BEHAVIOR OF RARE EARTH ELEMENTS IN BLACK SHALES DURING EARLY DIAGENESIS AND THERMAL MATURATION

Black shales are of interest because their geochemistry records information related to their origin, conditions of deposition, and early diagenetic history. The rare earth elements are abundant in black shales and have been used in many instances to infer provenance, to date these rocks and for paleoenvironmental studies. The utility of the REE for provenance studies and for paleoenvironmental reconstruction, hinges on the assumption that the REE are not significantly fractionated during diagenesis. Some studies have shown that the conservative behavior of the REE is the exception rather than the rule. Rather, the light REE are often significantly fractionated during late diagenesis.

We isolated various shale fractions (carbonate, silicate, phosphate and organic/sulfide fractions) using different techniques to determine the carrier phase of the REE and other trace elements. Analyses of the isolated fractions was done by ICP-MS. Our initial results show a trend of higher REE concentrations in the carbonate fraction, which is similar to typical seawater pattern. Other trace elements including the Rb, Zn, Mo, U, V and Cr are significantly depleted in the carbonate fraction relative to other shale fractions. The silicate fraction, dominated by clays, is slightly light REE enriched relative to the starting shale. The organic fraction contains low concentration of REE however in each case is, to varying degrees, middle REE depleted. This depletion may or may not be related to the organic fraction but could also be related to associated sulfides, which at this time, we are unable to separate with greater than 85% yield from the organic fraction.

These results seem to suggest that the REE signatures preserved in black shales represent the competing effects of the mineral fractions with the clay fraction dominating the REE chemistry. However the variations in the degree of light REE enriched/depletion in the mineral fractions suggest that even the minor amounts of light REE bearing organic matter/sulfides could alter the original REE pattern of the sediment if this fraction is lost during late diagenesis and thermal maturation.