Magmatic Conditions and Magma Ascent as Indicated by Hornblende Phase Equilibria and Reactions in the 1995-2002 Soufriere Hills Magma

Abstract

A study of amphiboles and associated minerals in samples of Soufrière Hills andesite erupted from 1995 to 2002 shows significant compositional variations within hornblende phenocrysts, a separate set of small pargasitic crystals in the groundmass, and two types of reaction rims on the phenocrysts. The composition of the amphiboles and coexisting phases defines the thermal history of the erupting magma. As many as seven zones (<200 µm wide) in the hornblende phenocrysts begin with a sharp increase in Mg and Si, and then change gradually to a more Fe- and Al-rich hornblende, a transition that is consistent with a temperature rise. Analyses of the hornblende phenocrysts and associated Fe–Ti oxides verify previous conclusions that the pre-eruption magma was at 130 MPa and 830 ± 10°C, but was variably heated before eruption. The heating occurred within ∼30 days of eruption for all magmas erupted, based on the width of Ti-rich rims on titanomagnetite phenocrysts. Experimental phase equilibria for the andesite confirm that the natural hornblende phenocrysts would be stable between 825 and 855°C at a PH2O of 130 MPa, and would be even more Al rich if crystallized at higher pressure. Pargasite is not stable in the andesite, and its presence, along with high-An plagioclase microphenocrysts, requires mafic magma mingling and hybridization with pre-existing andesite. Experimental melts of the andesite at 130 MPa and 830 and 860°C compare well with melt inclusions in quartz and plagioclase, respectively. Reaction rims on a few hornblende crystals in each andesite sample are rich in high-Ca pyroxene and are produced experimentally by heating the andesite above the stability limit for hornblende. Decompression-induced breakdown rims occur in some samples, and the rate of this reaction has been experimentally calibrated for isothermal andesite magma ascent at 830–860°C. The average ascent rate of magma during much of the 1995–2002 eruption has been >0·02 m/s, the rate that allows hornblende to erupt free of decompression-induced reaction rims.