Tracing mantle source variation through xenocrystic olivine in the Taupo Volcanic Zone, New Zealand: A role for lithospheric mantle in the shift from andesitic to rhyolitic compositions

The central Taupo Volcanic Zone (TVZ) in New Zealand, the most productive of silicic volcanic centres, is flanked by younger andesitic stratovolcanoes to the south. Small basaltic scoria cones occur along the length of the TVZ, the most primitive of which contain abundant xenocrysts, giving insight into the mantle feeding the TVZ. Here, we present major and minor element compositions of olivine from eleven mafic eruptions spanning the length of the central and south TVZ. Olivine compositions can be divided into three major groups, based on their texture, major and minor element composition, and zoning. Group 1 olivines are Fo88 in the most primitive cores, with minor element concentrations consistent with magmatic origin. Group 2 olivine compositions range from Fo72–88 but are homogeneous within individual crystals. CaO contents are consistently lower for a given forsterite content compared to group 1 olivines and are most consistent with a plutonic crystal mush origin. Group 3 olivines have core compositions of Fo93–90, and NiO and CaO (0.36–0.56 wt%,