Opaque oxide mineral crystallization in lunar high-titanium mare basalts

Compositions of the above minerals vary according to their textural setting. Late-stage, nearly pure ulvöspinel is also often present, especially in coarse-grained basalts. Subsolidus assemblages (1) Cr-ulvöspinel (host) + ilmenite ± native Fe and (2) ilmenite (host) + rutile + aluminian chromite ± native Fe and/or troilite are common in high-Ti mare basalts, particularly Apollo 17. Systematic lamellar orientation of the subsolidus oxide minerals suggests, for the most part, an origin by exsolution rather than by reduction. Compositional data indicate that host minerals undergo only minor changes in MgO and Fe/(Fe+Mg) as a result of subsolidus crystallization.

Composition of the magma, its cooling rate, and oxygen fugacity affect the stability, abundance, and order of crystallization of opaque oxides in high-Ti mare basalts. Cr-ulvöspinel is restricted to basalts with >0.3 weight percent Cr₂O₃. Armalcolite crystallizes from melts with ≥10 weight percent TiO₂ and ceases to crystallize when Fe/(Fe+Mg) of armalcolite reaches ∼0.70 and a reaction relation occurs. Slower cooling enhances reaction of armalcolite with the melt and results in decreased modal abundance. The modal abundance of Cr-ulvöspinel also decreases with slower cooling, probably because of increased Cr substitution in pyroxene and earlier nucleation of plagioclase. Ilmenite modal abundance is directly related to TiO₂ content of the magma and is independent of cooling rate. However, slower cooling tends to produce ilmenite grains of more magnesian composition through increased resorption of early olivine crystals. Absence of armalcolite in Apollo 11 low-K basalts and rarity of Cr-ulvöspinel in Apollo 11 high-K basalts may be due to crystallization at oxygen fugacities, respectively, above ∼10⁻¹³ or below ∼10⁻¹⁴.

Cr-ulvöspinel, armalcolite, and ilmenite are enriched in TiO₂ and Cr₂O₃ relative to high-Ti mare basalt magmas. Gravitational crystal settling of one, two, or all three of these minerals will produce derivative liquids increasingly poorer in TiO₂ and Cr₂O₃. Simultaneous fractionation of olivine depletes the derivative liquids in MgO. Compositional variations observed in high-Ti mare basalts can be accounted for by moderate (Apollo 17) to small (Apollo 11) amounts of fractionation involving early-crystallizing opaque oxide minerals and olivine