Rare earth element distribution in >400 °C hot hydrothermal fluids from 5°S, MAR: The role of anhydrite in controlling highly variable distribution patterns

Two submarine hydrothermal vent fields at 5°S, Mid-Atlantic Ridge (MAR) - Turtle Pits and Comfortless Cove - emanate vapor-phase fluids at conditions close to the critical point of seawater (407 °C, 298 bars). In this study, the concentration and distribution of rare earth element (REE) and yttrium (Y) has been investigated. Independent of the major element composition, the fluids display a strong temporal variability of their REE + Y concentrations and relative distributions at different time scales of minutes to years. Chondrite-normalized distributions range from common fluid patterns with light REE enrichment relative to the heavy REE, accompanied by positive Eu anomalies (type I), to strongly REE + Y enriched patterns with a concave-downward distribution with a maximum enrichment of Sm and weakly positive or even negative Eu anomalies (type II). The larger the sum of REE, the smaller Ce_CN/Yb_CN and Eu/Eu∗. We also observed a strong variability in fluid flow and changing fluid temperatures, correlating with the compositional variability.As evident by the positive correlation of total REE, Ca, and Sr concentrations in Turtle Pits and Comfortless Cove fluids, precipitation/dissolution of hydrothermal anhydrite controls the variability in REE concentrations and distributions in these fluids and the transformation of one fluid type to the other. The variable distribution of REE can be explained by the accumulation of particulate anhydrite (with concave-downward REE distribution and negative Eu anomaly) into a fluid with common REE distribution (type I), followed by the modification of the REE fluid signature due to dissolution of incorporated anhydrite. A second model, in which the type II fluids represent a primary REE reaction zone fluid pattern, which is variably modified by precipitation of anhydrite, can also explain the observed correlations of total REE, fractionation of LREE/HREE and size of Eu anomaly as well as Ca, Sr. The emanation of such a fluid may be favored in a young hydrothermal system in its high-activity phase with short migration paths and limited exchange with secondary minerals. However, this model is not as well constrained as the other and requires further investigations.The strongly variable REE fluid signature is restricted to the very hot, actively phase-separating hydrothermal systems Turtle Pits and Comfortless Cove at 5°S and has not been observed at the neighboring Red Lion vent field, which continuously emanates 350 °C hot fluid and displays a stable REE distribution (type I).