Fluid-present deformation aids chemical modification of chromite: Insights from chromites from Golyamo Kamenyane, SE Bulgaria

Abstract

Chemical signatures of chromitites are commonly used to track the evolution of the Earth's mantle. However, chemical modification during deformation may have important implications for the interpretation of chromites' signatures. Here, we describe the details of how deformation promotes chemical modification in chromite. Physicochemical characteristics of the chromites were quantified by measuring crystallographic orientation relationships using Electron Back-Scattered Diffraction (EBSD) and electron microprobe analysis (EMP). Chromites show porphyroclastic textures with coarse-grained porphyroclasts (ca. 0.2-5 mm) and fine-grained neoblasts (<200 mu m). Coarse-grained chromites are chemically zoned in terms of major elements from core to rim, preserving this initial igneous feature in the cores, while the outer rims reveal a metamorphic signature. Large chromite grains are characterized by local crystal-plastic deformation, exhibiting distinct inter-crystalline deformation including continuous crystal bending and subgrain boundaries as well as chemical modification in their outer, deformed parts. Two types of fine-grained chromite, F1 and F2, are present. While F1 exhibits a well-developed polygonal texture, straight grain boundaries and low intercrystal misorientation (<1 degrees), F2 shows low-angle boundaries and significant intercrystalline misorientation (2-8 degrees). Both F1 and F2 have higher Fe3+ and Cr and lower Mg# values than the cores of large grains. We interpret F1 and F2 to represent chromite recrystallized by heterogeneous nucleation and subgrain rotation recrystallization, respectively. Crystallographic preferred orientation (CPO) and misorientation data on the well-developed low-angle (subgrain) boundaries in coarse grains and F2 grains indicate that deformation in chromite was accommodated mainly by dislocation creep with the dominant activation of the {111}< 100 > slip system. The retrograde P-T exhumation path predicted by thermodynamic and chemical modeling suggests that these fine-grained chromites were produced when the initial chromitites reacted with oxidizing fluids during retrograde metamorphism (similar to 1.0 GPa and 500-700 degrees C). Our results show that deformation in the dislocation-creep regime in a chemically open system has induced chemical modification and homogenization within chromite aggregates as well as strain localization. This close physicochemical link offers new avenues of interpreting the chemical signatures of chromites, utilizing their microstructurally controlled variation or lack thereof.