Head, III, James - Lunar Regional Pyroclastic Deposits: Evidence for Eruption from Dikes Emplaced into the Near-Surface Crust

Abstract: 
Lunar pyroclastic deposits have several modes of occurrence; smaller, more isolated deposits suggest several modes of emplacement related to dike and sill emplacement, including strombolian, hawaiian, and vulcanian activity. The mode of emplacement of largest pyroclastic deposits (>1000 km2) has been less clear; for the Aristarchus Plateau deposits, very high effusion rate eruptions leading to sinuous rilles and associated pyroclastic emplacement have been implicated. A candidate modes of emplacement comes from analysis of the Orientale dark ring, a 154km diameter pyroclastic deposit that emanates from a linear depression interpreted to be a remnant elongated vent at the top of a dike; a wide dike stalled just below the surface, and the low-pressure environment led to gas buildup along the dike top, leading to eruption of an Io-like pyroclastic plume to produce the dark pyroclastic ring. Analysis of ascent and eruption of magma shows that the low-pressure environment associated with dike tip propagation could enhance formation of volatiles during dike ascent so that the dike arrives at the surface with the dike top already saturated with magmatic foam, and not requiring secondary buildup. Could this mechanism, arrival of volatile magmatic foam-laden dikes to the shallow subsurface, perhaps combined with further shallow crustal gas formation subsequent to stalling, lead to penetration of foams to the surface and eruption of magmatic foams to produce regional pyroclastic deposits?
In these cases, the low pressure always present in the propagating dike tip means that as dikes approach the surface their upper tips will consist of a cavity containing gas underlain by a region where gas bubbles concentrate into a foam. If the dike fails to break through to the surface, gas bubbles migrate up through the foam to increase the size of, and pressure in, the gas cavity. Additional foam is generated beneath the gas cavity if the dike is wide enough to allow convection to occur because this brings magma from depth to shallow enough levels for additional pressure-dependent gas release. The Orientale example showed that these processes acting in a ~500m-wide dike produced an ~25-fold gas concentration leading to an explosive eruption emplacing a ~150km diameter circular pyroclastic deposit. A generic example of this process would involve a 100 km long linear rille graben induced by a 300 m-wide dike; magmatic foam would occupy the upper ~ 8 km of the dike where the pressure was less than 40MPa. If the foam evolved the gas would ultimately represent ~5 mass % or ~50000ppm.  Foam release would produce an eruption speed of 487 m s-1 ejecting pyroclasts to ~147km. If this magma were deposited as pyroclasts over an area of 100 km (the rille length) x ~ 147km (the maximum range) with a bulk density on landing of 2000kg m-3, the resulting deposit thickness would be ~5m. We infer that essentially all of the observed dark mantle regional pyroclastic deposits on the Moon can be explained by minor variations on this scenario.