An experimental investigation of amphibole decomposition reaction rates is proposed which should yield estimates of magma ascent rates for many andesite, dacite and rhyolite magmas. Ascent rate estimates are used in conjunction with experimentally determined pre-eruption magma chamber conditions (P,T, fH2O)) to constrain models of eruption dynamics. Reactions to be studied are (1) decomposition of amphibole by reaction with melt to form pyroxenes, plagioclase, Fe-Ti oxides and a more evolved melt, and (2) associated plagioclase crystallization. Preliminary studies of natural tephra and dome rock samples indicate that these reactions have proceeded to different degrees in Mount St. Helens and long Valley caldera eruption products. The extent of reaction is indicated by measurements of the widths of amphibole breakdown rims, and by changes in plagioclase and melt composition. The extent of reaction is primarily a function of decompression (decreasing fH2O) at roughly constant temperature, judging from oxide geothermometry of recent Mount St. Helens dacite eruption products (1980-1986). Extent of reaction, as measured by amphibole breakdown rim width and plagioclase crystallization, will be experimentally calibrated as a function of decompression rate. Primary objectives of the research are (1) to estimate magma ascent rates for well-studied tephra and dome eruptions of Mount St. Helens and Long Valley caldera, and (2) to compare the ascent rate estimates with those derived from (a) crystal size distribution analysis and (b) earthquake focus migration analysis.
