Effect of thermal maturation on the K–Ar, Rb–Sr and REE systematics of an organic-rich New Albany Shale as determined by hydrous pyrolysis

Abstract

Hydrous-pyrolysis experiments were conducted on an organic-rich Devonian–Mississippian shale, which was also leached by dilute HCl before and after pyrolysis, to identify and quantify the induced chemical and isotopic changes in the rock. The experiments significantly affect the organic–mineral organization, which plays an important role in natural interactions during diagenetic hydrocarbon maturation in source rocks. They produce 10.5% of volatiles and the amount of HCl leachables almost doubles from about 6% to 11%. The Rb–Sr and K–Ar data are significantly modified, but not just by removal of radiogenic 40Ar and 87Sr, as described in many studies of natural samples at similar thermal and hydrous conditions. The determining reactions relate to alteration of the organic matter marked by a significant change in the heavy REEs in the HCl leachate after pyrolysis, underlining the potential effects of acidic fluids in natural environments. Pyrolysis induces also release from organics of some Sr with a very low 87Sr/ 86Sr ratio, as well as part of U. Both seem to have been volatilised during the experiment, whereas other metals such as Pb, Th and part of U appear to have been transferred from soluble phases into stable (silicate?) components. Increase of the K 2O and radiogenic 40Ar contents of the silicate minerals after pyrolysis is explained by removal of other elements that could only be volatilised, as the system remains strictly closed during the experiment. The observed increase in radiogenic 40Ar implies that it was not preferentially released as a volatile gas phase when escaping the altered mineral phases. It had to be re-incorporated into newly-formed soluble phases, which is opposite to the general knowledge about the behavior of Ar in supergene natural environments. Because of the strictly closed-system conditions, hydrous-pyrolysis experiments allow to better identify and even quantify the geochemical aspects of organic–inorganic interactions, such as elemental exchanges, transfers and volatilisation, in potential source-rock shales during natural diagenetic hydrocarbon maturation.