Feldspathic Mare Basalts at the Appollo 17 landing site, Taurus-Littrow

Abstract

The basalt target rocks that have been converted to regolith across the lunar maria are everywhere more feldspathic and less mafic than the basalt hand specimens recovered from four Apollo landing sites, an effect not due to either horizontal or vertical mixing with adjacent highland materials. These crushed target rocks need to be characterized by direct chemical and petrographic analysis of the lithic fragments of basalt in the regoliths and by determination of the phase equilibria in and adjacent to these compositions at low pressure. Such data are available for the basalts of Mare Crisium and Mare Nubium (Luna 16, 24) and for Very Low Titanium basalt, first defined by three lithic fragments from the Apollo 17 core. These are all feldspathic basalts, as are those from the Mare Tranquillitatis and Oceanus Procellarum soils (Apollo 11, 12). Such data are lacking for the principal basalt components at Mare Imbrium and Mare Serenitatis (Apollo 15, 17). The thoroughly investigated Apollo 17 landing site at Taurus-Littrow, SE Mare Serenitatis, provides an example where other published information may be used to arrive at estimates of the composition of the feldspathic mare basalt that was the principal target material for regolith formation. This crushed basalt composition is that of a liquid close to being in simultaneous equilibrium with all of olivine, plagioclase, calcium-rich pyroxene, spinel, armalcolite and ilmenite at low pressure. The simplest explanation would be that the basalt that dominated the formation of the regolith comes from a different flow unit that the hand specimens, but it strains credulity that not a single hand specimen can be positively assigned to that upper unit, and not a single soil sample can be positively identified as having formed principally from the unit that provides the hand specimens.