Discussion on ‘Magma storage region processes of the Soufrière Hills Volcano, Montserrat’, Geological Society, London, Memoirs, 39, 361-381.

Abstract

The recent study by Devine & Rutherford (2014), ‘Magma storage region processes of the Soufrière Hills Volcano, Montserrat’, published as Chapter 19 of the Geological Society of London Memoir, Volume 39, focused primarily on updating the record of magmatic temperatures recorded by Fe–Ti oxides in the andesite erupted from Soufrière Hills Volcano. However, a key result of the paper was that the compositions of some plagioclase-hosted melt inclusions indicate mixing of mafic magma components into the host andesite and trapping of the mixed melts within phenocrysts. This interpretation is consistent with our earlier work on the chemistry of melt inclusions (Humphreys et al. 2010) and of microlite crystal populations (Humphreys et al. 2009a, 2013). Humphreys et al. (2010) proposed this interpretation by showing that a subset of Soufrière Hills melt inclusions and matrix glasses has anomalous concentrations of K2O and/or TiO2, as have mafic inclusion matrix glasses (see Humphreys et al. 2010, fig. 2). However, in their Appendix, Devine & Rutherford (2014) suggest that enrichment in K2O can result only from decompression crystallization and not from magma mingling. They also assert that our melt inclusion dataset is fundamentally flawed and subject to faulty corrections for post-entrapment correction; and that our melt inclusions were actually matrix glasses. Finally, Devine & Rutherford (2014) doubt ‘whether or not one can look at melt-inclusion analyses and distinguish the effects of decompression crystallization … from the effects of mingling … with components derived from the injected mafic magma’.