Sulfur oxidation state and solubility in silicate melts

Abstract

Abstract We have determined the solubility of sulfur (S) as sulfide (S 2– ) for 13 different natural melt compositions at temperatures of 1473–1773 K under controlled conditions of oxygen and sulfur fugacities ( f O 2 and f S 2 , respectively). The S and major element contents of the quenched glasses were determined by electron microprobe. The sulfide capacity parameter (C S2– ) was used to express S 2– solubility as a function of the oxygen and sulfur fugacities according to the equation: $$\log C_{{S^{2 - } }} = \log S_{melt} \left( {wt\% } \right) + 0.5\log \left( {\frac{{fO_{2} }}{{fS_{2} }}} \right)$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mo>log</mml:mo> <mml:msub> <mml:mi>C</mml:mi> <mml:msup> <mml:mi>S</mml:mi> <mml:mrow> <mml:mn>2</mml:mn> <mml:mo>-</mml:mo> </mml:mrow> </mml:msup> </mml:msub> <mml:mo>=</mml:mo> <mml:mo>log</mml:mo> <mml:msub> <mml:mi>S</mml:mi> <mml:mrow> <mml:mi>melt</mml:mi> </mml:mrow> </mml:msub> <mml:mfenced> <mml:mrow> <mml:mi>w</mml:mi> <mml:mi>t</mml:mi> <mml:mo>%</mml:mo> </mml:mrow> </mml:mfenced> <mml:mo>+</mml:mo> <mml:mn>0.5</mml:mn> <mml:mo>log</mml:mo> <mml:mfenced> <mml:mfrac> <mml:mrow> <mml:mi>f</mml:mi> <mml:msub> <mml:mi>O</mml:mi> <mml:mn>2</mml:mn> </mml:msub> </mml:mrow> <mml:mrow> <mml:mi>f</mml:mi> <mml:msub> <mml:mi>S</mml:mi> <mml:mn>2</mml:mn> </mml:msub> </mml:mrow> </mml:mfrac> </mml:mfenced> </mml:mrow> </mml:math> . Sulfide capacities of silicate melts were found to increase with temperature and the FeO content of the melt. We combined our sulfide data at 1473–1773 K with (O’Neill and Mavrogenes, J Petrol 43:1049–1087, 2002) results at 1673 K, and obtained by stepwise linear regression the following equation for sulfide capacity $$\log C_{{S^{2 - } }} = 0.225 + \left( {25237X_{FeO} + 5214X_{CaO} + 12705X_{MnO} + 19829X_{{K_{2} O}} - 1109X_{{Si_{0.5} O}} - 8879} \right)/T{ }$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mo>log</mml:mo> <mml:msub> <mml:mi>C</mml:mi> <mml:msup> <mml:mi>S</mml:mi> <mml:mrow> <mml:mn>2</mml:mn> <mml:mo>-</mml:mo> </mml:mrow> </mml:msup> </mml:msub> <mml:mo>=</mml:mo> <mml:mn>0.225</mml:mn> <mml:mo>+</mml:mo> <mml:mfenced> <mml:mrow> <mml:mn>25237</mml:mn> <mml:msub> <mml:mi>X</mml:mi> <mml:mrow> <mml:mi>FeO</mml:mi> </mml:mrow> </mml:msub> <mml:mo>+</mml:mo> <mml:mn>5214</mml:mn> <mml:msub> <mml:mi>X</mml:mi> <mml:mrow> <mml:mi>CaO</mml:mi> </mml:mrow> </mml:msub> <mml:mo>+</mml:mo> <mml:mn>12705</mml:mn> <mml:msub> <mml:mi>X</mml:mi> <mml:mrow> <mml:mi>MnO</mml:mi> </mml:mrow> </mml:msub> <mml:mo>+</mml:mo> <mml:mn>19829</mml:mn> <mml:msub> <mml:mi>X</mml:mi> <mml:mrow> <mml:msub> <mml:mi>K</mml:mi> <mml:mn>2</mml:mn> </mml:msub> <mml:mi>O</mml:mi> </mml:mrow> </mml:msub> <mml:mo>-</mml:mo> <mml:mn>1109</mml:mn> <mml:msub> <mml:mi>X</mml:mi> <mml:mrow> <mml:mi>S</mml:mi> <mml:msub> <mml:mi>i</mml:mi> <mml:mrow> <mml:mn>0.5</mml:mn> </mml:mrow> </mml:msub> <mml:mi>O</mml:mi> </mml:mrow> </mml:msub> <mml:mo>-</mml:mo> <mml:mn>8879</mml:mn> </mml:mrow> </mml:mfenced> <mml:mo>/</mml:mo> <mml:mi>T</mml:mi> <mml:mrow/> </mml:mrow> </mml:math> . X MO is the mole fraction of the oxide of M on a single-oxygen basis, and T is in Kelvin. The sulfide capacity equation was combined with sulfate capacity (C S6+ ) data for similar compositions and at the same temperatures (Boulliung and Wood, Geochim Cosmochim Acta 336:150–164, 2022), to estimate the S redox state (S 6+ /S 2– ratio) as a function of melt composition, temperature and oxygen fugacity. Results obtained are in good agreement with earlier measurements of S 6+ /S 2– for basaltic and andesitic compositions. We observe a significant increase, however, relative to FMQ of the oxygen fugacity of the S 2– to S 6+ transition as temperature is lowered from 1773 to 1473 K. We used our results to simulate sulfur-degassing paths for basaltic compositions under various redox conditions (FMQ –2 log f O 2 units to FMQ + 2). The calculations indicate that, given an initial concentration of 0.12 wt% S in an ascending melt at 250 MPa, most of the S (&gt; 80%) will be degassed before the magma reaches 100 MPa pressure.

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