Chapter 17 Petrological and geochemical variation during the Soufrière Hills eruption, 1995 to 2010

Abstract

Citation for published item:ghristopherD F nd rumphreysD wFgFF nd frlyD tF nd qenreuD uF nd he engelisD FwFrF nd lilD wF nd honovnD eF @PHIRA 9etrologil nd geohemil vrition during the oufri ere rills eruptionD IWWS to PHIHF9D wemoirsFD QW F ppF QIUEQRPF Further information on publisher's website: he iruption of oufri ere rills olnoD wontserrt from PHHH to PHIHF ghpter IUF Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. ABSTRACT 22The andesite lava erupted at the Soufrière Hills Volcano is crystal rich with 33-63% phenocrysts, of 23 plagioclase (65%); amphibole (28%), orthopyroxene (7%) and minor Fe-Ti oxide and clinopyroxene 24 microphenocrysts. The andesite hosts mafic enclaves which have similar mineral phases to the andesite. 25The enclaves are generally crystal poor but can have up to 27% of inherited phenocrysts from the 26 andesite, the majority of which are plagioclase. The eruption is defined by discrete periods of extrusion 27 called phases, separated by pauses. The enclaves exhibit bulk geochemical trends that are consistent with 28 fractionation. We infer that the intruded mafic liquids of Phases I and II interacted and assimilated 29 plutonic residue remaining from the multiple prior mafic intrusions, while the basaltic liquids from Phases 30 III and V assimilated relatively little material. We also infer a change in the basaltic composition coming 31 from depth. The bulk Fe contents of both magma types are coupled and they both show a systematic inter-32 phase variation in Fe content. We interpret the coupled Fe variation to be due to contamination of the 33 andesite from the intruding basalt via diffusion and advection processes, resulting in the erupted andesite 34 products bearing the geochemical imprint of the syn-eruptive enclaves. 35 36The Soufrière Hills volcano is an andesitic dome complex located on the island of Montserrat in the 37Lesser Antilles arc. The present eruption began on the evening of July 18 th 1995 with ash venting 38 followed by phreatic explosions over the next weeks and months Robertson et al., 39 2000). Juvenile material arrived at the surface around November 15 th 1995 (Herd et al., 2005). The volcanic activity of Phase I is described in detail 49elsewhere (e.g. Miller et al., 1998;Calder et al., 2002; Norton et 50 al., 2002) (Herd et al., 2005). This resulted in the construction 55 of a larger volume dome than any extruded during Phase I. Dome collapses were less frequent than during 56Phase I but involved larger volumes, culminating in the largest volume collapse of the...

Chapter 1 An overview of the eruption of Soufrière Hills Volcano, Montserrat from 2000 to 2010

“…Comparison of glass, melt inclusion and spectroscopic gas data has shown that gas is sourced from both the rhyolitic interstitial melt within the andesite magma and the quenching of more mafic magma against the andesite . The issue of long-term petrological and geochemical change over the course of the eruption is reported by Christopher et al (2014) in this Memoir. They conclude that, although there was no progressive change of bulk composition with time, there was evidence that the Fe content of the mafic enclaves and the andesite host did change systematically.…”

Evolution of Mechanical Properties of Lava Dome Rocks Across the 1995–2010 Eruption of Soufrière Hills Volcano, Montserrat

“…Petrological studies of products throughout the eruption have shown that SHV has produced lavas of relatively similar composition hornblende-bearing andesites Christopher et al, 2014;Wadge et al, 2014), with an increasing proportion of mafic inclusions in later phases . Long-term petrology across the eruption was explored by Christopher et al (2014) and although they found systematic changes in Fe-content across time, they concluded that there was no progressive change of bulk composition, with SiO 2 content consistently between 56 and 62% throughout the eruption.…”

“…Petrological studies of products throughout the eruption have shown that SHV has produced lavas of relatively similar composition hornblende-bearing andesites Christopher et al, 2014;Wadge et al, 2014), with an increasing proportion of mafic inclusions in later phases . Long-term petrology across the eruption was explored by Christopher et al (2014) and although they found systematic changes in Fe-content across time, they concluded that there was no progressive change of bulk composition, with SiO 2 content consistently between 56 and 62% throughout the eruption. However, previous studies have documented that geomechanical rock properties of chemically indistinguishable lavas can vary broadly as a result of distinct pore structures (Kendrick et al, 2013;Schaefer et al, 2015;Heap et al, 2016a), local heterogeneities (Farquharson et al, 2016), anisotropy (Bubeck et al, 2017), and post-emplacement alteration (Pola et al, 2014;Siratovich et al, 2014;Coats et al, 2018).…”

Similarities and differences in the historical records of lava dome-building volcanoes: Implications for understanding magmatic processes and eruption forecasting

“…The time-averaged lava extrusion has been 3 m 3 s −1 , although rates exceeding 10 m 3 s −1 have characterised some phases of dome extrusion . The SiO2 content of historically erupted products has varied from to wt.% (Murphy et al, 2000;Zellmer et al, 2003b;Barclay et al, 2010;Christopher et al, 2014). The average SO2 emission rate from 1995 to 2010 was ∼530 t/d and largely decoupled from eruptive activity Edmonds et al, 2010;Christopher et al, 2015).…”