Armalcolite/ilmenite in lunar basalts : mineral chemistry, paragenesis, and origin of textures

Lunar high-titanium basalts contain ilmenite and armalcolite with various textural relationships. Differences in these textures have been attributed to differences in bulk rock composition, cooling rate and oxygen fugacity. These three parameters were investigated by controlled cooling-rate experimentation with: : (a) rocks. 70017 and 74275; (b) 4.5°C/hr and 0.5°C/hr cooling rates; and (c) I/W -0.2 and I/W -1.2 log atm f02 conditions. The rimming of armalcolite by ilmenite and the variety of textures so developed are largely a function of crystallization sequence, particularly involving pyroxene and ilmenite, which is dependent on f02. Experiments at low f02 tend to form "para-armalcolite" (i.e., unrimmed), whereas at fO2 near I/w, rimmed armalcolite ("ortho-") predominates. Compositions of ilmenite and armalcolite are dependent on the liquid composition, which in turn is a function of temperature and oxygen fugacity. The trivalent titanium content of armalcolite is also a function of f02, with greater Ti3 3+ contents representative of lower in armalcolite can be used as an indicator of the approximate oxygen fugacity prevailing at the time of crystallization.