Intrusive Magmatism and Explosive Volcanism on the Moon and Mercury: Insights from Floor-Fractured Craters

Abstract:

Lunar floor-fractured craters are a class of 170 craters characterized by shallow, deformed, and heavily fractured floors, with associated volcanic features commonly including deposits of mare material, pyroclastic deposits, and vents. The fracturing is interpreted to form in response to the stalling of a dike underneath the crater; resultant sill formation deforms the overlying crater floor. Morphologic measurements of these craters can be used to estimate sill dimensions, and from this we estimate the total volume of magma contained in all floor-fractured crater sills to be approximately 3 10⁶ km³, 30% of the total lunar mare volume. Thus floor-fractured craters represent a unique window into intrusive magmatic processes on the Moon, including magma degassing and explosive volcanism. On the Moon, explosive volcanism is frequently associated with the degassing of floor-fractured crater intrusions; the morphologies associated with this process include small vents commonly located along fractures, and dark halos, which are low albedo deposits surrounding vents and are interpreted to be pyroclastic in origin. Other explosive volcanic morphologies include regional-scale pyroclastic deposits for which no vents have yet been identified, and large vent structures, such as Sulpicius Gallus. In contrast to the numerous floor-fractured craters on the Moon, there have been no conclusively identified floor-fractured craters on Mercury. Instead, Mercury exhibits a large number of what are interpreted to be explosive volcanic deposits, on the basis of the orange/red color of the inferred pyroclastic material identified in color images. These deposits are often much more areally extensive than their lunar counter parts, and surround a wide range of large pit and vent-like morphologies, themselves up to 10s of km in diameter. Using existing databases of these features, we have divided the spectrally identified volcanic deposits into 6 initial morphologic subclasses. Using the formation mechanisms for explosive volcanic deposits on the Moon as a template, we explore possible formation mechanisms for the range of explosive morphologies on Mercury. We also explore how differences in the structure of the Moon and Mercury have led to differences in the expression of intrusive magmatism and explosive volcanism.