The lithium isotopic composition and geochemical implication of ultrahigh-pressure marbles from the Dabie-Sulu orogen, China

• Published in: Journal of Asian earth sciences Vol. 195; p. 104376
• Main Authors: Wan, Hongqiong, Xiao, Yilin, Sun, He, Liu, Haiyang, Wang, Yangyang, Li, Dongyong, Li, Wang-ye
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 15-06-2020
• Subjects: Dabie-Sulu orogen; Deep carbon cycling; Lithium isotopes; Subduction; UHP marble; Deep carbon cycling; Lithium isotopes; UHP marble; Dabie-Sulu orogen; Subduction
• ISSN: 1367-9120; 1878-5786
• DOI: 10.1016/j.jseaes.2020.104376

Subduction zones are important in materials’ cycling between Earth’s surface and mantle. As Li isotopic compositions of sedimentary carbonates (δ7Li = 5.2‰–31‰) differing markedly from those of the mantle (2.0‰–5.7‰), it is important to know if Li isotopic compositions of carbonates could be used to tracing deep carbon cycling. Here we report that meta-sedimentary carbonates could preserve their original heavy Li isotopic signatures (with δ7Li values up to 21.5‰) when subducted from surface to the mantle depths (the yellow arrow) and that the related carbonatitic or carbonate-rich silicate melts may retain various degrees of heavy Li isotopic signatures locally in the mantle (the yellow arrow). We infer that some mantle-derived magmas may have recorded the elevated δ7Li signatures induced by recycled carbonates (the yellow arrow) and thus Li isotopic geochemistry could be useful in tracing deep carbon cycling.
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•We firstly investigated the Li isotopic compositions of UHP marbles.•The δ7Li of UHP marbles is heavier than that of the upper mantle.•UHP marbles could form heavy Li isotopic heterogeneity in local mantle.•Li isotopes of UHP marbles are potential tools for tracing subduction and material-cycling.
Subduction zones are important in materials’ cycling between Earth’s surface and mantle. We report lithium (Li) contents and isotopic compositions (δ7Li) of ultrahigh-pressure (UHP) marbles of the Dabie-Sulu orogen, with the aim of elucidating the geochemical behavior of Li isotopes during deep subduction of surficial carbonates and exploring the application of Li isotopes in tracing subduction and material-cycling. Pure UHP marbles (with δ7Li values up to 21.5‰) show significantly heavier isotopic compositions than those of the mantle (2.0‰–5.7‰; average 3.7‰), while impure marbles containing some silicates have δ7Li values as low as 1.3‰. Weathering, fluid infiltration, and isotopic fractionation have negligible effects on the Li isotopic compositions of UHP marbles. Exhumed UHP marbles might be good samples to study those residual carbonates entering into the deep mantle. When subducted carbonates transport heavy Li isotopic signatures to mantle depths, they may result in local Li isotopic heterogeneity. As Li is a trace element and UHP marbles have significantly different δ7Li signatures to the mantle, Li isotopic geochemistry might be useful in tracing deep carbon cycling.