The interrelationship of biological marker maturity parameters and molecular yields during contact metamorphism

Abstract

Jurassic siltstone samples were collected, on a centimetre scale, as a function of increasing distance from a dolerite dyke (Isle of Skye, northwest Scotland). The constant origin and type of the organic matter in this single lithological horizon is indicated by organic petrological observations as well as organic geochemical analyses. Therefore, changes in biological marker distributions, with distance from the dyke contact, can be unequivocally correlated with the thermal influence of the intrusion. The vitrinite reflectance ( R 0 Average) values of these samples increase gradually from 0.35% to 3.60% as the dyke contact is approached. Four classical biological marker maturity parameters ( 20S (20S + 20R) , TA (TA + MA) , C 20 (C 20 + C 28) , and 22S (22S + 22R) ). have been measured from GCMS analyses of the sample extracts. Changes in these molecular parameters were compared with changes in the concentrations of the individual biological marker compounds, expressed relative to mass of rock extracted. This comparison revealed that these molecular parameters are not governed by the conventional productreactant relationships (chiral isomerisation, aromatisation or side-chain cracking reactions) operating solely in the free saturated or aromatic hydrocarbon structures. Release/formation from macromolecular and/or functionalised moieties (hydrocarbon or non-hydrocarbon) followed by compound loss provide an alternative paradigm in all four cases in this particular thermal regime. Direct chiral isomerisation in the free sterane or homohopane cannot, however, be completely ruled out as a contributor to an admixture of processes. Similarly, aromatisation in the free hydrocarbon fraction may account for a proportion of the triaromatic steroids.