Geocheminar fall-2012

The Taupo Volcanic Zone (New Zealand) is a location of extremely prolific crustal melt generation in an extensional convergent margin setting, and it has produced several large-magnitude eruptions throughout the Quaternary. Recurrent eruptions from the Okataina/Tarawera volcanic complexes between c. 45 ka and 1350 CE provide insights into processes of silicic magma accumulation and storage at depth. Zircon geochronological and geochemical data reveal that (1) the last voluminous caldera-forming eruption (Rotoiti) at 45 ka is 15 ka younger than previously thought, and (2) the silicic magma reservoir cycled through episodes of near-complete solidification and re-melting which occurred likely at time scales <1000 years. The thermo-mechanical conditions within the rhyolite magma storage zone are thus largely controlled by episodic inputs of fresh basaltic magma, which over time have partially erased the crystal record from earlier magmatic phases. Prograde crystal resorption can explain the paradoxically brief zircon crystallization intervals observed in many large-volume eruptions, whereas complex zircon age populations are preserved in rapidly melted and vented small-volume eruptions.