Fabrics and water contents of peridotites in the Neotethyan Luobusa ophiolite, Southern Tibet: Implications for mantle recycling in suprasubduction zones

Preservation of ultrahigh-pressure and super-reducing phases in the Neotethyan Luobusa ophiolite in Tibet suggests their deep origin down to the transition zone. Dunite and harzburgite core samples from the Luobusa Scientific Drilling Project show suprasubduction-zone geochemical signatures and equilibration temperatures at ~950– 1080 °C. Olivine developed the A-, B-, C- and E-type fabrics, as well as combination of the A- and E-type, or B- and E-type fabrics. TEM observations reveal straight dislocations and activation of multiple slip systems [100](010), [001](010), [001](100) and [100](001) in olivine. Different from hydrated peridotites above the mantle wedge, the average water contents in olivine, orthopyroxene (Opx) and clinopyroxene (Cpx) from 24 peridotite samples are 16±5 ppm, 90±21 ppm and 492±64 ppm, respectively. Trace-element compositions of Cpx excludes remarkable metasomatism after melt extraction. The high hydrogen partition coefficient between Cpx and Opx (D_H ^Cpx/Opx = 5.56±0.96) implies their equilibrium at high pressure and fast exhumation. Compared with deformation experiments, the B- and C-type fabrics might be formed in a subduction zone >200 km, whereas the A- and E-type fabrics were produced in the shallow mantle. Triggered by slab rollback, the Luobusa peridotites may have rapidly exhumed within a subduction channel and mixed with the fore-arc lithospheric mantle.