The potential of Ca isotopes to trace subducted marine carbonates in deep mantle

Marine carbonates, the major carrier of carbon upon the upper crust, can be subducted into the Earth’s interior along with oceanic crust, and then returned to the surface through magmatism, which constitute the deep carbon cycle. This process plays an important role in modulating the CO₂ concentrations in the atmosphere over geologic time, and thus the forming of the habitable earth. Therefore, identifying recycled marine carbonates in the mantle is critical to well understand the global deep carbon cycle. Calcium is one of the major constituent cations in marine carbonates and its isotopes may be a potential tracer for recycled marine carbonates in the mantle. To further evaluate the capability and challenges of Ca isotopes as such a geochemical tracer, we reviewed the Ca isotopic compositions in important reservoirs and the behavior of Ca isotopes during high-temperature geological processes that are related to the deep carbon cycle, including plate subduction, mantle metasomatism, mantle partial melting, magma differentiation, etc. Available studies show that carbonate-rich marine sediments have significantly lower δ^44/40Ca than the Earth mantle, and metasomatism by such recycled materials can cause lighter Ca isotopic compositions in deep mantle-derived rocks than those of the depleted mantle and mid-ocean ridge basalts. However, the Ca isotopic fractionation during partial melting of mantle peridotites is small (∼0.10‰) and the Ca isotopic fractionation during plate subduction and intermediate-mafic magma evolution is indistinguishable. These investigations suggest that Ca isotopes have great advances in tracing such recycled materials in the mantle. However, other processes (such as the influence by partial melts of eclogites) may induce similar effects on mantle-derived rocks as subducted marine carbonates but still remains debated, and thus further investigations are strongly needed in the future.