The effect of melt composition in synthetic lunar silicate melts on the behavior of trace elements during lunar magma ocean fractionation
Final Report Abstract
The main aim of Project to determine the behaviour of high-field strength elements (HFSE – Nb, Ta, Hf, Zr, W and Mo), Th and U, during fractional crystallization of a lunar magma ocean (LMO), and subsequent partial melting on lunar mantle cumulates. Specifically, it was the aim of the project to ascertain what effects, if any, the more TiO2-rich lunar basalt compositions, and the more reduced nature of lunar lithologies, has on trace element partitioning behaviour. In order to do this, a detailed and lengthy experimental campaign was carried out to study the partitioning behaviour of the HFSE, Th and U under lunar magmatic conditions and melt compositions. Experiments were carried out at controlled oxygen fugacities and temperature using gas mixing vertical muffle tube furnaces, and via CO-CO2 gas mixtures. The results of the experiments were then used to model how trace element ratios, such as Hf/W, U/W, Th/W, Ta/W and Mo/W varied in lunar melts, to test different lunar magma ocean crystallization models, and to estimate the range of conditions and residual mineral phases present in lunar mantle sources. Results show that both TiO2 content of lunar melts, and their more reduced nature affect trace element behaviour during lunar magmatism, in contrast to what takes place during terrestial magmatic processes. The elevated TiO2 content on lunar melts leads to an overall decrease in the mineral/melt partition coefficients for Ta, Hf, Zr, and Th, but an increase in the compatibility of redox sensitive elements like U, W and Mo, especially under more reducing conditions. When these new partitioning data were applied to melting of lunar cumulates, it became clear that in order to reproduce the aforementioned trace element ratios in lunar mare basalts (e.g. Ta/W, Hf/W, Th/W etc.), more reducing conditions are necessary (ca. IW-2), as well as the presence of residual metal, and an Fe-Ti oxide phase in the lunar mantle sources of high-Ti basalts when compared to those for low-Ti basalts. Moreover, new mineral/melt partitioning data produced at lunar f O2 suggest that Mo is exclusively tetravalent during lunar mantle melting, whereas W is still predominantly hexavalent (notwithstanding a small fraction of W4+). Because of this, Mo behaves as a weakly incompatible element during lunar mantle melting, which explains the lower Mo/W ratios of lunar basalts, when compared to those recorded by terrestrial basalts. Moreover, when these new partitioning data are applied to LMO fractionation models and lunar mantle melting, lunar basalt compositions are only reproduced when one assumes the LMO had siderohile element abundances similar to the bulk silicate Earth (BSE). The results of these models are in good agreement with stable isotope data for refractory elements (Ti, Si, Fe, etc.), which suggests the LMO is BSE-like. Moreover, they imply that, inasmuch as Mo and W are concerned, their nominally low abundances in lunar basalts can be explained by solely their retention during partial melting of a lunar mantle (namely for Mo) that is essentially BSE-like, meaning lunar core formation may not have impacted the abundances of these elements to a significant extent. Overall, all of initial objectives were achieved and the project was successful, both with regard to objectives met and scientific output in the form of publications. The results of the project, as well as the experimental protocols developed and refined during the project work, have been useful in follow-up work. The collective contribution to our understanding of lunar magmatism has been positive, particularly since it complements pre-exiting partitioning data on metal/silicate and sulfide/silicate systems. In particular we now have a better grasp of the processes, phases and conditions that determine HFSE, Th and U behaviour during partial melting and crystallization in the lunar interior. The work carried out within the scope of this project has also opened further avenues of research in the future.
Publications
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2016. The effect of titanium on the partitioning behavior of high-field strength elements between silicates, oxides and lunar basaltic melts with applications to the origin of mare basalts. Chemical Geology 440, 219-238
Leitzke, F.P., Fonseca, R.O.C., Michely, L.T., Sprung, P., Münker, C., Heuser, A. and Blanchard, H.
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2017. Competing effects of crystal chemistry and silicate melt composition on trace element behavior in magmatic systems: insights from crystal/silicate melt partitioning of the REE, HFSE, Sn, In, Ga, Ba, Pt and Rh. Contributions to Mineralogy and Petrology 172, 39
Michely, L.T., Leitzke, F.P., Speelmanns, I.M. and Fonseca, R.O.C.
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2017. Redox dependent behaviour of molybdenum during magmatic processes in the terrestrial and lunar mantle: Implications for the Mo/W of the bulk silicate Moon. Earth and Planetary Science Letters 474, 503-515
Leitzke, F.P., Fonseca, R.O.C., Sprung, P., Mallmann, G., Lagos, M., Michely, L.T. and Münker, C.
