Mixing relationship changes over the course of the eruption

The results from chapters 3 and 4 show that the erupted material from phase V has a complex mixing history and that the bulk geochemistry appears to have changed from earlier phases. This leads to question (3): Have these mixing and mingling mechanisms changed in their character through the course of the eruption? Petrological, textural and geochemical analysis demonstrates a change in the mixing relationship between the andesite and mafic magmas over the course of the eruption. The high proportion of the hybrid type B enclaves observed in the field (Fig. 3.6), the loss of the compositional gap in SiO2 between the andesite and mafic enclaves in phase V (Fig. 3.11), and geochemical modelling, all indicate that the mafic magma erupted in the form of enclaves has become more hybridised over the course of the eruption. This leads us to examine the possible reasons for the change in the ability of the two magmas to mix more efficiently.

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6.3.1 Influence of bulk geochemistry change?

The bulk geochemistry of the mafic enclaves shows that the mafic-end member may have changed, but whether this compositional change has enabled greater mixing between the two magmas is unclear. The bulk Fe concentrations in the mafic enclaves have decreased to become closer to the andesite concentrations (Fig. 3.11), but Mg, V and Sc concentrations have all increased, thus moving away from the andesite (Figs. 3.11; 4.5).

These changes in the trace and major element chemistry are probably not to a large enough degree to significantly alter the compositional contrast between the magmas and promote more efficient mixing. Compositional changes in the mafic end-member could only cause increased mixing between the magmas, if it was coupled with significant viscosity and temperature change. Therefore, it appears unlikely that compositional changes in the mafic enclaves across the length of the eruption have been solely the cause of increased mixing between the two magmas.

6.3.2 Influence of temperature change?

Increased mixing between the andesite and mafic magmas could have been promoted by a reduction in the temperature contrast between the magmas. A lower temperature contrast lowers the cooling rate and slows crystallisation of the intruding mafic magma thus allowing greater mixing (Sparks & Marshall, 1986). Any reduction in the temperature contrast between the two magmas would also have the effect of lowering the melt viscosity contrast. There is no strong evidence of a systematic temperature change in the intruding mafic magma from the mafic enclaves across the phases, but constraints on temperature from the mafic enclaves are poor. However, new results from Fe-Ti oxides geothermometry from phase V andesite, show a global temperature increase from 835°C in phase I to 845°C in phase V (Devine & Rutherford, 2014). This suggests that progressive heating of the andesite may have taken place over the course of the eruption since the initial mafic intrusion (Devine & Rutherford, 2014). The absence of quartz within the phase V andesite samples used in the above study is also cited as evidence for elevated heating (Devine & Rutherford, 2014); however, it should be noted that the results of this study show that quartz is not only present in the phase V andesite, but also in some mafic enclaves. A system wide increase in the temperature of the andesite magma over the course

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of the eruption could lead to a lower viscosity. For example, the viscosity of the SHV andesite rhyolite melt (using the empirical model of Giordano et al., 2008) can be calculated as 5.3 log Pa s (assuming 5 % wt H₂O) at 850 °C; a 10 °C increase reduces viscosity by 0.1 log Pa s; a 30 °C increase by 0.3 log Pa s. Any lowering of viscosity in the andesite would also increase the mobility of the gas phase through the andesite, thus advecting heat more efficiently (Bachmann & Bergantz, 2006) leading to more convection.

Although the high crystal content of the andesite will limit the effect of any melt viscosity changes to the effective viscosity (melt + crystals).

Evidence of a global increase in the andesite temperature using Fe-Ti geothermometry after repeated mafic intrusions (Devine & Rutherford, 2014) may result in a reduced temperature contrast between the magmas. This could promote more efficient magma mixing, which tallies with the observation of a more hybridised mafic magma in phase V. Nonetheless, a 10 °C increase in the andesite temperature is clearly not sufficient to reduce the temperature and viscosity contrast enough to alter magma interaction from mingling to mixing, as demonstrated by the continuing presence of mafic enclaves in the erupted andesite. Whether the andesite magma body as whole would have been uniformly heated is debatable. Murphy et al., (2000) suggest that the pre-eruptive andesite magma chamber could have been zoned in respect to temperature before eruptive activity commenced in phase I. It could be possible that chamber may have become more strongly zoned in respect to temperature with continuing transfer of heat from a mafic layer.

