INVESTIGATING MAXIMUM TEMPERATURES USING RAMAN SPECTROSCOPY OF CARBONACEOUS MATERIAL IN CONODONTS: A COMPARISON WITH THE CONODONT COLOR ALTERATION INDEX

Abstract

The conodont Color Alteration Index (CAI) has been widely used to provide estimates of maximum temperature in studies of thermal maturity. Despite this, several drawbacks of CAI have been identified. Firstly, there is significant uncertainty in the identification of CAI; methods have been proposed to make the quantification of CAI more rigorous, but they are not routinely applied. Secondly, CAI is not just related to temperature but also to duration of heating; as this duration is commonly unknown, there is significant uncertainty in estimates of maximum temperature based on CAI. Finally, CAI has been shown to be affected not just by heat, but also by diagenesis; although no mechanism by which diagenesis would affect CAI has been clearly defined. Raman spectroscopy of carbonaceous material (RSCM) has been extensively used to calculate maximum temperatures experienced by organic matter in sedimentary rocks. As carbonaceous matter is heated, it becomes progressively more graphitized, leading to an increase in structural order that can be measured by Raman spectroscopy. A set of 49 conodont specimens with varying CAI from several localities in British Columbia, Canada, were analysed with Raman spectroscopy, and maximum temperatures for these specimens were calculated independently from CAI by using the Iterative Fitting of Raman Spectra (IFORS) technique. Maximum temperatures calculated with IFORS often show significant deviation from the temperatures estimated by CAI alone; CAI thus appears to be an inconsistent indicator of maximum temperature. The greatest variation between IFORS and CAI temperatures occurs in specimens with CAI of 2.5, 4.5, and 6.0. Analysis of minor and trace element compositions of these specimens shows that they also have relatively high concentrations of exogenous cations. Additional investigation of the biomineral component of conodonts using Raman spectroscopy shows that diagenetic alteration is pervasive. We propose that conodont color is affected by the diagenetic uptake of exogenous cations into conodonts, and, as a result, CAI does not solely reflect changes in temperature. In contrast, RSCM with IFORS is a relatively inexpensive and minimally-invasive technique for the independent calculation of maximum temperatures, which can replace or supplement CAI.