ORIGIN OF NEGATIVE CE ANOMALIES IN MIXED HYDROTHERMAL-HYDROGENETIC FE-MN CRUSTS FROM THE CENTRAL INDIAN RIDGE

Layered Fe-Mn crusts from the off-axis region of the first segment of the Central Indian Ridge north of the Rodrigues Triple Junction were s tudied geochemically and mineralogically. Vernadite (delta-MnO2) is th e main mineral oxide phase. Th-230(xs) and Co concentrations suggest h igh growth rates of up to 29 mm/Myr and a maximum age of the basal cru st layer of 1 Ma. Whereas most of the major and minor elements show co ncentrations which are typical of hydrogenetic formation, Co, Pb, Ni a nd Ti concentrations are strikingly lower. Concentrations and distribu tion of the strictly trivalent rare-earths and yttrium (REY) are typic al of hydrogenetic ferromanganese oxide precipitates, but in marked co ntrast, the crusts are characterized by negative Ce-SN (shale-normaliz ed) anomalies and (Ce/Pr)(SN) ratios less than unity. Profiles through the crusts reveal only minor variations of the REY distribution and ( Ce/Pr)(SN) ratios range from 0.45 to 0.68 (compared to ratios of up to 2 for typical hydrogenetic crusts from the Central Indian Basin). The apparent bulk partition coefficients between the crusts and seawater suggest that for the strictly trivalent REY the adsorption-desorption equilibrium has been reached. Positive Ce anomalies in the partition c oefficient patterns reveal preferential uptake of Ce, but to a lesser extent than in normal hydrogenetic crusts. A new parameter (excess Ce, Ce-xs) to quantify the degree of decoupling of Ce from REY(III) is es tablished on the basis of partition coefficients. Ce-xs/Ce-bulk ratios suggest that the CIR crusts formed by precipitation of Fe-Mn oxides f rom a hydrothermal plume and that in hydrothermal plumes and normal se awater the enrichment of Ce results from the same oxidative sorption p rocess. The growth rates, calculated with Th-230(xs) data as well as w ith the Co formula, are inversely related to Ce-xs. (C) 1998 Elsevier Science B.V. All rights reserved.