HIGH-Ti VOLCANIC GLASSES: DERIVATION FROM A GARNET-BEARING SOURCE COUPLED WITH ASSIMILATION OF LATE-STAGE LMO

Introduction: The presence of garnet in the lunar mantle has been debated from many years (e.g., [1-3]). Data from the Apollo seismic network were unable to unambiguously resolve a garnet signature in the lower mantle. For example, higher seismic velocities in the lunar mantle (>500 km) could be indicative of an increased proportion of Mg-rich olivine [2,4], or be due to the presence of garnet (e.g., [3]). Longhi [5] used melting experiments (with starting compositions appropriate to terrestrial and lunar mantles) at 28-30 kbar (560-600 km) to argue against garnet being in the lunar mantle. However, other investigators have used several approaches to argue for the opposite. Thermodynamic modeling performed by [6-8] is consistent with up to 13 mol.% garnet being present at >500 km. Multi-saturation experimental evidence also supports garnet in the lunar mantle at depths greater than ~500 km [11-13] for both low-Ti (A-15 green) and high-Ti (A-14 black) glasses. In addition, certain volcanic glasses have trace-element signatures consistent with garnet being retained in their source regions [14-16]]. These glasses are the high-Ti A-12 Red and A-14 Black groups. They contain Sm/Yb, and Zr/Y ratios that are higher and Sc/Sm ratios that are lower than KREEP. Here, we investigate: 1) the high-Ti nature of these glasses; 2) if certain trace-element signatures in the high-Ti A-12 Red and A-14 Black glasses are indeed due to depletion of the garnet-loving elements because of garnet retention in the source. Modeling Parameters: We have modeled the