A new method to quantify the real supply of mafic components to a hybrid andesite

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

“…This would be consistent with similar observations made on the compositions of microlite crystal populations (Humphreys et al 2009a(Humphreys et al , 2013. In our original study, the key observation was that both K 2 O and TiO 2 anomalies are observed in melt inclusions and matrix glasses, and that they were apparently decoupled (see fig.…”

“…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 ) and of microlite crystal populations (Humphreys et al 2009a(Humphreys et al , 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 K 2 O and/or TiO 2 , as have mafic inclusion matrix glasses (see Humphreys et al 2010, fig.…”

“…Although Devine & Rutherford (2014) state that this may be due to analytical artefacts caused by inaccurate raster analysis of inhomogeneous groundmass, there are alternative explanations. One is that the groundmass is contaminated by disaggregation of mafic enclaves and transfer of small crystals into the andesite groundmass, including clinopyroxene and Ca-rich plagioclase (Humphreys et al 2009a(Humphreys et al , 2013. This is consistent with the interpretation that the anomalous glass compositions may be related to hybridization (see below; Humphreys et al 2010;Devine & Rutherford 2014).…”

The temporal evolution of chemical and physical properties of magmatic systems

“…In effect, chemical differentiation then became a question of how heat is lost from a single body of initially crystal-poor magma. In the last 20 years, petrology, thermal modelling, structural geology and geophysical monitoring of restless volcanoes have demonstrated that magmatic systems are constructed by the periodic injection of magma over prolonged periods of time (de Saint-Blanquat et al 2001;Coleman et al 2004;Glazner et al 2004;Annen 2009;Biggs et al 2011;Caricchi et al 2012;Humphreys et al 2012). Piecemeal emplacement complicates the temporal pattern of heat loss and, consequently, the inferences that can be drawn about the chemical evolution of the system and the probability of extracting melt of one composition versus another.…”

“…Volcanic eruptions of relatively crystal-poor magma testify to the existence of such bodies, but thermal models together with geochronological and geochemical data require that this is a relatively transitory physical state (Wilson & Charlier 2009;Cooper & Kent 2014;Wotzlaw et al 2014). Second, petrological studies of igneous rocks reveal a level of geochemical and textural complexity that cannot be reconciled with simple precipitation from a melt, instead suggesting physical and chemical interaction of crystal residues with melts over long periods of time in a thermally fluctuating mush environment (Paterson et al 2011;Humphreys et al 2012;Thomson & Maclennan 2012;. Intrusive bodies from small laccoliths to giant batholiths appear to have formed not in one large belch of magma, but via the amalgamation of many smaller, interacting pulses over time periods of more than 100 000 years Michel et al 2008;Schoene et al 2012).…”