Diagenetically Modified Buried Hydrothermal Manganese Crusts from the Lau Basin, S.W. Pacific

Abstract

Manganese crusts recovered from sediment sections at Ocean Drilling Programme (ODP) Sites 835 and 838 in the Lau Basin have been studied mineralogically and by bulk and partition geochemical techniques. Results show they are of hydrothermal rather than hydrogenous origin and are composed principally of stable 10 Å-manganite. The stratigraphic location of the crusts in the sediment sections would suggest they range between 0.2 and 3.5 million years old. The crusts were most likely formed at the sediment water interface before being incorporated into the sediment section by slumping and subsequent burial. Basal Site 835 crusts were most likely formed at or close to the Eastern Lau Spreading Center (ELSC), and those in the younger part of the sediment section were probably formed at a site of ''off-axis'' volcanism. Site 838 crusts were most likely formed on an intrabasin seamount. The crusts are thus the fossil equivalents of modern hydrothermal Mn crusts. Although the crusts show geochemical characteristics of their modern hydrothermal counterparts, they contain elevated concentrations of Ba, Cu, Ni, V, and Zn compared to their modern counterparts, and these cannot be accounted for by either included sedimentary material or hydrogenous inputs. The elevated concentrations of these elements in the crusts is thought to result from reaction with interstitial waters soon after burial, a phenomenon which has profound implications for our understanding of marine ferromanganese oxides in general, as it indicates that they can undergo postdepositional compositional changes over time.