Pre-eruptive Conditions of the Huerto Andesite (Fish Canyon System, San Juan Volcanic Field, Colorado): Influence of Volatiles (C–O–H–S) on Phase Equilibria and Mineral Composition

摘要

Crystallization experiments at 400 MPa, oxidized condition (ΔlogfO₂ = NNO + 1, where NNO is nickel–nickel oxide buffer) and over a range of temperatures (850–950°C) and fluid composition (XH₂O_in = 0·3–1) have been carried out to constrain the storage conditions of the sulphur-rich magma of the Huerto Andesite (an anhydrite, pyrrhotite, and S-rich apatite-bearing, post-Fish Canyon Tuff mafic lava). The results are used to evaluate the role of fluids released from the crystallization of magmas such as the Huerto Andesite on the remobilization of the largely crystallized dacitic Fish Canyon magma body. Experiments were performed using the natural andesitic bulk composition with and without added sulphur. The presence of sulphur slightly affects the phase equilibria by changing the phase proportions, stability fields of plagioclase, pyroxenes and ilmenite, and also affects the plagioclase composition. Phase equilibria and mineral composition data indicate that the magma may have contained 4·5 wt % water in the melt and that the pre-eruptive temperature was 875 ± 25°C. Assuming that the magma was in equilibrium with a fluid phase, the CO₂ concentration of the melt is estimated to be in the range 2000–4000 ppm (at 400 MPa). Before eruption, the andesite had an oxidation state very close to, or slightly within, the co-stability field of anhydrite–pyrrhotite at NNO + 1·1. At these conditions, the sulphur content in the melt is ∼500 ppm. Assuming open-system degassing resulting from continuing crystallization at depth, most of the CO₂ dissolved in the andesitic melt should be released after the crystallization of 10 vol. % of the magma, corresponding to a cooling from 875 to 825–850°C. Thus, the fluids released owing to crystallization processes should be mainly composed of water at temperatures below 825°C.