The Manicouagan Structure as a Window into Lunar Impact Melts

Recent exploration drilling activity on the central island of the Manicouagan impact structure has provided several deep (>1 km) sections through the impact melt sheet and underlying basement. One hole in particular (0608) comprises 1100 m of clast-free melt. This is located in the centre of the peak ring central region within an apparent trough-like structure. This thicker impact melt section has undergone differentiation into three zones: a lower monzodiorite (600 m), an upper quartz monzodiorite (250 m) and middle transition zone comprising quartz monzonite (250 m). Manicouagan's melt sheet has previously been considered to be up to 400 m thick or less. Moreover, the melt sheet-footwall boundary is typically interpreted to be flat in many crater models. The Manicouagan drill data indicate that this is not the case. The occurrence of a thicker, differentiated melt unit for this size of crater (~100 km) has implications for how we interpret lunar samples. Lunar impact melt samples of differing chemistry and mineralogy are typically interpreted to come from distinct impact basins. In the light of the recent and ongoing Manicouagan research, it is apparent that samples previously assigned to separate impact events may be differentiates of common impact melt sheets. Critically, this may occur at smaller diameters than previously considered. Future work should aim to test for consanguinity between lunar impact melts and impact-melt breccias, which has implications for cratering rates and the critical volumes of melt required to initiate differentiation. Examples are discussed with reference to Manicouagan impact melt lithologies and lunar materials.