Geochemical evolution of the Rabaul volcanic complex, Papua New Guinea - Insights from HFSE, Sr-Nd-Hf, and Fe isotopes

Abstract

The Rabaul volcanic complex, Papua New Guinea, is among the few calderas worldwide with ongoing volcanic activity. Its volcanic lithologies vary from mafic lavas from outer caldera cones to differentiated lavas within. In this study, representative lavas have been analysed for their major and trace element concentrations and radiogenic Sr-Nd-Hf isotope compositions to study the geochemical evolution of the magmatic system. Stable Fe isotopes and concentrations of high field strength elements (HFSE) complement the analyses as novel tools to assess the effect of high-temperature fractional crystallisation during ongoing differentiation. Major element systematics reveal a typical fractional crystallisation sequence of olivine, pyroxene, and plagioclase as the critical process controlling the magmatic evolution. A distinct increase of Zr/Hf from the basaltic (older) outer caldera lavas (~39) to the dacitic (younger) inner caldera lavas (~41–44) can be explained by fractionation of clinopyroxene and amphibole. Ratios of Nb/Ta tend to decrease with an increasing degree of differentiation, consistent with fractional crystallisation of amphibole but not clinopyroxene. Additional fractionation of Ti-magnetite and rather oxidising conditions are further supported by the Fe isotope compositions in the inner caldera lavas (δ57Fe from +0.03 to +0.22‰, ±0.04‰, 2 SD). The high Nb/Ta in more primitive outer caldera samples are coupled with increasing rare earth elements (REE) abundances, and slab melt-like subduction components can explain high Sr/Y and GdN/YbN in the magma sources. Complementary enrichments in fluid-mobile trace elements indicate that slab dehydration controlled the sub-arc enrichment of the inner caldera volcanism. Coupled Hf-Nd isotope compositions reveal the presence of the Indian-Australian mantle domain beneath Rabaul and a temporal trend towards sediment melt components overprinting inner caldera lavas. In conclusion, geochemical features show a temporal evolution controlled by (i) variable influence of partial slab melts vs slab fluids and (ii) a change in fractional crystallisation patterns from solely olivine and pyroxene-controlled to increasingly titano-magnetite and amphibole-controlled fractionation.