Pre-UHP titanite archives pro- and retrograde episodes of fluid-marble-interaction (Dabie Shan UHP unit, China)

Abstract

Deeply subducted crustal carbonate rocks are major transporters of carbon into the asthenosphere. However, mechanism and extent of the interaction of carbonate rocks at mantle depth with external fluids remain unclear. Carbonate interaction with external aqueous fluids derived from silicate rocks is of particular importance, because the associated decarbonation and dissolution reactions (i) influence the input into the subduction carbon cycle, (ii) influence rheology, and (iii) control isotope fingerprints of the interacting rocks, including the mantle.We investigate meta-carbonate rocks from the Dabie UHP unit, where marbles within a several km2 exposure show 87Sr/86Sr isotope signatures as low as 0.7037, which are unusual for crustal carbonates, but typical for mantle rocks. We focus on a sample with a large pre-UHP titanite, crystallized in a vein, with crustal 87Sr/86Sr signatures (87Sr/86Sri: 0.7071–0.7075) and its fluid-mediated replacement. The titanite survived subduction to mantle depth and was altered only at its margins during subduction and exhumation. This fluid-mediated replacement is characterized by (i) very low, mantle-like 87Sr/86Sr signatures (as low as 0.7045), both in the titanite reaction rim and the marble matrix, and (ii) significantly higher 87Sr/86Sr signatures (ca. 0.7085) related to allanite replacing titanite along discrete veins during retrogression.Combining mineral-scale Sr-isotope analyses with microfabric and mineral-chemical data of titanite, white mica, and calcite, allows for establishing five episodes (A to E) of fluid-mineral interaction linked with the pro- and retrograde reaction history: (A) prograde episode of interaction with aqueous fluid sourced from dehydration reactions in the impure marble or from subducting crustal rocks; titanite crystallized from fluid in a vein and acted as a sink for Ti, REE and Sr from the protolith, (B) fluid-interaction resulted in the replacement of titanite by rutile in the rim of the prograde titanite, ductile deformation lead to shape-preferred orientation of mineral relicts of the HP-UHP stage, (C) infiltration of a mantle-derived fluid is recorded by the back – reaction to titanite, whose 87Sr/86Sr signatures reflect mixing between the fluid from dissolved prograde titanite and the incoming fluid, (D) the continued interaction of the mantle-derived fluid with the marble resulted in pervasive replacement and mantle-like 87Sr/86Sr signatures of the minerals in the matrix, and (E) infiltration of a fluid sourced from crustal rocks or from metasomatized mantle fragments along grain boundaries and fractures at the retrograde episode E related to emplacement in the middle crust. We show that dissolution-precipitation reactions efficiently supported material transport and isotope chemical exchange between titanite and its meta-carbonate host rock, depicting Sr isotope mixing associated with dissolution and precipitation. Deformation was not a controlling factor. The fluid-mineral interaction episodes are related to map-scale metasomatism and the change of the isotope fingerprint of the marbles.