Feldspathic Mare Basalts at the Apollo 17 Landing Site, Taurus-Littrow

The basalt target rocks that have been converted to regolithacross the lunar maria are everywhere more feldspathic and lessmafic than the basalt hand specimens recovered from four Apollolanding sites, an effect not due to either horizontal or verticalmixing with adjacent highland materials. These crushed targetrocks need to be characterized by direct chemical and petrographicanalysis of the lithic fragments of basalt in the regolithsand by determination of the phase equilibria in and adjacentto these compositions at low pressure. Such data are availablefor the basalts of Mare Crisium and Mare Nubium (Luna 16, 24)and for Very Low Titanium basalt, first defined by three lithicfragments from the Apollo 17 core. These are all feldspathicbasalts, as are those from the Mare Tranquillitatis and OceanusProcellarum soils (Apollo 11, 12). Such data are lacking forthe principal basalt components at Mare Imbrium and Mare Serenitatis(Apollo 15, 17). The thoroughly investigated Apollo 17 landingsite at Taurus–Littrow, SE Mare Serenitatis, providesan example where other published information may be used toarrive at estimates of the composition of the feldspathic marebasalt that was the principal target material for regolith formation.This crushed basalt composition is that of a liquid close tobeing in simultaneous equilibrium with all of olivine, plagioclase,calcium-rich pyroxene, spinel, armalcolite and ilmenite at lowpressure. The simplest explanation would be that the basaltthat dominated the formation of the regolith comes from a differentflow unit than the hand specimens, but it strains credulitythat not a single hand specimen can be positively assigned tothat upper unit, and not a single soil sample can be positivelyidentified as having formed principally from the unit that providesthe hand specimens. KEY WORDS: cotectic; lithic fragment; lunar; target rock; regolith