EXPERIMENTAL STUDY OF CALC-ALKALINE ANDESITE DIFFERENTIATION AT MOUNT RAINIER, WASHINGTON

Intermediate and evolved magmas dominate the eruptive and mid-to-upper crustal plutonic portions of most subduction-related magmatic arcs. Inferring mantle-level processes, magma-crust interaction, and events leading to hazardous eruptions are aided by a quantitative understanding of the origins of intermediate and evolved arc magmas. Mt. Rainier has erupted a medium-K calc-alkaline basaltic andesite – andesite – dacite magmatic suite typified by abundant phenocrysts of plagioclase, 2 pyroxenes, and FeTi oxides, accompanied by accessory apatite and pyrrhotite, and in the more evolved samples minor amphibole. Melt inclusions indicate pre-eruptive dissolved H₂O of 3 – 4 wt% and FeTi oxides give fO₂s of mostly NNO+1 to +1.6.

Phase equilibrium experiments were conducted on representative Mt. Rainier basaltic andesite, andesite, silicic andesite, and dacite at 400 MPa saturated with 2:1 CO₂:H₂O gas, buffered at Re-ReO₂ (NNO+1.9). This pressure and gas composition give melt H₂O concentrations of ~3.5 wt%. The experiments reproduce the natural plagioclase – 2-pyroxene – FeTi oxide phenocryst assemblage and yield liquid compositions that vary in close agreement with the volcano's calc-alkaline magma series. Subtle differences between the compositions of synthesized liquids and the magma series are readily attributed to mixing between more and less evolved magmas sharing common or similar liquid lines of descent.

These results are strong support for crystallization-differentiation from basaltic andesite parents as the dominant cause of magmatic diversity at Mt. Rainier, and by implication, similar 2-pyroxene calc-alkaline suites elsewhere. Chemical and isotopic evidence for crustal assimilation is mainly restricted to the production of the basaltic andesite parents fueling Mt. Rainier and probably takes place at deep crustal levels involving primarily high-temperature mantle-derived magmas.