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Experimental phase equilibria constraints on pre‐eruptive storage conditions of the Soufriere Hills magma
Author(s) -
Barclay J.,
Rutherford M. J.,
Carroll M. R.,
Murphy M. D.,
Devine J. D.,
Gardner J.,
Sparks R. S. J.
Publication year - 1998
Publication title -
geophysical research letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.007
H-Index - 273
eISSN - 1944-8007
pISSN - 0094-8276
DOI - 10.1029/98gl00856
Subject(s) - plagioclase , amphibole , magma , geology , andesite , quartz , basalt , rhyolite , geochemistry , mineralogy , volcano , volcanic rock , paleontology
New experimental results are used to constrain the P, T, X(H 2 O) 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 fO 2 corresponding to ∼NNO+1 with P H 2 O and temperatures in the range 50 to 200 MPa and 800 to 940°C. Amphibole is stable at P H 2 O >115 MPa and temperatures <875°C. Quartz only becomes stable at low temperatures and after high degrees of crystallization (T <840°C, >72 wt% SiO 2 in residual melt) at P H 2 O >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% H 2 O and CO 2 contents <60 ppm. The evolved Soufriere Hills magma would therefore be H 2 O‐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 P H 2 O 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.