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

✍ Scribed by N. F. Rutherford; R. F. Heming

• Book ID: 104745874
• Publisher: Springer
• Year: 1978
• Tongue: English
• Weight: 961 KB
• Volume: 65
• Category: Article
• ISSN: 0010-7999
• DOI: 10.1007/bf00372287

✦ Synopsis

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, blot., 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 fO 2. Some ignimbrites contain the assemblage actinolite, gedrite, magnetite and hematite, reflecting post-eruption oxidation. The mineralogy also allows estimation of fs2 using pyrrhotite and thence fso~, fso3. The assemblage biotite-sanidine can be used to estimate fn~ and thence fn2s. Water fugacity is calculated in a variety of ways using both biotite and hornblende as well as the combining reaction H2+ 89

It is high and approaches PtotaZ 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~ 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.