Detrital and Diagenetic Illite Identified from Crystallite Thickness Measurements - A Tool for Paleo-temperature Estimate

Illite crystals in siliciclastic sediments are heterogeneous assemblages of detrital material coming from various source rocks and, at paleotemperatures >70°C, of superimposed diagenetic modification in the parent sediment. Only diagenetic rather than detrital phases should be used to estimate paleo-temperatures and to constrain the burial history of sedimentary successions that range in alteration intensity from diagenesis to low-grade metamorphism. The relative proportions of 2M1 detrital illite and diagenetic 1Md + 1M illite were distinguished by a combined analysis of crystal-size distribution and illite polytype quantification. It was found that the proportions of 1Md + 1M and 2M1 illite could be determined from crystallite thickness measurements by unmixing measured crystallite thickness distributions using theoretical and calculated lognormal and/or asymptotic distributions. In addition, quantitative polytype analysis was carried out using RockJock software for comparison. The two techniques gave comparable results which indicate that the unmixing method permits one to calculate the proportion of illite polytypes and, therefore, the proportion of 2M1 detrital illite, from crystallite thickness measurements that should not be used for paleotemperature evaluation.