Intrasample REE Variability in Pacific Ocean Fe-Mn Crusts: A Potential Paleoceanographic Indicator: ABSTRACT

Abstract

Recent studies of the microscale chemistry and morphology of marine Fe-Mn crusts suggest that prevailing oceanographic conditions at the time of formation are recorded in individual layers of the deposits. Using current radiometric crust growth-rate estimates of 1 to 5 mm per million years, thick Fe-Mn oxide accumulations may represent a growth record ranging from 10 to 100 million years. Detailed chemical and mineralogical analyses were performed on 5 mm layers of selected thick (5-10 cm) crusts from our collection. The analyses reveal significant variations in the distribution and abundance of the REE as a function of depth within the crusts. The trivalent REE generally exhibit sympathetic concentration changes in pure Fe-Mn oxides but display a greater degree of fractionation between the light and heavy REE in layers enriched in phosphatic matter. The compositional variability and the extent of REE fractionation can be explained partially by mineralogical control but are also believed to reflect differing depositional conditions and the composition of the seawater from which the deposits formed. Changes in the Ce anomaly throughout the crust correlates with Pt and Ir anomalies observed by other workers and appears to be a very promising indicator of major events. The potential usemore » of Ce as a paleoceanographic tracer stems from its electronic structure, which leads to a preference for the highly insoluble tetravalent CeO{sub 2}, whereas other REE exist principally in the trivalent oxidation state. Because Fe-Mn crusts accrete primarily under oxidizing conditions, variations in the Ce anomaly of crusts may be useful as a paleoredox indicator as previously suggested by other investigators. The REE patterns observed in this study will be discussed in terms of their relation to the major element composition and mineralogy of the crusts.« less