RARE EARTH ELEMENTS IN BLACK SHALES: ISOLATION AND ANALYSIS OF MINERAL FRACTIONS

The rare earth element (REE) chemistry of black shales is of interest because the relative abundance of these elements records information related to their provenance, conditions of deposition, and early diagenetic history. The utility of the REE in the reconstruction of paleoenvironments and provenance studies hinges on the assumption that the REE are not significantly fractionated during diagenesis. Earlier studies have shown that conservative behavior of the REE is the exception rather than the rule. Rather, the light REE are often significantly fractionated during late diagenesis.

We performed a series of sequential extractions on time-correlative shales of different thermal gradients to isolate various fractions (carbonate, silicate, phosphate and organic/sulfide fractions) to determine the carrier phase of the REE. The REE concentrations of these fractions were measured by ICP-MS. Our initial results show a trend of higher REE concentrations in the carbonate fraction, which is similar to typical seawater pattern. 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.

Our results 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.