The Role of Magmatism in Hydrocarbon Generation in Sedimented Rifts: a Nd Isotope Perspective from Mid-Cretaceous Methane-Seep Deposits of the Basque-Cantabrian Basin, Spain

Studies on the involvement of intrusive magmatism in hydrocarbon generation within sedimentary basins have gained momentum owing to increasing appraisal of the role that such processes may play in controlling global carbon cycle perturbations, and the exploration potential of the volcanic sedimentary basins. Nevertheless, for many areas the causal link between the intrusions and surrounding hydrocarbon systems remains disputed, encouraging a search for methods that could aid in identifying different hydrocarbon sources. Here, we have performed a multi-proxy geochemical study of the middle Cretaceous methane-seep deposits of the Basque-Cantabrian Basin, an early-stage, peri-cratonic rift marking the Mesozoic opening of the Bay of Biscay. Infilled by a thick sedimentary succession intruded by shallow-level igneous bodies, the basin shares analogies with modern young, sedimented rifts that sustain hydrocarbon seepage. We have applied a novel approach that uses the Nd isotope composition of the seep deposits to constrain the relationship between hydrocarbon seepage and igneous activity, and to explore the general potential of Nd isotopes to trace magmatic-influenced fluids in volcanic sedimentary basins. The Nd isotope data have been combined with rare earth element analyses and carbon and oxygen isotope measurements, providing broad insight into the former composition of the seeping fluids. For three out of four investigated seeps, the Nd isotope ratios observed in authigenic seep carbonates include signatures markedly more radiogenic than that reconstructed for background seawater-derived pore waters. The level of this Nd-143-enrichment varies both between and within individual deposits, reflecting spatial and temporal differences in fluid composition typical of seep-related environments. The radiogenic Nd isotope signals provide evidence of subseafloor interactions between the seeping fluids and mafic igneous materials, supporting the model of an igneous control on the mid-Cretaceous methane expulsion in the Basque-Cantabrian Basin. The thermogenic origin of the methane is in accord with the moderately negative delta C-13 values and paragenetic successions observed in the studied seep carbonates. For a single deposit, its relatively unradiogenic Nd isotope composition can be attributed to the smallest size and shallowest emplacement depth of the underlying intrusion, likely resulting in a short-lived character and limited hydrocarbon-generation potential of the associated contact metamorphism. The study demonstrates that Nd isotope analyses of seep carbonates offer a tool in disentangling methane fluxes from different organic matter alteration pathways for the numerous, both fossil and modern sedimented rifts for which the involvement of various methane sources remains insufficiently understood.This work was supported by the National Science Centre, Poland (grant No. 2016/23/D/ST10/00444; to MJ) , and the Eusko Jaurlaritza (Ikerketa Taldeak IT930-16) and the Spanish State Research Agency (project PID2019-105670GB-I00/AEI/10.13039/501100011033; both to LMA

Nd and Sr isotopic evidence for provenance of clastic material of the Upper Triassic rocks of Silesia, Poland

Nd and Sr isotope data were used to characterize the sources of the Upper Triassic (Keuper) siliciclastic rocks of Silesia in southern Poland. This continental succession, consisting predominantly of fine-grained mud- stones and siltstones, yields a remarkably uniform Nd isotopic composition. Nd model ages T2DM vary from 1.56 to 1.69 Ga and εNd values are in the range from –8.9 to –11.2, documenting old crust contribution in the provenance. In contrast, the Sr isotopic composition (87Sr/86Sr) of the clastics exhibits a relatively large variation from 0.710 to 0.723. The isotopic compositions indicate that the southern part of the Germanic Basin in Silesia was supplied with clastic material from the Bohemian Massif. The axis of the drainage area must have crossed from SW to NE the Saxothuringian units of the East Sudetes and most probably also the area of the Tepla–Barrandian Unit. There is no indication of any sediment transport from the Moravo-Silesian Belt and the Fore-Sudetic Block. It seems, that the Palaeozoic rocks of the latter domain must have been buried completely during Late Triassic times