Search

Separation of Fe–Ti oxides during magmatic differentiation plays an important role in controlling Fe isotopic evolution of residual magmas. Titanomagnetite (Tmag) is a common Fe–Ti oxide phenocryst phase in evolved basaltic lavas, but the effect of its separation on Fe isotopic evolution of residual melts remains poorly understood. Here we explore this issue with an Fe isotopic study on a suite of cogenetic alkaline volcanic rocks (range from picrobasalt to trachyandesite) and their titanomagnetite phenocrysts from St. Helena Island (South Atlantic). Results show that whole-rock δ57Fe values vary from 0.04 to 0.32‰ with decreasing MgO, and ulvöspinel-rich (X′usp ≈ 0.6) titanomagnetite phenocrysts have consistently lower δ57Fe than corresponding bulk samples with an average Δ57FeTmag−whole rock (δ57FeTmag − δ57Fewhole rock) value of − 0.05 ± 0.02‰ (2SD, N = 6). This value together with the speculated crystallization temperatures (~ 1100 ± 50 °C) of these titanomagnetite phenocrysts determine an equilibrium titanomagnetite-melt fractionation factor of Δ57FeTmag-melt = (− 0.094 ± 0.038) × 106/T2. Quantitative calculations involving this isotopic fractionation factor and previously suggested olivine-melt and clinopyroxene-melt isotopic fractionation factors can well reproduce the Fe isotopic evolution of St. Helena lavas. Specifically, the Fe isotopic variation before titanomagnetite saturation (MgO > 5 wt%) is dominated by fractional crystallization and accumulation of olivine and clinopyroxene, while that after titanomagnetite saturation is determined by fractional crystallization of multiphases including titanomagnetite, olivine and clinopyroxene. This study, combined with published mineral-melt fractionation factors for other Fe–Ti oxides, indicates that the removal of ulvöspinel-rich (X′usp > 0.5) titanomagnetite and near-pure ilmenite results in an increase of δ57Fe in evolved magmas, whereas separation of near-pure magnetite drives residual melt towards lighter Fe isotopic compositions. [ABSTRACT FROM AUTHOR]

• First study of Mg isotopic variation during evolution of cogenetic alkaline basalts. • Detectable Mg isotopic fractionation occurred during late-stage basalt differentiation. • Decrease of δ 26 Mg values is accompanied by decreasing TiO 2 content and Ti/Ti* ratio. • Segregation of Fe–Ti oxides significantly affects the δ 26 Mg value of MgO-poor magma. Magnesium isotopic fractionation during basalt differentiation is commonly thought to be negligible at current levels of analytical precision. However, Mg isotopic variation of ∼ 0.4 ‰ (δ 26 Mg) can be observed in some highly evolved volcanic rocks with MgO contents of <5 wt.%, suggesting that detectable Mg isotopic fractionation may occur during late-stage basalt differentiation. Here we examine this possibility with a Mg isotopic study of a suite of well-characterized cogenetic alkaline volcanic rocks from St. Helena Island (South Atlantic), which vary from primitive nepheline-normative basalt to highly evolved trachyandesite with MgO contents decreasing from 15.72 wt.% to 0.81 wt.%. Our results show that the basalt samples which only experienced segregation of olivine and clinopyroxene have a narrow δ 26 Mg range (−0.23‰ to −0.32‰), while the evolved rocks saturated with Fe–Ti oxides (MgO < 5 wt.%) display larger Mg isotopic variation with δ 26 Mg vary to higher (− 0.14 ‰) or lower (− 0.36 ‰) values relative to mantle value (δ 26 Mg = − 0.25 ± 0.04 ‰). For most of the Fe–Ti oxide saturated samples, their δ 26 Mg values are positively correlated with TiO 2 contents and Ti/Ti* ratios and negatively correlated with total-alkali contents. This indicates that detectable Mg isotopic fractionation occurred in MgO-poor samples, probably through fractional crystallization of Mg-bearing Fe–Ti oxides. Further Mg isotopic analysis of Fe–Ti oxide phenocrysts (titanomagnetite) in the evolved samples reveals that titanomagnetite has remarkably higher δ 26 Mg value (+0.15‰ to +0.52‰) than the corresponding bulk sample and silicate minerals. Fractional crystallization of such isotopically heavy titanomagnetite is thus expected to drive the residual magma progressively enriched in light Mg isotopes. In order to reproduce the observed Mg isotopic variation in St. Helena evolved samples, quantitative modeling requires a titanomagnetite-melt Mg isotopic fractionation factor (Δ 26 Mg Ti-Mgt−melt) of ∼0.6 ‰, which is among our measured δ 26 Mg difference value (0.41–0.73‰) between titanomagnetite and bulk samples. When combining our results with published data for alkaline lavas from the Antipodes volcano (New Zealand), contrasting patterns of Mg isotope fractionation emerge during late-stage basalt differentiation. Quantitative modeling shows that the positive and negative δ 26 Mg–MgO correlations shown by evolved lavas could be related to segregation of isotopically heavy titanomagnetite and isotopically light ilmenite, respectively, depending on redox state of the magma system. Therefore, this study highlights that basalt differentiation involving separation of Fe–Ti oxides may induce resolvable Mg isotopic fractionation, and the δ 26 Mg values of compositionally evolved rocks should be used with caution in studies of petrogenesis. [ABSTRACT FROM AUTHOR]