Silica-rich spinel harzburgite residues formed by fractional hybridization-melting of the intra-oceanic supra-subduction zone mantle: New evidence from TUBAF seamount peridotites

• Published in: Geochimica et Cosmochimica Acta
• Main Authors: Bénard, A., Müntener, O., Pilet, S., Arculus, R.J., Nebel, O.
• Format: Article in Journal/Newspaper
• Language: English
• Subjects: geo; art; Bismarck; Kamchatka
• Online Access: https://doi.org/10.1016/j.gca.2020.11.001; https://serval.unil.ch/resource/serval:BIB_688A675A21E0.P001/REF.pdf; http://nbn-resolving.org/urn/resolver.pl?urn=urn:nbn:ch:serval-BIB_688A675A21E04

Recent studies of serpentine-free, spinel peridotite xenoliths from the mantle lithosphere beneath the active Kamchatka and West Bismarck arcs have shown that these rocks are enriched in silica and highly depleted in incompatible elements in comparison with melting residues of either primitive or mid-ocean ridge mantle. It has been suggested that the silica-rich nature of peridotites from the intra-oceanic, fore- and sub-arc mantle lithosphere, collectively referred to as ‘Supra-Subduction Zone (SSZ) peridotites’, is primarily of residual origin and inherited from source processes during partial melting in the SSZ mantle asthenosphere (mantle wedge). However, quantifying the contribution of post-melting processes to the silica-rich nature of SSZ peridotites has remained challenging. Here we report petrological and major and trace element data for a new suite of spinel harzburgite xenoliths from the mantle lithosphere beneath TUBAF seamount, located in the fore-arc region of New Ireland (Papua New Guinea area). All samples are fresh peridotites displaying coarse-grained protogranular textures, and sometimes high orthopyroxene (up to ∼29 wt%) at low clinopyroxene (≤4 wt%) contents, which are typical for SSZ peridotites worldwide. TUBAF peridotites in this study have suffered very little post-melting metasomatism through the formation of ≤1 wt% amphibole, which subsequently experienced decompression-induced breakdown during the xenolith ascent. Otherwise, the rocks display a high degree of inter-mineral equilibration and melting signatures preserved through sub-solidus re-equilibration. The bulk-rock chemistry of TUBAF peridotites record a Fe-Al correlation along the 25–30% melting isopleths from ∼2 to <1 GPa, in combination with the distinctive enrichment in silica and (TiO2, Al2O3, Na2O)-depletion of SSZ peridotites. This strongly supports the melting origin of these ‘residual SSZ signatures’. Bulk-rock and mineral lithophile trace element compositions of TUBAF xenoliths are similar to those of other residual ...