Illite crystallinity: variability in vertical sections at different sub-metamorphic grades

Illite crystallinity, a measure of the thermal transformation of illite to muscovite, records the narrowing of the (001) diffraction peak up grade. However, it commonly varies to some degree with host-rock lithology, which can diminish its reliability as a paleothermometer. The Pennsylvanian-age Narragansett Basin contains argillaceous rocks and impure coals that reflect a wide range of thermal conditions (deep-burial diagenetic to staurolite grade) though over 3/4 of the area is underlain by sub-metamorphic rocks, as measured from coal rank and authigenic illite. Extensive use was made of cored coal-bearing sections that had been thermally altered in either diagenetic, lower anchizone (sub-chlorite) and upper anchizone (sub-biotite) realms in order to characterize the range of variability found at different argillaceous and bituminous horizons within each section. Ranges of illite crystallinity within individual cores vary inversely with grade but high variability is particularly characteristic of the diagenetic regime. Correlations to coal-rank parameters suggest that the most reliable illite geothermometers at this grade are found in low-organic mudstones that are not in close association with coal horizons. Illite from impure coals/bituminous shales have the lowest crystallinities under diagenitic conditions but such differences are minimal once anchimetamorphic conditions are attained. High octahedral Al/Fe+Mg and interlayermore » K appear to favor increased crystallinity at all sub-metamorphic levels.« less