The rainbow vent fluids (36°14′N, MAR): the influence of ultramafic rocks and phase separation on trace metal content in Mid-Atlantic Ridge hydrothermal fluids

Fluids were collected from the Rainbow vent field (36°14′N) on the Mid-Atlantic Ridge (MAR) during the 1997 diving FLORES cruise. This vent field, in ultramafic rocks at a depth of 2300 m, is composed of ∼10 black smokers emitting acidic (pH∼2.8) fluids at 365 °C. The low pH of the hot-temperature Rainbow fluids likely results from seawater–ultramafic rock interaction that releases H+ ions into reducing hydrothermal fluids. Fluid chemistry is strongly influenced by phase separation generating Cl-rich brines (ClEM=750 mM) strongly enriched with Mn, Fe, Co, Ni, Cu, Zn, Ag, Cd, Cs, Pb, Y, and rare earth elements (REE). REE and transition metal abundance (particularly Fe and Mn) in the Rainbow fluids is dramatically higher than in other MAR fluids. The abundance of trace element and REE enrichment is due to the greater solubility of these elements that is strongly favored by Cl-complexation at low-pH and high-temperature conditions. Chondrite-normalized REE patterns show strong LREE enrichment with evidence of the typical Eu anomaly. This REE partitioning suggests that either (1) ultramafic rocks represent only a part of rocks leached during hydrothermal alteration and/or (2) that the unique Rainbow fluid temperature, pH, and redox state issued from the ultramafic character of leached substratum can produce unique REE partitioning.