Zinc isotope evidence for carbonate alteration of oceanic crustal protoliths of cratonic eclogites

• Published in: Earth and planetary science letters Vol. 580; p. 117394
• Main Authors: Wang, Ze-Zhou, Liu, Sheng-Ao, Rudnick, Roberta L., Teng, Fang-Zhen, Wang, Shui-Jiong, Haggerty, Stephen E.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 15.02.2022
• Subjects: altered oceanic crust; carbonate recycling; cratonic eclogites; mantle metasomatism; zinc isotopes; carbonate recycling; zinc isotopes; altered oceanic crust; mantle metasomatism; cratonic eclogites
• ISSN: 0012-821X; 1385-013X
• DOI: 10.1016/j.epsl.2022.117394

•Zn isotopes in cratonic eclogites support their origin as carbonated oceanic crust.•Isotopic exchange of AOC protoliths with carbonates results in high-δ66Zn eclogites.•High-MgO eclogites were products of metasomatic overprinting of low-MgO eclogites.•Carbonated eclogites act as a potential source of high-δ66Zn intraplate basalts. Zinc isotopic compositions (δ66ZnJMC-Lyon) of low-MgO (<13 wt.%) and high-MgO (>16 wt.%) eclogites from the Koidu kimberlite complex, Sierra Leone, West African Craton, help constrain the origins of cratonic eclogites. The δ66Zn of low-MgO eclogites range from MORB-like to significantly higher values (0.21‰ to 0.75‰), and correlate inversely with Zn concentrations. Since marine carbonates are characterized by higher δ66Zn and lower Zn concentration than basaltic rocks, the low-MgO eclogites are suggested to originate from altered oceanic crustal protoliths that underwent isotopic exchange with carbonates within the crust during subduction. Compared to low-MgO eclogites, all but one of the high-MgO eclogites also have high δ66Zn (0.35‰ to 0.95‰), but they have lower Zn concentrations and Zn/Fe ratios, both of which are negatively correlated with MgO contents. These features point to formation of high-MgO eclogites via metasomatic overprinting of low-MgO eclogites through addition of secondary clinopyroxenes crystallized from infiltrating ultramafic melts. Thus, both low-MgO and high-MgO eclogites bear the imprint of subducted carbonate-bearing oceanic crust. Our study shows that the distinctively high-δ66Zn signatures of marine carbonates can be retained in deeply subducted oceanic crust that may contribute to mantle sources of intraplate alkali basalts with elevated δ66Zn and Zn/Fe. Therefore, Zn isotopes provide a viable means to trace carbonate recycling in the mantle.