Genesis of volatiles in suprasubduction basaltic melts from Tolbachik Volcano, Kamchatka

Vitreous basalts and plagioclase lapilli from Tolbachik Volcano studied by high-temperature gas chromatography reveal features of a fluid regime uncommon to suprasubduction melts. Prominent depletion in volatiles confirms the anomalous behavior of the Tolbachik fluid systems. Vitreous basalts contain minor amounts of water (0.16-0.27 wt.%) and carbon dioxide (95-440 ppm). New data on volatiles in the Tolbachik plagioclase lapilli show very low contents of CO (sub 2) and total gas (exclusive of H (sub 2) O) and enrichment in reduced fluids (CO and CH (sub 4) ) relative to the basalts. In general, analysis of basalts and plagioclase lapilli from different eruptions trace a progressive increase in reduced fluids (CO and CH (sub 4) ) and decrease in CO (sub 2) and total gas from past to present events. The concentrations of CO (sub 2) decrease, while those of CO and CH (sub 4) in basalts and plagioclase lapilli increase systematically with an increase in FeO/MgO ratios and K (sub 2) O contents in the lavas and in anorthite component in plagioclase.