Abstract

Smith (1979) suggested that all pyroclastic eruptions that exceed 1 km3 will show pronounced systematic compositional zonation. Since then, chemical and thermal zonation in large silicic magma bodies has become the expected condition in rhyolitic magma bodies. However, trace-element chemistry and Fe-Ti oxide geothermometry of Quaternary rhyolitic Plinian tephras from two volcanic centers in the Taupo Volcanic Zone, New Zealand, suggest that no strong systematic chemical or thermal zonation was present in either magma system. Of the Plinian tephra units erupted over the past 50 ka from the Taupo and Okataina volcanoes, 28 were sampled, and most were analyzed for Th, Pb, Rb, Sr, Y, Zr, and Nb, as well as Fe-Ti oxide composition. Taupo volcano tephras erupted over the past 10 ka have uniform compositions for all elements except Sr, and all these samples fall on the same T/fO2 buffer trend. However, pre-22 ka Taupo tephras are mineralogically and chemically distinct from the younger group and fall on a different T/fO2 buffer trend. This may be a result of the large eruption (>155 km3 of magma) from the Taupo volcano at 22.5 ka, and subsequent reequilibration of the magmatic system.

The Okataina volcano tephras erupted over the past 31 ka show slight systematic and considerable nonsystematic trace-element variation, but all fall on a single T/fO2 buffer trend. This may indicate slight preeruptive alteration of a single Okataina magma body.

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