Titanium isotope fractionation during lunar magmatism - the importance of redox

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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.