CAN APOLLINARIS-RELATED PYROCLASTIC DEPOSITS BE ATTRIBUTED TO CHANGING WATER: MAGMA INTERACTION RATIOS ON MARS?

Highland paterae represent a distinct class of Martian volcano that formed from explosive eruptions, which temporally transitioned into effusive activity. The cause of the explosive nature of these volcanoes is unclear, but paterae formation around Hellas basin has been attributed to water-magma interaction where the transition from explosive to effusive activity reflects a decrease in the amount of water present within the Martian crust. Here we postulate that Apollinaris Patera – a singular, isolated patera located along the crustal dichotomy near 8.5° S, 174° E – likely formed in a similar fashion. Specifically, Mars Orbiter Camera (MOC) and Context (CTX) imagery show that deposits comprising Apollinaris’ flanks exhibit various amounts of layering and erodibility. As one works up through Apollinars’ stratigraphic section, the deposits exhibit an overall increase in the amount of layering and an overall decrease in relative erodibility. Assuming that relative erodibility can be used as a qualitative proxy to the degree of welding within pyroclastic deposits, these observations imply that younger Apollinaris deposits are more welded than older deposits. This in turn suggests that the eruption temperature of Apollinaris deposits increased over the volcano’s lifetime; hotter eruption temperatures would have led to higher degrees of welding within the deposits. Using basaltic tuff cones and rings as an analogy (e.g., Wohletz and Sheridan, 1983), it is possible to explain these changing amounts of bedding and welding as due to decreasing water:magma ratios. To elaborate, larger water:magma ratios would result in cool (<100° C), wet eruptions resulting in non-welded and massive pyroclastic deposits whereas smaller water:magma ratios would result in hotter (>100° C), drier eruptions that could potentially result in welding and thinner beds. Although purely speculative, such a scenario for Apollinaris is consistent with morphologic evidence that past water may have existed within this region; in fact, the location of Apollinaris coincides with the Arabia shoreline of Clifford and Parker (2001) – perhaps the decreasing water:magma ratio at Apollinaris is linked to the recession of this postulated northern ocean. If true, the surrounding Medusa Fossae Formation could reflect sub-aqueous Apollinaris deposits.