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Zinc (Zn) isotope data for mid-ocean ridge basalts (MORB) are scarce. The possible effects of MORB differentiation and mantle compositional heterogeneity on MORB Zn isotope compositions are poorly understood. In this paper, we report Zn isotope data of MORB lavas from the East Pacific Rise (EPR) at 10°30′N, which are understood to have been derived by a similar extent of melting from a compositionally uniform mantle source but manifest significantly varying extents of magma differentiation (MgO = 7.38–1.76 wt%) to demonstrate unequivocal Zn isotope fractionation during MORB differentiation. These lavas have a small δ66Zn (relative to the JMC-Lyon standard) range of 0.21–0.32‰, with an average of 0.26 ± 0.07‰ (2SD, N = 24), overlapping with literature data. However, δ66Zn values show significant correlations with indices of magma differentiation (e.g., MgO and CaO), with the more differentiated samples having isotopically heavier Zn. We can thus conclude that MORB differentiation can lead to ∼0.1‰ δ66Zn elevation, which is similar to that observed in Hawaiian Kilauea Iki lava lake samples. Fractional crystallization of olivine and clinopyroxene at MgO > ∼3.5 wt% and clinopyroxene and Fe–Ti oxides dominated by ilmenite at MgO < ∼3.5 wt% results in the observed Zn isotope fractionation. The δ66Zn values for primitive N-MORB and MORB source mantle are estimated to be ∼0.22‰ and ∼0.17‰, respectively. Furthermore, we observe a positive correlation between δ66Zn and [La/Sm] N in primitive (MgO = 7.58–9.01 wt%) literature MORB, indicating the existence of possible Zn isotope heterogeneity in the oceanic upper mantle. We thus predict heavy Zn isotope enrichment in mantle sources for E-MORB samples with high [La/Sm] N , which requires future testing. [ABSTRACT FROM AUTHOR]

Redox-driven copper (Cu) isotope fractionation has been widely observed in low-temperature weathering and hydrothermal processes. However, how Cu isotopes may fractionate during magmatic processes remain unknown. To address this issue, we studied MORB samples from the East Pacific Rise (EPR) at 10°30′N to explore the Cu isotope behavior during magma differentiation. These samples are globally ideal because they are generated from a uniformly depleted sub-ridge mantle source but have experienced varying extents of fractional crystallization and sulfide segregation with large variations of MgO (1.76–7.38 wt%) and Cu (16.0–75.2 μg/g). Lavas with MgO ≥ 4.9 wt% involving olivine, clinopyroxene and plagioclase as the major liquidus minerals show homogeneous δ65Cu of 0.05 ± 0.03‰, suggesting little Cu isotope fractionation at this stage of magma differentiation. However, after Fe-Ti oxides appear on the liquidus, melt δ65Cu values first decrease rapidly to –0.41‰ at MgO of 3.9 wt% and then increase with the most evolved sample having δ65Cu of 0.08‰. Such a significant Cu isotope fractionation during magma differentiation has never been reported before. We suggest the large Cu isotope variation at MgO < 4.9 wt% reflect a redox change of MORB melt resulting from fractional crystallization of Fe-Ti oxides. The crystallization of ilmenite (Fe2+TiO 3), which is the first Fe-Ti oxide phase during MORB differentiation, causes a sudden increase of Fe3+/∑Fe in the residual melt and drives the redox reaction Fe3+ + Cu1+ → Fe2+ + Cu2+. As a result, sulfides segregated from the MORB melts have high Cu2+ content and heavy Cu isotope compositions with Δ65Cu Sulfide-Silicate melt > 0 because of the preferential bonding of heavy Cu isotopes (65Cu vs. 63Cu) with Cu2+, whose fractionation rapidly decreases δ65Cu of the residual melts. The increase of Fe3+/∑Fe will quickly drive the melt to be saturated in titanomagnetite (magnetite-Ulvöspinel solid solutions), whose crystallization decreases melt Fe3+/∑Fe and drives the redox reaction Fe2+ + Cu2+ → Fe3+ + Cu1+. As a result, the segregated sulfides after titanomagnetite saturation have decreasing Cu2+ content. These sulfides are also predicted to have low Ni content and exhibit Δ65Cu Sulfide-Silicate melt < 0, whose segregation raises δ65Cu in the residual melts. Therefore, we suggest a significant influence of redox states of Cu and abundance of Ni in the segregated sulfides on the Cu isotope fractionation during MORB differentiation. During mantle melting for MORB at ΔFMQ < 0, Cu isotope fractionation between melt and mantle sulfide is inferred to be limited, and the upper mantle has primitive MORB-like δ65Cu of 0.06 ± 0.05‰. [ABSTRACT FROM AUTHOR]

