THE INTERRELATIONSHIP OF BIOLOGICAL MARKER MATURITY PARAMETERS AND MOLECULAR YIELDS DURING CONTACT-METAMORPHISM

Jurassic siltstone samples were collected, on a centimetre scale, as a function of increasing distance from a dolerite dyke (Isle of Skye, n orthwest Scotland). The constant origin and type of the organic matter in this single lithological horizon is indicated by organic petrologi cal observations as well as organic geochemical analyses. Therefore, c hanges in biological marker distributions, with distance from the dyke contact, can be unequivocally correlated with the thermal influence o f the intrusion. The vitrinite reflectance (R0 Average) values of thes e samples increase gradually from 0.35% to 3.60% as the dyke contact i s approached. Four classical biological marker maturity parameters (20 S/(20S + 20R), TA/(TA + MA), C20/(C20 + C28), and 22S/(22S + 22R)), ha ve been measured from GCMS analyses of the sample extracts. Changes in these molecular parameters were compared with changes in the concentr ations of the individual biological marker compounds, expressed relati ve to mass of rock extracted. This comparison revealed that these mole cular parameters are not governed by the conventional product-reactant relationships (chiral isomerisation, aromatisation or side-chain crac king reactions) operating solely in the free saturated or aromatic hyd rocarbon structures. Release/formation from macromolecular and/or func tionalised moieties (hydrocarbon or non-hydrocarbon) followed by compo und loss provide an alternative paradigm in all four cases in this par ticular thermal regime. Direct chiral isomerisation in the free steran e or homohopane cannot, however, be completely ruled out as a contribu tor to an admixture of processes. Similarly, aromatisation in the free hydrocarbon fraction may account for a proportion of the triaromatic steroids.