Titanium isotope fractionation during lunar magmatism - the importance of redox
Final Report Abstract
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, and because of this, early lunar magmatism offers the earliest possible insights into the history of the fledgling Earth-Moon system. Early lunar evolution was characterized by the fractional crystallization of an early lunar magma ocean (LMO), followed by a period of intense meteorite impacts and mantle overturn. The latter resulted in the extensive melting of the lunar mantle, and the formation of large volumes of lunar basalts. These lunar mare basalts, which are found filling impact basins, 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. There is a precedent for redox-dependent stable isotope fractionation during magmatic processes. For example, terrestrial basalts tend to display higher δ56 Fe 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. If during the petrogenesis of high-Ti basalts, Ti3+ is similarly decoupled from Ti4+ , it could result in the fractionation of Ti isotopes during high-Ti basalt petrogenesis. The recent observation that high-Ti basalts show higher δ49 Ti than low-Ti basalts, whose Ti isotope composition is similar to BSE, appears to support this. However, the exact mechanism that enables this fractionation from taking place 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 analytical campaign to investigate the Ti isotope fractionation that results form 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 assess the potential of using 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.
Publications
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Ti K-edge XANES study on the coordination number and oxidation state of Titanium in pyroxene, olivine, armalcolite, ilmenite, and silicate glass during mare basalt petrogenesis. Contributions to Mineralogy and Petrology, 173(12).
Leitzke, F. P.; Fonseca, R. O. C.; Göttlicher, J.; Steininger, R.; Jahn, S.; Prescher, C. & Lagos, M.
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Unravelling lunar mantle source processes via the Ti isotope composition of lunar basalts. Geochemical Perspectives Letters, 13-18.
Kommescher, S.; Fonseca, R.O.C.; Kurzweil, F.; Thiemens, M.M.; Münker, C. & Sprung, P.
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The redox dependence of titanium isotope fractionation in synthetic Ti-rich lunar melts. Contributions to Mineralogy and Petrology, 176(3).
Rzehak, Laura J. A.; Kommescher, Sebastian; Kurzweil, Florian; Sprung, Peter; Leitzke, Felipe P. & Fonseca, Raúl O. C.
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Redox-dependent Ti stable isotope fractionation on the Moon: implications for current lunar magma ocean models. Contributions to Mineralogy and Petrology, 177(8).
Rzehak, Laura J. A.; Kommescher, Sebastian; Hoare, Liam; Kurzweil, Florian; Sprung, Peter; Leitzke, Felipe P. & Fonseca, Raúl O. C.
