Strontium isotopes and rare-earth element geochemistry of hydrothermal carbonate deposits from Lake Tanganyika, East Africa

At Cape Banza (North Tanganyika Lake), fluids and aragonite chimneys have b een collected many times since the discovery of this sublacustrine field in 1987. This sampling has been investigated here for the Sr isotopic composi tions and the rare-earth element features of the carbonates and a few fluid samples. The Sr-87/Sr-86 ratios Of the chimneys indicate that they have pr ecipitated from a mixture of lake water (more than 95%) and hydrothermal fl uids. No zoning in the chimneys was detected with our Sr data. For the rare -earth elements, the situation is more complex. The external walls of the c himneys are rare-earth-element-poor (La approximate to 500 ppb, Yb approxim ate to 200 ppb, La/Yb = 2 to 3.4). Their shale normalised rare-earth elemen t patterns suggest that they are in equilibrium with the inferred carbonate -depositing fluids. The rare-earth element concentrations of the internal w alls of the chimneys are significantly light rare earth elements (LREE)-enr iched with La contents sometimes up to 5 ppm. We suggest that they contain more vent-fluid rare-earth elements than the external wall samples, possibl y adsorbed on the surface of growing crystals or simply hosted by impuritie s. It was not possible to constrain the nature of these phases, but the var iations of the compositions of the internal wall materials of the active ch imneys with time, as well as data obtained on an inactive chimney indicate that this rare-earth element excess is mobile. Partition coefficients were calculated between the external wall aragonite and carbonate-depositing flu id. The results are strikingly similar to the values obtained by Sholkovitz and Shen (1995) on coral aragonite, and suggest that there is no significa nt biologic effect on the incorporation of rare-earth elements into coral a ragonite and that the various carbonate complexes involved Me(CO3+) complex es are the main LREE carriers in seawater (Cantrell and Byrne, 1987) instea d of Me(CO3)(2)(-) in Banza fluids) have the same behaviour during aragonit e precipitation. Copyright (C) 2000 Elsevier Science Ltd.