Vicky Hards

A geochemical investigation of fragmentation and physical fractionation in pyroclastic flows from the Soufrière Hills volcano, Montserrat

Abstract Geochemical analysis is used to investigate fragmentation and physical fractionation in pyroclastic flows. Bulk analyses of the matrices (

Magma hybridisation and diffusive exchange recorded in heterogeneous glasses from Soufrière Hills Volcano, Montserrat

Arc volcanoes commonly show evidence of mixing between mafic and silicic magma. Melt inclusions and matrix glasses in andesite erupted from Soufriere Hills Volcano, Montserrat, include an anomalously K2O-rich population which shows close compositional overlap with residual glass from mafic inclusions. We suggest that these glasses represent the effects of physical mixing with mafic magma, both during ascent and by diffusive exchange during the formation of mafic inclusions. Many glasses are enriched only in K2O, suggesting diffusive contamination by high-K mafic inclusion glass; others are also enriched in TiO2, suggesting physical mixing of remnant glass. Some mafic inclusion glasses have lost K2O. The preservation of this K-rich melt component in the andesite suggests short timescales between mixing and ascent. Diffusive timescales are consistent with independent petrological estimates of magma ascent time.

Periodic sulphur dioxide degassing from the Soufriere Hills Volcano related to deep magma supply

Abstract Soufrière Hills Volcano produced prodigious quantities of sulphur dioxide (SO2) gas throughout 1995–2013. An unprecedented, detailed record of SO2 flux shows that high SO2 fluxes were sustained through eruptive pauses and for two years after the end of lava extrusion and are decoupled from lava extrusion rates. Lava extrusion rates have exhibited strong 1- to 2-year cyclicity. Wavelet analysis demonstrates periodicities of c. 5 months and c. 2 years within the SO2 time series, as well as the shorter cycles identified previously. The latter period is similar to the wavelength of cycles in lava extrusion, albeit non-systematically offset. The periodicities are consistent with pressure changes accompanying deformation in a coupled magma reservoir system whereby double periodic behaviour may arise from limited connectivity between two reservoirs. During periods of lava extrusion SO2 is released together with the lava (yielding the c. 2 year period), albeit with some offset. In contrast, when magma cannot flow because of its yield strength, SO2 is released independently from lava (yielding the c. 5 month period). Our results have implications for eruption forecasting. It seems likely that, when deep supply of magma ceases, gas fluxes will cease to be periodic.

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

The recent study by Devine & Rutherford (2014), ‘Magma storage region processes of the Soufriere 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 Soufriere 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 . 2009 a , 2013). Humphreys et al . (2010) proposed this interpretation by showing that a subset of Soufriere 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’. Here we address the criticisms of the Devine & Rutherford (2014) Appendix, drawing on additional published data to support our arguments and explanations. We were unable to reproduce some of the key figures of Devine & Rutherford (2014) …