Volatile contents of obsidian clasts in tephra from the Taupo Volcanic Zone, New Zealand: Implications to eruptive processes

Abstract

Abstract Obsidian clasts in 6500–20,000-year-old rhyolitic tephra deposits from the Taupo Volcanic Zone, New Zealand, contain variable H2O (0.2–2.5 wt.%) and CI (0.12–0.18 wt.%) contents. These are elevated compared to degassed obsidian from rhyolitic domes (∼-0.1 wt.% H2O2, ∼0.1 wt.% Cl) which quenched at the surface. Major- and trace-element compositions of obsidian from tephra deposits suggest that they are co-genetic with associated pumice. The clear, glassy appearance of the obsidian, their high H2O release temperatures, and δ18O of +7‰ show that the water is magmatic, and was not absorbed into the obsidian after tephra deposition. Based on analysis of melt inclusions, recent primary Taupo Volcanic Zone rhyolitic magmas contain 4.3 wt.% H2O and 0.18 wt.% Cl. The obsidian in the tephra deposits, therefore, represents partially degassed, quenched primary magma. Water and Cl contents correlate in the obsidian, suggesting that the magmatic Cl partitioned into a H2O-rich fluid phase during eruptive degassing. The partitioning of Cl between the water-rich vapor phase and the residual magma can be modelled as open-system Rayleigh fractionation. Using a pre-eruptive H2O content of 4.3 wt.%, and Burnham's (1979) water solubility model, the pressure and depth of initial vesiculation of the Taupo magma is calculated to occur at about 0.9 kbar or ∼3.5 km depth. Initial fragmentation (assumed to occur at a vapor:melt ratio of 3:1) began at 0.1 kbar, or ∼500 m depth. Obsidian in most tephra deposits contains less than 1 wt.% H2O, and are calculated to have formed at The 2000-year-old Hatepe phreatoplinian tephra contains many obsidian fragments with > 2 wt.% H2O. The meteoric water which caused this eruption to be phreatomagmatic may have caused obsidian to quench at unusually great depths, resulting in higher H2O contents.