Pseudobrookite-group oxide solutions and basaltic melts

Orthorhombic, (Bbmm), R2(1−x)3+Rx2+Ti1+xO5 = [R3+,R2+,Ti]M1 [Ti,R2+,R3+]2M2O5 solid solutions were first recognized in lunar rocks and then identified as such on Earth. Even though they are less common than ilmenite and magnetite (s.l.), there is strong field, experimental, and theoretical evidence that they are not geological oddities. Besides, pseudobrookite oxide solutions may have had a critical role in the petrogenesis of the mare basalts and lunar picritic glasses as a source of TiO2, and may be present in other planetary bodies as well. As materials they are of particular theoretical and practical interest because of the cation order–disordering phenomena that affect their physical and mechanical properties, and participation in the TiO2-slag smelting process. X-ray crystallography and spectroscopy studies have shown that all the cations are in octahedral coordination occupying two nonequivalent sites with a 1:2 ratio. Evaluation of site fractions, configurational entropy in R2(1−x)3+Rx2+Ti1+xO5 solutions, and of the known properties of pseudobrookite-group MgTi2O5 suggests that R23+TiO5 solutions may reach cation random distribution and thus entropy maxima at lower temperatures than R2+Ti2O5-rich solid solutions. As a result, the entropy contribution to their thermodynamic stability may depend upon the R 2 + R 3 + ratio. On the basis of computations, experiments, and chemography of mineral equilibria it is inferred that (1) assemblages of pseudobrookite-group Ti oxide, ilmenite, rutile, olivine, and pyroxene(s) should be expected under lower crust and upper mantle conditions in the Earth, and, conceivably, in other planetary bodies including the Earthʹs Moon; (2) reactions between pseudobrookite-group Ti oxide and olivine, orthopyroxene and pseudobrookite-group Ti oxide or ilmenite can produce ilmenite- or olivine-saturated Ti-enriched liquids; (3) lack of either ilmenite or pseudobrookite-group Ti oxide saturation in melts, which are in equilibrium with orthopyroxene and olivine enriches these melts in TiO2; (4) the assemblage olivine + orthopyroxene + rutile has a large stability field and is probably replaced by olivine +orthopyroxene + ilmenite with increasing pressure and temperature, and olivine + orthopyroxene + pseudobrookite-group Ti oxide with decreasing pressure and increasing temperature; (5) lunar source regions, nominally saturated in olivine, orthopyroxene, and pseudobrookite-group Ti oxide or ilmenite may produce the range of TiO2 content in the lunar pristine glasses which, apparently, cannot be derived by partial melting of model late-stage Lunar Magma Ocean oxide + silicate assemblages or equivalent bulk compositions.