Olivine phenocryst origins and mantle magma sources for monogenetic basalt volcanoes in northern New Zealand from textural, geochemical and δ18O isotope data

Abstract

Olivine phenocrysts in basalts erupted in the Kaikohe-Bay of Islands volcanic field (KBIVF), a cluster of Quaternary monogenetic edifices in northern New Zealand, provide an opportunity to investigate magma source and assembly. The phenocryst population has both cognate and non-cognate origins with respect to the carrier magma. Common large subhedral-euhedral phenocrysts with compositions of Fo80–86 are autocrysts, having nucleated in a melt equivalent of the host whole-rock (~Mg#56–63). A subordinate population of large subhedral-euhedral phenocrysts that are Mg-rich (>Fo86) nucleated in a melt more mafic (>Mg#65). These are antecrystic and are likely to have a deep cumulate origin, perhaps the product of fractionation of a primitive parental magma. Smaller anhedral phenocrysts have compositions (~Fo70–75) that would have crystallised in a more evolved melt (~Mg#42–48). These are xenocrysts from stalled and fractionated crystal mush zones or intrusions in the mid and upper crust. Other small phenocrysts (Fo66–79) are euhedral and reversely zoned with mafic mantle zones (Fo80–85). They also nucleated in a more evolved crustal magma, but subsequently experienced magma-mixing with a more mafic magma. The olivine assemblage requires a vertically extensive magma system through the crust and upper mantle involving deep cognate crystallisation, and recycling of non-cognate crystals from deep and shallow intrusive bodies. This in accord with varied origins for the accompanying plagioclase and clinopyroxene populations in the rocks demonstrated in previous studies. This study presents the first δ18O analyses of olivines from monogenetic volcanoes in New Zealand. SIMS analysis of cognate and cumulate olivine phenocrysts in KBIVF basalts, and those from other volcanoes in northern New Zealand basaltic fields, show that they are non-zoned (δ18O ± 0.4‰). The inter-crystal range at most volcanoes is narrow (0.5‰), although the olivine population at a few volcanoes are more varied (range up to 1.25‰), requiring more than one source. The isotopic data for the more northern fields KBIVF (δ18O mean = 5.60‰; range = 5.12–6.20‰) and Whangarei volcanic field (δ18O mean = 5.50‰; range = 4.98–6.12‰) include values higher than that of olivine from MORB-HIMU sources, and overlap those associated with EMI/EMII sources or subduction zone magmas. Whole-rock trace element and Pb-isotope data from these fields have been interpreted as the result of subduction-induced contamination of the mantle wedge, a relic signal from now extinct Miocene-Pliocene arc volcanism. The magma compositions are also consistent with source depths above the garnet facies zone. This could explain the higher olivine δ18O values. In contrast, olivines from more southern fields do not display this O-isotope signature, consistent with postulated greater source depths.