Large Zn isotope variations in lower oceanic crust offer insights into oceanic crustal magma plumbing system

Constraining mid-ocean ridge magmatic processes is crucial for understanding the geochemical evolution of magmas traversing the oceanic crust. A crucial archive for investigating these magmatic processes is the chemical composition of lower oceanic crust. This study presents Zn isotope data for primitive to highly evolved cumulates from the Mid-Atlantic Ridge (Kane) and the Southwest Indian Ridge (IODP Hole U1473A), which are compared with published mid-ocean ridge (MOR) lava data. Previous studies have indicated that high-degree magma differentiation results in small Zn isotope fractionation (<0.1 ‰) in the mafic to felsic MOR lavas. Unlike the MOR lavas, significant δ⁶⁶Zn variations (0.1 − 0.5 ‰) are observed in the Kane and U1473A cumulates, which cannot be explained by crystal accumulation alone but reflect the variable incorporation of trapped melts. These trapped melts exhibit heavy δ⁶⁶Zn (>0.5 ‰) and strongly fractionated trace element characteristics, indicative of extensive melt-rock interactions. Furthermore, the bulk normal mid-ocean ridge basalt (N-MORB)-type lower oceanic crust has δ⁶⁶Zn values (0.24 ± 0.11 ‰) that are close to the average N-MORB (0.26 ± 0.06 ‰), suggesting minor Zn isotope fractionation during the crustal processing of MOR lavas. It is proposed that the small δ⁶⁶Zn variations in MOR magmas result from the homogenization and dilution of reactive signals within the eruptive reservoir. In contrast, prolonged processing of trapped melts through melt-rock interactions leads to significant Zn isotope fractionation in the lower oceanic crust. Thus, the evolution of oceanic crustal magma plumbing systems is more complex than inferred from MORB alone. Despite these complexities, efficient homogenization within eruptive reservoirs results in limited Zn isotopic variations in the erupted magmas. Hence, the Zn isotopic variations in MORB, except for the highly evolved ones, are predominantly linked to the source heterogeneity. Our findings also suggest heterogeneous δ⁶⁶Zn in subducted oceanic crusts, which has implications for mantle Zn isotope heterogeneity.