The effect of Fe–Ti oxide separation on iron isotopic fractionation during basalt differentiation
The effect of Fe–Ti oxide separation on iron isotopic fractionation during basalt differentiation
复制标题
DOI：
10.1007/s00410-022-01967-w
复制
发表时间：
2022-10
影响因子：
3.5
通讯作者：
Jian Zhao;Xiao‐Jun Wang;Li‐Hui Chen;T. Hanyu;Jin-hua Shi;Xiao-Wen Liu;H. Kawabata;Liewen Xie
中科院分区：
文献类型：
--
作者：
Jian Zhao;Xiao‐Jun Wang;Li‐Hui Chen;T. Hanyu;Jin-hua Shi;Xiao-Wen Liu;H. Kawabata;Liewen Xie
岩浆分异过程中铁钛氧化物的分离对残余岩浆铁同位素演化具有重要控制作用。钛磁铁矿（Tmag）是演化玄武岩中常见的一种铁钛氧化物斑晶相，但其分离对残留熔体Fe同位素演化的影响仍知之甚少。在这里，我们探讨这个问题与一套同生碱性火山岩（范围从picrobaltite粗安岩）和钛磁铁矿斑晶从圣赫勒拿岛（南大西洋）的铁同位素研究。结果表明，全岩δ 57 Fe值随MgO的减少而在0.04至0.32‰之间变化，富钛尖晶石（X′usp <$0.6）的钛磁铁矿斑晶的δ 57 Fe始终低于相应的整体样品，平均Δ 57 FeTmag −全岩（δ 57 FeTmag − δ 57 Fe全岩）值为− 0.05 ± 0.02‰（2SD，N= 6）。这个数值与这些钛磁铁矿斑晶的推测结晶温度（~ 1100 ± 50 °C）一起确定了平衡钛磁铁矿-熔体分馏因子Δ 57 FeTmag-melt =（− 0.094 ± 0.038）× 106/T2。定量计算涉及这个同位素分馏因子和以前建议的橄榄石熔体和单斜辉石熔体同位素分馏因子可以很好地再现圣赫勒拿熔岩的Fe同位素演化。在钛磁铁矿饱和前（MgO > 5wt%），Fe同位素变化主要由橄榄石和单斜辉石的分离结晶和富集作用决定，而在钛磁铁矿饱和后，Fe同位素变化主要由钛磁铁矿、橄榄石和单斜辉石的多相分离结晶作用决定。本研究结合已发表的其他Fe-Ti氧化物的矿物熔体分馏因子表明，去除富钛尖晶石（X′usp > 0.5）的钛磁铁矿和近纯钛铁矿导致演化岩浆中δ 57 Fe的增加，而近纯磁铁矿的分离使残余熔体朝向较轻的Fe同位素组成。
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.