Potential new minerals in corundum from the CR-11 chromatite orebody, Luobusa ophiolite, Tibet, China: Evidence for super-reduced mantle-derived fluids?

Titanium minerals enclosed in corundum separated from the Cr-11 orebody include native Ti, zamboite (FeTiSi2), osbornite (TiN)-khamrabaevite (TiC) solid solutions, TiSi2,, Ti5(Si,P)3, and TiB2, the last three are potentially new minerals. EELS, EDS and 3D electron diffraction were applied to study TiN-TiC, Ti5(Si,P)3, and TiB2 occurring in a single inclusion with an irregular outline up to 80 μm across. The osbornite-khamrabaevite solid solution forms an irregular mass up to 10 μm across having the composition Ti(N0.5C0.5). It is face-centered cubic cell with a=4.28(8) Å for which the space group Fm-3m is very probable. The Ti5(Si,P)3 phase with 22-25% of the Si replaced by P forms an incomplete overgrowth up to 20 μm thick around a rounded grain of TiB2 40 μm across. The Ti5(Si,P)3 phase has a primitive hexagonal cell with a=7.27(10) , c=5.03 (10) Å; space group P63/mcm; it is isostructural with Ti5Si3 and Ti5P3. TiB2 has a primitive hexagonal cell with a=3.01(6) , c=3.21(6) Å, space group P6/mmm; it is isostructural with synthetic TiB2. Corundum hosting the Ti minerals contains 0.2 – 1.3 atomic % Ti, whereas Cr and Fe contents < 0.05 at%. Cathodoluminescence (CL), which increases with decreasing Ti content, reveals both oscillatory and gradual zoning; contiguous corundum grains generally differ markedly in CL. Although the highly reduced Ti minerals all have analogues that can be synthesized at atmospheric pressure, these minerals could have formed at the high pressures implied by the presence of diamond recovered from the Cr-11 orebody. It has been suggested that the model proposed for highly reduced phases at Mount Carmel, Israel, including TiB2, can be applied to Tibet: local interaction of mantle-derived CH4±H2 fluids with basaltic magmas in the shallow lithosphere (depths of ∼30–100 km), resulting in precipitation of Ti3+-bearing corundum that entrapped alloy melts, some of which crystallized to the Ti-rich phases. Experimental work on the Ti-Fe-Si system indicates that minerals enclosed in corundum, Ti, FeTiSi2, TiSi2, and Ti5Si3, could have crystallized from the alloy melt at the lowest T accessible on the liquidus, i.e., 1034-1230 ºC in the ternary system (presumably lower in systems with B, P, C, N). The presence of TiB2 in three inclusions in Cr-11 suggests incorporation of crustal sediments in the subducted complex.