Determination of sulfur speciation and oxidation state of olivine hosted melt inclusions

Abstract

In order to better understand what controls sulfur speciation in melt inclusions, and how that pertains to the original basalt composition, we have conducted a series of heating experiments on naturally quenched and crystalline olivine-hosted melt inclusions. Sulfur speciation was determined from S Kα peak shift measurements by electron microprobe on the experimentally heated inclusions as well as a series of naturally quenched inclusions, and matrix glasses. Naturally quenched olivine-hosted melt inclusions record a similar but more variable sulfur speciation relative to matrix glasses, (up to 45–50% variation in S 6+/S total). Much of this range can be attributed to the effect of degassing which may either increase or decrease the S 6+/S total. In addition, olivine melt re equilibration and H diffusion out of the inclusion both potentially result in the oxidation of melt inclusions. Heating of melt inclusions can have different effects on the sulfur speciation under different conditions. A slight decrease in S 6+/S total and oxygen fugacity (∼0.1 log units) can occur from overheating of inclusions (above the temperature of entrapment), resulting from excess ferrous iron in the melt. An increase in heating times should result in an oxidation of the inclusion generated by increased H diffusion out of the inclusion. However, results of heating experiments on melt inclusions from an Izu backarc basalt for less than 30 min do not show a significant increase in sulfur oxidation. In addition, experiments conducted at both IW and FMQ have measured sulfur speciation consistent with naturally quenched inclusions suggesting that at experimental temperatures near that of olivine crystallization the furnace atmosphere does not exert significant control on the melt fO 2. By taking these parameters into account, sulfur speciation and oxidation state of basaltic melt trapped within inclusions can be accurately determined from both naturally quenched and heated olivine hosted melt inclusions.