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

Abstract

Abstract— Phase equilibrium and dissolution kinetics experiments on synthetic late‐stage magma ocean cumulates are used to place constraints on hypotheses for the 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 clinopyroxene‐ilmenite cumulates to form a hybrid mantle source. To satisfy the constraints of our experiments, we propose an alternative model that involves shallow‐level reaction and mixing of cumulates, followed by sinking of hybrid 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 models from the low solidus temperatures of clinopyroxene‐ilmenite cumulates. Partially molten clinopyroxene‐ilmenite cumulates become gravitationally unstable with respect to underlying mafic cumulates only when they have cooled to within ∼30°C of their solidus (∼1125°C at 100 km depth). At these temperatures, the viscosity of mafic cumulates is too high to allow for growth and descent of clinopyroxene‐ilmenite diapirs on the appropriate time scale. Reaction and mixing between late‐stage liquids and mafic cumulates at shallow levels would produce a refractory hybrid material that is negatively buoyant at higher temperatures and could sink more efficiently to the depths inferred for production of high‐Ti ultramafic glasses.