Abstract Titanium offers a burgeoning isotope system that has shown significant promise as a tracer of magmatic processes. Recent studies have shown that Ti isotopes display significant mass-dependent variations linked to the crystallisation of Fe-Ti oxides during magma differentiation. We present a comprehensive set of Ti isotope data for a range of differentiation suites from alkaline (Ascension Island, Afar and Heard Island), calc-alkaline (Santorini) and tholeiitic (Monowai seamount and Alarcon Rise) magma series to further explore the mechanics of Ti isotope fractionation in magmas. Whilst all suites display an increase in δ49/47Ti (deviation in 49Ti/47Ti of a sample relative to the OL-Ti reference material) during magma differentiation relative to indices such as increasing SiO2 and decreasing Mg#, our data reveal that each of the three magma series have contrasting δ49/47Ti fractionation patterns over comparable ranges of SiO2 and Mg#. Alkaline differentiation suites from intraplate settings display the most substantial range of variation (δ49/47Ti = +0.01 to +2.32‰), followed by tholeiites (−0.01 to +1.06‰) and calc-alkaline magmas (+0.06 to +0.64‰). Alkaline magmas possess high initial melt TiO2 contents which enables early saturation of ilmenite + titanomagnetite and a substantial degree of oxide crystallisation, whereas tholeiitic and calc-alkaline suites crystallise fewer oxides and have titanomagnetite as the dominant oxide phase. Positive slopes of FeO*/TiO2 vs. SiO2 during magma differentiation are related to high degrees of crystallisation of Ti-rich oxides (i.e. ilmenite). Bulk solid-melt Ti isotope fractionation factors co-vary with the magnitude of the slope of FeO*/TiO2 vs. SiO2 during magma differentiation.This indicates that the modal abundance and composition of the Fe-Ti oxide phase assemblage, itself is controlled by melt composition, governs Ti isotope fractionation during magma differentiation. In addition to this overall control, hydrous, oxidised calc-alkaline suites display a resolvable increase in δ49/47Ti at higher Mg# relative to drier and more reduced tholeiitic arc suites. These subparallel Ti isotope fractionation patterns are best explained by the earlier onset of oxide segregation in arc magmas with a higher oxidation state and H2O content. This indicates the potential of Ti isotopes to be utilised as proxies for geodynamic settings of magma generation.

中文翻译：

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熔体化学和氧化还原条件控制岩浆分异过程中的钛同位素分馏

摘要 钛提供了一种新兴的同位素系统，已显示出作为岩浆过程示踪剂的重要前景。最近的研究表明，Ti 同位素显示出与岩浆分化过程中 Fe-Ti 氧化物结晶相关的显着质量依赖性变化。我们提供了一系列来自碱性（阿森松岛、阿法尔和赫德岛）、钙碱性（圣托里尼）和拉斑岩（莫诺威海山和阿拉尔康隆起）岩浆系列的一系列分化套件的全面 Ti 同位素数据，以进一步探索岩浆中的 Ti 同位素分馏。虽然所有套件都显示在岩浆分化期间 δ49/47Ti（样品的 49Ti/47Ti 相对于 OL-Ti 参考材料的偏差）相对于指数（例如增加 SiO2 和减少 Mg#）的增加，我们的数据显示，在 SiO2 和 Mg# 的可比范围内，三个岩浆系列中的每一个都具有对比的 δ49/47Ti 分馏模式。来自板内环境的碱性分化套件显示出最大的变化范围（δ49/47Ti = +0.01 至 +2.32‰），其次是拉斑岩（-0.01 至 +1.06‰）和钙碱性岩浆（+0.06 至 +0.64‰） . 碱性岩浆具有较高的初始熔体 TiO2 含量，这使得钛铁矿 + 钛磁铁矿早期饱和，并有相当程度的氧化物结晶，而拉斑岩和钙碱性岩浆结晶较少的氧化物，并以钛磁铁矿作为主要氧化物相。在岩浆分异过程中，FeO*/TiO2 与 SiO2 的正斜率与富钛氧化物（即钛铁矿）的高度结晶有关。在岩浆分异过程中，固体-熔体 Ti 同位素分馏因子与 FeO*/TiO2 与 SiO2 的斜率大小共变。这表明 Fe-Ti 氧化物相组合的模态丰度和组成本身受以下因素控制熔体成分，在岩浆分异过程中控制 Ti 同位素分馏。除了这种总体控制之外，含水氧化钙碱性套件在较高 Mg# 下显示出 δ49/47Ti 的可分辨增加，相对于较干燥和更多减少的拉斑弧套件。这些次平行的 Ti 同位素分馏模式最好用氧化态和 H2O 含量较高的弧形岩浆中较早开始的氧化物偏析来解释。这表明 Ti 同位素有可能被用作岩浆生成的地球动力学环境的替代物。