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 Fo 88 in the most primitive cores, with minor element concentrations consistent with magmatic origin. Group 2 olivine compositions range from Fo 72 – 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 Fo 93 – 90, and NiO and CaO (0.36–0.56 wt%, <0.15 wt%, respectively) concentrations consistent with a primary magma or mantle-derived origin. Compositional variation within group 3 olivine allows for further characterisation of the mantle lithologies from which the crystals were sourced. Group 3 olivines from the central TVZ are lherzolite-derived, whereas group 3 olivines from the south TVZ are harzburgite-derived. However, recent whole-rock and melt inclusion data suggests that the most fertile mantle underlies the south TVZ, and the least fertile mantle underlies the inter-caldera regions of the central TVZ where caldera-forming volcanism has ceased. This is inconsistent with interpretations drawn from the olivine data. We therefore suggest that harzburgitic xenocrysts originate from old depleted sub-continental lithospheric mantle (SCLM), which is present at depths of ~30 km in the south TVZ. By contrast, rifting and crustal thinning in the central TVZ has removed the SCLM, explaining why only xenocrysts of lherzolitic origin are found. The presence of depleted SCLM may act as a boundary layer inhibiting ascent of melt from the mantle, providing an explanation for lower erupted volumes in the south TVZ. Dissolution of the orthopyroxene component in harzburgite in the presence of hydrous melt satisfies major element constraints on south TVZ basalt (high SiO 2, high MgO, low CaO/Al 2 O 3 ). Taken together, this is the first mineralogical evidence for a tectonic and lithospheric control on the shift from andesitic to rhyolitic activity in the TVZ, implying a direct link between rifting, magma composition and volcanic productivity.