Heterogeneous clasts as windows into magma mingling at Soufrière Hills volcano

A clast erupted from Soufrière Hills volcano during the syn-collapse Vulcanian explosions of July 2003 reveals several incorporated silicic zones within a more mafic host. These incorporated silicics display variable microlite contents that reflect different stages of magma crystallization. One (zone A) shows evidence of heating in several mineralogical features, including completely pseudomorphed hornblende crystals that broke down over a minimum time period of 37-78 days. The lower limit of this time period roughly correlates with the onset of hybrid seismic swarms prior to the 2003 dome collapse. Element X-ray maps reveal percolation of Ca-rich melt into the incorporated silicic margins, suggesting sustained contact that allowed isolation of portions of the resident magma. These characteristics suggest that magma mixing at Soufrière Hills may not only result from disaggregation of mafic inclusions, but also from disaggregation and incorporation of the resident magma into the mafic intrusion.