Lunar volcanism: A Geophysical perspective

Abstract

The lunar crust has preserved a record of the Moon’s volcanic and magmatic activity through time. While the extrusive maria dominate the volcanic record, little is known regarding their thickness and the details of their emplacement. Intrusive activity is even more elusive, with most intrusions expressing little to no surface signature. Here, we present a global investigation and a volumetric inventory of extrusive and intrusive volcanic materials (Broquet & Andrews-Hanna, 2024a, 2024b). Gravity and topography are inverted using a two-layer loading model under the premise of pre-mare isostasy to constrain mare and cryptomare thickness, as well as updated crustal thickness models. Substantial lateral variations in mare thickness are found, with averages 7.9 km within large mare basins compared to 1.6 km outside of these basins. This important thickness variation associated with minimal change in the surface topography can be explained by some combination of long-distance transport of low viscosity mare and/or a buoyancy control limiting mare eruptions to a constant level surface.

Our inversion predicts the shape of the lunar crust before it got obscured by mare materials. The pre-mare surface of the nearside Oceanus Procellarum region is ~2 km lower than the surroundings, and possible explanations, including a giant impact, pore space annealing, isostatic adjustment, and plume-induced crustal erosion, are discussed. The western part of Imbrium’s ring is not found in the pre-mare topography, implying that it never formed or that some processes erased its signature from gravity and topography. The feldspathic, pre-mare, crust is ~7 km thinner within large nearside basins than in models not accounting for the high-density mare. The pre-fill floor of these basins was ~6 km deeper than currently observed. These new insights have implications for impact simulations that try to reproduce the crustal structure of nearside mare basins.

Our preferred volumes of mare and cryptomare total to 20×106 km3. Investigation of crustal intrusions associated with linear gravity anomalies, floor-fractured craters, ring dikes, graben, and beneath volcanic constructs, yield a total volume of 9×106 km3. The major fraction of intrusive materials is in the form of ring dikes located at the margin of large basins in zones of flexural extension, which indicates an important control of lithospheric stress on magma ascent. Taken together, the total volume of the secondary crust corresponds to ~2% (and up to 5%), of the total lunar crust volume. The combined volume of intrusives and extrusives is found to be 3 times greater in the nearside than in the farside. Intrusive activity dominates in the farside (intrusive:extrusive ratio of 5:2), whereas extrusive volcanism is more pronounced in the nearside (1:5). Both are related to the lunar asymmetry in which the thinner crust and warmer subsurface beneath the Procellarum KREEP terrane enables enhanced melting and magma ascent. The strong asymmetry in melt production supports an early KREEP migration, which must have been established 100 s of Myr before nearside volcanism began to allow for the buildup of heat.