AN EVALUATION OF STRONTIUM ISOTOPIC DATING OF FERROMANGANESE OXIDES IN A MARINE HYDROGENOUS FERROMANGANESE CRUST

In order to assess the utility of dating marine, hydrogenous ferromang anese crusts with strontium isotopes, we conducted a series of leachin g experiments on samples selected from several stratigraphic layers of a well-preserved 9.5-cm-thick crust from the Pacific Ocean. Initially , using a method similar to that of previous studies, acetic-acid-leac hed residues of twelve subsamples were leached with HCl and the leacha tes analyzed for their isotopic composition. The results did not make stratigraphic sense, so we carried out two sequences of more detailed leaches on subsamples from four layers representing the chemical and t extural spectrum observed in the crust. In one sequence, mineral-speci fic leaches were selected for optimal extraction of strontium associat ed with the respective manganese and iron oxide phases, the main compo sitional endmembers within ferromanganese crusts. Strontium isotopic r atios for these leaches were compared with those from the other sequen ce, involving acetic acid plus HCl leaches on the same material. Resid ues remaining at the end of each leaching sequence were also analyzed. It is evident from the results that HCl extracts strontium associated with both phosphatic material and aluminosilicate detritus present wi thin the ferromanganese layers. Sr-87/Sr-86 ratios obtained on the man ganese oxides indicate that the strontium within the oxides exchanges with seawater strontium throughout the history of the deposit, regardl ess of the porosity of the layer. Assuming that this crust is represen tative of other marine, hydrogenous crusts, strontium isotopes appear to be of little value for the direct dating of ferromanganese oxide de posits.