Physics
Scientific paper
Aug 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993gecoa..57.3807n&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 57, Issue 15, pp.3807-3813
Physics
14
Scientific paper
We have determined the Fe 3+ / Fe 2+ ratio, S content, and the SO 2- 4 / S 2- ratio (by SK X-ray peak shift) for a suite of twelve mid-ocean ridge and backarc basin glasses. These samples range in oxidation state from two orders of magnitude below to 1.5 orders of magnitude above the Ni-NiO oxygen buffer. Sulfur speciation in the glasses is strongly dependent on magmatic oxidation state. We propose that the solubility mechanism for S in basaltic melts changes over the | O 2 range which characterizes convergent margin magmas. In low | O 2 , sulfide saturated basaltic melts, such as most MORBs, S is present largely as S 2- , and its concentration is controlled by sulfide melt-silicate melt equilibrium and varies positively with | O 2 . At higher | O 2 's, where sulfide melt becomes unstable, S concentration is controlled by melt-vapor equilibrium and decreases with increasing | O 2 . For the range of | O 2 's in the back arc basin basalts in this study (NNO-1 to NNO+1.5), S is dissolved in the silicate in both oxidized and reduced forms. For the lower | O 2 end of this range, S 2- dominates and equilibria involving reduced S-species control S melt concentration. At higher | O 2 's, the proportion of oxidized S increases and melt-vapor equilibria involving these species become more important. Because S partitions strongly into the vapor phase over the | O 2 range investigated, magmatic degassing may be effective at stripping S from the melt. The low S concentrations in island arc and backarc lavas do not necessarily reflect a low S source but rather result from higher | O 2 in the convergent margin environment.
Nilsson Kristen
Peach Cheryl L.
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