Diffusion-zoned pyroxenes in an isotopically heterogeneous mantle lithosphere beneath the Dunedin Volcanic Group, New Zealand, and their implications for intraplate alkaline magma sources

Abstract

An important task in assessing the magma source of an intraplate volcanic province is establishing the composition of the underlying lithospheric mantle. Pyroxenes in peridotite xenoliths from the ∼10,000 km2 Dunedin Volcanic Group in New Zealand reveal that the underlying lithospheric mantle is chemically and isotopically heterogeneous and has a complex thermal history. Portions of this mantle have light rare earth element–depleted clinopyroxene trace element concentrations and distinctly radiogenic Nd compositions (eNd(20 Ma) ≥ + 15.5) with model depleted mantle ages that are ≥100 m.y. older than the overlying Jurassic crust (type 1). The Nd isotopic composition of these moderately fertile domains is distinct from any Dunedin Volcanic Group magma, but the domains are embedded within enriched peridotitic mantle (type 2) that has formed through reaction with a light rare earth element–rich fluid that imparted an isotopic composition in strongly metasomatized xenoliths of 87Sr/86Sr(20 Ma) = 0.7028–0.7029, eNd(20 Ma) = + 5.0 to + 5.1, and 206Pb/204Pb = 19.9, 207Pb/204Pb = 15.5, and 208Pb/204Pb = 39.6. These isotope ratios overlap with the isotopically homogeneous high time-integrated U/Pb–like source signature of the host Dunedin Volcanic Group. However, all metasomatic and nonmetasomatic pyroxenes are zoned in temperature-sensitive elements (Al, Cr, Mg ± Ca), with trends indicating element exchange during cooling and the results of diffusion calculations implying that the zoning formed over hundreds of thousands to millions of years prior to late Oligocene–Miocene xenolith entrainment. These data, along with calculated pyroxene rare earth element homogenization diffusion rates, indicate that mantle metasomatism predated entrainment in the host magmas by millions of years. Furthermore, the presence of the cooling trends in all but one sample indicates that this upper lithosphere mantle preserves little or no sign of a rise in the geotherm at the time of magmatism. Zoning patterns in peridotite pyroxenes can therefore provide useful insight into the role of portions of the lithospheric mantle in formation of intraplate alkaline basalts.