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New Insights into the Origin of 14053 -- The Only Basaltic Rock Returned by Apollo 14

Introduction: The Apollo 14 Mission brought back a large number of lunar rocks, the majority of which were breccias [1]. Of the four rocks that were believed to be basaltic, 14310 and its pair, 14073, turned out to be our ‘type specimens’ of “melt rocks, at the time, a new concept for the lunar community. The remaining two, 14053 and its small-sized pair, 14072, are unique among the rocks in the lunar collection, in that they possess evidence for being the most reduced of all lunar basalts. It is to the origin of these unusual rocks to which this study is addressed. The sample studied, 14053, has a mass of 251g and is commonly used as an example of a pristine high-Al basalt. Rb-Sr studies yielded a well-defined age of 3.92 ±0.04Ga [2]; however, when 14053 was reinvestigated by Snyder and Taylor [3], the Sm-Nd data were determined to be disturbed, indicating that some of the components formed before the 3.92 Ga, Rb-Sr age. This is highly unexpected as Rb-Sr isotope systematics are almost always more easily disturbed then those of the Sm-Nd system [4]. These results led to the hypothesis that 14053 and other Apollo 14 hi-Al basalts actually had an impact-melt origin, with the pre-4.0 Ga ages representing melt rocks that were not reset after impact [3]. Likewise, unique petrographically observed, reduction features also led to questions about its origin. We have revisited these unusual features as they relate to the origin of 14053. Petrographic Description: As studied in thin section, 14053 has an ophitic texture and consists of predominantly pyroxene (50%) and plagioclase (40%). The remainder of the rock consists of ilmenite, chromite-ulvöspinel, silica, in the form of tridymite, FeNi metal, fayalite, troilite, K-Ba-rich glass, F-apatite, and baddeleyite. The rock contains typical basaltic mesostasis, consisting of fayalite, silica, glass, phosphates, and opaque minerals. Some of the fayalite shows reduction-breakdown to Fe metal and silica (Fig. 1). This rock also displays abundant examples of the extreme reduction of Cr-ulvöspinel to ilmenite, TiAl-chromite, and native Fe (Fig. 2). It is these reduction textures and mineral chemistry that are evidence for extreme reduction, the only lunar basalt with such assemblages. Results: Electron microprobe analyses of the opaque minerals in a thin section of 14053 confirm the earlier findings [1]. The typical sub-solidus reduction of ulvöspinel to ilmenite + Fe, seen is most mare basalts, involves ulvöspinel with relatively minor Cr2O3. The fO2 conditions are usually not sufficient low to reduce spinel that has appreciable Cr2O3 (e.g., CrFigure 1: Mesostasis showing breakdown of fayalite.