VOLATILE CONTENTS OF OBSIDIAN CLASTS IN TEPHRA FROM THE TAUPO VOLCANIC ZONE, NEW-ZEALAND : IMPLICATIONS TO ERUPTIVE PROCESSES

DUNBAR NW, KYLE PR

JOURNAL OF VOLCANOLOGY AND GEOTHERMAL RESEARCH
v. 49(#1-2) pp. 127-145 1992
(Title at LANL MAIN.)


Institutions:
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 Cl (0.12-0.18 wt.%) contents. These are elevated compared to degassed obsidian from rhyolitic domes (approximately 0.1 wt.% H2O, approximately 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 delta-O-18 of + 7-parts per thousand 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 approximately 3.5 km depth. Initial fragmentation (assumed to occur at a vapor:melt ratio of 3:1) began at 0.1 kbar, or approximately 500 m depth. Obsidian in most tephra deposits contains less than 1 wt.% H2O, and are calculated to have formed at < 0.1 kbar or approximately 500 m depth. Therefore, some obsidian in these tephra deposits may have formed from fragmented magma. The variable water contents of obsidian fragments in a single deposit indicates that quenching occurred over a range of depths. This, and the strong alignment of microphenocrysts in obsidian, suggests that fragmented magma was welded onto the conduit walls during eruption, cooled to form obsidian, then incorporated into the tephra. 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.
Keywords:
NEUTRON-ACTIVATION ANALYSIS; MELT INCLUSIONS; WATER; MAGMAS; H2O; GLASSES; ROCKS; ASH; CL; CONSTRAINTS