The magmatic driving force behind the most productive silicic volcanic system on Earth

Abstract:

The Taupō Volcanic Zone (TVZ) in the central North Island of New Zealand hosts numerous volcano types in geographically segmented areas. In the southern and northern ends of the TVZ, composite cones like Ruapehu and White Island erupt intermediate magmas and are considered the norm for an arc setting (Figure 1). In contrast the ~120 km-long central segment of the TVZ hosts huge caldera volcanoes which have collectively erupted >6000 km³ of silicic magma over the past 1.6 million years. However, the causes of this extreme magmatic productivity are not well understood, as the parental magmas (basalts) are extensively modified in the crust during ascent, overprinting their original mantle-derived signatures. We address this issue by applying forensic geochemical techniques to melt inclusions trapped within olivine crystals. In particular, we have developed a protocol for analysing inclusions within inherited olivines in mafic enclaves, quenched within rhyolitic magmas, to investigate primitive compositions feeding the TVZ caldera volcanoes. In the central TVZ, the compositions of primitive magmas feeding calderas contrast greatly with those that erupt outside of calderas as small scoria cones. Caldera basalts have lower abundances of incompatible elements, reflecting higher degree (10-30%) partial melts from a more depleted mantle source. In contrast, basalts erupted outside calderas are more similar to the globally typical composition of back-arc melts that reflect low degrees (3-10%) of decompression melting with low slab-fluid inputs. In turn, primitive magmas feeding intermediate cone volcanoes in the southern and northern TVZ have strong slab fluid signatures, reflecting flux melting of a depleted mantle source. We suggest that the style and productivity of magmatism in the central TVZ is related to several unique features of the TVZ setting including subduction of overthickened crust, deep seismicity, mantle flow and enhanced decompression melting.