OXYGEN ISOTOPES OF LUNAR ROCKS: DIFFERENT SOURCES FOR DIFFERENT HI-Ti BASALTS?

Introduction: Oxygen isotopes of planetary materials provide important constraints on the genesis and evolution of their parent planetary bodies. The general consensus about the formation of Earth-Moon system is that a Mars-sized planetary body impacted a proto-Earth off center and contributed 70%-90% of its material to the Moon [1,2]. The impactor (Thea) is not expected to have the identical oxygen isotope signature of the proto-Earth, yet all three-oxygen isotopes studies of lunar rocks fail to identify any deviation from terrestrial fractionation line [e.g. 3-6] and thus put severe constrains on the giant impact model for the origin of the Earth-Moon system. Exchange between proto-Earth and Thea is suggested to achieve the oxygen isotope equilibrium [7]. A recent model on the dynamics of the turbulent mixing between the terrestrial magma ocean and proto-lunar magma disk suggests that the equilibration can be achieved within 100-250 years [8]. Oxygen isotopes of lunar mare basalts can also provide information about the lunar mantle [5, 6]. High-Ti basalts have been determined to have lower δ 18 O (0.20-0.25 ‰) than low-Ti basalts [5, 6, 9], suggesting possible different sources for these basalt types. In addition the difference in δ