Chemical homogenization by fluid-present deformation in chromitites: An example from Golyamo Kamenyane chromitites, SE Bulgaria

Abstract:

Chromite is one of the main reservoirs of chromium in the Earth, and is an essential important accessory mineral in upper mantle peridotites preserved in ophiolites. Generally, chromite is better preserved than silicates of the host peridotitic rocks, because it is more resistant to fluid-related processes; this has led to the widespread use of chromitites to track the evolution of Earth’s mantle convection.

Deformed chromitites enclosed in serpentinized peridotites of Golyamo Kamenyane massif in SE Bulgaria have been studied as an example of chemical homogenization by deformation during metamorphism. Theses chromitites were affected by amphibolite-facies metamorphism and reacted with low fO₂ fluids and subsequent oxidizing Fe³⁺-rich fluids (~ 1.0 GPa and 500-700ºC)(Gervilla et al., 2012).

Here, we describe the details of how the microstructure was produced in chromite, showing the important role of deformation during metamorphism. They show porphyroclastic texture with coarse-grained porphyroclast (0.1 ~ 5 mm) and fine-grained neoblasts (> 100µm). Coarse-grained chromites show chemically zoned feature from core to rim, which preserve the initial igneous character, while at the rim a metamorphic signature is detectable. Our study using electron back-scattered diffraction reveals significant crystal-plastic deformation, such as inter-crystalline deformation defined by low-angle boundaries in the rim areas. Fine-grained chromites, both recrystallized and nucleated grains have higher Fe³⁺ and Cr and lower Mg# value compare to the core of coarse-grained chromites, which indicate that the fine- grained aggregates of chromite formed by both subgrain rotation and nucleation occurred in the presence of oxidizing fluids. Overall, deformation in a chemically distinct environment produced chemical homogenization of chromite.