Authigenic mineral growth in sedimentary and metasedimentary rocks enhances the degree of structural order of carbonaceous material

Abstract

The degree of structural order of carbonaceous matter (CM) in metasedimentary rocks is thought to largely depend on the maximum temperature the host rock experienced. This led to the widespread use of Raman spectroscopy of carbonaceous matter (RSCM) as geothermometer to determine maximum burial or metamorphic temperatures. The analytical certainty of most geothermometers based on RSCM is given as ±30–50 °C and the structural order of CM is often shown to be heterogeneous within individual samples, geological beds and units. To better understand the influence of authigenic mineral growth on CM maturation and heterogeneity we analyzed samples from three Precambrian CM-bearing stratigraphic units. Raman spectroscopic results indicate large heterogeneities in the degree of structural order of CM, resulting in ranges of up to 190 °C in calculated temperature within individual samples. Generally, within each sample, CM within authigenic minerals shows a significantly higher degree of structural order than CM in areas devoid of authigenic minerals. This results in calculated average temperatures from CM within authigenic minerals that are 27–94 °C higher than temperatures calculated from CM in neighboring areas devoid of authigenic minerals. Our results indicate that the structural order of CM within authigenic minerals is significantly enhanced. Thus, geothermometers based on RSCM must be used carefully with a thorough awareness of the petrographic context.