An evaluation of strontium isotopic dating of ferromanganese oxides in a marine hydrogenous ferromanganese crust

Abstract

In order to assess the utility of dating marine, hydrogenous ferromanganese crusts with strontium isotopes, we conducted a series of leaching 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-leached residues of twelve subsamples were leached with HCl and the leachates 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 textural spectrum observed in the crust. In one sequence, mineral-specific leaches were selected for optimal extraction of strontium associated with the respective manganese and iron oxide phases, the main compositional endmembers within ferromanganese crusts. Strontium isotopic ratios for these leaches were compared with those from the other sequence, involving acetic acid plus HCl leaches on the same material. Residues 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 within the ferromanganese layers. 87Sr 86Sr ratios obtained on the manganese oxides indicate that the strontium within the oxides exchanges with seawater strontium throughout the history of the deposit, regardless of the porosity of the layer. Assuming that this crust is representative of other marine, hydrogenous crusts, strontium isotopes appear to be of little value for the direct dating of ferromanganese oxide deposits.