Effect of Ti on structure of Ti-bearing sodium aluminosilicate melts using multi-nuclear NMR spectroscopy

Abstract

The evolution of the lunar mantle melts since the moon-forming giant impact event remains poorly understood. The mantle melts contain substantially more Ti compared to melts found on Earth. Therefore, evolution of the Moon might be affected by its Ti-rich mantle, as melt properties like viscosity and density are most likely influenced by these compositional changes. By incorporating Ti in simple Na trisilicate glasses and sodium aluminosilicate glasses, structural changes in this glass can be observed. This allows us to provide more information about the possible influence of Ti on the physical properties of the lunar melts. To obtain structural information from the Ti-bearing Na trisilicate glass, 29Si- 17O- and 27Al high resolution Nuclear Magnetic Resonance(NMR) spectroscopy were used. The 29Si NMR technique reveals detailed structures around the Si environment, particularly Qn species information where n is the number of Bridging Oxygen(BO) of a single Si tetrahedron. The 17O NMR technique provides detailed information about the O-bonds within the glass samples.

In general terms, Ti(network former) replaces the amount of Na(network modifier) in the glass while the amount of Si remains unchanged. For the endmember Na trisilicate(x=0), at -90 ppm and -104 ppm, two peaks appear on the 29Si NMR spectra that represent Q3 and Q4 species respectively where the Q3 peak at -90 ppm is more dominant. This Q3 peak has a Q fraction of around 0.6 while the peak representing Q4 makes up the remaining fraction. Overall, the Q4 peak increases at the expense of the Q3 peak with increasing the Ti content. When 18.75 mol% Ti is incorporated in the glass, the Q species fractionation has completely changed where the Q4 species clearly became the dominant fraction. This suggests that the number of Si surrounded by only BO would increase resulting in an increase of the network polymerization of the melt. From the 17O data a similar observation can be made. The Na-O-Si bonds are gradually replaced by Si-O-Si and Ti-O-Si bonds with increasing Ti content. Also, some fraction of Ti-O-Ti forms. For the aluminosilicate glasses, no Ti-O-Al bond was observed indicating a clear titanium and aluminium avoidance. The obtained structural knowledge regarding the incorporation of Ti on the network polymerization of the silicate glasses, allows us to make a direct link between the Ti content and the viscosity of the melt. Less upper mantle mixing could affect the previously believed timespan of magma ocean cooling to be much longer as heat is meanly being transferred with conduction. Because of the Lunar mantle overturn model driven by the higher density of the IBC layer, Ti coordination is believed to change from [4]Ti to [6]Ti. The Ti becomes a network modifier, lowering the viscosity of the melt with increased mantle mixing in the lower mantle. The simple melt used in this study lays a solid foundation for further research regarding lunar melts with a more realistic melt composition or environmental conditions and could therefore help to constrain the possibilities of lunar mantle dynamics and lunar magma ocean evolution.