Could rear-arc volcanism be driven by mantle instability?

Abstract

The drivers of rear-arc volcanism remain poorly understood. Taranaki volcano, in the North Island of New Zealand, offers an ideal location to investigate the causes of such volcanism. Taranaki erupts hydrous, amphibole-bearing, high-K calc-alkaline magmas with trace element and Sr-Nd-O isotope compositions characteristic of arc-like fluid-flux melting regimes. Its location 150 km behind the arc front and >250 km above the subducting slab, however, has led some geodynamic models to consider convective removal of lithosphere as an alternative driver of magmatic activity. Unlike typical arc magmas, late Pleistocene-Holocene eruptives from Taranaki preserve up to 40 % excesses of ²³⁰Th over ²³⁸U. These cannot be explained by crystal fractionation or assimilation processes and are unlikely to be imparted by sediment melting at the slab interface. We present a geochemical case to hypothesise that Taranaki magmas result from mantle instability associated with convective removal of lithosphere. In this model magmas represent variable mixtures of asthenospheric melts resulting from decompression melting with melts from downwelling lithosphere that have arc-like trace element signatures inherited from past (Miocene) metasomatism. Calculated upwelling rates for the asthenospheric component are similar to surface uplift and the timescales from melting to eruption must be less than 5 kyr, similar to eruptive periodicity. High-K rear-arc magmas with ²³⁰Th-excesses are common globally (e.g., Stromboli, Sangeang Api, Rishiri) suggesting mantle instability could be a more common scenario to explain rear-arc volcanism.