Abstract

Rare earth element data are reported for vent fluids collected from eight deep-sea hydrothermal areas, including samples from sediment-hosted and bare-rock hydrothermal systems. The MORB-normalized REE patterns from these diverse settings are remarkably similar to each other but distinctly different than seawater or basalt, being LREE-enriched and possessing a strikingly positive Eu anomaly. The REE concentrations making up these patterns vary widely, however, both on a spatial and temporal basis. When plotted against ionic radii, these fluid trends are remarkably linear and show the same fractionation patterns exhibited by plagioclase during magma segregation. This analysis suggests that the partitioning of the REE between fluid and rock during high-temperature hydrothermal alteration of MORB is dominated by crystochemical exchange during dissolution and recrystallization reactions at the exposed surfaces of plagioclase phenocrysts. A comparison of REE ionic radii in eight coordination indicate that the partitioning of + 3REE and + 2Eu are dominated by chemical substitution for Ca+2 and Sr+2, respectively. These considerations predict that the total +3REE content of vent fluid varies in a systematic way during the transition from An-rich MORB plagioclase to Ab-rich hydrothermal plagioclase and that these parameters can be used as indicators of the average extent of alteration inside active hydrothermal systems.