Titanium Oxidation State and Coordination in the Lunar High-titanium Glass Source Mantle

Introduction: Lunar high-titanium (HiTi) ultra-mafic glasses provide us with evidence of the mantle processes that led to the melting of the lunar magma ocean cumulates nearly one billion years after the magma ocean solidified. Constraints on the depth, temperature and melting processes that formed the HiTi glasses are crucial for understanding the melting history of LMO products. The Apollo 17 orange glass (A17O) and Apollo 15 red glass (A15R) represent two of the HiTi compositions in the spectrum of pristine ultramafic glasses returned from the moon [1]. The difference between these two compositions is that the A15R contains ~40% more TiO 2 than the A17O. The low f O2 of the ultramafic glass source regions allows for a certain amount of Ti 3+ in the source mineralogy; however the amount of Ti 3+ in the source and the host mineral for this element remain relatively unconstrained. In addition to the unknown mineralogy of the source region, the high amount of TiO 2 * and FeO* in the HiTi magmas makes the phase relations extremely sensitive to changes in the oxidation state of the source region. We have previously investigated the oxidation state effect on the olivine-orthopyroxene multiple saturations points of the A15R and A17O [2,3] and shown that the magnitude of the effect is proportional to the amount of Ti in the system. X-ray absorption near-edge spectroscopy (XANES) and extended X-ray absorption fine-structure (EXAFS) measurements have been made on minerals and glasses in experiments on synthetic analogues to the A17O and A15R. Our results show that Ti 3+ concentration does indeed affect the multiple saturation points, and is an important constituent in the lunar interior. Experiments and Analysis: Experiments were carried out in a piston cylinder apparatus using both graphite and spec-pure Fe-metal capsules. The starting material is a mixture of synthetic oxides, combined in correct proportions to recreate the bulk composition of A17O and A15R [1] and conditioned at iron-wüstite (IW) oxygen fugacity conditions. It has previously been shown that experiments in Fe-metal capsules buffer at a lower oxygen fugacity (IW-2) than that of graphite capsules (IW+1.2) [3]. Piston cylinder experiments with crystals were prepared as polished thick sections for analysis.