Quartz Resorption as a Geospeedometer in Peralkaline Rhyolites

Magma ascent rate affects eruptive style and intensity as it determines time available for syn-eruptive crystallization, vesiculation and permeable gas loss. The width of hornblende reaction rims has previously been used to estimate ascent rates for eruptions of andesitic volcanoes. Reactions between quartz and the coexisting melt could provide a similar proxy for peralkaline silicic magmas. Mayor Island, a peralkaline rhyolitic volcano in New Zealand, was used as a case study to investigate the use of quartz resorption as a geospeedometer. During the last 130 ka, Mayor Island has exhibited a wide range of eruptions, both with regards to intensity and volume. Previous studies have determined the pre-eruptive temperature to be around 750°C and pressure to be 100-125 MPa, and proposed that the magma chamber is saturated in water (Barclay et al., 1996). Neither the composition of the magma (72-74 wt% SiO2) nor the water content (4.4 wt%) have changed significantly between the different styles of eruptions, and the wide range of eruptive style was therefore attributed to variations in the ascent rate. In general, the quartz phenocrysts from the effusive eruptions are rounded, whereas those from the explosive eruptions are euhedral. Scaillet and Macdonald (2001) established that there are realistic conditions for which quartz in peralkaline rhyolites goes from stable to unstable to stable again during decompression. In this study, the stability fields of quartz were determined for a Mayor Island magma composition using an externally heated cold-seal pressure vessel. The rate of quartz resorption was assessed by carrying out time-series experiments. The pre-eruptive conditions were determined to be about 700-750°C from feldspar thermometry and phase equilibria. The results indicate that the magma was water under-saturated and consequently stored at higher pressures than previously calculated. The time-series experiments imply that magma that erupt explosively did not spend more than a couple of days at pressure between 150 and 50 MPa. REFERENCES Barclay, J., Carroll, M. R., Houghton, B. F. and Wilson, C. J. N. (1996). Pre-eruptive volatile content and degassing history of an evolving peralkaline volcano. Journal of Volcanology and Geothermal Research 74: 75-87. Scaillet, B. and Macdonald, R. (2001). Phase relations of peralkaline silicic magmas and petrogenetic implications. Journal of Petrology 42(4): 825-845.