Closure temperatures of intracrystalline ordering in anatectic and metamorphic hercynite, Fe2+Al2O4

The closure temperature, T(C), of the intracrystalline ordering of Mg-hercynite is estimated with a comparative crystal-chemical approach. The single crystals were selected front two distinct geological environments that represent extremely different cooling rates. The fast cooled setting, refers to allatectic metapelitic enclaves that occur in the high-K calc-alkaline lavas of the Neogene Volcanic Province of SE Spain. The slow cooled setting refers to metabauxite from the Anga metamorphic complex, Lake Baikal. Parameters sensitive to T(C) include the oxygen fractional coordinate (u) and the inversion parameter (i). Experimental equilibration data on the spinel and hercynite end-members and on their solid solution are fitted to equations where T(C) is given its a function of the hercynite content (Hc) of the solid solution and of u or i. The unavoidable simplifications made in this empirical approach are discussed. A reasonable value for T(C), similar to 400 degrees C, was obtained for the slow cooled metamorphic hercynite from the oxygen fractional coordinates. In contrast, an unreasonably high value of T(C) similar to 600 degrees C, was obtained from the inversion parameters. In the case of the last cooled anatectic samples, T(C) calculated from the two structural parameters are comparable; the five crystals show a rang in the calculated values for T(C) over similar to 250 degrees C, from similar to 700 to similar to 950 degrees C, which is reasonable considering the known diversity of cooling rates exhibited by their volcanie host-rocks.