Olivine-spinel re-equilibration in chromitites from the Alpine-Dinaride region

The crystal structure of spinels allows a wide range of cation substitutions, both in tetrahedral and in octahedral sites. In particular, when chromite is associated with olivine, these two minerals exchange Mg and Fe2+ ions under sub-solidus conditions. The exchange is temperature-pressure dependent. The dependence from T and (to a lesser extent) P, was used for the calibration of the olivine-spinel geothermometer (Ballhaus et. al., 1991), for the calculation of the re-equilibration temperature of the system. The application of this method depends on sub-solidus ions diffusivity, and in particular the possibility to reach equilibrium compositions. Usually, it is assumed that the equilibrium compositions of chromites and olivines are those analyzed close to intergranular borders. In order to better understand the spatial distribution of Mg and Fe2+ within these two minerals, we investigated their diffusion as a function of the distance from intergranular limits, with the aim of extending geothermometer applications to rocks with variable chromite to olivine ratios. For the evaluation of Mg-Fe2+ diffusion, we selected chromitite ores from two different geological contexts, characterized by fresh chromitites and dunites. The first area is located in the Finero Complex, Northern Italy, where sub-continental mantle peridotites host fresh chromitite bodies enveloped by dunites. The second area is the Iballe chromite mine, Northern Albania, where fresh chromitite bodies are hosted in the mantle section of the Mirdita Ophiolite. Mg and Fe2+ distribution between chromites and olivines was investigated through grids of EMP analyses (usually 7x7 or 9x9 with a mesh size of 0.1 or 0.2 mm) at variable chromite to olivine ratios. The most simple configuration, a tiny euhedral chromite grain surrounded by olivine, was used to model the ion diffusion curve in olivine. The Mg and Fe2+ concentrations are constrained by two boundary conditions: their values at the boundary and for a distance that tends to infinity. The best fit was attained using two curves: a hyperbole far from the boundary and an exponential close to the grain boundary, which can be used to infer re-equilibrated compositions, and hence temperatures. The same mathematical model, applied to chromitites, allows to extrapolate two different compositions, re-equilibrated and primary (magmatic?). This statistical methodology provides a way to derive re-equilibration compositions -and subsequently temperatures- of accessory spinel-bearing rocks more reliable than the usual approach based on analyses close to the boundary. It can be applied also to rocks that underwent widespread serpentinization, a common feature in podiform bodies within ophiolites. Finally, when applied to chromitites, it also returns primary compositions of both spinel and olivine allowing to deduce a primary, possibly magmatic, temperature. REFERENCES Ballhaus, C., Berry, R.F., & Green, D.H., 1991. High pressure experimental calibration of the olivine-orthopyroxene-spinel oxygen geobarometer: implications for the oxidation state of the upper mantle. Contributions to Mineralogy and Petrology, 107(1), 27-40.