Several sites of active hydrothermal flow have been found on the eastern
flank of the Juan de Fuca Ridge. These sites are typically located along
the edge of basaltic outcrops where sediment is thin. We present data on
Mn-oxides formed on such outcrops (Zona Bare and Grinin Bare). These
oxides are either black-layered crust or soft micro-concretions found in
partially altered sediments. X-ray diffraction (XRD) and scanning
electron microscopy (SEM) analyses of Mn crusts indicate the presence of
well-crystallized todorokite and birnessite encrusting detrital minerals
and replacing siliceous fossil. Transmission electron microscopy (TEM)
and energy dispersive X-ray spectroscopy (EDX) analyses were used to
identify amorphous and poorly crystallized Mn-rich phases in partially
altered sediments and crusts. TEM of impregnated samples showed textural
evidence suggesting that amorphous Mn oxides are incrusting cellular
structures that could be bacteria. The valence state of Mn in these
oxides was determined by parallel electron energy loss spectroscopy
(PEELS). Results indicate that todorokite and birnessite have an average
valence state of about 3.7 whereas the poorly crystallized Mn-rich
phases have a lower valence state. These data suggest that the formation
of hydrothermal Mn concretions occurs in several steps. The initial step
is the adsorption or precipitation of Mn, Fe, and Si around cell-wall
bacteria, extracellular polymers, and siliceous fossil remains. These
mineralizations are poorly crystallized phyllomanganates, which
progressively increase in size and crystallinity to give the final
birnessite and todorokite products. All of these Mn-rich phases are the
result of interactions between hydrothermal fluid and sediments and
formed in areas where hydrothermal fluids discharge through the
sediment.