The volatile component of quaternary ignimbrite magmas from the North Island, New Zealand

Abstract

Ignimbrites from the central North Island consist mainly of glass or its devitrified product (70–95%); their phenocryst mineralogy is varied and includes plag., hyp., ti-mag., ilm., aug., hblende, biot., san., qtz, ol., with accessory apatite, zircon and pyrrhotite. The Fe-Mg minerals can be used to divide the ignimbrites into four groups with hyp.+aug. reflecting high quench temperatures and biot.+hblende +hyp.+aug., low quench temperatures. Oxygen fugacities lie above the QMF buffer curve and even in ignimbrites with low crystal contents the solid phases apparently buffered fO₂. Some ignimbrites contain the assemblage actinolite, gedrite, magnetite and hematite, reflecting post-eruption oxidation. The mineralogy also allows estimation of $f_{{\text{S}}_{\text{2}} }$ using pyrrhotite and thence $f_{{\text{SO}}_{\text{2}} }$, $f_{{\text{SO}}_{\text{3}} }$. The assemblage biotite-sanidine can be used to estimate $f_{{\text{H}}_{\text{2}} }$ and thence $f_{{\text{H}}_{\text{2}} {\text{S}}}$. Water fugacity is calculated in a variety of ways using both biotite and hornblende as well as the combining reaction ${\text{H}}_{\text{2}} + \tfrac{1}{2}{\text{O}}_{\text{2}} = {\text{H}}_{\text{2}} {\text{O}}$. It is high and approaches P _total in most ignimbrites (~4kb) but is lower in unwelded pumice breccias. Comparison of temperature estimates using mineral geothermometers for the various phenocryst phases suggests that the ignimbrite magmas showed temperature differences of 60–100 °C and pressure differences of several kilobars. Individual magma chambers therefore, would have extended over several kilometres vertically. The chemical potential of water may have been constant through the magma.