Magmatic processes that generated the rhyolite of Glass Mountain, Medicine Lake volcano, N. California

Abstract

Glass Mountain consists of a 1 km 3 , compositionally zoned rhyolite to dacite glass flow containing magmatic inclusions and xenoliths of underlying shallow crust. Mixing of magmas produced by fractional crystallization of andesite and crustal melting generated the rhyolite of Glass Mountain. Melting experiments were carried out on basaltic andesite and andesite magmatic inclusions at 100, 150 and 200 MPa, H 2 O-saturated with oxygen fugacity controlled at the nickel-nickel oxide buffer to provide evidence of the role of fractional crystallization in the origin of the rhyolite of Glass Mountain. Isotopic evidence indicates that the crustal component assimilated at Glass Mountain constitutes at least 55 to 60% of the mass of erupted rhyolite. A large volume of mafic andesite (2 to 2.5 km 3 ) periodically replenished the magma reservoir(s) beneath Glass Mountain, underwent extensive fractional crystallization and provided the heat necessary to melt the crust. The crystalline residues of fractionation as well as residual liquids expelled from the cumulate residues are preserved as magmatic inclusions and indicate that this fractionation process occurred at two distinct depths. The presence and composition of amphibole in magmatic inclusions preserve evidence for crystallization of the andesite at pressures of at least 200 MPa (6 km depth) under near H 2 O-saturated conditions. Mineralogical evidence preserved in olivine-plagioclase and olivine-plagioclase-high-Ca clinopyroxene-bearing mag-matic inclusions indicates that crystallization under near H 2 O-saturated conditions also occurred at pressures of 100 MPa (3 km depth) or less. Petrologic, isotopic and geochemical evidence indicate that the andesite underwent fractional crystallization to form the differentiated melts but had no chemical interaction with the melted crustal component. Heat released by the fractionation process was responsible for heating and melting the crust.

Ambient and excess mantle temperatures, olivine thermometry, and active vs. passive upwelling

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“…Eqs. (2)- (5) were calibrated using 137 experimental data (selected from Falloon and Green, 1987;Falloon et al, 1988;Trönnes et al, 1992;Sisson and Grove, 1993a, b;Yang et al, 1996;Grove et al, 1997;Parman et al, 1997;Wagner and Grove, 1997;Kogiso et al, 1998;Walter, 1998;Montierth et al, 2000;Falloon et al, 2001;Gudfinnsson and Presnall, 2000;Longhi, 2002). The remaining 856 experimental data ( Fig.…”

The influence of water on the temperature of amphibole stability in melts

“…From this set we eliminated the experiments with low concentrations of MgO in the melt (< 0.5 wt %) in view of the large error caused by the parameter in these cases. A total of 44 composition pairs were selected from [7,13,15,16,19,20,24,28]. From these we calculated logarithms of the constants in the crystallization equations for tremolite and pargasite using Eqs.…”

An introduction to magma dynamics

“…Sisson & Grove 1993;Grove et al 1997;Pichavant et al 2002). However, on the basis of geochemical data, Hildreth & Moorbath (1988) proposed that magma diversity is acquired at the mantle-crust interface in MASH zones, i.e.…”