Late-stage Effusive Volcanism on Mercury: Evidence from Mansurian Impact Basins

Introduction: The globally extensive smooth plains of Mercury are believed to be mostly volcanic in origin [1]. Widespread effusive volcanism on Mercury is thought to have ended by ~3.5 Ga due to secular cooling of the planet’s interior, and contraction of its lithosphere [2]. As the planet cools and contracts, melt should be produced at a slower rate and in smaller volumes, so it will stall deeper and its escape routes will close. 3.5 Ga corresponds roughly with the end of Mercury’s Calorian system. Smooth plains younger than this have been reported, but are restricted to the interiors of impact basins, such as Rachmaninoff [3]. If widespread effusive volcanism on Mercury ceased in response to cooling and contraction during the Calorian, then Mansurian impact basins are good places to search for late-stage effusive volcanism. Effusive volcanism should be favoured in impact basins, because they remove overburden, promote uplift, temporarily reset the preexisting stress regime, propagate fractures and deposit heat [2]. If cooling and contraction were the main factors that controlled the decline of widespread volcanism on Mercury, then post-impact volcanism should similarly become less voluminous throughout the Mansurian. Smaller basins should have less post-impact volcanism because they produce shallower pathways for melt. Post-impact volcanism should also become less common throughout the Mansurian as Mercury continues to cool. Considering these expectations, we are conducting a global survey of Mansurian impact basins to study how effusive volcanism on Mercury waned as a consequence of global cooling and contraction. Methods: For our initial study, we include all Mansurian basins >100 km in diameter (n=43) [4]. We are examining the smooth infill of each basin to ascertain if it was emplaced as post-impact volcanism or impact melt. We will also determine the relative ages of these basins if possible to test if post-impact volcanism becomes rarer through the Mansurian. Some of the smooth infill of Rachmaninoff was determined to be post-impact volcanism on the basis of its resolvably younger crater size-frequency distribution compared to the rest of the basin material. The smooth infill also has a sharp colour boundary with the surrounding basin material, suggesting a volcanic, rather than an impact, provenance [3]. We will not use crater size-frequency statistics to determine relative ages for basin formation and infill emplacement. This is because the crater statistics for Rachmaninoff are probably dominated by secondary impacts [5]. Furthermore, Rachmaninoff is the largest basin in our study. All the other basins will have smaller count areas and fewer superposing craters, making ages derived from crater statistics more uncertain and probably indistinguishable. Instead, we will search for geological evidence that the smooth basin infill is a result of post-impact volcanism using the following criteria. Ghost craters. If ghost craters are visible within the basin fill, then it cannot be impact melt (Fig. 1). This is because impact craters must have had sufficient time to form on the basin floor before flooding in order to become expressed as ghost craters.