Knowledge of the behavior of magnesium (Mg) isotopes during magmatic differentiation is a prerequisite for applying Mg isotopes as a tracer of crustal recycling and mantle heterogeneity. Crystal fractionation of mafic silicate minerals leads to limited Mg isotope fractionation; hence, Mg isotopic variations in basalts largely result from contributions of subducted crustal materials into their mantle sources. However, recent studies have revealed that the segregation of iron oxides, such as chromite during early-stage basaltic differentiation and titanomagnetite and ilmenite during late-stage differentiation, can potentially modify Mg isotopic compositions of mantle-derived lavas. Titanomagnetite is much more commonly observed in terrestrial basaltic lavas than chromite and ilmenite, and can also form during early-stage basaltic differentiation. To constrain the effects of titanomagnetite crystallization on Mg isotopic systematics of basaltic magma, we report Mg isotopic data for a suite of well-characterized alkaline basaltic lavas (MgO wt% = 4.99 to 6.52) from Gerba Guracha, western Ethiopian Plateau, which have undergone extensive titanomagnetite fractionation. Their δMg values range from −0.29‰ to −0.17‰ and do not correlate with indicators of titanomagnetite fractionation such as TiO and V. These observations suggest that the segregation of titanomagnetite produced limited Mg isotope fractionation, possibly because of its composition-dependent, highly variable ΔMg (ΔMg = δMg − δMg) values. Considering that most iron oxides occur as complex solid solutions that are controlled by the oxygen fugacity of magmas, the effects of oxide segregation on Mg isotopic systematics of basalts require further evaluation.

中文翻译：

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玄武岩熔岩中钛磁铁矿分馏过程中的镁同位素行为

了解岩浆分异过程中镁 (Mg) 同位素的行为是应用镁同位素作为地壳循环和地幔异质性示踪剂的先决条件。镁铁质硅酸盐矿物的晶体分馏导致镁同位素分馏有限；因此，玄武岩中镁同位素的变化很大程度上是由于俯冲的地壳物质对其地幔源的贡献造成的。然而，最近的研究表明，铁氧化物的偏析，例如早期玄武岩分异期间的铬铁矿和晚期分异期间的钛磁铁矿和钛铁矿，可能会改变幔源熔岩的镁同位素组成。钛磁铁矿在陆地玄武岩熔岩中比铬铁矿和钛铁矿更常见，并且也可以在早期玄武岩分化过程中形成。为了限制钛磁铁矿结晶对玄武岩浆镁同位素系统的影响，我们报告了来自埃塞俄比亚高原西部 Gerba Guracha 的一系列特征良好的碱性玄武岩熔岩（MgO wt% = 4.99 至 6.52）的镁同位素数据，这些熔岩经历了广泛的钛磁铁矿分馏。它们的 δMg 值范围为 -0.29‰ 至 -0.17‰，与钛磁铁矿分馏指标（如 TiO 和 V）不相关。这些观察结果表明，钛磁铁矿的偏析产生了有限的 Mg 同位素分馏，可能是因为其成分依赖性、高度变量 ΔMg (ΔMg = δMg – δMg) 值。考虑到大多数铁氧化物以复杂固溶体形式存在，并受岩浆氧逸度控制，氧化物偏析对玄武岩镁同位素系统的影响需要进一步评估。