--> ABSTRACT: Diagenetic Resetting of Sm-Nd Isotope Systematics in Wilcox Group Sandstones and Shales, San Marcos Arch, South-Central Texas, by D. N. Awwiller and L. E. Mack; #91029 (2010)

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Diagenetic Resetting of Sm-Nd Isotope Systematics in Wilcox Group Sandstones and Shales, San Marcos Arch, South-Central Texas

D. N. Awwiller, L. E. Mack

Preliminary Sm-Nd analyses of sandstones and shales from the Wilcox Group (upper Paleocene-lower Eocene) suggest that the whole-rock Sm-Nd signature is changed by diagenesis. Samples are obtained from the San Marcos arch, south-central Texas, at depths ranging from outcrop to 15,000 ft. Samples from above the depth of intense illitization and feldspar reactions (about 10,000 ft) have whole-rock Nd model ages of about 1,500-1,400 Ma, whereas samples buried deeper than 13,000 ft have Nd model ages of about 1,700-1,500 Ma. The similar depositional age of all samples makes change in provenance an unlikely cause of the variation. Diagenetic modification is the only reasonable explanation for the observed differences.

Sm and Nd reservoirs within the shale change considerably with progressive diagenesis. Outcrop samples contain subequal concentrations of Sm and Nd in the acid-soluble and silicate fractions. The acid-soluble reservoir in deeper samples contains progressively less Sm and Nd and higher Sm/Nd ratios. In all samples, the acid-soluble Nd is more radiogenic than the Nd in the corresponding silicate fraction; the difference between the two reservoirs increases with depth.

Preferential loss of Nd relative to Sm in the silicate reservoir during diagenetic crystallization of illite and albite is the most likely explanation for the observed trend in Nd model age. These data imply that the earlier assumption that no fractionation takes place in the Sm-Nd system during diagenesis is incorrect.

AAPG Search and Discovery Article #91029©1989 AAPG GCAGS and GC Section of SEPM Meeting, October 25-27, 1989, Corpus Christi, Texas.