NEW INSIGHTS INTO SMALL LUNAR PYROCLASTIC DEPOSITS

Lunar dark-mantle deposits are high-priority sites for lunar science and exploration, partly because they are volatile- and metallic-element enriched remnants of ancient explosive or pyroclastic volcanic eruptions. New data from recent missions, including Kaguya, Chandrayaan-1, and Lunar Reconnaissance Orbiter (LRO), reveal details of topography, geomorphology and mineralogy that allow us to address major questions about the distribution, composition, volume and eruptive styles of lunar pyroclastic deposits. Of the ~100 known pyroclastic deposits, the majority are small (10s to 100s km²) and associated with irregular depressions, rilles, or non-circular volcanic vents located in floor-fractured craters. Telescopic spectral data suggest that these small lunar volcanoes contain olivine and pyroxene among the juvenile magmatic components and fragmented basalt and local host rock. Analyses of small pyroclastic deposits in five craters have focused on morphology, vent characteristics and other evidence that may allow us to identify separate eruptive episodes, pulses of magma intrusions, and changes in composition and volatility of source materials with time. Kaguya Terrain Camera and LRO Narrow Angle Camera images of these deposits show smooth surfaces with variations in surface texture and albedo that can be related to differences in eruption mechanism and/or duration. Evidence of layering at some sites suggests low-volume eruptions or multiple events. At Alphonsus and J. Herschel, smooth-surfaced, friable, asymmetric deposits, non-circular vent craters, and possible interior layering are observed; supporting an intermittently explosive, vulcanian eruption style. At Atlas and Oppenheimer, the deposits are more widespread, lower in albedo, and originate from irregular depressions. At Atlas, a thin layer of dark, friable material is covered in places by a cracked crust that may represent a late-stage eruption of hotter, more fluidized magma. At Lavoisier crater, small pyroclastic deposits are associated with small, low-relief deposits like those at Alphonsus, but lobed lava flows are also present, suggesting an association of effusive and explosive emplacement. These results suggest a greater diversity of emplacement styles of small explosive volcanic deposits than previously recognized.