Using wet pyroxenes to reconstruct magma degassing history

This proposal seeks to quantify the water content of parental mafic magmas in arcs, using a new method involving the measurement of H+ and Al in pyroxenes. The method utilises the linear relationship between the melt-pyroxene partition coefficient for water, and the Al content of the pyroxene, as determined experimentally. In the past, this method has been applied primarily to mantle xenoliths for inferring the water content of the mantle, with the exception of one study which looked at volcanic rocks from Central America. At Soufriere Hills Volcano and many other intermediate-silicic volcanoes in arc settings, mafic intrusion is thought to trigger and sustain eruptions. At SHV, the mafic magma supplies sulphur and heat to the crystal-rich resident andesite, provoking convective self-mixing and eruption. Gas emissions and ground deformation datasets all point to intrusion of magma at 6-12 km depth, and petrological studies show streaked and banded andesites, or rounded enclaves in the erupted products, suggesting that 'mingling', rather than mixing occurs between the two end member magmas. If the intruding magma has a high volatile content, it will supply a significant vapour phase to the system on quenching and degassing at the interface between the magmas. Magma with a high fluid content is highly compressible, damping ground deformation signals. Sulphur partitions strongly into vapour at depth in oxidised systems, so a high fluid content has a higher potential for 'stripping out' sulphur from the melt, segregating it and allowing it to migrate to the surface or to cause explosive eruption. This proposal seeks to compare pyroxenes from mafic magma (which occurs as enclaves) with those from the resident andesite, to assess the pre-eruptive H2O contents of both end member magmas and further, to evaluate whether it is possible to assess melt water contents through time using profiles across the pyroxenes and accounting for diffusive loss. The results will have broad implications for arc magmas in general.