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Serpentinites in subduction zones are important fluid reservoirs for crust-mantle interaction and arc magma formation. The serpentinite-derived fluids can carry fluid-mobile elements such as Ba and potentially affect the geochemical composition of the metamorphic rocks in the subduction zones. However, the Ba isotope behavior during interaction between serpentinite-derived fluids and metamorphic rocks is still poorly understood. The whiteschists of the Dora-Maira Massif in the Western Alps have experienced fluid infiltration by serpentinite-derived fluids during deep subduction. Here we report the Ba isotope data of the whiteschist and its protolith metagranite to investigate the Ba isotope behavior during fluid-rock interaction. The metagranites, which have experienced ultrahigh-pressure metamorphism, have a small variation of δ138Ba (−0.25 to 0.26‰), while the whiteschists display a large Ba isotope variation from −0.99‰ to 0.48‰. Such a large variation in whiteschists cannot be explained by chemical weathering, protolith heterogeneity, or kinetic effects. Instead, considering the negative correlations of δ138Ba with Ba and other fluid-mobile elements (e.g., Sr, Cs, Zn, and Cu) and the geochemical composition features of serpentinite-derived fluids, the large Ba isotope variation in whiteschists is inferred to be induced by the interaction between serpentinite-derived fluid and metagranite during intense fluid infiltration. This is consistent with the observation that the whiteschists with low Ba contents tend to exhibit heavier Mg-Fe isotope compositions but lighter Li isotope compositions, which indicates that the related fluids were derived from the dehydration of mantle wedge serpentinite. The fluid infiltrating process is well explained by a numerical model with high fluid-rock ratios of 4 to 12. Our modeling further suggests that serpentinite-derived fluid has a distinctly higher δ138Ba value than the depleted upper mantle. During fluid-rock interaction, heavy Ba isotopes prefer to be infiltrated into fluids with an apparent fractionation factor (Δ138Ba rock-fluid) of mostly −0.5‰ to −0.1‰ but can be down to −0.8‰ to −0.7‰ required for a few samples. A two-stage dehydration and metasomatism model at different depths can well explain the formation of Dora-Maira whiteschists. Whereas the Ba isotope composition of mantle wedge serpentinite is mainly dictated by crust-derived fluids from the subducting slab, an interaction between the serpentinite-derived fluids and the metagranite protolith resulted in a large Ba isotope variation of whiteschists. In case of a general involvement of serpentinites in subduction zones, serpentinite-derived fluids would significantly affect the Ba isotope compositions of subarc mantle peridotites and also arc lavas. [ABSTRACT FROM AUTHOR]

Subduction of low-density continental crust to subarc depths is generally associated with preceding subduction of the oceanic slab. However, the original oceanic signature is often overprinted by subsequent fluid metasomatism from the subducting continental crust. As a result, it is generally difficult to trace the geochemical processes of previous oceanic subduction in collisional orogens. This issue can be potentially resolved by applying B isotopes to metamorphic rocks from continental subduction zones. Here a combined study of whole-rock geochemistry and in situ tourmaline B isotopes was carried out for coesite-bearing whiteschist and phengite schist, as well as country rock metagranitoids from the Dora-Maira Massif in the Western Alps. While all these metamorphic rocks have a similar protolith of Permian granites, whiteschist and phengite schist experienced Mg-rich fluid metasomatism during the continental subduction, evidenced by their much higher MgO contents than the metagranitoids. Tourmaline in the metagranitoid (Tur-G) is schorlitic (X Mg = 15–55) with low δ11B values from −13 to −6‰, and is consistent with a magmatic origin. In contrast, tourmaline in metasomatic rocks (Tur-S) is mainly dravitic with the highest X Mg worldwide (X Mg = 90–98) and high δ11B values of −5 to +1‰. Integrated with whole-rock geochemistry and previous studies, Tur-S is interpreted to grow during the infiltration of external fluids that were highly enriched in MgO and relatively enriched in 11B. According to the B isotope compositions of precursor tourmalines, it is estimated that the external fluids had significantly higher δ11B values than +2.4‰. Based on B isotope compositions of both Tur-S and metasomatic fluids, our quantitative modeling suggests that the metasomatic fluids most likely originated from the mantle wedge serpentinite that was formed during the preceding oceanic subduction stage. Therefore, tourmaline B isotopes in the ultrahigh pressure metamorphic continental crust can be used to trace preceding fluid metasomatism at the interface between oceanic slab and mantle wedge. The mantle wedge serpentinite plays an important role in modifying the geochemical composition of deeply subducted supracrustal rocks and probably also the mantle sources of arc magmas. [ABSTRACT FROM AUTHOR]

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]

