Mobilizing high crystallinity magmas: Earthquake Flat eruption of the Okataina Volcanic Center, Taupo Volcanic Zone, New Zealand

Abstract:

High-crystallinity rhyolites are sparse in the rock record, but they may offer unique insights into mobilization of viscous crystal-rich magmas. It has been suggested that large-scale, crystal-rich eruptions are produced by wholescale rejuvenation of entire upper crustal batholiths, such as the >5000 km³ Fish Canyon Tuff. Here we present zircon data from the crystal-rich Earthquake Flat (EQF) rhyolite tuff, which was emplaced during the explosive eruption of 10 km³ of pyroclastic material from the Okataina Volcanic Center, Taupo Volcanic Zone, New Zealand. We show trace element and U-Th age data from EQF zircon unpolished surfaces (i.e. rims) and sectioned zircon interiors measured by sensitive high resolution ion microprobe - reverse geometry (SHRIMP-RG). The EQF zircon data show a restricted, yet evolved range in geochemical compositions, as well as evidence for cool storage (<700 °C; with most <650 °C). Within the EQF surface data, old rims are dominant (100 and 70 ka) suggesting that these zircons were stored in a highly crystalline mush, and that zircon growth was stopped by inclusion in major phases and/or slowed by low temperatures. Geochemical indices such as Y/P, an index believed to monitor apatite fractionation, show the development of two geochemically distinct populations of zircon within the crystal mush body. Further, since old U-Th ages and heterogeneous compositions are preserved in the surfaces, we conclude that zircon could not have remained in a homogeneous pre-eruptive melt body for long. Thus, the time between amalgamation of the melt body and eruption must have been short (<~2 ka). Our data show that the upper crustal storage region for the EQF melt, and possibly high-silica melts in general, is a complicated system of melt pockets suspended in a crystal mush of variable crystallinities, and that the degree of crystallinity in the reservoir will affect the geochemical record. Importantly, we conclude that the EQF eruption is an illustrative microcosm of how large crystal-rich magma bodies might be mobilized.