Chemical and lithological heterogeneity of upwelling mantle plumes inducing Zn isotope variation in Ocean Island Basalts

Abstract

Ocean Island Basalts, generated by partial melting of mantle plumes, show large δ⁶⁶Zn variations (up to 0.22 ‰). Modelling indicates δ⁶⁶Zn variations in melts generated by melting of solely CO₂-free peridotite, within melting range of OIBs (2–8 %), are very limited (∼0.02 ‰), excluding melting processes as the major control. Instead, large δ⁶⁶Zn variations observed in OIBs are more likely attributed to source heterogeneity. Here we compile Zn isotopes and elemental ratios (CaO/Al₂O₃ and Zn/Fe) of globally distributed, nearly primitive OIBs, including new data from Pitcairn and Samoa lavas, to further clarify the relationship between Zn isotopic variations in OIBs and chemical heterogeneity of mantle plumes. Regarding their Zn isotopic variability, global OIBs can be divided into three groups: (1) Low-δ⁶⁶Zn group, including Iceland and Kilauea lavas, displays MORB-like δ⁶⁶Zn (0.27 ± 0.03 ‰), CaO/Al₂O₃ and Zn/Fe, suggesting a source of CO₂-free peridotite. (2) Samples from Crozet, Madeira, Louisville and Samoa have slightly higher δ⁶⁶Zn (0.32 ± 0.06 ‰) and Zn/Fe than those of MORBs, indicating the presence of MORB-like pyroxenite in their sources. (3) OIBs from Cook-Austral, St. Helena, Koloa and Pitcairn show significantly higher δ⁶⁶Zn values (0.38 ± 0.05 ‰) than MORBs and other OIBs. Besides of their heavy Zn isotopes, they have distinct CaO/Al₂O₃ from MORBs and other OIBs, suggesting different source lithologies. Cook-Austral and St. Helena lavas have high CaO/Al₂O₃ and MORB-like Zn/Fe, suggesting carbonated peridotite source. Koloa lavas' high CaO/Al₂O₃ plus super-MORB Zn/Fe suggest carbonated pyroxenite source, whereas Pitcairn lavas' low CaO/Al₂O₃ and super-MORB Zn/Fe indicate a decarbonated pyroxenite source. Therefore, Zn isotopic variations in OIBs are predominantly controlled by recycled carbonate-related heterogeneity within mantle plumes.