PSRD: A Sample from an Ancient Sea of Impact Melt

Sophisticated computer modeling of the formation of lunar multi-ringed basins by impact indicate that substantial volumes of impact melt are produced, leading to melt bodies hundreds of kilometers in diameter and tens of kilometers deep. The impressively large bodies of magma created by the impact of a projectile 50 to 300 kilometers across might have differentiated, producing a zoned body with denser minerals concentrated towards the bottom and less dense minerals concentrated near the top, a process called fractional crystallization. Marc Norman (Australian National University) and colleagues at the University of Tennessee and the Johnson Space Center have studied a sample (67955) collected in the lunar highlands during the Apollo 16 mission. The overall texture, composition, and mineralogy of a clast (a fragment) in the rock indicate that it formed as an accumulation of crystals from a magma that was enriched in trace elements. Mineral compositions and crystal intergrowths suggest a similar depth of origin to lunar igneous rocks that formed more than 10 kilometers deep in the lunar crust, implying an impact melt pool at least as deep. Such a deep melt pool would have formed in an impact basin the size of Orientale, a multi-ringed basin whose inner ring is 480 kilometers across. Norman and co-workers also determined from samarium and neodymium isotopes that the igneous clast is 4.2 billion years old, clearly older than the typical age of 3.8–3.9 billion years assigned to visible lunar basins. The authors conclude that the clast in 67955 is a sample of a differentiated impact melt sea formed in an impact basin on the nearside of the Moon 4.2 billion years ago. The rock was part of a pile of ejecta thrown to the Apollo 16 site, possibly by the impact event that excavated the Imbrium basin.