Origin and Fate of Ferromanganoan Active Ridge Sediments

Major- and trace-element data, U- and Th-isotope relations, and element-accumulation rates suggest that the ferromanganoan active ridge sediments are only to a small extent due to leaching of oceanic rocks or deposition from ordinary sea water. The proportions of Fe, Mn, Ti, Al, Si, P, Ba, U and Th in such sediments are similar to those in carbonates of magmatic origin. This suggests that a CO2-rich phase of deep-seated origin is the carrier of these elements. High spreading rates should imply intense degassing and thus high accumulation rates of active ridge sediments; low spreading rates should imply the opposite. Such correlations have been observed. Active ridge sediments should not be confused with other Fe–Mn rich deposits on inactive ridges and on seamounts. Such deposits are often concretionary, form exceedingly slowly, and show much lower Fe/Ti and Fe/A1 ratios than do the active ridge sediments. Active ridge sediments are known since the Cretaceous, but some Fe–Mn–chert deposits on the continents that date back to the Archaean may be of a similar origin. However, many of these very old deposits rest on continental crust. Most old active ridge sediments must therefore have been subducted or metamorphosed beyond recognition; but may yet have played an important role during ore-forming processes.