Evidence from Mössbauer spectroscopy for intense localization of metamorphic fluid flow during the formation of spaced cleavage

Arenites in the low-grade metasedimentary rocks of the Stirling-Barren Group, Western Australia, contain discrete spaced cleavages. These in part are pure white mica, but up to 30% of the cleavage zones consists of fine-grained quartz resulting from crystal plastic deformation. A modified Gresens analysis shows that the cleavage domains formed by a 50% volume loss associated primarily with SiO₂ and MgO concentration decreases of 58% and 37%, respectively, and a 12% decrease in Fe₂O₃. Mössbauer spectroscopy shows that the Fe²⁺: Fe³⁺ ratio from lithon to cleavage changed from 50:SO to 15:85. This substantial difference cannot be accounted for by loss of iron. The data are best explained by the focusing of a highly oxidizing fluid through the cleavage domains where volumetric fluid rock ratios around 150 to 1 are indicated. In this example advectic processes appear to be important in low-grade regional metamorphism.