CHANGES IN RHYOLITE COMPOSITION OVER THE LAST 60,000 YEARS AT THE OKATAINA CALDERA COMPLEX, NEW ZEALAND RECORDED IN ZIRCON TRACE ELEMENT ZONATION

Using single crystals to map out the chemical evolution of a magmatic system has long been a goal of igneous petrology. This is especially true with crystals that can be utilized for in situ dating, where compositional and temporal information can be linked to understand the evolution of a magmatic system over part or all of its lifespan. By combining in situ U-Th age measurements in volcanic zircons with ion microprobe and micron-scale electron microprobe trace element traverses and comparing these results to the overall compositional changes of the magmatic system, in-roads can be made into unraveling the magmatic histories of specific magmas along with mapping the P-T-fO₂ changes within the magmatic system. In this way, zircons culled from single volcanic eruptions can be utilized to examine much of the history of the magmatic system from which they are derived. We directly connect the trace element compositions in single zircon and zircon populations to the temporal information provided through U-Th analyses of zircon from the ~1305 A.D. Kaharoa Rhyolite eruption at Mt. Tarawera, New Zealand. This record of micron-scale changes in the trace element composition within zircon over 10³ to 10⁵ years reflects the changes in source rhyolite from oxidizing/wet to reducing/hot and back over the last 60,000 years in the Okataina Caldera Complex (OCC). High Y, low Hf in single zircons and zones within zircons record the drier, reducing rhyolite of the Te Rere and Mangaone eruptions (23 and 30 k.y., respectively); conversely, low Y, high Hf in zircon reflects the wetter, oxidizing rhyolite of the pre-Mangaone and post-Te Rere (>40 k.y.; <20 k.y.) rhyolite. These results document the interconnectedness of the magmatic system in the OCC, suggesting that magmas in the OCC are not discrete pods of magma, but rather form a complex network of crystal mushes that are rejuvenated and sampled multiple times during the lifetime of the caldera. Zircon from these events can then be sampled in each successive eruption. This connection between micron-scale compositional changes and ages within zircon crystals and the overall temporal and compositional evolution of a magmatic system from which they are born opens the possibly of using other micron-scale analytical techniques to probe the compositional changes through time within magmatic systems by examining single zircon grains.