PRE-ERUPTIVE VOLATILE CONTENT AND DEGASSING HISTORY OF AN EVOLVING PERALKALINE VOLCANO

The volatile contents of melt inclusions trapped within phenocrysts of quartz and feldspar in peralkaline rhyolites from Mayor Island, New Z ealand, have been studied using Fourier transform infrared spectroscop y and ion microprobe analyses. The glass inclusions analyzed span the similar to 130 ka subaerial eruptive history of the island and come fr om volcanic deposits representing a wide range of eruptive styles (exp losive vs. effusive, magmatic vs. phreatomagmatic, low vs. high discha rge rates). The water content of all inclusions analysed is uniformly high at similar to 4.4 wt.% H2O, whereas CO2 contents are below the li mits of detection (similar to 50 ppm). Chlorine in melt inclusions ran ges from 2070 to 5200 ppm, while coexisting matrix glasses have genera lly lower Cl concentrations of 1700-4200 ppm; the apparent bulk distri bution coefficient describing Cl partitioning between melt and vapor p hase ([Cl](fluid)/[Cl](melt)) during degassing is 5-15. Fluorine appea rs to be less affected by eruptive degassing than is Cl, and melt incl usion and matrix glass F concentrations show significant overlap (1400 -2520 ppm F in inclusions, 1550-2890 ppm F in matrix glass). The obser ved invariance of melt inclusion water content with sample age contras ts with ion microprobe data on incompatible trace elements (e.g., Zr, Nb), which suggest similar to 35% fractionation between the oldest and youngest samples. This, along with the Cl partitioning behaviour, sug gests that at least the upper erupting portion of the Mayor Island mag ma chamber was water saturated throughout the volcano's 130 ka eruptiv e history. Furthermore, the large range of eruptive styles observed on Mayor Island are not due to differences in water concentration of the erupting magma but instead must reflect differences in rates of magma ascent and supply (which control the efficiency of non-explosive dega ssing).