GEOCHEMICAL IMPLICATIONS OF RARE-EARTH ELEMENT PATTERNS IN HYDROTHERMAL FLUIDS FROM MIDOCEAN RIDGES

Rare earth element data are reported for vent fluids collected from ei ght deep-sea hydrothermal areas, including samples from sediment-hoste d 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 ma king up these patterns vary widely, however, both on a spatial and tem poral basis. When plotted against ionic radii, these fluid trends are remarkably linear and show the same fractionation patterns exhibited b y plagioclase during magma segregation. This analysis suggests that th e partitioning of the REE between fluid and rock during high-temperatu re hydrothermal alteration of MORB is dominated by crystochemical exch ange 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, respectivel y. These considerations predict that the total +3REE content of vent f luid varies in a systematic way during the transition from An-rich MOR B plagioclase to Ab-rich hydrothermal plagioclase and that these param eters can be used as indicators of the average extent of alteration in side active hydrothermal systems.