffered olivine phyric magma supply; cooling to a cotectic assemblage is precluded by continuous mass addition. Trace element ratios deviate from those of the parent magma, with the magnitude a function of concentration, mixing masses, and bulk Kd. Outcomes of mixing of cooler (~1140°) multiply saturated magma with hotter magma are mass dependent. Recharge to resident mass mixing ratios of ~0.5 to 1 typically yield cpx followed by plagioclase. Incompatible trace element concentrations and ratios can be markedly different from those of endmember magmas, making inversion to parental compositions problematic. Smaller recharge to resident mass ratios yield olivine-saturated magma; cpx and plagioclase may resorb, and thus evidence of mixing may be difficult to detect. Mixing (±crystallization) make reconstruction of mixing endmembers difficult, and inferences about mantle sources and processes based on trace element inversion may be erroneous. Mass- and energy-constrained thermodynamic modeling places limits on phase equilibria, trace element, and isotopic heterogeneity introduced by RAFC; routine application of such models will not only improve quantitative understanding of crustal processes but also enhance the fidelity of mantle source reconstruction.