ORIGIN OF HYDROUS ARC MAGMAS: EXPERIMENTS TO DETERMINE THE EFFECT OF MELT COMPOSITION AND H2O CONTENT ON THE ACTIVITY OF SiO2

A series of “sandwich” -type experiments have been conducted to examine how H₂O and melt components (e.g. alkalis) influence the composition of mantle melts. In theory, a silicate magma has a constant silica activity when both olivine and orthopyroxene are present. Because H₂O has the effect of stabilizing olivine relative to orthopyroxene at magmatic conditions, increasing H₂O has the effect of decreasing the activity coefficient of SiO₂ in the melt. As a result, hydrous mantle melting may be capable of producing silica-rich melts such as high-Mg andesites (Carmichael, 2002). Our experiments involve a mantle “sandwich” comprised of olivine and orthopyroxene, separated from San Carlos peridotite, interlayered with mafic to intermediate compositions. Experiments have been run with variable amounts of H₂O and CO₂ in Au-Pd capsules at 1.0 GPa and 1200 degrees C. The run products were characterized by electron microprobe and FTIR to determine phenocryst and melt composition as well as H₂O and CO₂ abundance. The results confirm that H₂O does have a significant effect on the activity coefficient of SiO₂. Current work is focused on increasing the range of H₂O as well as varying the starting composition. These experiments will help quantify the influence of H₂O on the origin of arc magmas.