Abstract

Phase equilibria and isothermal, constant-rate decompression experiments conducted on a hawaiite from the Crater Flat volcanic zone near Yucca Mountain, Nevada, provide insight into the dynamics of magma transport for this magmatic province. H2O contents up to 4.6 wt% in melt inclusions, the lack of plagioclase phenocrysts, and the presence of amphibole all suggest that phenocrysts and melt in the Crater Flat volcanic zone were equilibrated at near-water-saturated conditions during ascent. Comparison of decompression-induced crystallization of plagioclase microlites in experiments with natural tephra implies that magma ascent rates for the region are >0.04 m/s if the magmas are water saturated. Estimates of minimum ascent rates increase if the magmas are water undersaturated. A two-stage process is needed to generate reaction rims found on natural amphibole, suggesting that small batches of magma were briefly stored (3–5 days) at depths as shallow as 800 m prior to eruption.

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