The Case for Lunar Maria as Potential Layered Extrusives

Harrison H. Schmitt
Engineering Physics, University of Wisconsin-Madison, Albuquerque, NM

Very little information exists on the thickness of magma cooling units within the lunar basaltic maria. Early photogeologic mapping identified thin, but extensive flows on the surface of mare regions; however, the thickness of cooling units deeper in the 2-4km thick basin filling units could not then be determined. The pervasive and deep, extensional fracturing of the pre-mare lunar crust beneath large impact basins, as well as elsewhere, would provide a structural environment conducive to extremely rapid initial eruption of mare magmas, provided that magma formed equally rapidly in the mantle. With rapid eruptions, thick cooling units may have formed. Fractional crystallization within such units likely would create classically layered extrusives, including possible mineral resource concentrations. Field observations and sampling by the author to approximate depths of ~150m during the Apollo 17 mission to the Valley of Taurus-Littrow on the Moon, combined with active seismic and gravity measurements, suggest that the mare basalt partially this valley consists of an approximately 1.1-1.4km thick, single cooling unit. No Apollo 17 samples collected and studied so far, however, indicate that gravitational layering is present in this unit. On the other hand, the author’s analysis of the sample suite from the Apollo 12 site indicate that gravitational differentiation occurred in the late, relatively thin olivine basalts erupted there.

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