Mechanisms of explosive volcanism on Mercury: Implications from its global distribution and morphology

Abstract

The identification of widespread pyroclastic vents and deposits on Mercury has important implications for the planet's bulk volatile content and thermal evolution. However, the significance of pyroclastic volcanism for Mercury depends on the mechanisms by which the eruptions occurred. Using images acquired by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging spacecraft, we have identified 150 sites where endogenic pits are surrounded by a relatively bright and red diffuse-edged spectral anomaly, a configuration previously used to identify sites of explosive volcanism. We find that these sites cluster at the margins of impact basins and along regional tectonic structural trends. Locally, pits and deposits are usually associated with zones of weakness within impact craters and/or with the surface expressions of individual thrust faults. Additionally, we use images and stereo-derived topographic data to show that pyroclastic deposits are dispersed up to 130 km from their source vent and commonly have either no relief or low circumpit relief within a wider, thinner deposit. These eruptions were therefore likely driven by a relatively high concentration of volatiles, consistent with volatile concentration in a shallow magma chamber prior to eruption. The colocation of sites of explosive volcanism with near-surface faults and crater-related fractures is likely a result of such structures acting as conduits for volatile and/or magma release from shallow reservoirs, with volatile overpressure in these reservoirs a key trigger for eruption in at least some cases. Our findings suggest that widespread, long-lived explosive volcanism on Mercury has been facilitated by the interplay between impact cratering, tectonic structures, and magmatic fractionation.

The Apparent Absence of Kilometer‐Sized Pyroclastic Volcanoes on Mercury: Are We Looking Right?

“…This finding is in agreement with (Brož et al, 2015;Wilson & Head, 2003) and also with the observations of large faculae (up to 260 km in diameter) surrounding putative volcanic vents on Mercury, which show little (<1°) or no topographic relief at all (Thomas et al, 2014a). For a fixed eruption volume, wider dispersal necessarily leads to a decrease in the height of the final shape and to proportionately shallower flank slopes.…”

“…The lower limit was chosen to resemble the typical volume of terrestrial scoria cones (determined from 986 edifices based on data from Pike, 1978, andHasenaka &Carmichael, 1985), and the upper limit of 40 km 3 was chosen as this is the median volume of putative large-scale explosive vents on the surface of Mercury (Thomas et al, 2014a). We chose volumes from 0.046 up to 40 km 3 with intermediate steps of 2.1, 4.2, 10, 20, and 30 km 3 .…”

Geology of the Raditladi quadrangle, Mercury (H04)

“…Peak ring crests were mapped as linear features where observed. Volcanic vents (rimless and irregular depressions, endogenic pits) were mapped integrating previous catalogs (Denevi et al, 2013;Thomas, Rothery, Conway, & Anand, 2014b). Thrust faults have been mapped using the triangular symbol located on the hangingwall while the graben symbology indicates only the trace of the structure.…”

Sulfur solubility in reduced mafic silicate melts: Implications for the speciation and distribution of sulfur on Mercury

“…The first hypothesis is consistent with the absence of spectral evidence for sulfide minerals in surface rocks (McClintock et al, 2008;Izenberg et al, 2014) while the second could explain the correlations between S and Ca-Mg observed in Mercurian lavas . Understanding the origin of high sulfur concentrations at the surface of Mercury is important to better constrain the structure of the planet and the distribution of sulfur amongst the different reservoirs (mantle, core and crust), the mechanisms of explosive volcanism (Kerber et al, 2009;Thomas et al, 2014a;Weider et al, 2016), and the formation of the hollows (sub-kilometer scale shallow depressions surrounded by bright deposits) which may have formed during sublimation of volatiles (Blewett et al, 2013;Thomas et al, 2014b).…”