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Phase Equilibria and Pre-eruptive P-T-fO2-fH2O-fS2 Conditions of Dacite from Volcán San Pedro (Chilean Andes)

Abstract : The pressure, temperature, oxygen fugacity, volatile content and compositions of subduction related silicic magmas is a highly investigated topic because of known climatic and societal potential hazardous effects, and the implications for the differentiation of the continental crust. Melt inclusions combined with thermobarometers provide a means of estimating such parameters, but a complementary approach is to perform phase equilibrium experiments at various P-T-fvolatile conditions. With the assumption that the bulk-rock composition is representative of a liquid, we have performed a phase equilibrium study of a dacite (66 wt% SiO2, 2.7 wt% K2O) from Volcán San Pedro, the Holocene edifice of the Quaternary Tatara-San Pedro complex (Chilean Andes). The dacite contains phenocrysts of plagioclase (Pl), hornblende (Hbl), biotite (Bt), clinopyroxene (Cpx), orthopyroxene (Opx), Fe-Ti oxides (Mt), pyrrhotite, and apatite. Coexisting Fe-Ti oxides gave a pre-eruptive temperature of ~850°C and an fO2 ~ NiNiO+1, whereas pyrrhotite composition indicates an fS2~ 0.02. We have performed experiments in IHPV at 800-950°C, 100-400 MPa, and with redox conditions from the Ni-NiO buffer to ~ 2.5 log units above. Water contents ranged from undersaturated (XH2O= 0.6) to saturation, and S from 0 to 1 wt% bulk. The full mineral assemblage (Pl+Cpx+Opx+Hbl+Bt+Mt) present in the lava is not stable at any of the investigated experimental conditions, notably by the lack of Cpx+Bt coexistence. Moreover, in S-free experiments Bt is only stable at < 825°C, which is lower than the Fe-Ti oxides temperature. In contrast, in experiments with 0.5-1 wt% added S, Bt is stable up to 875°C (+Hbl+Opx+Pl+anhydrite+pyrrhotite). This suggests that the San Pedro dacite magma was stored at ~ 6 km, at 850°C, close to water-saturated conditions, with a log fO2 of NNO+1 to NNO+2, and importantly, contained significant amounts of sulfur (0.1 < S < 1 wt% bulk).
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Submitted on : Thursday, August 10, 2006 - 4:00:52 PM
Last modification on : Friday, April 5, 2019 - 8:08:24 PM

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Fidel Costa Rodriguez, Bruno Scaillet, Michel Pichavant. Phase Equilibria and Pre-eruptive P-T-fO2-fH2O-fS2 Conditions of Dacite from Volcán San Pedro (Chilean Andes). EMPG IX - Ninth International Symposium on Experimental Mineralogy, Petrology and Geochemistry, 2002, Zürich, Switzerland. ⟨hal-00089147⟩

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