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The upper mantle is heterogeneous in Fe isotope compositions, but the origin of the heterogeneity needs understanding. Recent studies on oceanic basalts demonstrate that the upper mantle Fe isotope heterogeneity results from low‐degree melt metasomatism. However, whether this hypothesis is of global significance needs testing in continental settings. Here we present an Fe isotopic study on intraplate continental alkali basalts from continental China, which are shown to have derived from upper mantle containing more metasomatized lithologies. The results show first‐order positive correlations of δ56Fe with indices of low‐degree melt metasomatism (e.g., [La/Sm]N), which, together with the so‐far published mid‐ocean ridge basalts data, define global trends, substantiating the global significance of low‐degree melt metasomatism in causing upper mantle Fe isotope variation. We, thus, suggest that low‐degree melt metasomatism is a globally effective process to preferentially concentrate heavy Fe isotopes in metasomatic lithologies at upper mantle conditions both today and in Earth's history. Plain Language Summary: Previous studies have demonstrated that the upper mantle of the Earth has a heterogeneous iron isotope composition, but the cause to such a heterogeneity remains not well understood. By studying oceanic basalts from Mid‐Atlantic Ridge and East Pacific Rise, we suggest that oceanic upper mantle heterogeneous in iron isotope is the result of the mantle metasomatism, which is an enrichment process by low‐degree partial melt at upper mantle condition that progressively enrich more incompatible trace element as well as heavy iron isotopes (57,56Fe vs. 54Fe). However, whether this deep mantle process is globally widespread needs further testing using continental basalts. We here present an iron isotope study on intraplate alkali basalts from eastern continental China. The results are in line with the hypothesis based on the oceanic basalts, substantiating the global significance of melt metasomatism‐induced upper mantle iron isotope fractionation. We further suggest that this deep mantle process fractionate iron isotopes through preferentially concentrating heavy Fe isotopes in metasomatic lithologies at upper mantle conditions, both in Earth's history and at present. Key Points: Iron isotope study on continental alkali basalts verifies the global significance of low‐degree melt metasomatism in causing upper mantle Fe isotope fractionationLow‐degree melt metasomatism is a globally effective process to concentrate heavy Fe isotopes at upper mantle conditionsThe low‐degree melt metasomatism associated Fe isotope fractionation could be both recent and ancient [ABSTRACT FROM AUTHOR]

• New Sr-Nd-Hf isotopic data on MORB lavas from OH-1 and OH-3 segments at the Mid-Atlantic Ridge 33–35°N. • Three mantle components are identified in the mantle source of these MAR lavas. • A Sr-Nd-Hf isotopic view on the origin of geological differences in OH-1 and OH-3 segments. We present new Sr-Nd-Hf isotopic data on mid-ocean ridge basalts from two ridge segments (OH-1, OH-3) between the Oceanographer and Hayes fracture zones at the Mid-Atlantic Ridge 33–35°N to constrain the nature and origin of upper mantle heterogeneity beneath the Mid-Atlantic Ridge. Together with the major and trace elements data (Niu et al., 2001), the new Sr-Nd-Hf isotopic data illustrate that the mantle sources of these lavas comprise three components, i.e., a depleted mantle with high radiogenic Hf isotopic compositions (ADM), an incompatible elemental enriched component with radiogenic Sr and un-radiogenic Nd-Hf isotopic compositions (E-type I), and an incompatible element depleted component with variably enriched Sr-Nd-Hf isotope compositions (i.e., E-type II). The ADM and E-type I components may be understood as representing ancient mantle melting residues and metasomatic veins developed at the base of the thickening oceanic lithosphere, respectively. The unique E-type II component is best explained as recent mantle melting residues of a geochemically enriched mantle, probably associated with the Azores mantle plume to the north. Taking together, we interpret the E-MORB dominated OH-1 lavas as resulting from a relatively higher degree melting of a mantle source composed of ADM matrix and E-type I material, whereas the N-MORB dominated OH-3 lavas resulting from lower degree melting of the mantle source primarily comprising ADM and E-type II components. Such a petrological and geochemical understanding explains the contrast in crustal thickness, ridge morphology and tomography, and mantle Bouguer anomalies between the two ridge segments. [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]