HR: 10:35h
AN: V31G-02    [PDF]
TI: Pre-eruptive volatile contents of the 26.5 ka Oruanui magma, Taupo Volcanic Zone, New Zealand
AU: Anderson, A T
EM: canderso@midway.uchicago.edu
AF: University of Chicago, Department of Geophysical Sciences, 5734 S. Ellis Ave, Chicago, IL 60637 United States
AU: * Liu, Y
EM: yangl@geosci.uchicago.edu
AF: University of Chicago, Department of Geophysical Sciences, 5734 S. Ellis Ave, Chicago, IL 60637 United States
AU: Wilson, C J
AF: Institute of Geological \& Nuclear Sciences, PO Box 30368, Lower Hutt, 6315 New Zealand
AB: The Taupo Volcanic Zone (TVZ) in New Zealand is exceptional worldwide for its large-scale Quaternary silicic magmatism. The 26.5 ka, $\sim$530 km$^{3}$ (magma) Oruanui event is the largest young TVZ eruption. Although showing geochemical variability, Oruanui rhyolite compositions show no systematic strata-bound variations. Our study of the Oruanui deposits can test the hypothesis that a magma body with a stable density stratification exists before eruption and, for this, we determined the volatile contents in Oruanui melt inclusions. Quartz phenocrysts from both pumice fall deposits and ignimbrite were doubly polished to expose melt inclusions, reentrants, and hourglass inclusions. In total, 69 melt inclusions and 32 reentrants were analyzed using FTIR spectroscopy. Average dissolved H$_{2}$O and CO$_{2}$ ($\pm$1$\sigma$) in melt inclusions are: 4.9$\pm$0.3 wt% H$_{2}$O and 100$\pm$42 ppm CO$_{2}$ (Fall Unit 1), 5.0$\pm$0.1 wt% H$_{2}$O and 100$\pm$10 ppm CO$_{2}$ (Fall Unit 2), 5.0$\pm$0.1 wt% H$_{2}$O and 110$\pm$40 ppm CO$_{2}$ (middle ignimbrite), and 5.2$\pm$0.3 wt% H$_{2}$O and 200$\pm$70 ppm CO$_{2}$ (upper most ignimbrite or late-erupted ignimbrite). Minimum pre-eruptive pressures of each melt inclusion, derived from the solubility model for mixed H$_{2}$O-CO$_{2}$ fluid (Liu et al. 2003), vary from $\sim$110 to 220 MPa. The H$_{2}$O and CO$_{2}$ contents of melt inclusions from late-erupted ignimbrite are generally $>$4.9 wt% and 200 ppm, greater than those from other samples. The higher volatile contents in the late-erupted samples are consistent with a deeper origin of these samples in the magma chamber, supporting gas-saturation crystallization. However, there is an increase in both CO$_{2}$ and H$_{2}$O contents from early- (Fall Unit 1 and 2, middle ignimbrite) to late-erupted samples. Although the correlation is weak, the positive trend may indicate gas-undersaturated crystallization. The H$_{2}$O and CO$_{2}$ contents of reentrants from single pumices vary widely, but are generally lower than in melt inclusions from the same pumice, reflecting late stage pre-eruptive/eruptive degassing.
DE: 1000 GEOCHEMISTRY (New field, replaces Rock Chemistry)
DE: 3600 MINERALOGY AND PETROLOGY (replaces
DE: 8404 Ash deposits
DE: 8414 Eruption mechanisms
DE: 8439 Physics and chemistry of magma bodies
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting