Mechanism of formation of podiform chromitite: Insights from the oxidation states of podiform chromitites and host peridotites from the Luobusa ophiolite, southern Tibet

Although the mechanism of formation of podiform chromitite is debated, oxygen fugacity (fO(2)) is likely one of the controlling factors, although the detailed mechanisms are poorly understood. To provide insight into this issue, we determine the Fe3+/Sigma Fe ratio of chromite via Mossbauer spectrometry, and from this calculate the fO(2) of chromitites, dunite envelopes, dunite lenses, and their host peridotites in the Luobusa ophiolite in southern Tibet. The fO(2) (-1.49 to -0.52 log units relative to the fayalite-magnetite-quartz buffer) and mineral chemistry of the host peridotites are similar to those of both abyssal and forearc peridotites. Higher fO(2) values of the dunites and chromitites (-0.65 to -0.33, 0.69 to 1.13, and -0.46 to 0.64 log units for the dunite lenses, dunite envelopes, and chromitites, respectively) relative to those of host peridotites must have been inherited from their parental magmas. Both chromite compositions and the fO(2) of the dunites and chromitites are similar to those in boninite-like rocks. Importantly, there is a previously unrecognized but marked decrease in fO(2) from the dunite envelopes to the chromitites. This decrease is intimately related to the chromitite-forming processes and preferably explained as the result of the exsolution of oxidized fluids from high-fO(2) magmas during subduction initiation.

Automated summary

The study reveals that the fO(2) values of dunites and chromitites in the Luobusa ophiolite in southern Tibet are higher than those of their host peridotites, likely inherited from their parental magmas. The formation of chromitites may be related to the exsolution of oxidized fluids from high-fO(2) magmas during subduction initiation.