The geochemistry of the opaque minerals in Apollo 14 crystalline rocks

Abstract

The opaque minerals in rock 14310, a high-alumina fine grained basalt consist of ilmenite, and optically anisotropic chromium ulvöspinel, native FeNi metal, baddeleyite, tranquillityite, schreibersite, and a Fe, Ni sulfide (pentlandite or chalcopyrrhotite?). The opaque assemblage of rock 14053, a high-Fe medium grained basalt with subophitic texture consists of ilmenite, a titanium-chromite, chromium ulvöspinel, troilite, FeNi metal, fayalite with breakdown products including pure Fe, tridymite and silica glass. In rock 14310 only one spinel was found; it is a chromium ulvöspinel which occurs in pyroxene and the groundmass and is similar in composition to the ulvöspinels formed during late-stage crystallization of Apollo 12 rocks. Spinels encountered in 14053 are a titanium aluminium rich chromite, chromium ulvöspinel and titanium picotite. Their composition is similar to spinels found in the Apollo 12 microgabbro 12018. The Fe Ni metal in 14310 contains 1.5 to 37 wt % Ni and 0.5 to 1.9 wt % Co and occurs middle to late in the paragenetic sequence. These Ni contents cannot be correlated with the mineral assemblage in which the metal phase occurs. Two coexisting iron phosphide minerals, schreibersite, one a Ni-free variety and the other containing 23 wt % Ni, were found in FeNi metal and troilite. This is the first reported occurrence of schreibersite as an indigenous mineral in the lunar rocks and casts suspicion on the criterion for meteoritic contamination used with the Apollo 11 and 12 samples. The zirconium dioxide, baddeleyite, is commonly associated with the opaque minerals, mainly ilmenite. Electron microprobe analyses indicate the presence of minor Zr in both ilmenite and ulvöspinel (0.2 up to 0.57 wt % ZrO 2 in ilmenite versus 0.03 to 0.19 in ulvöspinel). i.e. Zr is preferentially concentrated in the ilmenite structure. The presence of schreibersite in 14310 and the late stage breakdown of fayalite to pure Fe + silica are strong evidences supporting crystallization under extremely low ƒ: 0 2 .