Titanium isotope fractionation during lunar magmatism - the importance of redox

钛是一种难熔的亲石元素，正因为如此，它不受行星分化过程中核心形成和挥发性元素耗尽的影响。因此，在块状硅酸盐地球和月球（BSE和BSM）中任何可观察到的Ti同位素变化都应该是这些天体运行过程的结果。与地球相比，月球的地质历史更简单，因此其早期的月球岩浆活动提供了对初出茅庐的地月系统历史的最早可能的见解。月海玄武岩又分为低钛和高钛两种。而低钛玄武岩的部分熔融产物的橄榄岩类月球地幔来源，高钛玄武岩被认为是由于熔融的杂交，铁钛氧化物丰富的源区。重要的是，高钛玄武岩的地幔源被认为比低钛玄武岩的地幔源还原得更少，以至于除了Ti 4+之外，还可能存在显著比例的Ti 3+。钛在硅酸盐熔体和月球地幔矿物中的形态和配位的任何氧化还原依赖的变化可能会影响钛同位素在月球岩浆活动期间的断裂。如果在高钛玄武岩的成岩过程中，Ti ~（3+）与Ti ~（4+）发生解耦，则可能导致Ti同位素的分馏。最近的观察，高钛玄武岩显示更高的d49 Ti比低钛玄武岩似乎支持这一点。在岩浆过程中，氧化还原相关的稳定同位素分馏是有先例的。例如，陆地玄武岩倾向于显示出比其地幔源更高的d56 Fe。这一观察结果被解释为玄武岩的三价铁/二价铁比值与其地幔源相比存在差异的结果。然而，可能使Ti同位素分馏的确切机制没有得到很好的约束。也就是说，氧化还原的作用和什么阶段是能够分馏Ti同位素在部分熔融的高Ti月球地幔sources.It的目的是在这里提出的研究进行联合实验和地球化学活动，调查Ti同位素分馏，结果从月球岩浆活动。具体来说，它是本研究的目的是再现月球玄武岩的熔融和结晶的条件，然后测量的Ti同位素组成的所有阶段参与高Ti玄武岩岩石成因。有了这些数据，将有可能确定在月球玄武岩中观察到的Ti同位素分馏的确切机制。这项研究的总体目标是探索潜在的使用Ti同位素的阶段从分化的行星机构在太阳系中作为一个代理的氧化还原条件，主持熔化和结晶在这些机构。

Titanium is a refractory lithophile element, and, because of this, it is unaffected by core formation and volatile element depletion during planetary differentiation. As such, any observable Ti isotope variations in the bulk silicate Earth and Moon (BSE and BSM) should be the result of processes operating in these bodies. When compared to Earth, the Moon has a simpler geological history, thus its early lunar magmatism offers the earliest possible insights into the history of the fledgling Earth-Moon system. Lunar mare basalts are sub-divided into low- and high-Ti varieties. Whereas low-Ti basalts are the partial melting products of peridotite-like lunar mantle sources, high-Ti basalts are thought to result from melting of hybridized, Fe-Ti oxide-rich source regions. Importantly, the mantle sources of high-Ti basalts are thought to be more reduced than those of low-Ti basalts, so much so that a significant fraction of Ti3+ could be present in addition to Ti4+. Any redox-dependent changes on how Ti is speciated and coordinated in silicate melt and lunar mantle minerals may affect how Ti isotopes fractionate during lunar magmatism. If during the petrogenesis of high-Ti basalts Ti3+ is decoupled from Ti4+ it could result in the fractionation of Ti isotopes. The recent observation that high-Ti basalts show higher d49Ti than low-Ti basalts appears to support this. There is a precedent for redox-dependent stable isotope fractionation during magmatic processes. For example, terrestrial basalts tend to display higher d56Fe than their mantle sources. This observation has been interpreted to be the result of differences between the ferric/ferrous iron ratios of basalts when compared to their mantle sources. However, the exact mechanism that potentially enables Ti isotope fractionation is not well constrained. Namely, the role of redox and what phases are capable of fractionating Ti isotopes during partial melting of high-Ti lunar mantle sources is not known.It is the aim of the research proposed here to carry out a combined experimental and geochemical campaign to investigate the Ti isotope fractionation that results from lunar magmatism. Specifically, it is the aim of this research to reproduce the conditions of melting of lunar basalts and their crystallization, and then to measure the Ti isotope composition of all phases involved in high-Ti basalt petrogenesis. With these data, it will be possible to identify the exact mechanism responsible for the observed Ti isotope fractionation in lunar basalts. The overarching objective of this research is to explore the potential use of Ti isotopes in phases from differentiated planetary bodies in the solar system as a proxy for the redox conditions that preside over melting and crystallization in these bodies.