Volatiles contents, degassing and crystallisation of intermediate magmas at Volcan de Colima, Mexico, inferred from melt inclusions

Abstract

In volatile-saturated magmas, degassing and crystallisation are interrelated processes which influence the eruption style. Melt inclusions provide critical information on volatile and melt evolution, but this information can be compromised significantly by post-entrapment modification of the inclusions. We assess the reliability and significance of pyroxene-hosted melt inclusion analyses to document the volatile contents (particularly H 2 O) and evolution of intermediate arc magmas at Volcán de Colima, Mexico. The melt inclusions have maximal H 2 O contents (B4 wt%) consistent with petrological estimates and the constraint that the magmas crystallised outside the amphibole stability field, demonstrating that pyroxenehosted melt inclusions can preserve H 2 O contents close to their entrapment values even in effusive eruptions with low effusion rates (0.6 m 3 s -1 ). The absence of noticeable H 2 O loss in some of the inclusions requires post-entrapment diffusion coefficients (B1 9 10 -13 m 2 s -1 ) at least several order of magnitude smaller than experimentally determined H ? diffusion coefficient in pyroxenes. The H 2 O content distribution is, however, not uniform, and several peaks in the data, interpreted to result from diffusive H 2 O reequilibration, are observed around 1 and 0.2 wt%. H 2 O diffusive loss is also consistent with the manifest lack of correlations between H 2 O and CO 2 or S contents. The absence of textural evidence supporting post-entrapment H 2 O loss suggests that diffusion most likely occurred via melt channels prior to sealing of the inclusions, rather than through the host crystals. Good correlation between the melt inclusion sealing and volcano-tectonic seismic swarm depths further indicate that, taken as a whole, the melt inclusion population accurately records the pre-eruptive conditions of the magmatic system. Our data demonstrate that H 2 O diffusive loss is a second-order process and that pyroxene-hosted melt inclusions can effectively record the volatile contents and decompression-induced crystallisation paths of vapoursaturated magmas.

Blowing Off Steam: Tuffisite Formation As a Regulator for Lava Dome Eruptions

“…The lavas generally contain ∼30 vol.% phenocrysts, 25-50 vol.% microlites and 14-45 vol.% "dry" rhyolitic glass (Reubi and Blundy, 2008;Savov et al, 2008;Lavallée et al, 2012;Kendrick et al, 2013) with a bimodal porosity distribution peaking at 12 and 26% . Geothermometry of pyroxenes suggests relatively deep magma storage at temperatures of 960-1020 • C (Savov et al, 2008), while melt inclusions verify this, indicating trapping conditions of 10-150 MPa and 959-1015 • C (Reubi et al, 2013). The andesite is largely degassed when it extrudes at the surface, and conduit temperature is estimated to exceed 940 • C Reubi et al, 2013).…”

“…Geothermometry of pyroxenes suggests relatively deep magma storage at temperatures of 960-1020 • C (Savov et al, 2008), while melt inclusions verify this, indicating trapping conditions of 10-150 MPa and 959-1015 • C (Reubi et al, 2013). The andesite is largely degassed when it extrudes at the surface, and conduit temperature is estimated to exceed 940 • C Reubi et al, 2013). Outgassing has been shown to concentrate at the vent, with some geochemical indicators of impending activity recorded at summit fumaroles (Varley and Taran, 2003).…”

“…We then chose our experimental temperature based on the conduit temperature estimate of >940 • C (Reubi et al, 2013), rheological work on the dome lavas and calorimetric measurements on the rhyolitic interstitial glass that show the T g is 746-751 • C (at a cooling rate of 10 • C/minute; Kendrick et al, 2013). The samples were then placed into a preheated Carbolite 1400 furnace to sinter at a target temperature of either 940 or 980 • C for 3, 72, 120, or 168 h and then cooled at 4 • C per minute to room temperature (to minimize differential cooling that could cause damage to the sample).…”

Pre- and syn-eruptive degassing and crystallisation processes of the 2010 and 2006 eruptions of Merapi volcano, Indonesia

“…3a), with elevated CO 2 /H 2 O compared to equilibrium degassing trends, has frequently been noted in melt inclusions from subduction systems (Atlas et al 2006;Johnson et al 2008;Vigouroux et al 2008;Blundy et al 2010;Berlo et al 2012;Nadeau et al 2013;Reubi et al 2013). This feature is commonly interpreted as being the result of magma mixing (e.g.…”

“…This feature is commonly interpreted as being the result of magma mixing (e.g. Atlas et al 2006), complex degassing histories, involving CO 2 fluxing (Johnson et al 2008;Vigouroux et al 2008;Blundy et al 2010;Nadeau et al 2013) or non-equilibrium degassing (Gonnermann and Manga 2005), or alternatively, due to post-entrapment diffusive loss of H 2 O or H + (Berlo et al 2012;Reubi et al 2013). At Merapi, elevated CO 2 /H 2 O concentrations in amphibole and pyroxene-hosted melt inclusions have previously been observed and interpreted to reflect CO 2 fluxing and the liberation of CO 2 -rich gas via crustal carbonate assimilation (Nadeau et al 2013).…”