Empirical and experimental constraints on Fe-Ti oxide-melt titanium isotope fractionation factors
Empirical and experimental constraints on Fe-Ti oxide-melt titanium isotope fractionation factors
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Fe-Ti 氧化物熔体钛同位素分馏因子的经验和实验限制
DOI：
10.1016/j.gca.2022.02.011
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发表时间：
2022
影响因子：
5
通讯作者：
Hoare L
中科院分区：
文献类型：
--
作者：
Hoare L
在岩浆分异过程中，长英质岩石的Ti同位素组成较镁铁质岩石重，碱性岩浆的Ti同位素组成较其他岩浆系列重。这两个观察结果被解释为反映优先螯合轻钛同位素的铁钛氧化物，如金红石，钛铁矿和钛磁铁矿。然而，这样的解释，到目前为止，依赖于全岩研究同生岩浆样品和详细的机制的氧化物熔体平衡的Ti同位素组成的岩浆是约束不足。为了解决这个问题，我们已经测量了Ti同位素组成的共存的铁钛氧化物和基块或硅酸盐熔体在两个天然熔岩从对比构造环境（赫德岛和圣托里尼岛），和实验运行的产品（金红石熔体）。所有的Fe-Ti氧化物相的同位素一致地比它们各自的基质或硅酸盐熔体轻，Δ49/47 TiO_x-melt的量值从金红石到钛铁矿和钛磁铁矿增加。在金红石-熔体实验和钛铁矿-基质对之间观察到的Ti同位素分馏的差异主要反映了它们的Tisingle键O键长的小差异，钛铁矿同位素较轻（Δ49/47钛铁矿-熔体超推至1000 K = −0.600 ± 0.035‰）与金红石相比（Δ49/47铁金红石-熔体外推至1000 K = −0.404 ± 0.099‰），这是由于钛铁矿中的Tisingle键O键稍长。相比之下，在钛磁铁矿-基质对之间观察到的Δ49/47 Ti的变化随着钛磁铁矿中TiO 2含量的增加而增加（增加钛尖晶石组分），在富钛磁铁矿（21-23 wt.%）中，Δ49/47 Ti钛磁铁矿-熔体值外推至1000 K范围为−0.811至−1.451‰Heard Island的TiO 2; Usp 66 -73），而Santorini的为−0.673‰至−0.863（14-15 wt.% TiO2; USP 45 -49）。我们解释这是由于一个较弱的和扭曲的晶格，由于在局部阳离子环境的变化，导致交换较小的Fe 3+离子与较大的Fe 2+离子在磁铁矿ulvöspinel固溶体。我们的结果是一致的分馏因素推断的矿物分离的Ti同位素分析和从头计算。我们使用这些分馏因子结合Ti同位素分馏因子的硅酸盐矿物重新计算从以前的研究，沿着与岩石学信息来模拟的行为，Ti同位素在地球地幔部分熔融，以及再现观察到的变化δ49/47 Ti的分化岩浆从不同的地球动力学设置。
The Titanium (Ti) isotope compositions of felsic rocks are heavier than their mafic counterparts, and alkaline magmas develop heavier Ti isotope compositions compared to other magma series during magmatic differentiation. Both observations are interpreted to reflect the preferential sequestration of light Ti isotopes in Fe-Ti oxides, such as rutile, ilmenite and titanomagnetite. However, such interpretations so far rely on whole rock studies of cogenetic magmatic samples and the detailed mechanics of oxide-melt equilibrium on the Ti isotope composition of magmas is poorly constrained. To address this, we have measured the Ti isotope composition of co-existing Fe-Ti oxides and groundmass or silicate melt in both natural lavas from contrasting tectonic settings (Heard Island and Santorini), and experimental run products (rutile-melt). All Fe-Ti oxide phases are consistently isotopically lighter than their respective host groundmass or silicate melt, with the magnitude of Δ49/47Tioxide-meltincreasing from rutile to ilmenite, and titanomagnetite. The observed difference in Ti isotope fractionation between rutile-melt experiments and ilmenite-groundmass pairs is primarily reflective of small differences in their Tisingle bondO bond length, with ilmenite being isotopically lighter (Δ49/47Tiilmenite-meltextrapolated to 1000 K = −0.600 ± 0.035‰) compared to rutile (Δ49/47Tirutile-meltat extrapolated to 1000 K = −0.404 ± 0.099‰) due to slightly longer Tisingle bondO bonds in ilmenite. In contrast, the variation in Δ49/47Ti observed between titanomagnetite-groundmass pairs increases as a function of increasing TiO2content (increasing ulvöspinel component) in titanomagnetite, with Δ49/47Tititanomagnetite-meltvalues extrapolated to 1000 K ranging from −0.811 to −1.451‰ in Ti-rich titanomagnetite (21–23 wt.% TiO2; Usp66-73) from Heard Island compared to −0.673‰ to −0.863 in Santorini (14–15 wt.% TiO2; Usp45-49). We interpret this to result from a weaker and distorted crystal lattice due to changes in the local cationic environment resulting from exchange of smaller Fe3+ions with larger Fe2+ions during magnetite-ulvöspinel solid solution. Our results are consistent with fractionation factors inferred from Ti isotopic analyses of mineral separates and ab-initio calculations. We use these fractionation factors in combination with Ti isotope fractionation factors for silicate minerals recalculated from previous studies, along with petrologic information to model the behaviour of Ti isotopes during partial melting of Earth’s mantle as well as reproducing the observed variation in δ49/47Ti of differentiated magmas from distinct geodynamic settings.