Controls on variations in cristobalite abundance in ash generated by the Soufrière Hills

The Soufrière Hills Volcano (SHV) crystallizes cristobalite (crystalline silica) in its lava domes, and inhalation of cristobaliterich ash may pose a chronic respiratory hazard. We investigate the causes of variation in cristobalite abundance (measured by X-ray diffraction) in ash from dome collapses, explosions and ash venting from 1997 to 2010. Cristobalite abundance in bulk dome-collapse ash varies between 4 and 23 wt%. During periods of slow lava extrusion (,5 m s), cristobalite is abundant (7–23 wt%), which we attribute to extensive devitrification in slow-cooling lava; it can also form rapidly (15 wt% in 2 months), but we find no correlation between cristobalite abundance and dome residence time (DRT). By contrast, during rapid extrusion (.5 m s), cristobalite abundance is low (4–7 wt%, similar to that associated with Vulcanian explosions), and correlates strongly with DRT. We attribute this correlation to progressive vapour-phase mineralization or devitrification, and the lack of contamination by older lava. Cristobalite abundance is expected to be .7 wt% for collapse of slowly extruded lava, for ash venting through a dome or for incorporation of hydrothermally altered edifice during explosions; cristobalite abundance is expected to be ,7 wt% for collapse of rapidly extruded lava, for ash venting without dome incorporation and from Vulcanian explosions at SHV. Gold Open Access: This article is published under the terms of the CC-BY 3.0 license. Cristobalite is a high-temperature, low-pressure crystalline silica polymorph that may crystallize as a meta-stable phase in dome lavas, and persists at ambient conditions. In industrial settings, the silica polymorphs of quartz, cristobalite and tridymite are capable of causing silicosis, a fibrotic lung disease. Crystalline silica is also classed as a Group 1 human carcinogen by the International Agency for Research on Cancer (IARC 1997). The discovery of cristobalite in the volcanic ash from the 18 May 1980 eruption of Mount St Helens, USA, prompted intensive research to determine the silicosis risk from inhaling the ash (e.g. Dollberg et al. 1986), but the evidence from a series of toxicological, epidemiological and clinical studies at the time was inconclusive on this point (see Horwell & Baxter 2006 for a review). In the event, exposure was short-lived and this substantially reduced public concern. Today, the chronic pathogenicity of cristobalite in volcanic ash is still under debate (Horwell et al. 2012). The Soufrière Hills Volcano (SHV), Montserrat began its current eruption in July 1995. Lava dome growth started in late 1995 and has continued, intermittently, in a series of five phases (Wadge et al. 2014). Lava domes are inherently unstable, and are prone to partial or full collapses that generate pyroclastic density currents (PDCs) and associated co-PDC ash plumes. There have also been more than 100 Vulcanian explosions during the eruption. Baxter et al. (1999) observed that cristobalite is generated within the Soufrière Hills dome and is abundant in the co-PDC ashfall. Horwell et al. (2003) characterized the cristobalite in different grain-size fractions and found that the cristobalite was concentrated in the finest, respirable fractions (,4 mm diameter). Over the course of the Soufrière Hills eruption, numerous mineralogical analyses and risk assessments for respiratory health have been carried out, based on quantification of cristobalite abundance by X-ray diffraction (XRD) and Si magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy (see the review in Baxter et al. 2014). Initial analyses were carried out on ash from the first major dome-collapse PDC on 3 April 1996, where 25 wt% cristobalite was found in the sub-10 mm fraction. Similar abundances were found in dome-collapse ash deposited in 1996 and 1997 (10–27 wt%, four samples), in 1999 (30 wt%, one sample) and in August–September 2001 (24–29 wt%, three samples) (Baxter et al. 1999, 2014). A routine assessment in 2009–2010 found that cristobalite abundance had dropped during dome growth in Phase V to ,5 wt% in the bulk ash. The current study was devised to quantify cristobalite abundance in samples spanning the entire eruption, using a single XRD technique for consistency, in order to determine whether this decrease in cristobalite abundance was real or an artefact of technique progression over time. Eruption parameters, such as dome residence time (DRT), lava extrusion rate and SO2 flux, were then correlated with the data to investigate the causes of fluctuations in cristobalite abundance. Assessment of cristobalite abundance in volcanic ash is complicated by the presence of other minerals, in particular, plagioclase feldspar, which is often the dominant mineral in andesitic ash, and makes rapid quantification by XRD challenging owing to a peak overlap with cristobalite. Early studies of Mount St Helens and Soufrière Hills ash addressed this problem by employing the Talvitie method (Talvitie 1951), in which all minerals are dissolved except crystalline silica (Baxter et al. 1999). This method, which involves boiling the ash in phosphoric acid for 8 min, is difficult to perform consistently since the final mineral From: Wadge, G., Robertson, R. E. A. & Voight, B. (eds) 2014. The Eruption of Soufrière Hills Volcano, Montserrat from 2000 to 2010. Geological Society, London, Memoirs, 39, 399–406. http://dx.doi.org/10.1144/M39.21 # 2014 The Authors. 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