New Mexico Geological Society Annual Spring Meeting — Abstracts

Obsidian, crystals and pumice occur in many tephra layers from the Taupo and Okataina volcanic centers of the Taupo Volcanic Zone, New Zealand. Based on morphology and petrographic investigation, obsidian fragments are considered to be cogenetic with the bulk of the tephra, rather than dome derived lithic material. Analyses of water contents in aidan obsidian give values of 0.4 to 4.1 weight percent. These values support the conclusion of primary origin, as does one D/Hvalue of -70 permil. A dome obsidian sample from the same area contaoins 0.29 wt % H₂O, close to the experimentally determined value of 0.2 wt % H₂O for atmospheric equilibrium of water in silicic glass (Taylor et al, 1983).

Temperature and oxygen fugacity calculated from co-existing magnetite and ilmenite compositions were determined for most of the tephra units, giving ranges of 750-915°C and log fO₂ of -11 to -15. The Taupo and Okataina centers cover roughly the same span of values, but fall on two distinct buffer trends, indicating derivation from separate magma types. The highly explosive Taupo Plinian unit give higher temperatures than deposits from other associated plinian eruptions. Heating stage geothermometry studies on two phase melt inclusions yield melt temperatures close to those from Fe-Ti oxides. Decrepitation of inclusions occurs soon after melting, probably indicating that the trapping pressure was high.

Electron microprobe analyses of glass inclusions in the cores of ferromagnesian silicate grains show similar major element chemistry to obsidian from the same eruption, but are enr iched in the volatile el ement Cl. The glass inclusions, which approximate pre-eruptive concentrations, average 0.19 wt %Cl, while the more degassed obsidians contain about 0. 15 wt %. The major element chemistry of glass erupted from the Taupo Volcanic Center over the last 20,000 years is essentially constant. Apparently, chemical zonation of the magma chamber has not occurred.