Nd isotope composition of seep carbonates: Towards a new approach for constraining subseafloor fluid circulation at hydrocarbon seeps

Abstract

The carbonates forming at deep-sea hydrocarbon seeps provide an archive for the source and migration pathways of the seeping fluids. Interpretation of the archived isotopic and elemental signatures is, however, not straightforward because of mixing between the signals of fluids and ambient seawater, limited understanding of subseafloor fluid circulation, and effects of diagenetic alteration. The Nd isotope system is among the most widely used tracers of circulation patterns in modern and past oceans, but to date Nd isotopes have been rarely used in investigations of fluid migration pathways at hydrocarbon seeps. To test the sensitivity of Nd isotope signals of seep carbonates to record past interactions between the seeping fluids and Nd-143-enriched, volcanic-derived mineral components, we performed Nd, Sr and stable isotope analyses of the Lower Cretaceous (Barremian) seep carbonates found in Bagka (Czech Carpathians). These seep deposits are directly underlain by a complex of mafic igneous rocks, which the methane-charged fluids must have passed through on their way to the seafloor. The epsilon(Nd)(t) values measured in the seep carbonates are significantly more radiogenic than the inferred signal of contemporaneous local seawater, and shifted towards the composition typical of the underlying basaltoids. This is in agreement with the Sr-87/Sr-86 ratios, which are markedly lower that the coeval seawater signature. Consistent co-variance trends are observed among the measured epsilon(Nd)(t), Sr-87/Sr-86 and delta C-13 values, attesting increasing role of the volcanic-derived fluid component with increasing contribution of methane-derived carbon. The results of the present approach reveal the potential of Nd isotopes to fingerprint exotic fluid end-members present in plumbing systems of both fossil and modern methane discharges. The method can be of particular use in studies of seeps associated with plate margins, where Nd-143-enriched igneous materials are commonly encountered in the basement or in the sediment column.