Fabrics and water contents of peridotites in the Neotethyan Luobusa ophiolite, southern Tibet: implications for mantle recycling in supra-subduction zones

The preservation of ultra-high-pressure and super-reducing phases in the Neotethyan Luobusa ophiolite in Tibet suggests their deep origin near the mantle transition zone. Dunite and harzburgite core samples from the Luobusa Scientific Drilling Project show supra-subduction zone geochemical signatures and equilibration temperatures of c. 950–1080°C. Olivine shows A-, B-, C- and E-type fabrics, and combinations of A- and E-type or B- and E-type fabrics. Transmission electron microscopy observations show straight dislocations and the activation of multiple slip systems [100](010), [001](010), [001](100) and [100](001) in olivine. The mean water content in olivine, orthopyroxene (Opx) and clinopyroxene (Cpx) from 24 peridotite samples was 16 ± 5, 90 ± 21 and 492 ± 64 ppm, respectively, which is different from the water content of hydrated peridotites above the mantle wedge. The trace element compositions of Cpx exclude significant metasomatism after melt extraction. The high hydrogen partition coefficient between Cpx and Opx ( D H Cpx/Opx  = 5.56 ± 0.96) implies equilibrium at high pressures and rapid exhumation. Based on deformation experiments, the B- and C-type fabrics could be formed in a subduction zone at depths >200 km, whereas the A- and E-type fabrics were produced in the shallow mantle. In a process triggered by slab rollback, the Luobusa peridotites may have been rapidly exhumed within a subduction channel and mixed with the lithospheric mantle of the forearc. Supplementary material: Major oxide contents in Opx, Cpx and spinel, trace element concentration in Cpx, micrographs and TEM images of peridotite are available at https://doi.org/10.6084/m9.figshare.c.4307828