Subducted Mg-rich carbonates into the deep mantle wedge

Recent studies have concluded that subducted calcium (Ca-) rich carbonates could be dissolved in slab derived aqueous fluids and transported upwards into the shallow mantle wedge (75-120 km), while magnesium (Mg-) rich carbonates could be delivered to a greater depth (i.e., the mantle transition zone, similar to 410 km), melted, and recycled into the convective upper mantle. However, it remains unknown whether or not Mg-rich carbonates can be transferred to the deep mantle wedge (>similar to 120 km) by subduction-zone fluids, which, if true, is important for tracing deep carbon. In this paper, we report a comprehensive mineralogical, geochemical, stable (Mg and O) and radiogenic isotopic (zircon U-Pb) study of garnet clinopyroxenites from the Maowu ultra-mafic massif (a slice of the mantle wedge) in the Dabie orogeny, Central China. Whole-rock and mineral trace elemental features and zircon U-Pb ages reveal evidence of mantle wedge metasomatism by a slab-derived melt or supercritical fluid from the subducted rutile-bearing eclogitic Paleo-Tethys oceanic crust, in addition to subsequent metamorphism occurring during the Triassic collision between the South and North China blocks. Combined with the results of previous works, the high Th/U ratios of both whole rocks and metasomatized zircons with no oscillatory zoning lead us to infer that a supercritical liquid rather than a melt was the metasomatic agent during oceanic subduction at peak conditions (5.3-6.3 GPa and similar to 800 degrees C, 160-190 km). Abundant carbonate mineral inclusions (including calcite, dolomite and magnesite) and the high delta O-18(VSMOW) values of the metasomatized zircons (up to 12.2 parts per thousand) indicate that sedimentary carbonates were leached by the supercritical fluid. Furthermore, whole-rock delta Mg-26 values (-0.99 parts per thousand to -0.65 parts per thousand) that are lower than normal mantle values (-0.25 +/- 0.07 parts per thousand) imply that the incorporated carbonates contain not only calcites but also a certain amount of dolomites (approximately 1-10 wt.% of the metasomatic supercritical liquid). The dissolved Mg-rich carbonates in the slab-derived supercritical liquid could effectively modify the Mg isotope composition of the deep mantle wedge. Our study represents a critical step towards achieving a broad understanding of the behaviours of recycled carbonate during slab subduction. (C) 2018 Elsevier B.V. All rights reserved.