RARE-EARTH ELEMENT GEOCHEMISTRY OF HYDROTHERMAL DEPOSITS FROM THE ACTIVE TAG MOUND, 26-DEGREES-N MID-ATLANTIC RIDGE

The rare earth element (REE) geochemistry of various phases from the a ctive TAG hydrothermal mound has been examined and related to their mi neralogy and fluid chemistry. The mound deposits range from black and white smoker chimneys, massive anhydrite/sulphide mixtures, oxides, an d ochres. All phases, except black smoker chimney anhydrite, demonstra te a positive Eu anomaly when normalised to chondrite REE values. REE substitution into sulphide and sulphate phases appears to be strongly influenced by crystallographic control for all REE other than Eu. Prec ipitation of anhydrite within the TAG mound is the major mechanism for removal of REE during mound circulation and 0.15-0.35 g anhydrite is inferred to precipitate from every kg of fluid venting from the white smoker chimneys. Oxides from the mound fall into three different categ ories with distinct REE patterns: oxide rims on sulphides, atacamite-b earing oxides, and silica-rich Fe-oxides and ochres. The oxide rim pha ses contain sulphide and seawater derived REEs whereas the atacamite-b earing oxides and the ochreous material exhibit no seawater signature which suggests precipitation from, or alteration by, a modified hydrot hermal fluid.