IMPLICATIONS OF VOLATILE EXSOLUTION IN ISLAND-ARC LOCATIONS: ANALYTICAL MODELING OF SURFACE DEFORMATION AND PRESSURE-VOLUME RELATIONSHIPS AT SOUFRIERE HILLS VOLCANO, MONTSERRAT W.I

Extensive research has been conducted on the implications of recharge in intermediate to silicic composition magmatic systems. Many studies suggest interactions between newly injected material and resident magmas can lead to extensive exsolution of volatiles and eruption before geochemical or thermal equilibrium. Mafic recharge of felsic composition magmatic systems is commonly observed in island-arc locations and has been observed petrologically and stratigraphically at Soufriere Hills Volcano located on the island of Montserrat in the Caribbean (16.72N, 62.18W). After showing some signs of unrest, notably an increase in shallow seismicity starting in 1992, the Soufriere Hills Volcano has been erupting periodically since July of 1995. What is not clearly defined, despite the extensive evidence for mafic recharge, is the relative contribution of recharge and exsolved volatile contents on the dynamic pressurization of the magmatic system. The findings presented in this work are: (1) exsolved volatiles from the cooling and degassing of mafic recharge pulses at depth cause little to imperceptible amounts of volume change which could be observed in the surface deformation field; and (2) the volume change caused by the mafic recharge itself is the dominant physical process producing overpressures at SHV. The novel approach attempted in this work is the correlation of deep sub-surface physical magmatic processes with far-field geodetic deformation data. Modeling of volatile exsolution overpressures with mafic flux volumes at SHV implies several orders of magnitude difference between pressurization caused by exsolved volatiles (~0.1%) and that caused by the injections themselves (~100%). A first-order observation of the vast difference between pressurization due to volatile exsolution (even in extreme 60% crystallization situations) and that produced by the volume of injected material leads me to conclude that at Soufriere Hills Volcano, volatile exsolution is not a plausible physical process for the production of magma chamber pressures inferred from ground-deformation GPS data. The results of this research suggest that for mid-crustal magmatic systems in island-arc locations volatile exsolution correlates little with eruptive potential if recharge is significant and ongoing.