Conversely, although decreasing the temperature contrast between the magmas would lead to more efficient mixing, it could also lead to slower reheating rates (Bachmann &

Bergantz, 2006).

The transition between explosive and extrusive behaviour at arc volcanoes may be in part linked to temperature changes in the host magma caused by mafic magma intrusion (Ruphrect & Bachmann, 2010). The differing eruption styles of the effusive 1846-47 and plinian 1932 eruptions at Quizapu, Chile have been attributed to the host magma temperatures (Ruphrect & Bachmann, 2010). The effusive 1846-47 eruption showed evidence for magma mixing, where the intrusion of a hotter mafic magma led to significant reheating of host magma, elevating temperatures from 830 to 1000 °C (Ruphrect &

Bachmann, 2010). The increase in the host magma temperature is interpreted to have enhanced magma degassing, but reduced rapid magma expansion and explosive behaviour

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(Ruphrect & Bachmann, 2010). In contrast, the plinian 1932 eruption showed no evidence for magma reheating by mafic intrusion; therefore, it is concluded that the gases were trapped longer in magma during ascent, leading to explosive behaviour (Ruphrect &

Bachmann, 2010). However, despite an apparent increase in the global andesite temperature at SHV (Devine & Rutherford, 2014), phases IV and V were as explosive as earlier phases, with some of the largest vulcanian explosions to date (Wadge et al., 2014).

6.3.3 Influence of mafic magma supply change?

Over the course of the eruption the proportion of mafic enclaves observed in the erupted deposits has increased since phase I (1 to 8 %; Murphy et al., 2000; Komorowski et al., 2010, Mann, 2010; Barclay et al., 2010; this study). This may indicate that perhaps the volume of the intruding mafic magma has increased significantly. In the ‘gas sparging’

model proposed by Bachmann & Bergantz, (2006), it is suggested that remobilisation of the andesite mush via advection from gas requires 0.5 km³ of gas at magma chamber pressures sourced from ~2–3 km³ of mafic magma. Thus far from 1995 to 2010, only

~0.06 km³ of mafic magma in the form of mafic enclaves has been erupted suggesting a large volume of the mafic magma remains unerupted at depth, which is corroborated by continuing ‘excess’ sulphur emissions to date (e.g Edmonds et al., 2014). There is no clear evidence from gas emissions or geophysical data to suggest an overall increase in the rate of mafic magma supply. The higher proportion of mafic enclaves could just reflect a portion of eruptible melt reservoir containing more enclaves is being progressively accessed. In section 6.1 the possibility that type A enclaves may be more susceptible to disaggregation after formation than the more hybridised type B enclaves was discussed.

Therefore, perhaps as the mafic magma becomes more hybridised over the course of the eruption, the increased volume of mafic enclaves erupted is just a function of a lesser susceptibility to disaggregation. To test this hypothesis, future work examining the proportion of disaggregated mafic material in the andesite in phases I and II is required.

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Figure 6.1: Schematic summary model of mafic-andesite interaction and flux of material at SHV (not to scale). Mafic magma intrusion at depth remobilised the andesite mush. Transfer of heat either via advection or bubble transfer led to convection and remobilisation of the andesite (Bachmann & Bergantz, 2006). Mafic magma may be mingled into the andesite via mafic plumes (type A enclaves) and detachment from or breakup of a vesiculating hybridised layer (type B enclaves). Progressive mafic magma intrusion has resulted in more hybridised enclaves (type B) over the course of the eruption as mafic-andesite interaction has changed. Fluids rich in H₂O, CO₂ and S are transferred from persistent mafic intrusion at depth (e.g.

Edmonds et al., 2001) and disaggregation of enclaves (Edmonds et al., 2014). The decoupled metal-rich vapour phase may be degassed through shear fractures in the shallow conduit or dome.

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