Ultra-refractory mantle xenoliths from ocean islands: How do they compare to peridotites retrieved from oceanic sub-arc mantle?

Ultra-refractory harzburgitic mantle is abundantly sampled as xenoliths in ocean island magmas. Its occurrence has potentially important implications for both geochemistry and geodynamics, and we therefore need to understand how these unusually refractory compositions form. In this contribution, we compare ultra-refractory spinel harzburgite xenoliths from ocean islands with peridotites collected along active arcs, based on available major element data on rocks and their minerals. The compositions of arc peridotites vary from ultra-refractory to fertile and estimated equilibration temperatures range from about 800 to > 1300 °C. These variations are not directly related to setting (ocean–ocean or ocean–continent collision; forearc, arc or back-arc). In general, however, fertile compositions seem to be more common in continental arcs and to be accompanied by higher equilibration temperatures. Ultra-refractory peridotites appear to be most common along oceanic forearcs and give generally lower temperatures in the range 950–1050 °C. Ultra-refractory harzburgites sampled in subduction settings are chemically very similar to the ultra-refractory peridotites found as xenoliths in ocean islands. Both rock types compare quite well with the residues from experiments on the formation of primitive arc magmas. The formation of such magmas and their ultra-refractory harzburgite residues requires melting beyond the stability of clinopyroxene. Such high degrees of partial melting are probably achieved in two stages. The first stage is likely to take place as decompression melting along mid-ocean ridges or in back-arc spreading centers, whereas the second stage seems to require fluid-fluxed melting in the mantle wedge. Large degrees of melting may also be achieved in other types of settings if temperatures are sufficiently high. However, the common occurrence of ultra-refractory harzburgites together with melts generated from a strongly depleted precursor in active arcs is an argument in support of a subduction related origin of ultra-refractory harzburgites both in arc settings and in ocean islands. This implies that ultra-refractory arc material is common and may be recirculated by mantle convection. Because of their low density relative to “normal”, more fertile asthenospheric material, these ultra-refractory residues may rise as diapirs and intrude shallower parts of the asthenosphere, where they can accrete to younger oceanic plate.