Computer Science
Scientific paper
May 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992gecoa..56.1863w&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 56, Issue 5, pp.1863-1874
Computer Science
70
Scientific paper
The concentration of S in basaltic magmas at 1 atm pressure is strongly dependent on temperature, the fugacities of oxygen ( f O 2 ) and sulfur ( f S 2 ), and bulk composition. Microprobe analyses of total S in rapidly quenched, submarine basalt glasses, used in conjunction with wet chemical analyses of Fe 2 O 3 / FeO and its relationship to f o 2 , allow direct calculation of f s 2 using an expression which relates dissolved sulfur content to sulfur fugacity. The relationship between S fugacity and dissolved S in a silicate liquid at 1 bar total pressure can be represented by the expression ln X s = a ln f s 2 - b ln f o 2 + c ln X FeO + d / T + e + f i X i , where X s is the mole fraction of dissolved S, a through f i are experimentally calibrated regression coefficients, and the summation is over melt components, i . Back calculation of the input data yields a standard error of 0.026 wt% S for a magma with 0.1 wt% of S. Prediction of the immiscible Fe-S-O liquid saturation surface for basaltic liquids in T - X - f o 2 - f s 2 space is made through consideration of the heterogeneous equilibrium 1/2 S 2( gas ) + FeO ( silicate melt ) = FeS ( sulfide melt ) + 1/2 O 2( gas ) , using standard state thermodynamic data. Data from natural basalt glasses demonstrate that during the differentiation and Fe enrichment of basaltic magmas, the increases in S content which are observed require the ratio f o 2 / f s 2 to decrease by 3 log 10 units over the temperature range 1260 to 1050°C. This decrease is equivalent to the enthalpy change of the above reaction. Application to basalts and gases from Kilauea volcano demonstrates that during ascent, S is depleted in the magma as a result of shallow effervescence, and the gases which are evolved are in equilibrium with the magma during fire fountaining.
Carmichael Ian S. E.
Wallace Paul
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