Cosmogenic ³ He chronology of postglacial lava flows at Mt Ruapehu, Aotearoa / New Zealand

Abstract. Accurate volcanic hazard assessments rely on a detailed understanding of the timing of past eruptions. While radiometric methods like 40Ar/39Ar or K/Ar are by far the most conventional lava flow dating tools, their low resolution for young (<20 ka) deposits interferes with the development of precise chronologies of recent effusive activity on most volcanoes. Mt Ruapehu (Aotearoa / New Zealand) has produced many lava flows throughout its history, but the precise timing of many recent eruptions remains largely unknown. In this study, we use cosmogenic 3He exposure dating to provide 23 eruption ages of young lava flows at Ruapehu. We then compare our results with existing 40Ar/39Ar and paleomagnetic constraints, highlighting the value of cosmogenic nuclide exposure dating in refining recent eruptive chronologies. Of the 23 sampled flows, 16 provided robust eruption ages (5 %–20 % internal 2σ; n≥3) between ca. 20 and 8 ka, except for one lava flow that erupted at around 43 ka, and their age distribution indicates that, during the last 20 kyr, effusive activity at Ruapehu peaked at 17–12 ka and at 9–7.5 ka. Nearly identical eruption ages of lavas located in different flanks of the volcanic edifice suggest concurrent activity from multiple vents during relatively short time intervals (0–2 kyr) at around 13, 10, and 8 ka. We analysed four individual lava flows previously dated by 40Ar/39Ar, two of which yield eruption ages older than the older limit of the 2σ interval of the radiometric dates, but the good clustering of individual samples from our sites suggests that our results better represent the real eruption age of these flows. Our 3He-based chronology shows excellent agreement with paleomagnetic constraints, suggesting that production rate uncertainties are unlikely to impact the accuracy of our eruption ages. This study demonstrates how cosmogenic nuclide dating can provide greater detail on the recent effusive chronology of stratovolcanoes, helping to resolve the low resolution of and difficulty in applying radiometric dating methods to young lava flows.