Using Olivine Diffusion Chronometry Analysis to Determine the Eruptive History of Rangitoto

Beneath the city of Auckland, New Zealand, sits one of the world's most complex monogenetic magmatic systems known as the Auckland Volcanic Field (AVF). The largest, youngest, and lone polygenetic volcano in the AVF is Rangitoto, which is known for its controversially complicated magmatic plumbing system, and uncertain eruptive history. It is believed to have undergone two eruptive events, beginning with a voluminous shield-building event approximately 650-550 years ago, and culminating with a more explosive eruption phase that produces samples of scoriated basalts near the summit of the volcano (Linnel et al., 2016). However, recent research conducted by Bryan '24, suggests that there may have been a third, discernable eruption event, triggered by a mafic injection of primitive magma in the AVF, adding to the complexity of the system. Using olivine diffusion chronometry, we can estimate magma ascent rates associated with eruptions on Rangitoto, and draw conclusions about its eruptive history, which could provide key insights into volcanic hazard profiling for Auckland. Using the methodology from Brenna et al. (2018), we can analyze diffusion profile gradients to estimate magmatic ascent rates. Assuming brightness/greyscale counts of BSE images was solely a function of Fe-Mg exchange, we used Brenna's equation C=(C0-Cint)erf(xsqrt(D)t)+C (assuming a temperature of 1100o C) to estimate ascent rates of the sample by fitting our data to a theoretical model. Coupled with data from Bryan '24, we analyzed four different samples, two of which were from the summit region. As discussed by Bryan '24 basal lava flows indicate two zones of ascension rates corresponding to separate sequences of volcano activity. The primary finding from the work done by Bryan '24, was the presence of reversely zoned olivine crystals in summit samples of Rangitoto, which provide evidence for a potential third eruption phase related to triggers seen in polygenetic magmatic systems. However, this conclusion was made from a singular summit sample. It is the goal of this project to examine the robustness of the work done by Bryan '24, by analyzing additional summit samples, and estimating the associated ascent rates. To this point, we have only found normal zoning in each of our samples, including one derived from the summit, with ascension times spanning 50 to 135 days. These results could provide key insights to hazard response in the populous city of Auckland, and a potential shift in th