Abstract

White mica is an important reservoir of boron (B) in subducted metasedimentary rocks. To quantify the effect of phengite breakdown on B release and transfer, we investigated the replacement processes of phengitic mica and associated element redistribution in exhumed ultra-high pressure (UHP) impure marbles from the Dabie terrane, China. Three types of microstructures, which are related to the liberation, transport, and precipitation of B, are recognized: (1) Type-I is the pseudomorphic replacement of phengite by the assemblage of biotite + plagioclase ± quartz ± epidote. The assemblage maintains a flaky shape of pre-existing phengite, which only rarely occurs as a relict phase in the product assemblages. Phase equilibria modelling indicates that the conditions of replacement reactions are 1.2–1.3 GPa at ~600 °C, with a relatively low XCO2 of 0.04–0.16. (2) Type-II is characterized by the local occurrence of fine-grained tourmaline (Tur-F) along the outer margin of Type-I texture domains, which represents the in situ precipitation of B released by the replacement reactions. (3) Type-III texture involves the irregularly shaped intergrowth of tourmaline (Tur-In) + quartz in the calcite matrix, isolated from the Type-I and Type-II texture domains. The Type-III minerals are in equilibrium with diopside and plagioclase and most likely represent the precipitation from a “transported”, B-bearing fluid.Mineral trace element analyses indicate that relict phengite in Type-I texture has significantly higher B contents (284–464 μg/g) than all of the product minerals (2–26 μg/g). A mass balance estimate indicates that the majority (>90%) of B in the reactants is lost during the process of phengite replacement. The calculations in terms of mineral-fluid partition indicate that the fluid involved in the replacement reactions has B contents of 1893–6858 μg/g. In situ B isotopic analyses indicate that Tur-F and Tur-In have a similar variation range of δ11B (+4 ~ +7‰). Both types of low-pressure tourmaline have lower δ11B values than the previously reported eclogite-facies tourmaline (reaching +15‰) from phengite-bearing marbles in the same study area. This result indicates two episodes of tourmaline growth and B transfer. Thus, a two-stage model of B loss from phengite during slab exhumation is proposed. Our results reveal that the breakdown of white mica in exhumed metasedimentary rocks can cause a strong release of B, which would largely influence the B budget at convergent plate margins.

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