Optical properties and chemistry of graptolite periderm following laboratory simulated maturation

Abstract

The optical properties and chemistry of graptolite periderm have been investigated following laboratory induced maturation and compared to vitrinite subjected to the same experimental procedures. With increasing maturation there is a systematic increase in maximum, minimum and random reflectance and bireflectance of the graptolite periderm. Infrared spectroscopy demonstrates that the graptolite periderm comprises principally an aromatic structure with aliphatic groups. With progressive maturation the graptolite periderm undergoes dealkylation with a loss of aliphatic C-H containing moieties (CH 1, CH 2 and CH 3); at high levels of maturation the aromatic system undergoes structural transformation towards a more highly conjugated and condensed aromatic ring structure. The hydrogen and oxygen indices of the graptolite periderms, determined by Rock-Eval pyrolysis, are similar to Type II kerogen and exhibit a progressive decrease in hydrogen index with increasing maturation. There is a direct correlation between the maximum and random reflectamce of graptolite periderm and vitrinite of the same maturation level. The graptolite periderm has consistently lower mean minimum reflectance and higher brieflectance than vitrinite which may expressthe biaxial negative optical properties and greater aromaticity of the graptolite periderm as compared to the vitrinite. Comparison of infrared spectra suggests that the graptolite periderm comprises fewer aliphatic structures and is more highly aromatic than vitrinite of the same maturation. The optical and chemical properties of graptolite periderm are sensitive indices of organic maturation, and their maximum and random reflectance can be correlated with vitrinite reflectance. Measurements of the optical or chemical properties of graptolites provides a precise index of organic maturation and is of particular utility in pre-Upper Silurian strata or marine rocks in which vitrinite and palynomorphs are rare or absent.