Redox and mineral controls on Fe and Ti isotopic fractionations during calc-alkaline magmatic differentiation
Redox and mineral controls on Fe and Ti isotopic fractionations during calc-alkaline magmatic differentiation
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DOI：
10.1016/j.gca.2023.06.016
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发表时间：
2023-08
影响因子：
5
通讯作者：
Aleisha C. Johnson;Zhe J. Zhang;N. Dauphas;R. Rudnick;J. Foden;Magali Toc
中科院分区：
文献类型：
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作者：
Aleisha C. Johnson;Zhe J. Zhang;N. Dauphas;R. Rudnick;J. Foden;Magali Toc
印度尼西亚林贾尼火山钙碱性分异套中熔岩和相应矿物分离物的钛和铁同位素组成表明，矿物和熔体之间的铁和钛同位素分馏低于其他套中记录的所有分异阶段的含量。Ti的有限同位素分馏可能是由于低Ti磁铁矿和单斜辉石是Rindjani熔岩中Ti的主要载体，因为这些矿物被认为相对于硅酸盐岩浆具有有限的平衡Ti同位素分馏。其他岩浆分异套控制的去除富钛磁铁矿和单斜辉石的作用较小的特点有较大的Ti同位素分馏。这种效应是钙碱性岩浆如Rindjani的Fe 3 +/Fe 2+比值升高的间接结果，这促进了Fe 3+以Fe 2 +-Ti4+对为代价掺入磁铁矿中，从而增加氧逸度将抑制全球岩浆系列中的Ti同位素分馏。同样，我们发现可以忽略不计的铁同位素分馏Rindjani散装岩石和矿物分离，与以往的研究不同。这也可能是由于Rindjani分异套件的氧化性质，这导致熔体和矿物中相似的Fe 3 +/Fe 2+比，并降低了整体矿物熔体Fe分馏因子。因此，配对的Ti和Fe同位素分析可能是一个强大的工具，以评估氧逸度在分化过程中，独立于喷发样品的Fe 3+测定。
Titanium and Fe isotopic compositions of lavas from a calc-alkaline differentiation suite and corresponding mineral separates from the Rindjani Volcano, Indonesia show that Fe and Ti isotopic fractionations between minerals and melts are lower than those recorded in other suites at all stages of differentiation. The limited isotopic fractionation for Ti is likely due to low-Ti magnetite and clinopyroxene being the dominant carriers of Ti in Rindjani lavas, as these minerals are thought to have limited equilibrium Ti isotopic fractionation relative to silicate magmas. Other magmatic differentiation suites controlled by removal of Ti-rich magnetite and characterized by a lesser role of clinopyroxene have larger Ti isotopic fractionations. This effect is an indirect consequence of the elevated Fe3+/Fe2+ratio of calc-alkaline magmas such as Rindjani, which promotes Fe3+incorporation into magnetite at the expense of Fe2+-Ti4+pairs, such that increased oxygen fugacity will subdue Ti isotopic fractionation in global magmatic series. Similarly, we find negligible Fe isotopic fractionation in Rindjani bulk rocks and mineral separates, unlike previous studies. This is also likely due to the oxidized nature of the Rindjani differentiation suite, which leads to similar Fe3+/Fe2+ratios in melt and minerals and decreases overall mineral-melt Fe fractionation factors. Paired Ti and Fe isotopic analyses may therefore represent a powerful tool to assess oxygen fugacity during differentiation, independent from Fe3+determinations of erupted samples.