Abstract
Mg-Fe2+ diffusion patterns in olivine and chromite are useful tools for the study of the thermal history of ultramafic massifs. In the present contribution, we applied the exponential modeling of diffusion patterns to geothermometry and geospeedometry of chromitite ores from two different ophiolite contexts. The Iballe ophiolite (Northern Albania) hosts several chromitite pods within dunites. Primary and re-equilibrated Mg#, estimated by using an exponential function, provided re-equilibration and primary temperatures ranging between 677 and 996 °C for chromitites and between 527 and 806 °C for dunites. Cooling rates for chromitites are higher than for dunites, suggesting a different genesis for the two lithologies, confirmed also by spinel mineral chemistry. Chromitites with MORB affinity formed in a SSZ setting at a proto-forearc early stage, explaining the higher cooling rates, while dunites, with boninitic affinity, were formed deeper in the mantle in a more mature subduction setting. At the Nea Roda ophiolite (Northern Greece) olivine in chromitites do not show Mg-Fe variations, and transformation into ferrian chromite produced “fake” diffusion patterns within chromite. The absence of diffusion patterns and the low estimated temperatures (550–656 °C) suggest that Nea Roda chromitites were completely re-equilibrated during an amphibolite-facies metamorphic event that obliterated all primary features.
Highlights
Published: 4 January 2022Spinels crystallize in a wide range of solid solutions at different PT conditions
To minimize the potential error in the characterization of primary and re-equilibrated values, we used an exponential function (Equation (1)) to model Mg# diffusion patterns, following the methodology implemented by Bussolesi et al [11]
Results show that Iballe cooling rates are different for dunites and chromitites
Summary
Spinels crystallize in a wide range of solid solutions at different PT conditions. They are refractory and resistant to alteration, and in the case of chromite, among the first phases to crystallize [1,2,3]. Due to their properties, they are used as petrogenetic indicators [2,4,5], as their mineral chemistry is a function of parental melt composition and geodynamic setting, as well as temperature and pressure. The olivine-spinel geothermometers potentially work over a wide range of temperatures, from high-T conditions down to at least 650 ◦ C, until the cessation of the elemental exchange [8,9,10]
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