Sulfur Release Associated with Eruption of the Washilla Ridge Member of the Columbia River Flood Basalts

Several estimates of S release associated with eruption of Columbia River Basalts (CRB) have been published. These estimates employ the 'petrologic method' which depends upon analysis of glassy melt inclusions within phenocrysts. Quantification of sulphur release from the most voluminous CRB formation, the Grande Ronde Basalt (~70% of total CRB volume), has proved to be an elusive target due to its perceived aphyric nature, leading to the development of predictive models based on global S abundance in basalts [1]. Here we present analyses of glassy melt inclusions within microphenocrysts of plagioclase and magnetite in glassy phreatomagmatic tephra produced during eruption of the Wapshilla Ridge Member of the Grande Ronde. Melt inclusion S concentrations are 1080 - 1880 ppm with an average of 1500 ppm. The variable S content suggests simultaneous microphenocryst growth and degassing. Sulfur contents in host glassy lapilli are 1290 - 240 ppm with an average of 410 ppm. Variable S in lapilli glasses indicates quenching before degassing was complete, consistent with a phreatomagmatic eruption style. Using the least degassed melt inclusions (representative of pre-eruptive magma) and the most degassed lapilli glasses (representative of degassed lava), we find the released sulfur yield to be 1640 ppm, somewhat in excess of 1190 ± 260 ppm given by the Blake et al. [1] predictive model. The volume of the Wapshilla Ridge Member is 40,000 km3 [2], and its chemical composition exhibits limited variability. Assuming that the tephra we have analyzed is representative, the total mass of S released during the eruption was 180 Gt. Injection would have peaked at 3.6 Mt of S per day during the peak stages of the Wapshilla Ridge eruption, estimated at 0.8 km3 magma/day [2]. This study demonstrates the significance of analysis of glassy tephra associated with the major CRB units toward yielding valuable petrologic information otherwise mostly inacessible from the lavas. References: [1] Blake et al. (2010) EPSL 299, 328-338; [2] Reidel and Tolan (2012) GSA Special Paper, in press.