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The mobility of redox sensitive elements such as iron, carbon and sulfur in subduction zone fluids is important for redox transfer between the subducting slab and mantle wedge. However, the speciation of Fe during its mobility in subduction zone fluids at forearc depths is still unclear. Here, we studied the Fe isotope compositions of high-pressure (HP) fluid-metasomatized rocks (Mg-rich leucophyllite and gneiss, which is mineralogically transitional between leucophyllite and metagranite) from the Eastern Alps and found evidence for two distinct types of Fe species during Fe mobility in subduction zone fluids. The protolith of both types of rocks is metagranite, which shows δ56Fe values of 0.12–0.25‰. Compared to the metagranite, the transitional gneiss shows slightly higher Fe 2 O 3t contents, lower Fe3+/∑Fe ratios and significantly lower δ56Fe values of −0.03–0.16‰, whereas the leucophyllite exhibits much lower Fe 2 O 3t contents, comparable or slightly lower Fe3+/∑Fe ratios and generally higher δ56Fe values of up to 0.55‰. These observations indicate that the granitic protolith has experienced two types of fluid metasomatism. Combined with previous results, we infer the mobility of Fe in subduction zone fluids at forearc depths via two kinds of Fe species, Fe2+-(CO 3), and Fe2+-(HS) or Fe2+-Cl. The Fe mobility was dominant in the form of the Fe2+-(CO 3) complex in a carbonate-rich fluid during the formation of transitional gneiss, consistent with reported radiogenic Sr and isotopically light Mg isotope compositions, whereas Fe loss in the form of Fe2+-(HS) or Fe2+-Cl complexes via dissolution of biotite and phlogopite can account for the formation of leucophyllites. According to the tectonic evolution of the Eastern Alps, both types of fluids possibly originated from the dehydration of the sediment-serpentinite mélange produced by the subducting oceanic slab of the Alpine Penninic unit. The diversity of ferrous iron species in the subduction zone fluids at forearc depths probably reflects the variation in the redox properties of fluids derived from carbonate-rich sediment and serpentinite under such conditions. Our results provide an excellent example of tracing Fe mobility with distinct Fe species in subduction zone fluids. [ABSTRACT FROM AUTHOR]

Marble-hosted eclogite is volumetrically minor in collisional orogens, but its geochemistry has great bearing on the origin of deeply subducted crustal rocks and the fluid mobility of subduction zones. This paper presents a combined study of whole-rock major-trace elements and Sr Nd isotopes, mineral O isotopes, carbonate C and O isotopes, and zircon U Pb ages and Lu Hf isotopes for marble-hosted ultrahigh-pressure metamorphic eclogites from Rongcheng and Sanqingge in the Sulu orogen. The results provide insights into the protolith nature of eclogites and the fluid mobility of subduction zones. Zircon U Pb dating yields consistent middle Triassic ages for the two occurrences of eclogites, indicating new growth of metamorphic zircon during continental collision. The Sanqingge eclogite shows LREE-enriched patterns and negative ε Nd (t) of − 16.6 to − 14.3 for whole-rock and negative ε Hf (t) of − 27.1 to − 15.2 for metamorphic zircon. A few relict zircon domains show middle Neoproterozoic U Pb ages and negative ε Hf (t) of − 35.2 to − 15.5. Thus, the Sangqingge eclogite was metamorphosed from a mafic rock that was derived from partial melting of an anciently enriched mantle source. In contrast, the Rongcheng eclogite exhibits flat or even LREE-depleted patterns with negative ε Nd (t) values of − 12.2 to − 1.0 for whole-rock but positive ε Hf (t) values of 5.4 to 10.4 for zircon. The occurrence of interstitial and highly cuspate plagioclase along grain boundaries indicates the presence of partial melting in the eclogite. Thus, its positive zircon ε Hf (t) values are ascribed to the eclogite protolith of juvenile origin, whereas the LREE depletion is due to extraction of LREE-rich anatectic melt from the eclogite during the Triassic continental collision. As such, the Rongcheng eclogite was metamorphosed from a mafic rock that was derived from partial melting of a less enriched mantle source. All the eclogites from both areas show variably high δ 18 O values of 9.4‰ to 19.5‰. Oxygen isotope fractionations between mineral pairs mostly yield eclogite-facies temperature of 600 to 800 °C, suggesting that the high δ 18 O signature was inherited from their protoliths before the Triassic subduction. In combination with the field relation between the eclogite and marble, it is inferred that the eclogite protolith is probably basaltic tuff and its high δ 18 O value would be acquired together with the marble protolith during their deposition from the surface water. Therefore, there would be the limited isotopic exchange between marble and eclogite during continental collision. [ABSTRACT FROM AUTHOR]

