The volatile component of quaternary ignimbrite magmas from the North Island,New Zealand期刊界 All Journals 搜尽天下杂志传播学术成果专业期刊搜索期刊信息化学术搜索

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

Authors:	N F Rutherford R F Heming

Institution:	(1) Geology Department, University of Auckland, Auckland, New Zealand;(2) Present address: Union Corporation Australia Pty Ltd, 55, Macquarie Street, 2000 Sydney, NSW, Australia

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.

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