Co–Ni decoupling indicates fluid exsolution during the formation of podiform chromitites: Insights from the Luobusa ophiolite, southern Tibet

Abstract

Podiform chromitites typically occur in harzburgites near the petrological Moho in ophiolite sections, and are petrologically and economically significant. Hydrous fluids may have an important role in the formation of podiform chromitites, although unambiguous evidence for the involvement of such fluids is rare. To examine whether hydrous fluids are involved in the formation of podiform chromitites, we measured the Ni, Co, Mn, and Zn contents of olivines and chromites in representative samples from the Luobusa ophiolite, southern Tibet, taking account of the different geochemical behaviors of these elements in melts and hydrous fluids. In the harzburgites, the olivine (Ol) has a limited compositional range (2605–2919 ppm Ni, 117–135 ppm Co, 736–1005 ppm Mn, and 28.9–35.7 ppm Zn), whereas the chromite (Chr) has a relatively variable composition (718–1239 ppm Ni, 364–493 ppm Co, 1314–2733 ppm Mn, and 1411–1900 ppm Zn). From the dunite lenses to the dunite envelopes, and to the chromitites, both olivine and chromite exhibit significant decreases in Co (Ol = 137 to 45 ppm; Chr = 542 to 168 ppm), Mn (Ol = 889 to 273 ppm; Chr = 2426 to 862 ppm), and Zn (Ol = 37.7 to 4.0 ppm; Chr = 1746 to 206 ppm) contents, and an increase in Ni contents (Ol = 3009 to 5946 ppm; Chr = 413 to 1462 ppm). These features cannot be fully explained by subsolidus re-equilibration, partial melting, fractional crystallization, and melt–rock reactions. Olivine relicts in chromite indicate the dissolution of pre-existing olivine in dunitic channels. The olivine dissolution may have been due to the water-rich nature of the parental melts of the chromitites. These observations and mass-balance calculations suggest that, during the formation of podiform chromitites, chromite crystallization was accompanied by olivine dissolution and exsolution of a hydrous fluid phase. This resulted in the preferential transfer of Ni into the melt and olivine relicts, and Co, Mn, and Zn into the fluid phase. As such, the chromite and olivine in the chromitites became Ni-rich and Co-, Mn-, and Zn-poor, which led to Ni–Co decoupling.