ABSTRACT: Sm/Nd Ratio as a Diagenetic Tracer, Paleogene, Texas Gulf Coast

Lawrence E. Mack, David N. Awwiller

Illitization of smectite during burial diagenesis apparently fractionates Sm from Nd. In the Paleocene-Eocene Wilcox Group of the central Texas Gulf Coast and the Oligocene Vicksburg and Frio formations of south Texas, whole-rock shale has normal crustal Sm/Nd ratios of 0.174-0.210. In the < 5-µm shale fraction, Sm/Nd is depleted relative to the corresponding whole rock by 0.011 to 0.051. Because fine clay from modern Mississippi River mud has a normal Sm/Nd ration of 0.191, and is similar to the whole sample (Sm/Nd = 0.188), we infer that depleted Sm/Nd in fine-fraction illite is a diagenetic effect.

Sm/Nd in modern pore fluids from Wilcox and Vicksburg sandstones can be extremely elevated (as high as 0.73), apparently due to smectite illitization. Because of the relative rare earth element concentrations in shale vs. water (approximately 30 ppm Nd in rock, approximately 10^-4 ppm Nd in pore fluids), a small depletion in Sm relative to Nd in diagenetic illites causes a very large enrichment of Sm/Nd in the pore fluids.

Thus, Sm/Nd is a geochemical tracer specific both as to source (shale) and timing (syn- or post-illitization), and is potentially valuable as a tracer of pore fluid movement in sedimentary basins. Sm/Nd ratios also constrain paragenetic sequences by relating times of formation of authigenic phases to the time of illitization. In the Wilcox sandstone, calcite cement has Sm/Nd ratios of 0.187-0.204 (pre-dating illitization), whereas ankerite cement has Sm/Nd ratios of 0.39-0.71, and post-dates illitization.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990