Distinct Cooling Histories of Apollo 15 Basalts

Abstract

THE ridges and valleys of the Fra Mauro region on the Moon consist of a blanket of ejecta from the impact which created the Imbrium basin approximately 1,200 km away. The estimated thickness of the blanket at the Apollo 14 landing site is 40 to 80 m (ref. 1). The rocks returned from the landing site are predominantly friable breccias which include fragments of Imbrium basalt mixed with regolith. Some of the fragments may be older breccias which were formed by impacts before the Imbrium event. The two largest basaltic rocks returned are 14310 (3,439 g) and 14053 (251 g). Basalt 14310 has an intersertal texture (grain size 0.1 to 0.5 mm) and is at least 60% plagioclase (Ab5–20Or1–4 An75–95), 35 to 40% ortho and clinopyroxene; there are also small amounts of ilmenite, troilite, ulvospinel, glass metallic iron and so on3,4. This rock is chemically similar to the feldspathic basalts which may be lunar highland material5. It has also been suggested that it may have been crystallized at Fra Mauro in near-surface conditions from a remolten pocket in the ejecta blanket6. Basalt 14053 has an ophitic texture (grain size 0.5 to 2 mm) and contains 40 to 45% plagioclase (Ab10–20Or1–5 An75–90), 45 to 50% clinopyroxene and small amounts of olivine, ilmenite, cristobalite, troilite, ulvospinel, metallic iron, glass and so on3. This rock is probably a clast from a boulder of diameter 1 to 2 m which is a breccia2. Texture and composition are similar to those of Mare basalts as returned, for example, from Oceanus Procellarum.