Abstract : The studied samples are from gravity cores recovered during the Ticoflux II expedition offshore of the Pacific coast of Costa Rica in 2002. They were collected on the north-western slope of a seamount called “Dorado”, which is an area of diffuse low-temperature hydrothermal activity. The study is focused on core GC50 composed of altered hemipelagic muds containing detrital clays and authigenic minerals such as zeolites, Fe/Mn oxides, and phosphates.
The mineralogical (XRD, SEM, TEM-EDS) and geochemical (major and trace elements, Rare Earth) characterizations of the newly formed minerals in GC50 sediments allowed to distinguish two alteration phases. A first phase of diagenesis. This first step was followed by the precipitation of apatite and Mn and/or Fe oxides, which all are most abundant close to the basalt-sediment contact. Neodymium and strontium isotopes, as well as REY distribution patterns, allowed to determine the nature of the fluids that led to this second phase of alteration. All these data suggest an interaction with an upward flowing low temperature hydrothermal fluid which has not significantly exchanged with the basalts during its preceding circulation within the oceanic crust.
A more detail study (EXAFS, HRTEM) of the samples rich in Fe/Mn oxides allowed to propose a model for Mn oxides formation and evolution, in this off-axis hydrothermal context. This model demonstrates that the formation of todorokite (tectomanganate), which is an ubiquitous mineral phase in marine environments, requires the presence of a precursor, which is vernadite (phyllomanganate) in the present case.