V034-0006
Experimental Calibration of an Fe3+/Fe2+-in-Amphibole Oxybarometer and its Application to Transcrustal Magmatic Processes at Shiveluch Volcano, Kamchatka

Monday, 14 December 2020
Poster
Andrea Goltz1, Mike Krawczynski1, Melinda Darby Dyar2, Molly C McCanta3, Antonio Lanzirotti4 and Matthew Newville4, (1)Washington University in St Louis, St. Louis, MO, United States, (2)Mount Holyoke College, South Hadley, MA, United States, (3)University of Tennessee, Earth and Planetary Sciences, Knoxville, TN, United States, (4)Center for Advanced Radiation Sources, University of Chicago, Chicago, IL, United States
Abstract:
Oxygen fugacity is a key intrinsic parameter for magma genesis, but is difficult to estimate for primitive arc magmas, which rarely crystallize two oxides and have to traverse the entire crustal column before eruption. We here use Fe3+/Fe2+ of igneous amphibole as an indicator of magmatic oxygen fugacity using experimentally-determined amphibole-melt partition coefficients (D), and apply our oxybarometer to amphiboles crystallized throughout the crustal column beneath Shiveluch Volcano, Kamchatka.

Amphiboles synthesized in piston cylinder and cold seal apparatus experiments were analyzed using μ-X-Ray Absorption Spectroscopy (μ-XAS) and a calibration employing standards with known Fe3+ and Fe2+. Amphibole-melt partition coefficients for those cations were determined for basalts, andesites, and dacites at a range of pressures, temperatures, oxygen fugacities, and water contents analogous to amphibole crystallization at arc settings.

Amphibole-melt Fe3+/Fe2+ partition coefficients have a strong dependence on both composition and oxygen fugacity and range almost an order of magnitude. At intermediate oxygen fugacities from NNO to NNO+2, D is composition-dependent and decreases from basalt to dacite, potentially due to changing melt structure. At higher fO2 (Ru-RuO2), all starting compositions have similar D, and Fe3+ behaves slightly compatibly.

These experimentally-determined values for D were used to interpret measurements of Fe3+ in amphiboles from mafic enclaves and tephra from Shiveluch, which are petrogenetically related to the basaltic D values. Natural amphibole Fe3+/Fe2+ is representative of melt oxygen fugacity from ~NNO+2.3 to ~NNO+4.5. The lower end of the fO2 estimates is based on analyses of a lower crustal amphibole-clinopyroxene cognate xenolith and of basaltic-andesite tephra grain mounts, and we interpret the fO2 of these samples to be representative of the fO2 of primitive melts at this volcano. The higher Fe3+/Fe2+ ratios of amphibole in mafic enclaves may reflect transcrustal processing such as crystallization from a different, more oxidized parental magma or of post-crystallization oxidation (dehydrogenation) of the amphiboles in mafic enclaves during mingling of basaltic and andesitic magmas prior to or during eruption.

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