Concentrations, sources, and losses of H 2 O, CO 2 , and S in Kilauean basalt

Details Concentrations, sources, and losses of H 2 O, CO 2 , and S in Kilauean basalt Concentrations, sources, and losses of H 2 O, CO 2 , and S in Kilauean basalt

Jun 1983

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1983gecoa..47.1139h&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 47, Issue 6, pp.1139-1150

Physics

15

Scientific paper

Basaltic glasses included in olivine phenocrysts from Kilauea volcano contain concentrations of H 2 O, CO 2 , and S similar to glassy Kilauean basalt dredged from the deep sea floor and greater than vesicular, subaerial Kilauean basalt. Our result contrasts with earlier reports that inclusions of basaltic glass in phenocrysts have little or no H 2 O and large ratios of CO 2 / H 2 O . Our analysed inclusions of glass are larger than 100 micrometers thick and similar in chemical composition to the host glass surrounding the olivine crystals indicating that the trapped melts are representative of the bulk liquid from which the crystals grew. Crystallization of about 2-8% of olivine from the melts after they were trapped is indicated by slight departures from the experimentally established equilibrium distribution of Mg and Fe between olivine and liquid. The measured concentrations of CO 2 correspond to phenocryst crystallization pressures of about 1.3 kbar for a subaerial basalt and about 5 kbar for a submarine basalt, consistent with geophysical models of Kilauea volcano. The compositions of volcanic gas predicted from our analyses are consistent with restored compositions of actual Kilauean gases. The rate of sulfur emission predicted from our analyses is greater than the sulfur dioxide emission rate observed during repose, but probably consistent with total degassing including eruptive episodes. The concentrations of H 2 O, K 2 O, Cl, and P in parental Kilauean basalt can be derived from upper mantle phlogopitic mica, pargasitic amphibole and apatite with compositions close to those of natural primary minerals in ultramafic xenoliths from continental kimberlites, or solely from apatite and phlogopitic mica with H 2 O / K 2 O near 0.47 ± 0.03, slightly higher than the range of values reported. The amounts of phlogopitic mica and pargasitic amphibole contributing volatiles to Kilauean tholeiite is about 10 percent by mass of the parental liquid, or about 5% if the source does not include amphibole. In view of an estimated 20% of partial melting of mantle source rock to produce Kilauean tholeiites, there may be about 2 weight percent of mica plus amphibole in part of the mantle beneath Kilauea, or about 1 weight percent of phlogopitic mica if amphibole is absent.

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