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

Abstract

The basalt target rocks that have been converted to regolith across INTRODUCTION the lunar maria are everywhere more feldspathic and less mafic than The conventional interpretation of lunar petrogenesis the basalt hand specimens recovered from four Apollo landing sites, viewed the mare basalt hand specimens as representing an effect not due to either horizontal or vertical mixing with primary magma compositions. These were postulated to adjacent highland materials. These crushed target rocks need to be have originated by small mass fractions of renewed partial characterized by direct chemical and petrographic analysis of the melting of (plagioclase-saturated) mantle cumulate selithic fragments of basalt in the regoliths and by determination of quences produced during consolidation of a global the phase equilibria in and adjacent to these compositions at low magma ocean, which was itself generated by impact pressure. Such data are available for the basalts of Mare Crisium heating during initial accretion of the Moon. This inand Mare Nubium (Luna 16, 24) and for Very Low Titanium terpretation requires revision (O’Hara, 2000), because basalt, first defined by three lithic fragments from the Apollo 17 the Al/Si ratios of >3·5 × 10 tonnes of mare basalt core. These are all feldspathic basalts, as are those from the Mare remotely sensed from orbit cannot be explained if the Tranquillitatis and Oceanus Procellarum soils (Apollo 11, 12). erupted magma composition was similar to that of the Such data are lacking for the principal basalt components at Mare hand specimens. Spectral reflectance studies would apImbrium and Mare Serenitatis (Apollo 15, 17). The thoroughly pear to extend this conclusion beyond the limited tracks investigated Apollo 17 landing site at Taurus–Littrow, SE Mare of the orbital X-ray spectrometer to the greater part of Serenitatis, provides an example where other published information the nearside maria surfaces (Pieters, 1978; and see Basaltic may be used to arrive at estimates of the composition of the Volcanism Study Project, 1981, section 2.2.1, and plates feldspathic mare basalt that was the principal target material for 2.8 and 2.9). Liquids of hand-specimen composition have regolith formation. This crushed basalt composition is that of a high-pressure phase equilibria totally inconsistent with liquid close to being in simultaneous equilibrium with all of olivine, their derivation as low mass fraction partial melts from plagioclase, calcium-rich pyroxene, spinel, armalcolite and ilmenite originally plagioclase-saturated, even if now completely at low pressure. The simplest explanation would be that the basalt plagioclase-free, mantle assemblages (Longhi, 1982; that dominated the formation of the regolith comes from a different O’Hara, 2000, fig. 4, and notes 93 and 96), or some more general mantle bulk composition close to saturation flow unit than the hand specimens, but it strains credulity that not with an alumina-rich phase. a single hand specimen can be positively assigned to that upper The pronounced negative europium anomalies in the unit, and not a single soil sample can be positively identified as basalts have been attributed to europium depletion of having formed principally from the unit that provides the hand their source region due to flotation of plagioclase crystals specimens. from that magma ocean to form the lunar highland crust. There is, however, no positive europium anomaly in the average lunar highlands (Korotev & Haskin, 1988; O’Hara, 2000, fig. 3, and notes 61 and 62). In the absence of that anomaly, there is no compelling evidence for plagioclase flotation, for the former existence of a