Experimental phase equilibria constraints on pre-eruptive storage conditions of the Soufreiere Hills magma

J. Barclay, M. R. Carroll, M. J. Rutherford, M. D. Murphy, J. D. Devine, J. Gardner, R. S. Sparks

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Abstract

New experimental results are used to constrain the P, T, X(H2O) conditions of the Soufriere Hills magma prior to ascent and eruption. The experiments were performed on a powdered andesite erupted in January, 1996, at an fO2 corresponding to ∼NNO+1 with PH2O and temperatures in the range 50 to 200 MPa and 800 to 940°C. Amphibole is stable at PH2O >115 MPa and temperatures <875°C. Quartz only becomes stable at low temperatures and after high degrees of crystallization (T <840°C, >72 wt% SiO2 in residual melt) at PH2O >115 MPa. Analyses of rhyolitic glass inclusions in quartz and plagioclase from recently erupted samples indicate melt water contents of 4.27±0.54 wt% H2O and CO2 contents <60 ppm. The evolved Soufriere Hills magma would therefore be H2O-saturated at pressures <130 MPa.

These results suggest that the Soufriere Hills magma containing the stable assemblage amphibole, quartz, plagioclase, orthopyroxene, magnetite and ilmenite was stored at PH2O of 115-130 MPa, equivalent to a minimum depth for a water-saturated magma chamber of 5–6 km depth. Magma temperatures were initially low (820–840°C). Quartz is believed to have been destabilised by a heating event involving injection of new basaltic magma. The stability field of hornblende provides a useful upper limit (∼880°C) for the extent of this reheating.
Original languageEnglish
Pages (from-to)3437-3440
Number of pages4
JournalGeophysical Research Letters
Volume25
Issue number18
DOIs
Publication statusPublished - 15 Sep 1998

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