Titanium isotope fractionation during lunar magmatism - the importance of redox

钛是一种难熔的亲石元素，因此，它不受岩心形成和行星分化过程中挥发性元素耗尽的影响。因此，在大块硅酸盐地球和月球（BSE和BSM）中任何可观察到的钛同位素变化都应该是这些天体中运行过程的结果。与地球相比，月球的地质历史更简单，因此它早期的月球岩浆活动提供了对羽翼未成熟的地月系统历史的最早认识。月海玄武岩分为低钛和高钛两种。低钛玄武岩是类橄榄岩月幔源的部分熔融产物，而高钛玄武岩则被认为是由富铁钛氧化物杂化源区熔融而成。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更少，以至于除了Ti4+之外，还可能存在很大一部分Ti3+。在硅酸盐熔体和月幔矿物中，钛如何形成和协调的任何依赖氧化还原的变化都可能影响月球岩浆活动中钛同位素的分馏。如果在高钛玄武岩成岩过程中Ti3+与Ti4+解耦，则可能导致钛同位素分馏。最近的观察表明，高钛玄武岩的d49Ti含量高于低钛玄武岩，这似乎支持了这一观点。在岩浆过程中有依赖氧化还原的稳定同位素分馏的先例。例如，陆相玄武岩的d56Fe含量往往高于其地幔源。这一观察结果被解释为玄武岩的铁/亚铁比率与地幔来源的差异。然而，可能使Ti同位素分馏的确切机制尚未得到很好的约束。也就是说，在高钛月幔源部分熔融过程中，氧化还原的作用和哪些相能够分馏出钛同位素尚不清楚。这里提出的研究目的是开展一项结合实验和地球化学的运动，以调查月球岩浆活动导致的钛同位素分馏。具体而言，本研究的目的是再现月球玄武岩的熔融和结晶条件，进而测量高钛玄武岩岩石成因所涉及的所有相的Ti同位素组成。有了这些数据，就有可能确定月球玄武岩中观察到的钛同位素分馏的确切机制。本研究的首要目标是探索来自太阳系不同行星体的Ti同位素的潜在用途，作为这些体中熔化和结晶的氧化还原条件的代理。

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.