Nitrogen under super-reducing conditions: Ti oxynitride melts in xenolithic corundum aggregates from Mt Carmel (n. Israel)

Titanium oxynitrides (Ti(N,O,C)) are abundant in xenolithic corundum aggregates in py-roclastic ejecta of Cretaceous volcanoes on Mount Carmel, northern Israel. Petrographic observa-tions indicate that most of these nitrides existed as melts, immiscible with coexisting silicate and Fe-Ti-C silicide melts; some nitrides may also have crystallized directly from the silicide melts. The TiN phase shows a wide range of solid solution, taking up 0–10 wt% carbon and 1.7–17 wt% oxygen; these have crystallized in the halite (fcc) structure common to synthetic and natural TiN. Nitrides coexisting with silicide melts have higher C/O than those coexisting with silicate melts. Analyses with no carbon fall along the TiN–TiO join in the Ti–N–O phase space, implying that their Ti is a mixture of Ti³⁺ and Ti²⁺, while those with 1–3 at.% C appear to be solid solutions between TiN and Ti0.75O. Analyses with >10 at% C have higher Ti²⁺/Ti³⁺, reflecting a decrease in fO2. Oxygen fugacity was 6 to 8 log units below the iron–wüstite buffer, at or below the Ti2O3–TiO buffer. These relation-ships and coexisting silicide phases indicate temperatures of 1400–1100 °C. Ti oxynitrides are prob-ably locally abundant in the upper mantle, especially in the presence of CH4–H2 fluids derived from the deeper metal-saturated mantle.