Sr–Nd isotope decoupling at Cretaceous hydrocarbon seeps of the Basque-Cantabrian Basin (Spain): Implications for tracing volcanic-influenced fluids in sedimented rifts

Abstract Strontium isotopes have seen common use as a tracer of volcanic-influenced fluids at hydrothermal vents and cold seeps hosted by sedimented rifts. However, for some fluid emissions, no apparent contribution of volcanogenic Sr has been observed, despite geological or seismic evidence suggesting magmatic involvement in the fluid expulsion. Here, we explore the behaviour of Sr during fluid-rock interactions in a sedimented rift based on the isotope and elemental composition of Albian hydrocarbon-seep carbonates of the Basque-Cantabrian Basin, Pyrenees. The basin represents a peri-cratonic rift formed during the opening of the Bay of Biscay and infilled with organic-rich siliciclastics affected by subvolcanic intrusions. The intrusions stimulated thermogenic hydrocarbon generation in the sediments and resulted in enrichment of the associated seep carbonates in volcanic-derived, radiogenic Nd. In contrast, none of the 87Sr/86Srcarbonate ratios observed in this study provide evidence for subseafloor interactions between the seeping fluids and igneous materials. Instead, all four studied seep deposits show various degrees of 87Sr-enrichment, bearing the fingerprint of interactions with the rift-filling siliciclastics, rather than a volcanogenic signature. Furthermore, for three deposits, the observed unusual, positive correlation between the Sr and Nd isotope ratios attests to a presence of an evolved, deep-seated fluid end-member enriched in both 87Sr and 143Nd. This isotope decoupling can be explained by the different responses of Nd and Sr to subseafloor fluid-volcanic interactions, with a moderate degree of elemental exchange able to modify the fluid Nd isotope signal, but insufficient to affect the fluid Sr isotope signature. The study shows that for many sedimented seeps and vents, and especially for low-temperature, hydrocarbon-dominated emissions, volcanic-influenced fluids can be more reliably traced using the Nd isotope system, whereas Sr isotopes cannot provide standalone evidence for their absence.