Ergosterol

A comparison of whole rock SiO2 and crystalline SiO2 shows that dome rocks of all magmatic compositions are able to crystallise cristobalite (Figure 4.20). The data, however, show that samples above an intermediate composition (e.g., ~59-60 wt. %) contain substantially more cristobalite than samples below. High viscosities (e.g., dacitic magmas at dome temperatures have viscosities around 10⁷ Pa s, whereas andesites are nearer 10⁴ Pa s (Spera, 2000)), and correspondingly slow extrusion rates, result in prolonged dome residence times (DRT; the duration for which a magma packet resides in a dome), where samples could be held at suitable crystallisation conditions for longer durations.

No constraints on vapour phase depositional kinetics are available; however, for devitrification cristobalite, crystallisation occurs by a continuous growth mechanism following linear growth kinetics (Wagstaff, 1969). Therefore, the abundance of devitrification cristobalite for a magma packet held at suitable crystallisation conditions could be partially constrained by time. This is supported by the decompression study of Hammer and Rutherford (2002), for which there was a greater abundance of groundmass cristobalite (labelled quartz; Figure 4.19) for longer duration experiments. Therefore, the abundance of devitrification cristobalite would be inversely proportional to the extrusion rate. Accordingly, in a concurrent study on ash samples from Soufrière Hills volcano, Horwell et al. (submitted-a; Appendix 4) found that ash sourced from slowly extruded domes is more abundant in cristobalite than ash from rapidly extruded domes, and show a correlation between DRT and cristobalite content for samples resulting from faster extrusion rates.

Analogously, a difference in viscosities could be sufficient to suppress the crystallisation of devitrification cristobalite; or, there may be a minimum viscosity for which cristobalite can crystallise (with further relevance for the plastic cryptodome samples). Supporting this,

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Horwell et al. (post review; Appendix 4) note that the cristobalite content in Soufrière Hills dome rock is predominantly vapour phase in origin, whereas samples from Santiaguito and Mount St. Helens show extensive devitrification. Dome rock samples from a rhyolitic dome (e.g., Chaitén, Chile) as well as additional samples from the current, less-felsic sample suite (e.g., Unzen, Colima and Merapi) would substantially strengthen this viscosity-dependent interpretation.

Figure 4.20: Comparison of cristobalite content determined by XRD with whole-rock SiO2

content determined by XRF for all locations studied. Data for Soufrière Hills are included as an intermediate composition, and are taken from Horwell et al. (post review; Appendix 4).

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A comparison of DRT with cristobalite abundance for the well-constrained sample suite from Mount St. Helens exhibits a visually asymptotic line for dome rock samples, with an average for asymptote-nearing samples of approximately 14 wt. % (Figure 4.21, dashed line). This correlation is somewhat tenuous as different sections throughout the relationship are comprised of data from different eruptive periods; however, the general shape of the curve may imply that cristobalite content increases with residence time until a critical value is reached. DRTs were estimated for the 2004-2006 samples by Pallister et al. (2008) using collection locations and known lineal rate of extrusion to track samples back to the vent.

Residence times for the 1980-1986 samples were taken from the USGS sample record (unpublished). All cryptodome samples have been assigned a maximum DRT of 62 days, representative of the duration from March 17, 1980, the approximate date on which the first seismic activity was recorded (Endo et al., 1990), and May 18, 1980, the day of the eruption.

Curiously, a possible recharge of the magmatic system by hotter magma at depth recorded in Spine 4 (2004-2008; Blundy et al., 2008) did little to influence the cristobalite abundance (samples SH.309.1A, SH.315.4, SH.315.5) relative to other samples. Two samples (circled) which do not conform to the paradigm are the plastic 1980 cryptodome samples. Extrusion rate may better correlate with cristobalite content than DRT (as determined for ash samples in Horwell et al., submitted-a; Appendix 4), since DRT does not directly reflect activity;

however, data were not available for a sufficient number of samples.

