Mineralogy and chemical composition of ferromanganese crusts from the Cruzeiro do Sul Lineament - Rio Grande Rise, South Atlantic

Samples dredged from the Cruzeiro do Sul Lineament exhibit ferromanganese crusts precipitated above phosphate-rich substrates (phosphorites, phosphate-impregnated volcanic rocks and older phosphatized ferromanganese crusts). Co-chronology of the crusts, paleontological data and 87Sr/86Sr ratios of the phosphatized substrate indicate that the last phosphatization event in the region happened between 17.6 and 14.6 Ma ago. When the event ceased, oxic conditions were resumed and hydrogenetic crust precipitation started at a maximum rate of 1.6–3.0 mm/Ma, forming layers of Fe-vernadite above the phosphorites. These crusts differ in composition from crusts that formed prior to the phosphatization, which were impregnated by carbonate fluorapatite and recrystallized forming a 10 Å Mn-phase, in expense of a lower crystallinity Mn-phase. Mn/Fe ratios of non-phosphatized crusts vary from 1.05 to 1.41. The Mn/Fe ratios as well as the metal content (Co = 0.65–1.04 wt%, Zn = 0.04–0.06 wt%, Cu = 0.02–0.06 wt% and Ni = 0.29–0.48 wt%) are similar to hydrogenetic crusts described in other parts of the world