Seminar

The fluid-rock interactions strongly control the mantle compositions and heterogeneity in subduction zones. The Luobusa ophiolite in South Tibet is a remnant of Neo-Tethys oceanic lithosphere that formed above a newly-formed subduction zone in Eocene. Its mantle sequence is dominated by harzburgites with minor dunites and chromitites included, and record multi-stage fluid/melt modifications, making the Luobusa ophiolite an ideal objective for studying the mantle heterogeneity in subduction settings. Olivines from the mantle sequence have large δ⁷Li variations, ~ 4-13‰ in harzburgites and ~ 1-11‰ in high-Cr chromitites. Clinopyroxenes in the harzburgites have prevalent finger-like protrusions and left-sloping REE patterns comparable to those of clinopyroxene in abyssal peridotites, and show high Li concentrations (~ 2-8 ppm) and negative δ⁷Li values (~ -15 to -8‰), obviously beyond the ranges of normal upper mantle (< 1.3 ppm and 4±2‰). Such a dataset suggests that the Luobusa mantle wedge was penetrated by slab-derived fluids at an early stage, leading to the high δ⁷Li values of olivine. The hydrated harzburgites were later refertilized by depleted mantle-derived melts, generating the irregular LREE-depleted clinopyroxenes with light δ⁷Li values and converting previously hydrated harzburgites at deeper levels into non-hydrated ones. Subsequent melting in both hydrated and non-hydrated harzburgitic mantle regimes generated the Mg-rich magmas. These magmas passed through and reacted with the mantle harzburgites above, locally converting them into dunites and chromitites. Slab dehydration and asthenospheric upwelling together contributed to the heterogeneous mantle wedges in newly-initiated subduction zones.