Magnesium isotopic fractionation during basalt differentiation is commonly thought to be negligible at current levels of analytical precision. However, Mg isotopic variation of $\sim 0.4\mathrm{‰}$ (${\delta }^{26}$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 ${\delta }^{26}$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 ${\delta }^{26}$Mg vary to higher ($-0.14\mathrm{‰}$) or lower ($-0.36\mathrm{‰}$) values relative to mantle value (${\delta }^{26}\mathrm{Mg}=-0.25\pm 0.04\mathrm{‰}$). For most of the Fe–Ti oxide saturated samples, their ${\delta }^{26}$Mg values are positively correlated with TiO₂ 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 ${\delta }^{26}$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 (${\mathrm{\Delta }}^{26}$Mg_{Ti-Mgt−melt}) of ∼0.6 ‰, which is among our measured ${\delta }^{26}$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 ${\delta }^{26}$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 ${\delta }^{26}$Mg values of compositionally evolved rocks should be used with caution in studies of petrogenesis.

在目前的分析精度水平上，玄武岩分化过程中的镁同位素分馏通常被认为可以忽略不计。但是，镁同位素的变化$〜0.4\mathrm{‰}$ （${\delta }^{26}$MgO含量小于5 wt。％的一些高度演化的火山岩中可以观察到Mg），这表明在玄武岩后期分化过程中可能发生可检测到的Mg同位素分馏。在这里，我们通过对来自圣赫勒拿岛（南大西洋）的一系列特征明确的成岩碱性火山岩的镁同位素研究来检验这种可能性，该岩岩从原始霞石-规范玄武岩到高度演化的菱锰矿，其MgO含量从15.72 wt％降低。 0.1％至0.81％重量。我们的结果表明，仅经历了橄榄石和clinopyroxene分离的玄武岩样品具有较窄的${\delta }^{26}$Mg范围（-0.23‰至-0.32‰），而Fe-Ti氧化物（MgO <5 wt。％）饱和的演化岩石显示出较大的Mg同位素变化， ${\delta }^{26}$镁含量更高（$-0.14\mathrm{‰}$）或更低（$-0.36\mathrm{‰}$）相对于地幔值（${\delta }^{26}\mathrm{镁}=-0.25\pm 0.04\mathrm{‰}$）。对于大多数Fe-Ti氧化物饱和样品，${\delta }^{26}$Mg值与TiO ₂含量和Ti / Ti *比呈正相关，与总碱含量呈负相关。这表明在贫MgO样品中发生了可检测到的Mg同位素分级分离，这可能是由于含Mg的Fe-Ti氧化物的分级结晶所致。析出样品中的Fe-Ti氧化物隐晶（钛磁铁矿）的Mg同位素分析表明，钛磁铁矿显着更高${\delta }^{26}$Mg值（+ 0.15‰至+ 0.52‰）高于相应的大块样品和硅酸盐矿物。因此，预期这种同位素重的钛磁铁矿的分数结晶将驱使残余的岩浆逐渐富含轻的Mg同位素。为了重现在圣赫勒拿岛演化样品中观察到的Mg同位素变化，定量建模需要使用钛磁铁矿-熔解的Mg同位素分馏因子（${\mathrm{\Delta }}^{26}$Mg _Ti-Mgt-melt）约为0.6‰，这是我们测得的值${\delta }^{26}$钛磁铁矿和块状样品之间的镁差值（0.41-0.73‰）。当将我们的结果与来自Antipodes火山（新西兰）的碱性熔岩的公开数据相结合时，在后期玄武岩分化过程中会出现对比的Mg同位素分馏模式。定量建模表明，正面和负面${\delta }^{26}$根据岩浆系统的氧化还原状态，演化的熔岩显示出的Mg-MgO相关性可能分别与同位素重的钛磁铁矿和同位素轻的钛铁矿的偏析有关。因此，这项研究强调了涉及Fe-Ti氧化物分离的玄武岩分化可能诱导可分辨的Mg同位素分馏，并且${\delta }^{26}$在成岩研究中，应谨慎使用成分演化的岩石的Mg值。