Origin of lunar high-titanium ultramafic glasses: Constraints from phase relations and dissolution kinetics of clinopyroxene-ilmenite cumulates

Phase equilibrium and dissolution kinetics experiments on synthetic late-st age magma ocean cumulates are used to place constraints on hypotheses for t he origin of lunar high-Ti ultramafic glasses. Models for the production of high-Ti lunar magmas have called for either (1) assimilation of late-stage clinopyroxene-ilmenite cumulates at shallow levels or (2) sinking of clino pyroxene-ilmenite cumulates to form a hybrid mantle source. To satisfy the constraints of our experiments, we propose an alternative model that involv es shallow-level reaction and mixing of cumulates, followed by sinking of h ybrid high-Ti materials. This model can fulfill compositional requirements imposed by the pristine lunar glass suite that are difficult to satisfy in assimilation models. It also avoids difficulties that arise in overturn mod els from the low solidus temperatures of clinopyroxene-ilmenite cumulates. Partially molten clinopyroxene-ilmenite cumulates become gravitationally un stable with respect to underlying mafic cumulates only when they have coole d to within similar to 30 degrees C of their solidus (similar to 1125 degre es C at 100 lan depth). At these temperatures, the viscosity of mafic cumul ates is too high to allow for growth and descent of clinopyroxene-ilmenite diapirs on the appropriate time scale. Reaction and mixing between late-sta ge liquids and mafic cumulates at shallow levels would produce a refractory hybrid material that is negatively buoyant at higher temperatures and coul d sink more efficiently to the depths inferred for production of high-Ti ul tramafic glasses.