The potential of Nd isotopes in disentangling fluid sources at hydrocarbon seeps: a recent perspective

In recent years, Nd isotopes have seen increasingly common use in studies of hydrocarbon seeps. Given the distinct Nd isotope signature of mafic igneous rocks, particular emphasis in these investigations has been on reconstructing former interactions between the seeping fluids and volcanic materials. The results of our case studies on ancient seeps underlain by mafic volcanic bodies, including Cretaceous seep carbonates of the Outer Carpathians (Czech Republic) and Basque-Cantabrian Basin (Spain), as well as Eocene seeps of the Cascadia convergent margin (Washington) consistently document their significant enrichment in volcanogenic Nd, reflected in their increased Nd isotope ratios (ε_Nd values). The extent of this ¹⁴³Nd-enrichment varies depending on the geological context of given seeps, most notably the ε_Nd signatures and thickness of the volcanics and overlying sedimentary piles, and the Nd isotope signal of background local pore waters. The highest ε_Nd values are observed in seep carbonates very shallowly underlain by thick mafic volcanics: the Cretaceous seep of the Carpathians and Eocene seeps of Cascadia. For the former, the ε_Nd values are up to 7.5 units higher than the signature of coeval non-seep pore water, whereas for the latter the ε_Nd values are as high as +1.9, close to the highest value ever recorded for seawater. More moderate ¹⁴³Nd-enrichment typifies the Cretaceous seeps of Spain, for which the volcanic intrusions were emplaced at considerable depths below the seafloor. In such cases, the Nd isotope signature of the fluid-volcanic interactions was partially obscured by subsequent interactions between the fluids and the overlying sediments. Rather than focusing solely on exploring the new geochemical tool, the primary aim of our studies was to address broader questions regarding the tectonic architecture and geological history of the sedimentary basins that host given seep deposits. For the Eocene seeps of Washington, the Nd isotope data served to document interactions between the methane-rich fluids and the volcanic terrane of Siletzia, which underlies the Cascadia forearc; these results placed important stratigraphic and structural constraints on the activation and earliest history of convergence in Cascadia, following Siletzia accretion. For the studied Cretaceous seeps, all hosted by early, sediment-covered rifts, the studies demonstrated that Nd isotopes offer a valuable new tool of deconvolving methane fluxes from different organic matter alteration pathways for the very complex, sedimentary-magmatic systems of incipient rifts. At the same time, these studies emphasized important limitations of Sr isotopes, the system most commonly used to document interactions between the seeping fluids and igneous rocks. Because of the much higher Sr/Nd ratios observed in pore waters than in igneous rocks, the potential of Sr isotopes to record fluid-volcanic interactions is considerably lower than that of Nd isotopes. Thus, broader use of Nd isotopes can assist in identifying potential volcanogenic fluid endmembers for the numerous sedimented rifts for which evidence for magmatic involvement in the fluid expulsion remains equivocal.

This work was supported by the National Science Centre, Poland, grant No. 2016/23/D/ST10/00444