Williamson et al. (2010) have suggested that cristobalite can form rapidly at the Soufrière Hills dome, perhaps between hours to days after a magma package is injected into the dome base. The best constraints on dome samples from Mount St. Helens, 1980-1986 suggest up to 8 wt. % can form in three days (MSH.67) and up 18 wt. % in less than three weeks (e.g., MSH.326.2 and MSH.315.5; Table 5). Dome collapse ash samples from Soufrière Hills confirm cristobalite abundances of 5 wt. % after just 1 day of growth and 15 wt. % after 1 month (Horwell et al., submitted-a; Appendix 4), indicating that even a short residence time can result in cristobalite-rich ash. We note that, for the case of ash, it is difficult to constrain the presence of cristobalite to a newly formed dome since the source may include older material, especially at Soufrière Hills which has been active since 1995 (and historically active for much longer). Similarly, Horwell et al. (2010a) found 16 wt. % cristobalite in ash after three months of dome growth at Chaitén volcano, Chile. Dome rock samples from Chaitén were not available for the present study to consider whether this rapid crystallisation was devitrification or vapour-phase driven (with additional implications for Figure 4.20);

however the Soufrière Hills sample used by Williamson et al. (2010), MVO287, was found to have both devitrification and vapour phase cristobalite (Horwell et al., post review;

Appendix 4).

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Figure 4.21: Comparison of cristobalite abundance with dome residence time (DRT) for dome rock samples from Mount St. Helens. The two data points circled are discussed further with respect to Figure 4.24. The dashed line is an average of all cristobalite values above 10 wt. %.

A sample from the Mount St. Helens cryptodome (MSH.BD.4) shows evidence of a final decompression event (Figure 4.22), recorded by frozen, burst melt inclusions in feldspar, analogous to those observed by Williamson et al. (2010). This is a sample of ‘grey’ dacite, which Hoblitt and Harmon (1993) suggest underwent a second vesiculation event during the 18 May 1980 lateral blast. This sample, along with MSH.BD.03 (another ‘grey’ dome rock sample), had substantially less cristobalite than texturally-identifiable ‘black’ cryptodome samples which did not undergo a secondary vesiculation event (3-6 wt. % versus 11-13 wt.

%). It is possible that the grey dacite originated from a fresher batch of magma or was sufficiently ductile until the moment of depressurisation to limit sub-solidus cristobalite crystallisation. Both possibilities would limit the ‘real’ (fresher batch) or ‘effective’

(sufficiently ductile) dome residence time, and may account for the seemingly anomalous results of the two samples circled in Figure 4.21. This plasticity would also account for the flow alignment observed for microlites in cryptodome samples (Figure 4.13). Hoblitt and Harmon (1993) observed no difference in chemistry between ‘grey’ and ‘black’ samples, and Cashman (1988) identified no qualitative difference in microlite abundance or quantitative difference in crystal size distribution, concluding microlite nucleation was probably triggered

Cristobalite Formation 4

by a single intrusion of material. Cashman (1988) also suggests that microlite crystallisation occurred at depth rather than in the dome, which is supported here by incorporation of microlites within patches of devitrification cristobalite (e.g., Figure 4.6b). Therefore, based on these studies, material was likely too ductile to facilitate crystallisation.

In magmas, in general, prolonged cooling results in few, large crystals whereas rapid cooling results in many, smaller crystals. A similar distribution for size frequencies was qualitatively observed for cristobalite crystals in dome rock; however, insufficient data was available to compare mean crystal size to dome residence time and/or extrusion rate. The ‘massive’

crystals identified in samples from Mount St. Helens and Santiaguito lend credence to the notion of prolonged growth and warrant further research, considering the Brujo dome was active from 1959-1963, without experiencing a major dome collapse, and Mount St. Helens cryptodome samples were at ~900 °C (Cashman, 1992; Rutherford and Hill, 1993) for ~60 days (Pallister et al., 2008).

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Figure 4.22: Scanning electron micrographs of ruptured plagioclase phenocryst melt inclusions (decompression features) in MSH.BD.4.

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Table 4.5: Dome residence times for samples from Mount St. Helens.

Volcano Sample Origin Dome Residence

Time (days)

MSH.103 Federal building 2

MSH.127 Talus 5

Edifice MSH.ED.1 Pre-1980 edifice --

Dyke MSH.DY.1 Toutle River valley --

*All cryptodome samples are assigned a maximum possible duration of 62 days.

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