Melt extraction and subduction zone fluid infiltration can produce light Fe isotope compositions of arc lavas relative to the MORBs. However, the Fe isotope signature of subduction zone fluid is poorly constrained, which has hampered our understanding of Fe cycling in the subduction zone. Jadeitite veins in forearc serpentinite provide a unique record for the characteristics of subduction zone fluids. Here we report Fe isotope data for Myanmar jadeitites and associated rocks to constrain the Fe isotope signature of subduction zone fluids. Two types of jadeitites from Myanmar display distinct δ56Fe much lower than typical igneous rocks. White jadeitites, directly precipitated from fluids, have variable δ56Fe from −0.60‰ to −0.04‰, while green jadeitites, formed by fluid-chromite interaction, have δ56Fe (−0.44‰ to −0.19‰) similar to the associated amphibole-rich blackwalls (−0.47‰ to −0.23‰). The light Fe isotope signature of the jadeitites is not induced by weathering, kinetic fractionation, dissolution of Fe-rich carbonate, or mineral dissolution-reprecipitation process. Instead, it most likely reflects the Fe isotope variation of serpentinite-derived fluids. Correlations of δ56Fe with REEs, Sb, and U in white jadeitites suggest that the jadeitite-forming fluid is a mixture of serpentinite-derived and sediment-derived fluids. In contrast, correlations of δ56Fe with Si, Al, and Cr in green jadeitites record the interaction of low-δ56Fe fluids with chromite. Considering the abundance of the serpentinite-derived fluid, it can effectively influence the Fe isotope compositions of the ambient mantle and arc lavas. • Myanmar jadeitites record signature of subduction zone fluids. • Both white and green jadeitites have variably low-δ56Fe. • Low δ56Fe of jadeitites reflect serpentinite-derived fluid signature. • Subduction zone fluid can influence the δ56Fe of ambient mantle and arc lavas. [ABSTRACT FROM AUTHOR]

Serpentinites entering subduction zones show selective enrichment in trace elements and are the most important carriers of B in this setting. The evolving B isotopic composition (δ11B) of serpentinite is used in studying water-mediated interactions between mantle and crust in subduction zones, but we lack a full understanding of the processes controlling B isotope fractionation during serpentinization that would allow this tracer to be better utilized. Boron isotope fractionation between lizardite/forsterite/diopside and aqueous fluids is investigated using quantum mechanics (density functional theory, DFT) and the first-principles molecular dynamics simulation (FPMD) here. The 11B enrichment sequence in minerals follows the order of forsterite > clinopyroxene (diopside) > white mica > phlogopite > lizardite. On the basis of boron configurations in aqueous fluid acquired from FPMD simulations based on DFT, the reduced isotopic partition function ratio (β-factor) of aqueous B(OH) 3 and B(OH) 4 − are 1000lnβ B(OH)3 = −20.1 + 37.07(1000/T) + 8.2(1000/T)2 and 1000lnβ B(OH)4- = −13.94 + 24.43(1000/T) + 9.12(1000/T)2 at the P-T range of 0–500 MPa and 298–1000 K. This theoretical approach gives an equilibrium B isotope fractionation of Δ11B lizardite-fluid = 15.89–22.88(1000/T) - 0.19(1000/T)2 between lizardite and neutral serpentinization fluid in the temperature range from 298 to 673 K. By contrast, the boron isotope fractionation between minerals formed as the slab descends (e.g., forsterite, diopside) and fluid released from subducted crustal lithologies is limited, indicating that secondary olivine minerals may inherit the δ11B signatures of the dehydrated serpentinites. This study provides new insights into how B isotopes can constrain boron sources and pathways in subduction zones, and trace phases recycled into the deep mantle and their later recycling in ocean island basalts. [ABSTRACT FROM AUTHOR]