Chapter 16 Pre-eruptive vapour and its role in controlling eruption style and longevity at Soufrière Hills Volcano

Abstract

Citation for published item:idmondsD wF nd rumphreysD wFgFF nd ruriD iF nd rerdD F nd dgeD qF nd wsonD rF nd veddenD F nd lilD wF nd frlyD tF nd eiuppD eF nd ghristopherD F nd qiudieD qF nd quidD F @PHIRA 9reEeruptive vpour nd its role in ontrolling eruption style nd longevity t oufri ere rills olnoF9D wemoirsFD QW F ppF PWIEQISF Further information on publisher's website: 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. Schmidt and Poli, 1999). The generation of mafic magmas in the "hot 52 zone" at the base of the arc crust is fundamentally controlled by the H 2 O content of the melts, 53 which act to "flux" the amphibolite crust and cause assimilation (Annen et al,. 2006). In 54 shallow storage areas, the exsolved fluid content of stored magma directly influences the 55 compressibility and hence response of the magma body to changes in pressure or volume, 56 ultimately determining magma ascent rates and eruption style, as well as eruption duration 57 and size (Huppert and Woods, 2002). The rate of crystallisation of magma in crustal 58 reservoirs is influenced by the opposing effects of fluxing by CO 2 -rich gases from depth, 59 which act to "freeze" the magma (Blundy et al., 2010), and new batches of incoming hot, 60 volatile-rich magma, which act to "defrost" it, by heating and transferring H 2 O-rich fluids 61 (e.g. Bachmann and Bergantz, 2006). Sulphur partitions into this fluid (e.g. Scaillet et al., 62 1998;Zajacz et al., 2012), further enhancing the proportion of vapour coexisting with the 63 magma prior to the eruption, and acting as a convenient tracer to measure in volcanic gases 64 (e.g. Gerlach et al., 2008). In the conduit, transitions between lava dome building and 65 explosive Vulcanian activity are controlled by finely-balanced feedbacks involving the 66 development of permeability during degassing, and the increase in viscosity and consequent 67 retardation of bubble growth caused by crystallisation and H 2 O exsolution from the melt 68 (Melnik and Sparks, 1999;Sparks et al., 2000;Melnik and Sparks, 2002; Clarke et al., 2007). 69 CarmichaelThe arc magma system is complex; petrological and geochemical evidence points to the 70 erupted porphyritic andesite being a hybrid, the result of perhaps countless recharge events by 71 internal energy associated with dissolved volatiles can be released into the magma chamber, 87 and this is a mechanism for inducing complex eruption cycles on long timescales for volatile-88 rich magmas (Huppert and Woods, 2002). One mechanism for this is the presence of a large 8...

Crustal‐scale degassing due to magma system destabilization and magma‐gas decoupling at Soufrière Hills Volcano, Montserrat

“…The principal controls on sulfur degassing are and the exsolved gas fraction, which is in turn related to the initial H 2 O content. The parental basaltic andesites on Montserrat are estimated to have contained at least 9 wt% H 2 O initially [ Edmonds et al ., ]. This is somewhat higher that the values used in the experiments shown in Figure (−7 wt%), which will serve to drive the maximum sulfur release to higher pressures.…”

“…Petrological and geochemical data also support a vertically extensive magmatic system from 5 km to at least 10 km [ Edmonds et al ., ]. Phenocryst mineral assemblages and the compositions of most melt inclusions indicate that the SHV andesite magma assembled at low‐pressure at the top of the magma system at 5 km [ Barclay et al ., ; Edmonds et al ., ; Rutherford and Devine , ].…”

“…Phenocryst mineral assemblages and the compositions of most melt inclusions indicate that the SHV andesite magma assembled at low‐pressure at the top of the magma system at 5 km [ Barclay et al ., ; Edmonds et al ., ; Rutherford and Devine , ]. However, there is evidence of magma residence and crystallization at greater depths, including some plagioclase and pyroxene‐hosted melt inclusions with >6 wt% H 2 O as well as water contents of orthopyroxene that indicate melts with as much as 9% H 2 O and depths of crystallization >10 km [ Humphreys et al ., ; Edmonds et al ., ]. Finally, the andesite magma shows the signature of extensive amphibole fractionation [ Zellmer et al ., ; Christopher et al ., ] at depths well below the shallow magma chamber.…”

“…Very water‐rich and vapour saturated andesite magmas were resident in the middle crust [ Edmonds et al . ] and could be an additional source for some of the excess sulfur.…”

Volatiles in subduction zone magmatism

“…Under conditions of high fO 2 , sulphur partitions strongly into vapour even at high pressures (Scaillet & Pichavant 2003); this has the result that for evolved melts stored in the upper crust, the bulk of the sulphur may be stored as vapour (Wallace & Gerlach 1994;Wallace & Edmonds 2011). Alternatively, or perhaps in addition for some systems, mafic magma underplating supplies volatiles through a process of 'gas sparging' (Bachmann & Bergantz 2006) or quench crystallization, vesiculation and mingling (Edmonds et al 2014b). The result of these vapourdominated processes is that, in order to understand the magnitude of the sulphur and carbon budgets of arc magmas, it is necessary to measure their fluxes as gases at the surface.…”