22-EPN3-025: Isotope Geochemistry Traces Magma–Shale Interaction

Visit by Manfredo Capriolo, University of Oslo (Norway), to TA2.10 Stable, Rare Gas and Radiogenic Isotope Facility at CRPG (France).
Dates of visit: 17 – 28 April 2023

Report Summary: The research project aims to constrain and quantify magma–shale interaction via rhenium-osmium isotope analyses of whole rock samples from different geological settings in the Oslo Rift (about 300 Ma). Magma–shale interaction can produce massive amounts of greenhouse gases when magmas are injected into sedimentary basins, with potential implications for sulphur sequestration as sulphides. Here, 8 magmatic rock samples (of both intrusive and effusive origin) and 2 host shales were analysed in the laboratories of the Stable Rare Gas and Radiogenic Isotope Facility (CRPG, Nancy, France).

Among the investigated magmatic ones, 7 rock samples are alkali basalt sills with variableextents of hydrothermal overprint, and 1 rock sample is an evolved lava. The latter has a very low (< 5 ppt) Os concentration, making its magmatic Os isotope ratio prone to overprinting by crustal assimilation. The 7 alkali basalt sills display ¹⁸⁷Os/¹⁸⁸Os_299Ma values ranging from 0.14 to 1.15, reflecting different (from negligible to strong) degrees of shale contamination. Interestingly, the 2 host shales of Cambrian age display ¹⁸⁷Os/¹⁸⁸Os_299Ma values of 1.12 and 1.42. The rhenium-osmium isotope data from this research project indicate that magma–shale interaction via shallow magmatic intrusions in sedimentary basins can vary in intensity, implying a different extent of thermogenic degassing. These results improve the understanding of degassing scenarios from past events, open new perspective on the comparison with present-day atmospheric changes, and suggest interesting implications for economic resources, such as hydrocarbon reservoirs or sulphide ore deposits.

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