Studies of mid-ocean ridge basalts (MORB) have revealed a heterogeneous asthenospheric mantle in chemical elements and radiogenic isotopes. Here we report that MORB mantle is also heterogeneous in Fe isotopes through studying the glass samples from seamounts flanking the northern East Pacific Rise between 5° and 15°N. These samples show large Fe isotope variation with δ56Fe values (+0.03‰ to +0.36‰) exceeding the known range of MORB (+0.05‰ to +0.17‰). Such highly varied δ56Fe values cannot be well explained by seafloor alteration, fractional crystallization or partial melting processes, but instead require a source mantle significantly heterogeneous in Fe isotope compositions. Importantly, the δ56Fe values of these basalts correlate significantly with major and trace elements and Sr-Nd-Pb-Hf radiogenic isotopes, reflecting melting-induced mixing of a two-component mantle with the enriched component having heavy Fe isotope compositions dispersed as physically distinct domains in the depleted mantle matrix. The major and trace element characteristics of the enriched mantle component, as inferred from these basalts, are consistent with a low-degree melting origin. Such low-degree melts with heavy Fe isotope compositions most likely formed at sites such as the lithosphere-asthenosphere boundary beneath ocean basins, which can metasomatize the overlying oceanic lithosphere by crystallizing dikes/veins of garnet pyroxenite lithologies. Recycling of these dikes/veins with isotopically heavier Fe can readily contribute to the Fe isotope heterogeneity in the MORB mantle. However, the extremely high primitive δ56Fe values of the two alkali basalts (up to 0.34‰) require an enriched source component with unusually high δ56Fe values. We suggest that partial melts from the recycled dikes/veins of garnet pyroxenite lithologies can react with the ambient peridotitic mantle and generate a secondary garnet pyroxenite with heavier Fe isotope compositions than, but similar radiogenic isotope compositions as its precursor. Melting-induced mixing between these garnet pyroxenites (recycled and newly formed) and depleted mantle matrix can readily explain the compositional variations in elements, radiogenic isotopes and Fe isotopes observed in these seamount lavas. These new data and correlated variations offer a new dimension for understanding the origin of mantle chemical and isotopic heterogeneity in the context of chemical differentiation of the Earth. [ABSTRACT FROM AUTHOR]

Whether the Earth's mantle has a chondritic δ56Fe (deviation in 56Fe/54Fe from the IRMM-014 standard in parts per thousand) value or not remains under debate. The current view is that the observed average δ56Fe of mid-ocean ridge basalts (MORB) cannot be explained by partial melting of mantle source with chondritic value alone. Here, we report Fe isotope compositions on 29 MORB glasses sampled along a flowline traverse across the East Pacific Rise (EPR) axis at 10°30′N. These glasses show large MgO variation (1.8–7.4 wt.%) that forms a compositional continuum resulting from varying extent of fractional crystallization, which is accompanied by systematic Fe isotopic variation. Fractional crystallization modeling suggests that early crystallization of olivine, pyroxene and plagioclase gives rise to an iron enrichment trend and an increase in δ56Fe. Once Fe-Ti oxides appear on the liquidus and begin to crystallize, the FeOt and TiO 2 contents of the residual melt decrease rapidly, which lead to a slight decrease in δ56Fe. These observations indicate that significant Fe isotope fractionation can indeed take place during MORB melt evolution. Hence, δ56Fe values of variably evolved MORB melts do not represent those of primary MORB melts and thus cannot be used to infer mantle source Fe isotope compositions. Importantly, δ56Fe values of primary MORB melts after correction for the effect of fractional crystallization can be well reproduced by mantle melting. Therefore, our study supports the idea that the Fe isotope composition of the accessible Earth is close to be chondritic. We note that conclusion would assume that the core, which takes up ∼90% of the Earth's Fe, must have a chondritic Fe isotope composition. [ABSTRACT FROM AUTHOR]

Studies of mid-ocean ridge basalts (MORB) show a variable Fe isotope composition of the oceanic upper mantle. To test a recent hypothesis that heavy Fe isotope enrichment in the MORB mantle results from the same process of incompatible element enrichment, we conduct an Fe isotope study of well-characterized MORB samples from a magmatically robust segment (OH-1) of the Mid-Atlantic Ridge (MAR) at ∼ 35°N. The data show large Fe isotope variation (δ 56 Fe = +0.03 to +0.18‰) that correlates well with the abundances and ratios of more-to-less incompatible elements and with Sr-Nd-Hf isotopes. Our findings in support of the hypothesis can be detailed as follows: (1) the oceanic upper mantle has a heterogeneous Fe isotope composition on varying small spatial scales with isotopically heavy Fe (high- δ 56 Fe) preferentially associated with pyroxenite lithologies; (2) such lithologies, which are also enriched in the progressively more incompatible elements, are of low-degree (low-F) melt metasomatic origin; (3) with all the conceivable processes considered, the low-F melt metasomatism takes place at the lithosphere-asthenosphere boundary (LAB) beneath ocean basins through crystallization of incipient (Low-F) melt in the seismic low velocity zone (LVZ) at the base of the growing oceanic lithosphere (i.e., LAB) over the Earth's history since the onset of plate tectonics, forming composite lithologies with geochemically enriched pyroxenite veins dispersed in the depleted peridotite matrix; (4) such mantle of composite lithology when transported to beneath the present-day ocean ridges will undergo decompression melting and produce MORB melts with geochemical trends of "melting-induced mixing" as observed at the MAR and global MORB; (5) we predict all this to be a globally common process and widespread. • Large Fe isotope compositional variation is observed in OH-1 MORB lavas from Mid-Atlantic Ridge at ∼35° N. • Heavy Fe isotope enrichment in the MORB mantle results from the low-degree melts metasomatism. • The heavy Fe isotope enrichment process of low-degree melt mantle metasomatism is globally widespread. • Large δ 56 Fe variations in global MORB results from melting-induced mixing of the heterogenous upper mantle. [ABSTRACT FROM AUTHOR]