Fine-scale temporal recovery, reconstruction and evolution of a post-supereruption magmatic system

Abstract

Supereruptions (>1015 kg ≈ 450 km3 of ejected magma) have received much attention because of the challenges in explaining how and over what time intervals such large volumes of magma are accumulated, stored and erupted. However, the processes that follow supereruptions, particularly those focused around magmatic recovery, are less fully documented. We present major and trace-element data from whole-rock, glass and mineral samples from eruptive products from Taupo volcano, New Zealand, to investigate how the host magmatic system reestablished and evolved following the Oruanui supereruption at 25.4 ka. Taupo’s young eruptive units are precisely constrained chronostratigraphically, providing uniquely fine-scale temporal snapshots of a post-supereruption magmatic system. After only ~5 kyr of quiescence following the Oruanui eruption, Taupo erupted three small volume (~0.1 km3) dacitic pyroclastic units from 20.5 to 17 ka, followed by another ~5-kyr-year time break, and then eruption of 25 rhyolitic units starting at ~12 ka. The dacites show strongly zoned minerals and wide variations in melt-inclusion compositions, consistent with early magma mixing followed by periods of cooling and crystallisation at depths of >8 km, overlapping spatially with the inferred basal parts of the older Oruanui silicic mush system. The dacites reflect the first products of a new silicic system, as most of the Oruanui magmatic root zone was significantly modified in composition or effectively destroyed by influxes of hot mafic magmas following caldera collapse. The first rhyolites erupted between 12 and 10 ka formed through shallow (4–5 km depth) cooling and fractionation of melts from a source similar in composition to that generating the earlier dacites, with overlapping compositions for melt inclusions and crystal cores between the two magma types. For the successively younger rhyolite units, temporal changes in melt chemistry and mineral phase stability are observed, which reflect the development, stabilisation and maturation of a new, probably unitary, silicic mush system. This new mush system was closely linked to, and sometimes physically interacted with, underlying mafic melts of similar composition to those involved in the Oruanui supereruption. From the inferred depths of magma storage and geographical extent of vent sites, we consider that a large silicic mush system (>200 km3 and possibly up to 1000 km3 in volume) is now established at Taupo and is capable of feeding a new episode or cycle of volcanism at any stage in the future.