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Diagenetic Resetting of Sm-Nd Isotope Systematics in Wilcox Group Sandstones and Shales, San Marcos Arch, South-Central Texas
David N. Awwiller, Lawrence E. Mack (1)
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 burial diagenesis. The samples are from the San Marcos arch, south-central Texas, at depths ranging from outcrop to 15,000 feet. Sandstones from above the depth of intense feldspar reactions (ca. 10,000 feet) have whole rock Nd model ages of 1400 - 1500 Ma, whereas sandstones buried deeper than ca. 10,000 feet have Nd model ages of 1500 - 2000 Ma. Subsurface shale samples have whole rock model ages that increase from 1300 Ma at 5,000 feet of burial to 1500 Ma at 15,000 feet. Model ages increase most between 5,000 and 10,000 feet, within the zone of intense smectite illitization. The similar depositional age of all samples makes change in provenance an unlikely cause of the observed Nd model age variation.
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. In all samples, the acid-soluble Nd is more radiogenic than the Nd in the corresponding silicate fraction, and the difference between the two reservoirs increases with depth.
Diagenetic enrichment of Sm relative to Nd in the authigenic phases is the most likely explanation for the observed increase in Nd model ages in the sandstones. Decrease in Nd causes the increase in model age in the shales.
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