Processes and timescales of magmatic rejuvenation and residence prior to post-caldera rhyolitic eruptions: Ōkataina Volcanic Centre, Aotearoa New Zealand

Abstract

Understanding the timescales and processes through which magma bodies form and develop in the crust prior to eruption is vital for volcanic monitoring and interpreting future unrest signals, particularly at caldera volcanoes. Here we present the results of a case study on Ōkataina Volcanic Centre, New Zealand, into the timescales of pre-eruptive processes operating during young intra-caldera eruptions, using Fe-Mg interdiffusion profiles in orthopyroxene. Three eruptions covering the three different vent regions, the 15.6 ka Rotorua episode from the Ōkareka Embayment, the 14.0 ka Waiohau episode from Tarawera, and the 5.5 ka Whakatāne episode from Haroharo have broadly similar rhyolitic compositions but show distinctions in their mineralogy and crystal zoning profiles. Back-scattered electron imaging coupled with major-element analyses of orthopyroxene cores and rims imply that open-system mixing and disequilibrium is a common process in the early stages of magma assembly at Ōkataina. Through interpreting mineral chemistry and zoning patterns, we infer that melt segregation, magma body assembly from the source mush and residence in the upper crust takes place on the order of decades up to several centuries, or (if prematurely triggered) as short as months. Priming events such as heating by basaltic injection (common in the Ōkataina rhyolite eruptions) occur over decades prior to eruption. Our findings highlight that multiple processes can dictate the assembly, residence and eruption of melt bodies from the crust and that most magmatic processes occur over timescales that are short enough to be directly relevant to geophysical imaging of magma reservoirs and monitoring initiatives.