Mineral and whole-rock trace elements and non-traditional stable isotopes (Fe-Zn-Cu isotopes) of serpentinites from different tectonic settings

Serpentinite is essential in chemical exchanges between Earth's interior and surface, and plays a fundamental role in cycling of redox sensitive elements. Fluid-mobile element (FME) of serpentinite is critical for revealing the processes of mantle rock hydration, dehydration, and element recycling. Here we compile mineral and whole-rock trace elements and non-traditional stable isotopic compositions (Fe, Zn, Cu) of serpentinites from various tectonic settings. Trace elements of serpentine indicate that the occurrence of both LREE-enriched and LREE-depleted patterns in serpentines experienced by different degrees of serpentinization. The contents of REE and trace element of chrysotile are compared with those of lizardite and antigorite, while the contents of HREE and FME of lizardite are slightly higher than those of antigorite. The moderately incompatible elements together with REE could be useful to discriminate serpentines formed after olivine and pyroxene. Serpentine originated from pyroxene is enriched in compatible elements such as Sc, Zn, Cr, Y and Ti, while serpentine generated from olivine displays nearly flat REE patterns. The whole-rock trace element and REE patterns are overlapped for serpentinites formed among different settings, but there are also some differences. The REE and trace elements of the serpentinite from the slow-spreading Indian Mid-Ocean Ridge are higher than those from the Atlantic Mid- Ocean ridge and the Pacific Mid-Ocean ridge. Compared with serpentinite and serpentinized dunite, the Mariana serpentinite mud has higher REE and trace elements, while the serpentinized dunite has lower REE and fluid-immobile element. It is difficult to distinguish serpentinite in different environments by using absolute element concentrations. However, the alkali-U element ratios can distinguish mid-oceanic serpentinite from forearc serpentinite. We summarize the data of Fe, Zn and Cu isotopes of serpentinite. The fractionation of Fe and Zn isotopes of ophiolitic serpentinite is closely related to serpentinite devolatilization. The δ56Fe values of these serpentinites progressively increased during serpentinite devolatilization as bulk Fe3+/ΣFe decreased, while the δ66Zn and Zn contents show a positive correlation. These features suggest that serpentinite can release oxidized fluids. Compared to peridotite, serpentinite has significantly lower δ65Cu values, which can be explained by the addition of oxidation of sulfur-bearing fluids during serpentinization. Fe, Zn, and Cu isotopes composition of the serpentinite have great potential in tracing fluid properties and redox states, which is of great significance to the chemical cycle of the crustmantle system. © 2022 Geological Society of China. All rights reserved.