Lunar Regional Pyroclastic Deposits: Evidence for Eruption from Dikes Emplaced into the Near-surface Crust

Introduction: Lunar pyroclastic deposits can be subdivided into several modes of occurrence [1], suggesting different modes of emplacement. For example, some smaller pyroclastic deposits have been interpreted to have erupted during strombolian/hawaiian activity [2], while others are linked to vulcanian activity related to dike and sill emplacement below crater floors [3,4]. The largest pyroclastic deposits (Orientale dark ring, Aristar-chus Plateau, Sinus Aestuum, Rima Bode, Mare Va-porum, Sulpicius Gallus, and Taurus Littrow) cover regions >1000 km 2 [1], and their mode of emplacement has been less clear than that of the smaller, more isolated deposits. In the case of the Aristarchus Plateau pyroclas-tic deposits, very high effusion rate eruptions leading to sinuous rilles [5] and associated pyroclastic emplace-ment have been implicated. The location of specific vents, and thus eruption styles, has been less clear for the remainder of the large deposits, partly due to burial and obscuration by post-pyroclastic deposit effusive volcan-ism [6]. Important evidence related to candidate modes of emplacement comes from analysis of the Orientale dark ring, a 154 km diameter pyroclastic deposit that was shown to emanate from a linear depression interpreted to be the remains of an elongated vent at the top of a dike [7]. In this interpretation, a wide dike stalled just below the surface, and the low-pressure environment led to gas buildup along the top of the dike, ultimately leading to the eruption of an Io-like pyroclastic plume to produce the dark pyroclastic ring. The central elongate depression thus represents evidence of this shallow dike emplacement-related activity. Additional evidence for the association of pyroclastics with dike-emplacement activity comes from comes from the analysis of the ascent and eruption of magma [8], where it was shown that the low-pressure environment associated with dike tip propagation could enhance formation of volatiles during dike ascent so that the dike could arrive at the surface with the top of the dike already saturated with magmatic foam, and not requiring secondary buildup as in the vul-canian [3] or the Orientale dark halo [7] cases. Could this mechanism, arrival of volatile magmatic foam-laden dikes to the shallow subsurface [8], perhaps combined with further shallow crustal gas formation [8] subsequent to stalling, lead to the penetration of foams to the surface and eruption of magmatic foams to produce regional py-roclastic deposits? Distribution and Characteristics of Linear Features: The association of linear rilles (interpreted to be the surface manifestation of stress fields …