Tourmaline composition probes serpentinite-derived fluid mobility in subduction zones

Serpentinite dehydration in subduction zones plays a pivotal role in geochemical cycling on Earth. A number of geochemical studies on arc magmas have elucidated the contributions of serpentinite-derived fluids to mantle sources. However, due to complex geological overprints during subduction zone processes, discerning serpentinite signatures in exposed metamorphic rocks within fossil subduction zones remains challenging. In this study we address these difficulties through in-situ investigations of tourmaline, the geochemistry of which reflects the host environment as well as potential fluid-induced processes. The presence of zonations in tourmaline makes it an excellent recorder of consecutive geological events. Integrated major and trace elements along with in-situ boron isotopes of tourmaline from the high-pressure Sopron area (Hungary) in the Eastern Alps were used to unravel fluid action sourced from serpentinite. Despite the presence of color zoning, tourmaline in the orthogneiss (Tur-G) has low X_Mg [Mg/(Mg + Fe)] of ca. 0.3–0.6 and δ¹¹B values of around −11 ‰, along with variable trace element compositions. Petrological observations and geochemical analyses suggest that the inner domains of Tur-G are of igneous origin, while the outer rims are likely affected by subsequent metamorphic events. Tourmaline in metasomatized kyanite-quartzite (Tur-K) veins exhibits distinct geochemical zoning, and preserves metamorphic cores and fluid-induced rims. The inner domains of Tur-K display low X_Mg (<0.6), relatively high trace element concentrations and δ¹¹B values of less than −10 ‰, whereas the overgrowths exhibit extremely high X_Mg values (>0.99), low trace element concentrations and high δ¹¹B values reaching up to +21 ‰, clearly indicating the incorporation of serpentinite-derived Mg-¹¹B-rich fluids. Through comparison with other metamorphic and metasomatic tourmalines in (ultra)high-pressure rocks globally, we establish that tourmaline with high X_Mg > 0.85 and δ¹¹B values >0 ‰ may serve as an effective proxy for detecting serpentinite-derived fluids in subduction zones.