OXIDE-SILICATE INTERACTION IN PLANETARY INTERIORS: PSEUDOBROOKITE TITANIUM OXIDES

Pseudobrookite Ti oxides represent a great source or sink of Ti in Earth and Planetary systems, moreover, their presence is controlled by the response of mineral reactions to the specific tectonic path(s) followed by a rock unit which makes them potentially good petrogenetic or geodynamic indicators. Cation-disordering and configurational entropy contributions may enhance the stability of Psd oxides rich in trivalent cations relative to ones that are rich in divalent cations at low to moderate T in general and, depending on the trivalent cations at either high or low fO2 conditions in particular. Geochemically, their significance lies in their direct or indirect effect on the speciation of HFSE and REE (e.g., Lu-Hf). Computations and experiments anticipate primary Ilm+Ol+Opx, Rt+Ol+Opx, and Psd oxide+Ol+Opx assemblages in peridotites which in this order appear to mark isothermal decreases in P. Melting of Ol+Opx+Ti oxide can produce Ti-enriched melts in equilibrium with Ol+Opx or Ol+Ti oxide whereas Opx+Ti oxide assemblages are metastable relative to Ol+Liq. The crystallization of Opx+Psd oxide/Ilm from Ol-saturated mafic melts may require extreme Ti-enrichment, perhaps, even greater than that documented in the lunar picritic glasses (16.4 wt%) which, not concidentally, lack liquidus Ilm, Psd oxide, or Rt. Only melt compositions that fall within an Ol-Opx-Ti oxide join have the potential to be Ti-enriched and coprecipitate Ol, Opx, and either Psd oxide, Ilm, or Rt. During cooling early Psd oxide is likely to react with melt which may eliminate any evidence of its former presence in mafic systems. Lunar data suggest that early Cr-spinel may also react with melt or Opx to produce Opx+Ti oxide or Ol+Ti oxide, respectively. Compositional and phase equilibrium constraints, dilution, and Ti oxide fractionation may restrict the presence of Ti-rich melts or Ti-enriched regions in the Earth’s mantle whereas the source region(s) of the lunar picritic glasses may have contained Ti oxides in general and Psd oxide in particular. This possibility may obviate the need to invoke mixing of late-stage shallow level Fe-rich Ilm-bearing materials with Mg-rich Ol+Opx-dominated lunar mantle sources. The lunar picritic glasses may represent nearly eutectic or small degree melts of nominally Ol+Opx+Ti oxide-saturated sources and/or primary Opx+Psd oxide/Ilm assemblages.