A SHALLOW MANTLE SOURCE FOR THE CHANG'E 5 LAVAS REVEALS HOW TOP-DOWN HEATING PROLONGED LUNAR MAGMATISM

The 2 billion year old basalts collected by the Chang’e 5 mission are younger than any other sampled lunar igneous rock. These lavas provide critical insight into the evolution of the Moon at a time when magmatism was waning and represent a key data point for understanding how rocky bodies cool. Knowledge of the depth of origin of basaltic melts is crucial for unraveling the heat sources for mantle melting and their bearing on the thermal evolution of the Moon. We present high-P-T experiments and phase equilibrium modeling performed on a Chang’e 5 basalt parental liquid composition with the goal of determining the P-T conditions of its liquidus multiple saturation point and the depth of melting. We couple the results with thermal modeling of the effects of a sub-crustal KREEP layer on the temperature of the upper mantle at 2 Ga.

Our experimental results and constraints from the mineralogy and geochemistry of the samples themselves show that some of the Chang’e 5 samples represent a parental liquid composition and that parental magma formed in the shallow mantle, at ~75 – 130 km depth and 1200-1225 °C. The magma did not undergo significant fractional crystallization after separation from its source region and its mantle source contained olivine, CPX, an Fe-Ti oxide, and plagioclase. Furthermore, the Sr-Nd isotopic compositions of the Chang’e 5 basalts show that the heat-producing KREEP reservoir did not physically interact with the magma.

Models for prolonged lunar magmatism fall into two categories, deep heating vs. shallow heating, and differ as to whether the Th, U, and K-enriched KREEP reservoir is carried into the mantle during cumulate overturn or remains at the base of the crust. The shallow source for the Chang’e 5 melt at 2 Ga, and Sr-Nd isotopic evidence for the lack of the heat-producing KREEP reservoir in sources of the Chang’e 5 basalt and high-Ti basalts collected by Apollo, demonstrate that KREEP was not carried into the deep mantle to generate prolonged melting. Rather, we show that a sub-crustal KREEP layer conductively heating the nearside mantle from the top down is likely responsible for prolonged lunar magmatism. Our thermal modeling shows that the presence of a 5-10 km thick sub-crustal KREEP layer can keep the shallow upper mantle warmer by ~250 – 500 °C. Therefore, top-down heating of the mantle by KREEP is the likely heat source for prolonged lunar magmatism.