Approach to chemical equilibrium in diagenetic chlorites

Abstract

Electron microprobe analyses were made on diagenetic chlorites in sandstones and mudstones from two deep wells according to the petrographic character of the chlorite occurrence: as pseudomorphic phases, rims on quartz or glauconite or as distinct phases in the clay matrix. Chlorite compositions do not depend upon crystallization site (reacting phases) making it apparent that new chlorites can form in an approach to chemical equilibrium at or near the surface (40° C, 1 km depth). Comparison of this data with that for late diagenetic and early metamorphic chlorites indicates that the compositional range for different grains in the same thin section is similar for the samples throughout the 40°–270° C temperature span. Compositional range decreases upon further metamorphism.

Al content appears to be a more reliable indicator of temperature variations than other substitutions in the chlorite structure. The clay mineral assemblage which indicates sedimentary facies affects the trends in composition (Al increase or decrease) as a function of temperature. The octahedral site occupancy show a general increase in going from diagenesis to metamorphic conditions in pelitic rocks. The range of Fe-Mg ratios seems to depend more on the chemistry of each sample than the temperature of formation of the minerals.