Rare earth elements in mussel shells of the Mytilidae family as tracers for hidden and fossil high-temperature hydrothermal systems

Bathymodiolus mussels of the Mytilidae family live in ecological niches at marine vent sites where hydrothermal fluids discharge at the seafloor and mix with ambient seawater. We report the first concentration data for rare earth elements and yttrium (REY) of Bathymodiolus shells and of low-temperature diffuse hydrothermal fluids venting in their respective habitat at three hydrothermal vent fields along the Mid-Atlantic Ridge (MAR) and for littoral Mytilus edulis shells from the western North Atlantic, Dingle Bay, Ireland, and from the southern North Sea, German Bight, Germany. Similar to high-temperature hydrothermal solutions expelled in the respective area, the low-temperature diffuse fluids from the hydrothermal vent sites display positive Eu anomalies. These indicate that the fluids carry a component previously involved in high-temperature water–rock interaction, as decoupling of Eu from neighbouring REY requires temperatures above about 200 °C. While the Bathymodiolus shells from the hydrothermal vent fields reflect these positive Eu anomalies, the littoral M. edulis shells studied for comparison, do not show Eu anomalies, consistent with the lack of any high-temperature hydrothermal activity in their coastal environments. Shells of mussels of the Mytilidae family, such as Bathymodiolus and M. edulis, therefore, are archives whose REY distribution can be used as a proxy for the Eu anomaly of the waters in which the mussels grew. Although the Ca carbonate of a mussel shell is precipitated from the extrapallial fluid of the mussel and not directly from seawater, the Eu/Eu* ratio of seawater is not significantly modified under the low-temperature conditions prevailing during vital processes and biomineralization. This allows one to use the positive Eu anomalies in the REY distribution patterns of Bathymodiolus shells as a tracer for hidden or fossil high-temperature hydrothermal systems where other indicators of high-temperature hydrothermalism are not available.