The May 18, 1980, eruption of Mount St. Helens. 1. Melt composition and experimental phase equilibria.

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The Mt St Helens, Washington State, pumice of 18 May is a dacite containing 60 wt.% glass and phenocrysts of plagioclase, orthopyroxene, amphibole, titaniferous magnetite and ilmenite. The glass is uniform in composition, a rhyodacite with 73 wt.% SiO2; the phenocrysts are also uniform in composition except for the plagioclase, which has cores averaging An57 and rims averaging An49. Analyses of seven pairs of coexisting Fe-Ti oxides in a representative sample of the light pumice were recast using various mineral calculation procedures; they yielded T ranging from 920o-940oC and a -log fO2 of 10.3-10.1. Microprobe analyses of 57 glass inclusions trapped in plagioclase phenocrysts in the light pumice showed little deviation from an average rhyodacitic composition (69.90 + or - 0.87 wt.% SiO2) when special care was taken to account for Na loss during analysis. The difference between the average total of these glass inclusion analyses and 100% is 4.6 + or - 1 wt.%, which is interpreted to be volatiles dissolved in the glass. On an anhydrous basis the average glass inclusion composition is identical to the matrix glass, indicating that neither underwent significant fractionation after melt was trapped by the plagioclase. Experimentally determined phase relations for the representative dacite sample place limits on conditions in the 18 May Mt St Helens magma chamber, assuming that the dissolved volatiles were 4.6 + or - 4 wt.% and the T was 920o-940oC. Hydrothermal experiments over a range of P, T and fO2 indicate that at no pressure is the observed phase assemblage and residual melt chemistry produced when PH2O = Ptot. Experiments using CO2-H2O fluids to achieve PH2O's less than Pfluid did reproduce the observed residual melt chemistry and an An50 plagioclase at a specific set of conditions, i.e. at fO2's between the NNO and MNO buffers, at a Pfluid of 220 MPa (2.2 kbar), and at a PH2O = 110 MPa (all at 920o-940oC). Amphibole was not stable under these conditions but possibly would be if the PH2OO/Pfluid ratio was raised to 0.7 or if F were added to the experimental system. It is concluded that just prior to eruption, the upper part of the Mt St Helens magma chamber was at a P of 220 + or - 30 MPa, corresponding with a depth of 7.2 + or - 1 km, PH2O was 0.5 to 0.7 Ptotal, and T was 930 + or - 10oC. (Authors' abstract)-C.N.

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Journal of Geophysical Research