Spectral reflectance of conodonts; a step toward quantitative color alteration and thermal maturity indexes

Texas Wesleyan University, Department of Physics, Fort Worth, TX, United States

Changes in the color of conodonts have long been used to assess thermal maturity. Color is a subjective measure, and color changes in conodonts are related to a subjective scale, the conodont alteration index or CAI. In this paper, we propose a simple, nondestructive method for objectively determining CAI and relating CAI to thermal maturity, the spectral reflectance of conodonts (SRC). The diffuse reflectance of about 30 large conodont fragments arranged on a barium-sulfate slide was determined with a total reflectance spectrophotometer in the wavelength range of 300-850 nm. By examining conodonts that ranged from a CAI of 1 to a CAI of 6 we found that the average slope of the reflectance curve from 550 to 800 nm is a good proxy for CAI. A second-order regression equation estimates CAI from this slope with high accuracy (correlation coefficient = 0.99). These estimates appear most accurate for a CAI of 1 to a CAI of 4, where the slopes change most rapidly, but give reasonable results up to a CAI of 6. Based on the results of our analysis of two samples with known thermal maturities from the Valles Caldera region of New Mexico, we propose a preliminary relationship among SRC slope, CAI, and in-situ alteration temperature.

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