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

Eric H. De Carlo

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. This implies that well preserved Fe-Mn crusts may provide a record of major paleoceanographic events reaching back to the onset of Fe-Mn oxide precipitation on the substrate material. 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 I anomalies observed by other workers and appears to be a very promising indicator of major events. The potential use of Ce as a paleoceanographic tracer stems from its electronic structure, which leads to a preference for the highly insoluble tetravalent CeO₂, 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. It is hoped that the use of nannofossil biostratigraphy will help constrain the growth history of the crusts and allow the reconstruction of paleoceanographic conditions from REE stratigraphic analyses.

AAPG Search and Discovery Article #90097©1990 Fifth Circum-Pacific Energy and Mineral Resources Conference, Honolulu, Hawaii, July 29-August 3, 1990