Ultra-refractory mantle in the oceanic domain
|Location||International Geological Congress,oslo 2008|
|Author||Neumann, Else-Ragnhild; Simon, Nina S.C.|
|Holding Date||29 September 2008|
Ultra-refractory harzburgitic mantle is abundantly sampled as xenoliths in ocean island magmas and along magmatic arcs. Its occurrence has potentially important implications for both geochemistry and geodynamics, and we therefore need to understand how these unusually refractory compositions form. Ultra-refractory harzburgites sampled in subduction settings are chemically very similar to the ultra-refractory peridotites found as xenoliths in ocean islands, but are on average significantly more refractory than peridotites dredged and drilled from mid-ocean ridges and fracture zones. Ultra-refractory harzburgites are characterized by the absence of primary clinopyroxene, low whole rock Al2O3, CaO and HREE concentrations, low Al2O3 in orthopyroxene (<3 wt%), high Cr# in spinel (0.3-0.8) and high forsterite contents in olivine (>91.5). The formation of 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. Due to their whole rock composition, ultra-refractory harzburgites would have low densities and very high viscosities relative to normal, more fertile asthenospheric material. Thus, they might be preserved as fragments in the convecting mantle over long periods of time and would tend to accumulate at the top of the convecting mantle, where they can accrete to younger oceanic plate.