Lunar Volcanism: Factors Controlling Intrusion Geometries and Eruption

Introduction: Partial melting occurs in planetary bodies due to (1) temperature rise as heat is produced from radioisotope decay; or decompression as parts of the mantle rise due to (2) thermal convection or (3) overturn driven by compositional instability. Case (1) dominated early-forming differentiated asteroids heated by short half-life isotopes [1, 2] but is not relevant to the Moon. There is still debate as to the relative importance of the latter two processes as a function of depth in the Moon and geologic time [3], but both cause progressive accumulation of melt with time. Melt migration occurs by slow percolation along grain boundaries, but magma geochemistry on Earth suggests [4] that melt migrates more quickly in fracture networks [2] in the upper mantle [5] when the strain rate applied to the host rocks overlying a zone of partial melting drives the rocks from a plastic to an elastic rheological response [6]. Once a fracture is initiated, melt drains into it to form a dike. If the melt viscosity is small enough, melt flow from the source region into the dike may be fast enough to cause the fast-moving dike tip to continue to fracture the host rocks in a brittle fashion. We used this concept to model giant dike swarms inferred to underly graben radiating from Tharsis, Mars [6] and now apply the same principles to the Moon. Intruded dikes: Figure 1 shows a cross-section of the geometry of a partial melt zone in the mantle feeding a dike propagating along a neutral buoyancy level (NBL) at depth D defined by the density change at the base of the crust. The dike driving pressure Pd is provided by the buoyancy [g H Δρ] of the column of melt extending to the base of the source region at depth H below the NBL (g is the acceleration due to gravity and Δρ is the difference between mantle and magma densities). The dike grows both up into the crust and down into the upper mantle. The NBL does not have to be the base of the crust: Wieczorek et al. [7] have shown that some lunar basalts are less dense than the lower crust and so may be neutrally buoyant within the crust. With the appropriate choice of D the same principles apply.