Time scales of olivine storage and transport as revealed by diffusion chronometry at Waitomokia Volcanic Complex, Auckland Volcanic Field, New Zealand

Abstract

Pre-eruptive time scales of magma storage and transport play a critical role in understanding the magmatic processes leading to an eruption. Moreover, these processes still need to be better constrained for mafic volcanic centres in continental intraplate settings as being a source of significant hazards for human populations and infrastructure due to their limited predictability in space and time. We conducted a detailed petrological study to investigate the time scales of olivine storage and transfer throughout the entire evolution of the Waitomokia Volcanic Complex eruption, which encompassed the transition from the initial phreatomagmatic to the final magmatic stage. This volcanic complex is situated within the Auckland Volcanic Field (New Zealand), which was active approximately 250,000 years ago. Olivine crystal textures and compositions were determined from stratigraphically-constrained deposits of the volcanic complex. Olivine crystals are typically skeletal, <300 μm in length, and zoned in forsterite (Fo = 100*Mg/[Mg + Fe]; mol%), CaO, MnO and NiO wt% contents. We classified olivine into three major groups based on their Fo core compositions: (1) normally zoned crystals with high Fo content (Fo > 85), (2) crystals with intermediate Fo contents (84–81), and (3) reversely zoned crystals with lower Fo core content (<80). The diffusion profiles of olivine were modelled in the context of a specific magmatic environment linked with changes in thermodynamic variables during storage (temperature, pressure, and oxygen fugacity). We propose that the normally zoned olivine crystals grew in one magmatic environment (ME1), which subsequently intruded into a more evolved environment (ME2), where they interacted and were stored for up to 135 days before the eruption. Moreover, during the magma ascent to the surface, a second magma mixing event occurred between ME2 and magma within a third magmatic environment (ME3), forming reversely-zoned olivine crystals yielding notably shortener ascent times of approximately a few days. The rocks from the opening phreatomagmatic eruption stage show a larger range in olivine group types compared to the final magmatic activity, where those from the deeper ME1 are more abundant. The short time scales of magma transport obtained in our study, on the order of days to months, should be informative of the warning times that may be encountered between the onset of volcanic unrest and an eruption in the Auckland Volcanic Field.