Magmatic intrusion-related processes in the upper lunar crust: The role of country rock porosity/permeability in magmatic percolation and thermal annealing, and implications for gravity signatures

Abstract

Shallow crustal country rock on the Moon is demonstrably more fractured and porous than deeper crustal bedrock, and Gravity Recovery and Interior Laboratory (GRAIL) mission gravity data have shown that deeper crustal bedrock is more porous than previously thought. This raises the question of how crustal porosity and permeability will influence the nature of magmatic dike intrusions in terms of: 1) the ability of intruding magma to inject into and occupy this pore space (shallow magmatic percolation), 2) the influence of the intruded magma on annealing of this porosity and permeability (thermal annealing) both 1 and 2 densify the country rock), and 3) the effect of crustal porosity on favoring sill formation as a function of depth in the lunar crust. We analyze quantitatively the emplacement of basaltic dikes and sills on the Moon and assess these three factors in the context of the most recent data on micro- and macro-scale porosity of lunar crustal materials. For the range of plausible micro/macro-scale porosity and permeability determined by crack widths (mm to cm) and open crack lateral continuity (mm to tens of cm), we find that 1) rapid conductive cooling of injected magma due to the very large surface area to volume ratio restricts magmatic percolation to very limited zones (extending for at most several tens of cm) adjacent to the ascending dike or intruded sill, even in the upper several hundred meters of the lunar crust; 2) the conductive heat loss from intruded dikes and sills results in a thermal wave decay rate that is predicted to limit the extent of intrusion-adjacent thermal annealing to less than ~6% of the thickness of the intruded body; 3) the extremely rapid rise rate of magma in dikes originating from sources in the lunar mantle disfavors the lateral migration of dikes to form sills in the crust, except in specific shallow crustal locations influenced by impact crater-related environments (e.g., floor-fractured craters). We conclude that, although magmatic percolation and thermal annealing in association with lunar mare basalt magmatic dike and sill emplacement should be taken into consideration in interpreting gravity signatures, the effects are likely to be minor compared with the density contrast of the solidified basaltic magmatic intrusion itself.