Ian Ernest Masterman Smith

The geothermal system at Iamalele in the D’Entrecasteaux Islands of southeastern Papua is hosted by late Quaternary high-silica ignimbrite, air-fall tuff and associated volcaniclastic rocks together with lava flows and domes. These volcanic rocks are part of the subduction-related Papuan Arc and are interpreted as the eruption products from a caldera. Whole rock chemical compositions range from andesite to rhyolite; the occurrence of rhyolitic rocks is a unique feature within in the Papuan Arc. Geothermal activity occurs over 30 km3 of the Iamalele area. Chemical analyses of hot spring discharges indicate a near-surface acid sulphate fluid and a deeper reservoir that probably has a significant seawater component. Hydrothermal alteration identified in a ~200 m drill core indicates that the geothermal reservoir is well-zoned and contains trace element signatures characteristic ofshallow epithermal precious metal deposits. With increasing depth there are mineral assemblages characteristic of advanced argillic, intermediate argillic and potassic alteration. The presence of an active geothermal system within a caldera structure raises the possibility of large-scale explosive volcanic activity in southeastern Papua which needs to be factored into future hazard assessment in the area.

A major period of subduction‐related volcanism in northern New Zealand began about 20 million years ago; its locus of activity migrated southward during the late Cenozoic in response to changes in convergence between the Australian and Pacific tectonic plates. As the volcanic arc migrated, six discrete fields of small‐scale basaltic volcanoes were established behind the subduction system. These volcanoes show geochemical characteristics that reveal patterns of mantle behaviour linked to the rapidly evolving plate tectonic environment of Zealandia. Two northern volcano fields began activity about 8 million years ago and have been active into recent times; their chemical compositions reflect partial melting of shallow mantle and their spatial relationships an essentially static source environment. In contrast, four southern fields are dominated by alkalic magmas that likely ascended rapidly from garnet‐bearing asthenospheric sources in a northward migrating pattern of discrete temporal episodes (∼1 ma duration) since 3 Ma. The Northland fields were sourced in mantle that became coupled to the lithosphere and have remained active during the last 8 Ma. The Auckland fields are interpreted to reflect independent actively rising asthenospheric mini‐plumes linked to mantle convection associated with but geochemically distinct from the subduction system beneath central North Island New Zealand.

Eruptions of basaltic material in small-scale volcanic fields located in intraplate settings display a very diverse range in physical and chemical characteristics. Despite its relevance to the understanding of volcanic hazards, the relationship between physical properties of eruptions (explosivity, volume, location) and chemical composition of erupted products has, to date, not been investigated. Here we present a relationship between mantle heterogeneity and extents of partial melting, and both erupted volumes and eruptive style from the Auckland Volcanic Field (New Zealand), and we suggest that this provides a general model for small-scale “monogenetic” magmatic systems globally. Small volcanic centers consistently take the form of nephelinitic tuff rings and scoria cones, whereas larger centers are produced from effusive eruptions of less alkalic magmas. Nephelinitic melts are generated by melting of a deep, carbonated source, whereas less alkalic melts are the products of melting of a shallower, noncarbonated source. U-Th-Ra isotope data from eruptions closely paired in space and time show that mixing between magmas is extremely limited as a consequence of different ascent mechanisms due to differential segregation of melts from varying sources (early, carbonated melts ascending by higher porosity channels, and later, uncarbonated melts by a more diffusive regime). This suggests that extraction of melt is nearly instantaneous in these environments. Our results stress the importance of melting and magma dynamics in determining the size and style of eruptions in small volcanic fields, and suggest that mantle controls should be an important consideration in volcanic hazard assessment.