Changes in magma storage conditions following caldera collapse at Okataina Volcanic Center, New Zealand
Large silicic volcanic centers produce both small rhyolitic eruptions and catastrophic caldera-forming eruptions. Although changes in trace element and isotopic compositions within eruptions following caldera collapse have been observed at rhyolitic volcanic centers such as Yellowstone and Long Valley, much still remains unknown about the ways in which magma reservoirs are affected by caldera collapse. We present 238U–230Th age, trace element, and Hf isotopic data from individual zircon crystals from four eruptions from the Okataina Volcanic Center, Taupo Volcanic Zone, New Zealand, in order to assess changes in trace element and isotopic composition of the reservoir following the 45-ka caldera-forming Rotoiti eruption. Our data indicate that (1) mixing of magmas derived from crustal melts and mantle melts takes place within the shallow reservoir; (2) while the basic processes of melt generation likely did not change significantly between pre- and post-caldera rhyolites, post-caldera zircons show increased trace element and isotopic heterogeneity that suggests a decrease in the degree of interconnectedness of the liquid within the reservoir following collapse; and (3) post-caldera eruptions from different vents indicate different storage times of the amalgamated melt prior to eruption. These data further suggest that the timescales needed to generate large volumes of eruptible melt may depend on the timescales needed to increase interconnectedness and achieve widespread homogenization throughout the reservoir.
KeywordsRhyolite Taupo Volcanic Zone Hf isotope Zircon 230Th–238U
This work was partially supported by National Science Foundation awards EAR-0738749 and EAR-1144945 to Kari Cooper, as well as by Durrell research grants from UC Davis and a Geological Society of America graduate student research grant to AER. We would also like to thank Jorge Vazquez and Matt Coble for their assistance on the SHRIMP-RG at the USGS/Stanford University facility, as well as Justin Glessner for his assistance with multicollector ICP-MS analyses and Mark Stelten for his valuable insights and help with sample preparation.
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