HIGH SULFUR CONCENTRATIONS FROM OLIVINE-HOSTED ABSAROKITE MELT INCLUSIONS, CASCADE RANGE, OREGON: SIGNS OF AN OXIDIZED SOURCE?

      Melt inclusion sulfur concentrations provide potentially important constraints on the oxidation state of the magma during the early stages of magma genesis.  In a suite of basalt lavas from the central Oregon Cascades, we have identified olivine-hosted (Fo₈₀ to Fo₉₂) inclusions characterized by a range of sulfur concentrations (1000 to 8000 ppm).  Based on the trend of the Fe-S sulfide saturation line (extrapolated to 19 wt% Fe oxide), total dissolved sulfur, present as sulfide (S^-2), should not exceed ~2700 ppm in these absarokite/shoshonitic compositions.  We have interpreted the sulfur above the extrapolated sulfide saturation line to be as present as sulfate (S⁺⁶), constituting as much as 60% of the dissolved sulfur.  Based on this interpretation, the magma at the time of crystallization and melt inclusion entrapment was oxidized, with up to 5500 ppm dissolved sulfur as sulfate.  Sulfur Ka peak positions were determined on an electron microprobe for high sulfur melt inclusions using a modified version of the experimental procedure described by Carroll and Rutherford (1988) and Wallace and Carmichael (1994).  Calculated S peak shifts, measured relative to pyrite (FeS₂) using a 5mm beam diameter with a 30 nA current, suggest fO₂ varies from QFM+1 to QFM+1.2.  Oxygen fugacity, determined from the S peak shift corresponds to fugacity estimates determined graphically, based on a correlation between percent sulfate versus oxygen fugacity.  Additionally, this procedure agrees well with previous calculations of oxygen fugacity for Cascade absarokite magmas, based on olivine-liquid equilibria, of QFM+1 to QFM+2 (Conrey et al, 1997). 

      The oxidized state of the magma, and high alkali content, suggests addition of an oxidized fluid to the mantle source, possibly derived from the subducting Juan de Fuca Plate.  This inference of a significant fluid component appears in contrast to the interpretation that the Cascade volcanic arc represents a dry system, where dehydration occurs early in the subduction process.