How to make a geothermometer for high-resolution measurements of the reflectance of small vitrinite particles

Abstract

Vitrinite reflectance geothermometer was originally developed for use in oil exploration, but it has various applications; e.g., estimating the depth of burial in sedimentary basins, analysis of the thermal structure of accretionary complexes, and detecting evidence of frictional heat in fault rocks. However, use of the vitrinite reflectance geothermometer is not widespread. This paper describes a modified apparatus that was specifically designed to analyze small vitrinite particles in accretionary complexes and fault rocks. The original apparatus was designed to illuminate large coal samples over a wide field of view (>100 μm), and measured reflectance with a high-voltage photomultiplier. A wide field of illumination is required for petrographic identification of vitrinite; however, reflections from other bright minerals around the small vitrinite particles generate a high level of background noise. The apparatus described here has a dual illumination system: a homogeneous beam illuminates a wide field of view to allow identification of vitrinite, and a concentrated spot beam of only 1.6 μm diameter is used to measure reflectance. The spot illumination system measures reflectance with a semiconductor photodiode sensor. The avalanche-type photodiode has a sensor cooling system, provides high-resolution, low-noise reflectance measurements, and is simpler to use than the photomultiplier.