Evidence for diverse lunar melt compositions and mixing of the pre-3.9 Ga crust from zircon chemistry

Abstract:

The first 600 million years of the lunar magmatic history (pre-Nectarian; >3.92 Ga) record a mix of endogenous (evolution of the Lunar Magma Ocean and formation of the lunar crust) and exogenous processes (impacts resulting in the formation of large basins) that can be used to better understand both planetary evolution and the early impact history of the inner solar system. However, distinguishing between these processes is challenging because most lunar samples are impact-derived breccia or regolith composed of rocks and minerals having various origins. While some intact igneous fragments can be found within pre-Nectarian samples, they rarely exceed 1 cm in size and often show evidence for impact recrystallization. The mineral zircon (ZrSiO₄), forming in differentiated felsic melts during both endogenous and exogenous processes on the Moon, can retain age information as well as chemical and isotopic signatures for billions of years. In this study, we combined U-Pb geochronology of Apollo 14 zircons (²⁰⁷Pb–²⁰⁶Pb ages from 3.93 to 4.36 Ga) with zircon trace element chemistry and thermodynamic models to better constrain the pre-Nectarian magmatic history. Our data show variable U, Th, and Al concentrations in pre-4.0 Ga zircons grains suggesting that: 1) the early lunar crust could have been quite diverse in composition, and 2) the distribution of heat-producing radioactive elements in the melt sources forming the lunar crust was heterogenous. Al-in-zircon in these pre-4.0 Ga grains show at least two different magma sources for those early crustal rocks, the first one enriched in aluminum and potentially derived from assimilation of anorthosite-rich rocks (e.g., Feldspathic Highlands Terrane; FHT), the second one poorer in aluminum and possible derived from fractionated KREEP basalts from the Procellarum KREEP Terrane (PKT). Melt diversity seems to disappear after 4.0 Ga, with zircons recording one single magma composition falling in-between the two groups evidenced for the pre-4.0 Ga grains. This apparent homogeneous mixture of PKT and FHT-like rocks is consistent with the upper 15 km of lunar crust being thoroughly mixed, re-melted and homogenized by the large basin-forming impacts occurring during the pre-Nectarian period.