The Nature and Origin of Boulder 1, Station 2, Apollo 17

The Boulder 1 breccias are similar in composition to other Taurus-Littrow massif samples and therefore probably derived from the same source, undoubtedly the Serenitatis basin. However, they are substantially different in texture from other Apollo 17 massif rocks, indeed are very nearly unique among the rocks returned by all Apollo missions. The boulder is set apart by its content of dark, rounded inclusions or bombs, up to several tens of centimeters in dimension, consisting largely of very fine, angular, mineral debris, welded together by a lesser amount of extremely fine-grained material that appears to be devitrified glass. To account for these uncommon structures, a phase of the basinforming impact event is sought that would produce relatively small amounts of debris and deposit them on or near the basin rim. It is suggested that the components of the boulder might represent very early, high angle ejecta from the Serenitatis event, and that the dark breccia inclusions are accretional structures formed from a cloud of hot mineral debris, melt droplets, and vapor that was ejected at high angles from the impact point soon after penetration of the Serenitatis meteoroid. This small amount of early high-angle ejecta would have remained in ballistic trajectories while the main phase of crater excavation deposited much larger amounts of deeper-derived debris and melt-rock on the rim of the basin, after which the early ejecta was deposited as a cooler (∼450°C) stratum on top. The matrix of this breccia gained its modest degree of coherency by thermal sintering as the capping stratum cooled. The boulder is a fragment of this layer, broken out and rolled to the foot of the South Massif ⩽ 55 m.y. ago.