Elemental and isotopic (C, 0, Sr, Nd) compositions of Late Paleozoic carbonated eclogite and marble from the SW Tianshan UHP belt, NW China: Implications for deep carbon cycle

摘要

Graphical abstractDisplay OmittedHighlights•Marbles have variable δ18O values caused by infiltration of external H2O-rich fluids.•Carbonated eclogites have Sr-Nd isotopic signatures inherited from their precursor.•Carbonated eclogites have low δ13C values due to the contribution of organic carbon.AbstractSubduction zones are important for understanding of the global carbon cycle from the surface to deep part of the mantle. The processes involved the metamorphism of carbonate-bearing rocks largely control the fate of carbon and contribute to local carbon isotopic heterogeneities of the mantle. In this study, we present petrological and geochemical results for marbles and carbonated eclogites in the Southwestern Tianshan UHP belt, NW China. Marbles are interlayered with coesite-bearing pelitic schists, and have Sr-Nd isotopic values (εNd(T=320Ma) = −3.7 to −8.9,87Sr/86Sr(i) = 0.7084–0.7089), typical of marine carbonates. The marbles have dispersed low δ18OVSMOWvalues (ranging from 14 to 29‰) and unaffected carbon isotope (δ13CVPDB = −0.2–3.6‰), possibly due to infiltration of external H2O-rich fluids. Recycling of these marbles into mantle may play a key role in the carbon budget and contributed to the mantle carbon isotope heterogeneity.The carbonated eclogites have high Sr isotopic compositions (87Sr/86Sr(i) = 0.7077–0.7082) and positive εNd(T = 320 Ma)values (from 7.6 to 8.2), indicative of strong seafloor alteration of their protolith. The carbonates in the carbonated eclogites are mainly dolomite (Fe# = 12–43, Fe# = Fe2+/(Fe2+ + Mg)) that were added into oceanic basalts during seafloor alteration and experienced calcite – dolomite – magnesite transformation during the subduction metamorphic process. The uniformly low δ18O values (∼11.44‰) of carbonates in the carbontaed eclogites can be explained by closed-system equilibrium between carbonate and silicate minerals. The low δ13C values (from −3.3 to −7.7‰) of the carbonated eclogites most likely reflect contribution from organic carbon. Recycling of these carbonated eclogites with C isotope similar to typical mantle reservoirs into mantle may have little effect on the mantle carbon isotope heterogeneity.