Lunar magma transport phenomena

Abstract

An understanding of the most significant events during the first Ga or so of lunar history—global differentiation associated with a Lunar Magma Ocean (LMO) and the later generation of about 10 7 km 3 of mare basalts—depends profoundly on magma transport dynamics. LMO evolution is dominated by the characteristics of hard-turbulent convection of a fluid undergoing phase change. Quench protocrust, formed at the ocean surface in early times, is repeatedly disrupted by bolide impact, ocean body tides, and associated crustal foundering. During this epoch, the ocean temperature is set by the balance between convective heat brought to the surface and brown-body radiation from the surface. At later times, the convective flow is organized into a number of regimes. From top to bottom these include conductive crustal lid, a viscous sublayer, an inertial core, a lower viscous sublayer, and a mushy cumulate region. Crystal settling and flotation is restricted to the viscous sublayer regions. Dilatancy pumping may prevent the efficient expulsion of intercumulus melt during cumulate formation. Typical velocities within the inertial region are of order 10 m/s. Because late-stage Ti-rich cumulates are dense relative to the earlier Mg-rich cumulates, hyper-to-sub-solidus convection tends to mix LMO crystallization products. Numerical simulations that account simultaneously for compositional and thermal buoyancy effects indicate a characteristic mixing time ∼ 200 Ma. Statistical measures of mixing (variance and spatial correlation length) quantify the details of mixing. The Darcy percolative and vein-drain models of melt extraction are briefly discussed. Because compaction lengths are small, ~ 10 2 m, the rate of melt extraction is governed by the balance between melt buoyancy and Darcy friction. Differential (melt/matrix) velocities are of order 10 km/Ma for nominal values appropriate for mare basalt melt extraction from a heterogeneous (mixed) LMO source. Transport of magma through the lunar lithosphere via a connected crack network seems highly probable. For the range of inferred discharge rates applicable for Lunar Mare Volcanism (LMV; 10 3 to 10 7 m 3/s) typical fracture widths and ascent speeds are 10 −1 to 50 m and 10 −1 to 10 m/s, respectively. A simple model is proposed to evaluate the extent of fractionation as magma traverses cold lunar lithosphere. If Apollo green glasses are primitive and have not undergone significant fractionation en route to the surface, then mean ascent rates of 10 m/s and cracks of widths > 40 m are indicated. Lunar tephra and vesiculated basalts suggest that a volatile component plays a role in eruption dynamics. The predominant vapor species appear to be CO, CO 2, and COS. Near the lunar surface, the vapor fraction expands enormously and vapor internal energy is converted to mixture kinetic energy with the concomitant high-speed ejection of vapor and pyroclasts to form lunar fire fountain deposits such as the Apollo 17 orange and black glasses and Apollo 15 green glass.