Depositional environment, thermal maturity and irradiation effects on porphyrin distribution: Alum Shale, Sweden

The Swedish Alum Shale, which was deposited from Middle Cambrian to Lower Ordovician time in the shallow marine waters of the Lapetus Ocean, contains abundant uranium. The sedimentary organic matter associated with these shales has undergone irradiation, which has affected both the insoluble and soluble portions. Rock extracts from the Alum Shale contain exclusively vanadyl porphyrins indicating deposition under a marine anoxic environment. The decrease in the concentration of porphyrins with increasing uranium content indicates that irradiation has destroyed porphyrins. Comparison of the detailed vanadyl porphyrins distribution does not show any preferential destruction of particular vanadyl porphyrin, i.e., DPEP or ETIO, due to irradiation. Hence the porphyrin maturity parameter, which is based on the changes in the relative concentration of ETIO and DPEP porphyrins, can be used for estimating the maturity of the Alum Shale regardless of its uranium concentration. This also suggests that the effect of irradiation on porphyrins is different from that of heating.