First terrestrial occurrence of tistarite (Ti2O3): Ultra-low oxygen fugacity in the upper mantle beneath Mount Carmel, Israel

The minimum oxygen fugacity (f_{O 2} ) of Earth's upper mantle probably is controlled by metal saturation, as defined by the iron-wüstite (IW) buffer reaction (FeO → Fe + O). However, the widespread occurrence of moissanite (SiC) in kimberlites, and a suite of superreduced minerals (SiC, alloys, native elements) in peridotites in Tibet and the Polar Urals (Russia), suggest that more reducing conditions (f_{O 2} = 6-8 log units below IW) must occur locally in the mantle. We describe pockets of melt trapped in aggregates of corundum crystals ejected from Cretaceous volcanoes in northern Israel which contain high-temperature mineral assemblages requiring extremely low f_{O 2} (IW < -10). One abundant phase is tistarite (Ti₂O₃), previously known as a single grain in the Allende carbonaceous chondrite (Mexico) and believed to have formed during the early evolution of the solar nebula. It is associated with other reduced phases usually found in meteorites. The development of super-reducing conditions in Earth's upper mantle may reflect the introduction of CH₄ + H₂ fluids from the deep mantle, specifically related to deep-seated volcanic plumbing systems at plate boundaries.