Sr and Nd isotopic variations in ferromanganese crusts from the Central Pacific: Implications for age and source provenance

Isotopic analyses of two hydrogenetic ferromanganese (Fe-Mn) crusts from volcanic edifices in the central Pacific Ocean reveal systematic variations in 87Sr86Sr">⁸⁷Sr⁸⁶Sr and 143Nd144Nd">¹⁴³Nd¹⁴⁴Nd, with both ratios decreasing as a function of depth into the Fe-Mn crusts. Leaching experiments suggest that Sr in the crusts is contained in at least two discrete sites. A loosely bound Sr, dominated by modern marine Sr, is removed by leaching with a 10 percent acetic acid solution. The 87Sr86Sr">⁸⁷Sr⁸⁶Sr ratio of the residue is significantly less than the 87Sr86Sr">⁸⁷Sr⁸⁶Sr ratio of the unleached material. The Sr-isotope ratios of leached samples are compared with the temporal variation in seawater to provide ages for layers within the Fe-Mn crusts. These data suggest that the oldest crust layers began to accrete in the early to middle Miocene. Correlated to the 87Sr86Sr">⁸⁷Sr⁸⁶Sr variations, 143Nd144Nd">¹⁴³Nd¹⁴⁴Nd ratios suggest that the Nd-isotope composition of central Pacific Ocean seawater also changed systematically over this time interval, or that the Fe-Mn crusts simply incorporated Nd from various parts of an isotopieally heterogeneous ocean as the crusts were carried along with the oceanic plate. In contrast to the layered ferromanganese crust, the phosphatized volcaniclastic substrates have Sr and Nd isotope compositions that are consistent with their volcanic origin.