Lunar pyroclastic deposits and effusive constructs: Petrology, eruption styles, and spectral properties.

摘要

We have combined results from sample studies of the Apollo 17 volcanic beads with Clementine remote sensing observations of deposits composed of these beads in order to understand and model the erruption that emplaced the beads. A study of metal blebs in the Apollo 17 74001/2 core shows that the magma experienced at least three reactions: (1) an initial reduction at an oxygen fugacity of IW-1.3 at 4 km depth to form CO gas (1320{dollar}spcirc{dollar}C); (2) oxidation, perhaps related to reactions that produced the volatiles on the surfaces of the beads; and (3) reduction reactions in the volcanic plume during bead cooling and crystallization. The textures within the beads are dominantly a function of their location within the plume and the diversity of mineral textures and compositions within the beads indicates several cooling regimes were possible in the plume. A brown texture seen in all the orange glasses at the bottom of the core is interpreted to be from devitrification occurring as the glasses fell back through the optically dense plume after being ejected to the outer portions of the plume.; Clementine UVVIS data was used to study the spectral properties of seven regional Dark Mantle Deposits (DMDs). Dark patches in the Sinus Aestuum deposit represent one extreme with the weakest UV/VIS slope and glass absorption while the Aristarchus Plateau deposit has the strongest slope and glass absorption. Other regional deposits fall between these two extremes because they have mixtures of the glasses and beads, along with local soils. Regional DMDs are thought to form from Hawaiian style fire fountains where submillimeter clasts are expelled tens of kms to form continuous deposits while larger clasts land near the vent to coalesce and form mare lava flows. The Orientale Ring deposit on the western limb, which is the only annular regional DMD identified on the Moon, is interpreted to have erupted from a 20-km high umbrella-shape plume with average ejection velocities of 360 m/s and glasses expelled to an average range of 80 km from the vent.; Mare domes, such as those in Marius Hills, Rumker Hills, and in northern Mare Tranquillitatis, were spectrally identical to adjacent mare. In contrast, volcanic cones of Marius Hills and Mons Esam were spectrally distinct, with lower reflectances, UV/VIS slopes, and weaker mafic absorptions, all attributed to fine-grained crystals in the spatter. The mare domes are interpreted to have formed at low effusion rates that would allow a small shield to develop. Most lunar cones are aligned linearly, supporting eruptions from near-surface dikes. The Marius Hills cones show no similar alignment and are therefore considered to be from strombolian activity marking the transition from effusive eruptions that produced the mare and domes to more explosive activity at the terminal stages of these eruptions.