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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]

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]

We present geochemical and age data for coeval basalt and rhyolite in the Yinshan Block, western North China Craton, to study the origin of the ‘Daly Gap’ in this bimodal volcanic suite. SHRIMP zircon U-Pb dating for felsic rocks and bulk rock 40 Ar/ 39 Ar dating for mafic rocks yield their ages of 107–133 Ma. The basalts show elevated abundances of incompatible elements with [La/Sm] N  = 2.65–3.54 and enriched Sr-Nd-Hf isotopic signatures ( 87 Sr/ 86 Sr(t) = 0.7054–0.7084, 143 Nd/ 144 Nd(t) = 0.5119–0.5123, 176 Hf/ 177 Hf(t) = 0.2824–0.2826), consistent with their derivation from low degree melting of previously metasomatized lithospheric mantle. The rhyolites display similar enrichment in alkalis and other incompatible elements, but have obvious Nb-Ta depletion. Combined with their more enriched Nd-Hf isotopic compositions (ε Nd (t) = −12 to −18, ε Hf (t) = −2 to −14), these rhyolites are inferred to have been generated by mixing of the mantle-derived basaltic magma with felsic melts derived from lower continental crust anataxis induced by the basaltic magma. Furthermore, the extremely low Ba, Sr, Eu contents, high Rb/Sr ratios and unexpectedly high 87 Sr/ 86 Sr (0.7920–0.7159) in these felsic samples also suggests that their parental magma experienced protracted fractional crystallization of plagioclase (K-feldspar to a lesser extent) en route to the surface. This rock association and the unique geochemical characteristics of these volcanic rocks offer a new view on the petrogenesis of bimodal volcanism: the mafic rocks originated from partial melting of the ancient lithospheric mantle. Underplating of the basaltic magmas triggered melting of lower continental crust. The extensive fractional crystallization of the hybridized magma resulted in the highly fractionated rhyolites. This mechanism explains the “Daly Gap” exhibited in this rock association in a continental setting and also the observation that both the lithospheric mantle and crust in East China are thinner than that in West China. [ABSTRACT FROM AUTHOR]