OLIVINE DIFFUSION CHRONOMETRY REVEALS RAPID ASCENT RATES AND COMPLEX ERUPTIVE HISTORY OF RANGITOTO, AUCKLAND VOLCANIC FIELD, NEW ZEALAND

Rangitoto is the youngest and largest volcano in the Auckland Volcanic Field (AVF), last erupting ~500–550 cal yr BP. Located at a Quaternary basaltic intraplate setting on New Zealand’s North Island, the AVF consists of 51 cinder cones including Rangitoto, which is approximately ~6 km wide. Olivine diffusion chronometry may be the key to understanding eruptive histories and magma ascent rates at Rangitoto. Eleven lava samples from Rangitoto’s lower lava field exhibited vesicular textures and a crystal-rich mineral assemblage dominated by reversely zoned olivines with predominantly sharp rims ranging from ~10 µm to ~200 µm in thickness. Several smaller unzoned olivine crystals (~20µm) were also present, with brightness similar to the rims. Zoned and unzoned clinopyroxene and unzoned plagioclase were high in abundance. Brenna et al. (2018), used several techniques to study reversely zoned olivine crystals in the AVF. This included major and minor element diffusion profiles which revealed two magma batches, indicating magma mixing and rapid ascent. Using the methodology from Brenna et al. (2018), and assuming brightness in BSE was solely a function of Fe-Mg exchange, we calculated diffusion profiles of zoned olivine crystals using the equation: C = (C₀-C_int)erf(x/√Dt)+C and a temperature of 1100 ˚C, coupled with ImageJ software to determine the duration of magma ascent. Diffusion models created from BSE images of olivine crystals provide a wide range of ascent durations, though most of the data suggest a rapid ascent prior to eruption, as recorded by the Fe-rich crystal rims. Diffusion models of 30 olivines across 2 samples record a bimodal distribution of diffusion rates, with a median of 14 days. Sharp 10 µm rims suggest a duration of ~1 day, while thick rims (100 - 400 µm) correlate with durations greater than 100 days. A third of olivines are oscillatory zoned and may have experienced multiple stages of diffusion, with one model suggesting a recent ascent duration of ~2 days, and a prior ascent spanning ~56 days. These rapid ascent rates may be a sign of a more complex eruptive history and possibly a polygenetic magmatic system, unlike typical cinder cones that erupt only once. Understanding the magmatic history and plumbing system of Rangitoto could provide insights into potential future hazards that would affect Auckland.