The distribution of rare-earth elements in deeply buried Gulf Coast sediments

Abstract

Analyses of rare-earth elements (REE) in Gulf Coast sediments indicate that the absolute REE content of the bulk sample is nearly constant between 174 and 165 ppm at depths of 1.81 and 3.67 km, respectively; but it increases perceptibly to 212 ppm at a depth of 4.77 km. Similarly, the absolute REE contents in the clay-sized fractions range between 175 and 205 ppm at depths of 1.81 and 4.42 km but they increase moderately to 272 ppm at a depth of 4.77 km. The ratios of the light to heavy rare-earths (LREE/HREE) of the bulk samples and their clay-sized fractions are nearly constant between 7.2 to 8.4 for depths between 1.81 and 4.42 km, whereas the ratio decreases to about 5.5 and 6.6 for sediments at 4.77 km. Both the absolute REE content and the LREE/HREE ratio of the sediments most likely reflect the character of the provenance(s) of the sediments, and they are influenced only to a minor extent by diagenesis. The Eu/Sm ratios of all samples cluster very tightly around the value of 0.21, similarly to that of the North American Shale composite. A nearly constant Eu/Sm ratio among samples at all depths may have been inherited primarily at the provenance(s) of these Gulf Coast sediments.