Hydrothermal scavenging of rare-earth elements in the ocean

Abstract

SUSPENDED participate samples collected from the TAG hydrothermal vent field, at 26^° N on the Mid-Atlantic Ridge, contain ˜10% of the total rare-earth element content (by volume) of ambient sea water. Shale-normalized distribution patterns show both positive europium anomalies and negative cerium anomalies, indicating that the rare-earth elements in these hydrothermal precipitates come from both hydrothermal vent fluid and seawater sources. Rare-earth/Fe concentration ratios in the suspended p articulate material increase at increasing distances from their hydro-thermal source, indicating that rare-earth elements must be continuously extracted from sea water as hydrothermal precipitates are dispersed through the water column. Therefore, although high-temperature vent fluids escaping from the sea floor are typically enriched 10–10,000 times in rare-earth elements relative to sea water^1–4, hydrothermal systems must nevertheless act as a net sink in the global ocean budget of the rare-earth elements. But as the maximum rare-earth/Fe ratios observed for suspended particles are ˜10 times lower than previously reported values for hydrothermal sediments^5–8, it seems that most of the uptake of rare-earth elements from sea water must occur only after hydrothermal precipitates have settled to the underlying sediments.