Fluids play a crucial role in element mobility and mass transfer at the slab-mantle interface in subduction zones. However, tracing the source and chemical composition of subduction zone fluids still remains challenging. High-pressure (HP) metamorphic leucophyllites, mainly composed of quartz, muscovite/phengite and Mg-chlorite, occur in several localities in the Eastern Alps and experienced significant Mg-metasomatism at forearc depths during the Eoalpine orogeny. They thus provide a good opportunity to explore the origin of Mg-rich fluids in the continental subduction zone. The leucophyllites are rich in Mg and poor in Fe, Na, and Ca and occur as lenses and layers in the country rocks of metagranites. Both leucophyllites and metagranites show similar whole-rock REE distribution patterns, and magmatic zircons from them exhibit similar U-Pb ages of ∼271 Ma, suggesting that both types of rock have the same protolith of early Permian granite. One metagranite that preserves magmatic composition shows a low δ26Mg value of −0.64‰, suggesting a contribution of carbonate-rich sediment when granitic melts were produced. A profile of samples from metagranite through transitional gneiss to leucophyllite in Hungary shows two trends in δ26Mg values. In the Vashegy sections, δ26Mg values firstly decrease from −0.64‰ to −1.30‰ during gneiss formation, and then increase to 0.29‰ during leucophyllite formation. In a second locality, the TV tower section, the δ26Mg values firstly decrease from −0.29‰ to −0.89‰ during gneiss formation, and then remain constant during leucophyllite formation. Two leucophyllites in Austria exhibit higher δ26Mg values of 0.05 to 0.09‰ compared to their country rocks of about −0.20‰. Based on the Mg isotope systematics and their relationships to whole-rock geochemical compositions, we propose two types of fluid in the subduction zone: (1) a low δ26Mg (<−1.3‰) fluid produced by dissolution of mainly Mg-calcite at HP conditions; (2) a high δ26Mg (>0.3‰) fluid derived from dehydration of talc-rich serpentinite. This is for the first time to find subduction zone fluids with distinct δ26Mg values at forearc depths. Both fluids possibly originated from dehydration of the sediment-serpentinite mélange at the slab-mantle interface. The fluids were heterogeneous in δ26Mg values and they metasomatized slices of continental crust along shear zones. The presence of fluids with δ26Mg values as low as −1.3‰ suggests that the dissolution of Mg-rich carbonates can be significant at forearc depths. Our results provide an excellent example of tracing different sources of subduction zone fluids by coupling Mg isotopes with other petrological and geochemical variables. They may also have great implications for the generation of magmas with variable Mg isotope compositions at convergent plate boundaries. [ABSTRACT FROM AUTHOR]

Fluids released by subducting slabs are important to the geochemical evolution of Earth's crust-mantle system. However, it is still not well constrained for the mobility of redox sensitive elements (like Fe, C or S) in subduction zone fluids, and its effect on the redox property of the overlying mantle wedge and its derived arc magmas. Iron isotope fractionation is sensitive to the redox state and speciation of Fe in fluids, which can potentially provide clues on the oxygen fugacity (f O 2) of slab-derived fluids. Whiteschists at the Dora-Maira Massif in the Western Alps have experienced ultrahigh-pressure metamorphism at subarc depths. They are rich in both SiO 2 (mostly >65 wt.%) and MgO (4–10 wt.%), and mainly composed of pyrope, quartz/coesite, talc, phengite and kyanite. They were demonstrated to have a granite protolith, but metasomatized by serpentinite-derived Mg-rich fluids at the slab-mantle interface in a subduction channel. Thus these rocks provide an excellent target to explore the fluid-rock interaction by Fe isotopes. The whiteschists show extremely high δ56Fe values of +0.32 to +1.22‰, being much higher than those of +0.10 to +0.38‰ for the surrounding metagranites. The Fe isotope composition of whiteschists significantly deviates from the igneous differentiation trend defined by rocks with basaltic to rhyolitic compositions. The heavy Fe isotope compositions must be produced by a metasomatic process. The much lower Fe 2 O 3 and FeO t contents and Fe3+/∑Fe ratios but higher δ56Fe values for the whiteschists relative to their protolith suggest the reduction of Fe3+ to Fe2+ and the loss of isotopically light Fe, probably in the form of Fe(II)-Cl and/or Fe(II)-(HS) complexes, through saline fluids and/or HS- bearing fluids. The reduction of Fe3+ to Fe2+ and the possible presence of HS- suggests the occurrence of relatively reduced fluids. Such reduced fluids were probably derived from serpentinite dehydration at the slab-mantle interface in the subduction channel. Therefore, the Fe isotope results demonstrate that the f O 2 at the slab-mantle interface can be locally highly heterogeneous, leading to various oxidation states in the mantle wedge and arc magmas. [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]

Whole rock lithium (Li) and Li isotopic compositions of coesite-bearing whiteschists and their country rock metagranites from the Dora-Maira Massif in the Western Alps were investigated to understand their behavior during fluid-rock interaction in mantle depths of continental subduction zones. The whiteschists reveal relatively low δ7Li values (−6.4‰ to +2.4‰, with an average value of −2.1‰) in comparison to the country rock metagranites (δ7Li = −1.2‰ to +3.6‰, with an average value of +1.4‰). The δ7Li values of whiteschists do not show any correlations with major elements (e.g. Mg, Fe), Li, or isotopes (i.e., δ26Mg and δ18O values) and cannot be explained by kinetic fractionation. Hence, the low δ7Li values of whiteschists result from interactions with metasomatic fluids containing isotopically light Li that occurred in deep portions of a continental subduction zone. Fluid-rock interaction modeling suggests that the metasomatic fluids contain low δ7Li values (down to −4‰) and moderate Li concentrations (50 to 150 ppm), which were probably derived from the dehydration of serpentinites in the subduction channel. The presence of such low δ7Li fluids in deep subduction zones could cause Li isotopic heterogeneity in the earth's interior. • The whiteschists have relatively low δ7Li values (−6.4‰ to +2.4‰). • Isotopically light Li compositions resulted from metasomatism with fluids. • The metasomatic fluids were probably derived from the dehydration of serpentinites. • Such low δ7Li fluids could cause Li isotopic heterogeneity in the Earth's mantle. [ABSTRACT FROM AUTHOR]