The composition of fluids generated during deep continental subduction estimated from fluid inclusions in UHP rocks.

Composition and provenance of slab-derived hydrous fluids and melts represent a key parameter in understanding the geochemical variations of subduction-related magmas. Chemical and physical characteristics of the slab components may be strongly reliant upon both the thermal structure of the mantle wedge, and the nature of the subducted lithosphere. While fluids are well known to play a key role in element recycling during subduction of oceanic crust, it is less clear how dehydration and/or melting occur during subduction of continental crust. A direct approach to get information on the nature of fluids is provided by fluid inclusion analysis. Two examples of peak fluid inclusions in UHP metamorphic rocks from the Su-Lu Terrane (China) and the Dora-Maira Massif (Alps) are discussed. In the Su-Lu terrane, primary multiphase solid (MS) inclusions within UHP quartzites represent fluids internally generated through dehydration reactions at pressure above 3.5 GPa. Inclusions contain paragonite, muscovite, sulfates, carbonates, phosphates, and oxides. Calculated compositions indicate aqueous fluids (H₂O = 25 - 50 wt %) containing tens of wt % of Si, Al, and alkalies. Trace element patterns show enrichments in large-ion lithophile elements (LILE; K, Sr, Rb, Cs, Ba, Pb, U) and light rare earth elements (LREE). In the Dora-Maira whiteschists, MS inclusions formed at 730°C and ≤ 4GPa have rather complex compositions and consist of Mg-chlorite, Na-phlogopite, Cl-rich apatite, Zn-rich pyrite, and chlorides, with subordinate talc and magnesite. Although water is absent, IR-synchrotron-radiation mapping revealed significant H⁺ diffusion from MS inclusions to host garnet. Peak aqueous solutions are Si, Al, and Na-rich, and contain significant amounts of Mg, Ca, P, S, and LILE, supporting an open system fluid generation with significant contribution from external brines derived from dehydration of oceanic lithosphere during subduction. Although the studied rocks have been collected from two distinct mountain belts, fluid data show remarkable similarities. Intermediate aqueous solutions (H₂O ≤ 50 wt %) may be produced during dehydration reactions of subducted continental lithosphere at depths > 120-150 km. A remarkable feature of this fluid is represented by the dominant alkali-alumino-silicate character of the solutes. Further, solutions are concentrated, resulting in an unusual composition, which is intermediate between a fluid and a melt. Deep fluids show trace element distribution characterized by significant Ti and Nb (and Ta) depletions and LILE enrichments, resulting in patterns close to upper continental crust composition. Following subduction at depth, aqueous solutions released from continental lithologies may interact with the peridotitic mantle, imprinting their enriched signatures. Addition to mantle rocks of fluid-transported upper continental crust components could result in a composition close to EM2 (enriched mantle 2) mantle end-member, supporting a major role of continental subduction in fluid mediated element recycling.