Determination of pre-eruptive H2O, F and Cl contents of silicic magmas using melt inclusions, Taupo Volcanic center, New Zealand

Dunbar, N.W., Hervig, R.L., and Kyle, P.R.

Volatile contents of melt inclusions within phenocrysts in three rhyolitic tephras from the Taupo volcanic center, New Zealand, were measured by microbeam techniques. Direct measurements of H2O and F were made by ion microprobe, and Cl was determined by electron microprobe. We examined the 0.02 ka old Taupo and Hatepe plinian tephras, and the ~22 ka old Okaia tephra. Major and trace element chemistry of the inclusions is similar to that of bulk rock, supporting our assumption that volatile contents of inclusions are representative of the magma in which the crystals grew. Inclusions from the Taupo plinian tephra contain a mean of 4.3 wt.% H2O, 450 ppm F, and 0.173 wt. Cl, from the Hatepe plinian tephra 4.3 wt.% H2O, 430 ppm F, and 0.173 wt.% Cl, and from the Okaia tephra 5.9 wt.% H2O, 470 ppm F, and 0.207 wt.% Cl. Sulfur was determined by electron microprobe and was below the detection limits of 200 ppm in all cases. The constant H2O, F and Cl from a number of stratigraphic horizons in the tephra deposits, suggest that the Taupo and Hatepe plinian tephras ( 8.2 km3 DRE) were derived from a magma body which did not contain a strong volatile gradient. Virtually no major element zonation is seen in this eruptive set (Dunbar et al., in prep). The higher volatile contents and distinct chemistry of the Okaia tephra show that it was derived from a different, and possibly deeper, magma body.