Petrogenetic implications of chromite-seeded boninite crystallization experiments: Providing a basis for chromite-melt diffusion chronometry in an oxybarometric context
Abstract
Boninites are rare high magnesium andesites that often contain trace chromites. These chromites precipitate from primitive boninitic melts and are thought to be carried to the surface in melts that continue to crystallize a significant volume of silicate minerals. Such magmatic differentiation drives primitive chromite out of equilibrium with the residual melt with respect to both divalent and trivalent cation proportions. Diffusion then operates to alter primitive chromite toward a composition in equilibrium with residual melt. To simulate this process, we have performed internally heated pressure vessel experiments, providing insights into the processes of chromite-melt re-equilibration through time. While Fe-Mg exchange at magmatic temperatures equilibrates the tetrahedrally coordinated divalent cations in chromite in less than 6 hours, equilibration of trivalent cations in octahedral coordination with residual melts is slower. Our experimental results show that chromite Al concentrations are ubiquitously lower, and Fe2+/Fe3+ values are ubiquitously higher than modelled equilibrium values, indicating that there was insufficient time for significant Al and Fe3+ replacement of Cr. Similar observations can be made for natural chromite compositions in the extrusive sequence of the Troodos ophiolite (Cyprus). Based on a simple model of diffusive equilibration, we estimate that microphenocrystic chromites up to 60 μm in diameter take between 60 days and c. 170 years to equilibrate under conditions analogous to the physiochemical state of melt immediately prior to eruption. For the Troodos ophiolite extrusive sequence, this implies that mafic magmas are erupted less than c. 170 years after extraction from the mantle for disequilibrium textures and compositions to be preserved.
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