Evolution of Intrusions in Lunar Floor-Fractured Craters: Degassing, Solidification and Relationship to Tectonic and Volcanic Features

Abstract:

Lunar floor-fractured craters are a class of 170 craters characterized by their anomalously shallow, heavily fractured floors; associated floor morphologies include deposits of mare material, pyroclastic deposits, and vents. Floor-fractured craters are located in close proximity to surface mare deposits, and are also closely associated with both the interiors and edges of lunar basins. The interior volcanic features, in conjunction with the morphologic and morphometric characteristics of the craters, suggest a formation consistent with subcrater magmatic intrusion and sill formation. Morphometric data suggests that the areal extent of the intrusion mirrors the dimensions of the crater floor, and that the intrusion does not extend past the crater wall region. The intrusion thickness can be calculated by comparing the observed depth of the craters with the predicted depth of a crater of similar diameter, and the averaged intrusion thickness is ~ 1 km. Thus these intrusions represent large subsurface magmatic provinces. We investigate the evolution of these large magmatic intrusions with emphasis on how the degassing of the intrusion leads to pyroclastic eruptions in certain craters, and whether these eruptions utilize fractures created by the tectonic deformation of the crater floor, or if they instead form from subsidiary diking off of the intrusion. We also investigate the amount of volatiles necessary to produce the pyroclastic eruptions, considering both inherent volatiles in the magma, and volatiles generated by reactions during the shallow subsurface evolution of the magma. The craters Alphonsus and Humboldt serve as ideal study cases to compare and contrast floor morphology, fracture location, and observed volcanic deposits. The results of this analysis have importance for why pyroclastic eruptions occurred in certain locations, and it also has implications for the volatile budget of lunar magmas.