Generation, ascent and eruption of magma on the Moon: New insights into source depths, magma supply, intrusions and effusive/explosive eruptions (Part 2: Predicted emplacement processes and observations)

We utilize a theoretical analysis of the generation, ascent, intrusion and eruption of basaltic magma on the Moon to develop new insights into magma source depths, supply processes, transport and emplacement mechanisms via dike intrusions, and effusive and explosive eruptions. We make predictions about the intrusion and eruption processes and compare these with the range of observed styles of mare volcanism, and related features and deposits. Density contrasts between the bulk mantle and regions with a greater abundance of heat sources will cause larger heated regions to rise as buoyant melt-rich diapirs that generate partial melts that can undergo collection into magma source regions; diapirs rise to the base of the anorthositic crustal density trap (when the crust is thicker than the elastic lithosphere) or, later in history, to the base of the lithospheric rheological trap (when the thickening lithosphere exceeds the thickness of the crust). Residual diapiric buoyancy, and continued production and arrival of diapiric material, enhances melt volume and overpressurizes the source regions, producing sufficient stress to cause brittle deformation of the elastic part of the overlying lithosphere; a magma-filled crack initiates and propagates toward the surface as a convex upward, blade-shaped dike. The volume of magma released in a single event is likely to lie in the range 10 2 km 3 to 10 3 km 3 , corresponding to dikes with widths of 40–100 m and both vertical and horizontal extents of 60–100 km, favoring eruption on the lunar nearside. Shallower magma sources produce dikes that are continuous from the source region to the surface, but deeper sources will propagate dikes that detach from the source region and ascend as discrete penny-shaped structures. As the Moon cools with time, the lithosphere thickens, source regions become less abundant, and rheological traps become increasingly deep; the state of stress in the lithosphere becomes increasingly contractional, inhibiting dike emplacement and surface eruptions. In contrast to small dike volumes and low propagation velocities in terrestrial environments, lunar dike propagation velocities are typically sufficiently high that shallow sill formation is not favored; local low-density breccia zones beneath impact crater floors, however, may cause lateral magma migration to form laccoliths (e.g., Vitello Crater) and sills (e.g., Humboldt Crater) in floor-fractured craters. Dikes emplaced into the shallow crust may stall and produce crater chains due to active and passive gas venting (e.g., Mendeleev Crater Chain) or, if sufficiently shallow, may create a near-surface stress field that forms linear and arcuate graben, often with pyroclastic and small-scale effusive eruptions (e.g., Rima Parry V). Effusive eruptions are modulated by effusion rates, eruption durations, cooling and supply limitations to flow length, and pre-existing topography. Relatively low effusion rate, cooling-limited flows lead to small shield volcanoes (e.g., Tobias Mayer, Milicius); higher effusion rate, cooling-limited flows lead to compound flow fields (e.g., most mare basins) and even higher effusion rate, long-duration flows lead to thermal erosion of the vent, effusion rate enhancement, and thermal erosion of the substrate to produce sinuous rilles (e.g., Rimae Prinz). Extremely high effusion rate flows on slopes lead to volume-limited flow with lengths of many hundreds of kilometers (e.g., the young Imbrium basin flows). ∗ Corresponding author. Tel: + 1 (401) 863-2526. E-mail address: [email protected] (J.W. Head). http://dx.doi.org/10.1016/j.icarus.2016.05.031 0019-1035/© 2016 Elsevier Inc. All rights reserved. J.W. Head, L. Wilson / Icarus 283 (2017) 176–223 177 Explosive, pyroclastic erupti gating dike crack-tips can caus crustal depths both the smeltin mare basalt magmas contribute the dike as it erupts at the sur eruption types whose manifes strombolian-style eruptions ma style eruptions map to broad posits); (3) gas-rich ultraplinia (e.g., many isolated glass bead magma in the dike tip, buildup mixed country rock (e.g., Alph gas buildup in wide dikes, ene Orientale dark ring); (6) multip deposits (e.g., Rima Bode, Sinu accompanied by continuing pyr a broader pyroclastic ‘spatter’ e sufficiently long, an associated and Aristarchus Plateau dark m The asymmetric nearside-fa crustal thickness differences; in the thinner nearside crust. Seco tions in lithospheric thickness a KREEP Terrain). Differences in mony to a laterally and vertical flux of mare basalts is a result apiric rise and mantle melting, creasingly contractional, all fact magma. Late-stage volcanic eru volume, high-effusion rate erup tic of deep source regions below generation, ascent, intrusion an tation of the lunar volcanic rec their relation to mantle heterog