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Did the Orientale Impact Melt Sheet Undergo Large-scale Igneous Differentiation by Crystal Settling?

James Cassanelli
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Abstract Text: 

Theoretical scaling laws predict the generation of significant quantities of impact-derived melt during basin-scale impact crater formation. Given the known distribution of large impact basins across the surface of the Moon, these predictions suggest that a considerable portion of the upper lunar crust may be comprised of impact melt products. Due to the potentially significant volumetric contribution of impact melt to the upper lunar crust, understanding the nature and evolution of impact melt sheets is of fundamental importance to understanding the geology and petrology of the Moon. While the volumes of melt produced through impacts are reasonably well constrained by theory and observations, relatively little is known about the processes involved in melt sheet cooling and solidification subsequent to initial formation.
Following the impact event, melt created through shock heating will collect within the excavated crater and immediately begin rapid radiative heat loss to space. This radiative top-down cooling leads to the establishment of a large temperature gradient across the impact melt sheet producing a configuration characterized by very high Rayleigh numbers. Under these conditions, the impact melt sheet will undergo vigorous convection which will govern continued heat loss from the system. As heat is lost from the melt sheet and temperatures drop below the liquidus, crystals will begin to nucleate within the melt. The fate of the crystals nucleated during cooling will determine the petrology of the final impact melt deposit. In general, two end-member cases are possible: (1) Nucleated crystals may separate from the melt through fractional crystallization, causing igneous differentiation and producing an altered crustal stratigraphy, or (2) Nucleated crystals may remain entrained within the melt and solidification may occur by equilibrium crystallization, causing homogenization of the crustal stratigraphy.
Here, we perform a case study on the Orientale basin and explore the possibility for igneous differentiation of the Orientale impact melt sheet by assessing the thermal and physical processes driving cooling and crystallization, focusing on crystal settling. Results show that efficient convective cooling results in complete solidification of the ~15 km thick Orientale basin impact melt sheet within ~5.5 kyr. The rapid cooling and vigorous convection prevent crystal settling up to a solid fraction of ~0.6, at which point the increased viscosity and lower thermal gradients slow convection enough to allow efficient crystal settling at the non-convective boundaries of the system. However, at this solid fraction, a sharp rheological transition is predicted, beyond which the melt-crystal mixture takes on a solid-like behavior, subduing both convection and settling motions. Therefore, equilibrium crystallization is predicted to dominate during solidification of the Orientale impact melt sheet, and large-scale igneous differentiation is not anticipated. This result is in agreement with remote sensing observations and petrologic predictions and suggests that up to ~5% by volume of the lunar crust may have been homogenized through impact melt processes.

J. Trimble (ARC)
SSERVI Identifier: 

Recognizing that science and human exploration are mutually enabling, NASA created the Solar System Exploration Research Virtual Institute (SSERVI) to address basic and applied scientific questions fundamental to understanding the Moon, Near Earth Asteroids, the Martian moons Phobos and Deimos, and the near space environments of these target bodies. As a virtual institute, SSERVI funds investigators at a broad range of domestic institutions, bringing them together along with international partners via virtual technology to enable new scientific efforts."