Sm-Nd model ages of sandstones and shales from the Oligocene Frio Formation of south Texas and the Paleocene-Eocene Wilcox Group of south-central Texas increase with increasing depth of burial. Depleted-mantle model ages (Tdm) of Frio shales increase from ca. 800 Ma at 1 km of burial to ca. 1000 Ma at ∼4km; in Frio sandstones, Tdm increases from ca. 750 Ma at 1 km to over 1200 Ma at 4 km of burial. In Wilcox shales, Tdm increases from ca. 1300 Ma to ca.1500 Ma over a burial depth of 1.5 to 4.5 km; Tdm of Wilcox sandstones increases from ca. 1450 Ma at 2.5 km of burial to as much as ca. 2000 Ma at 4.0 km. The sampling scheme employed minimizes, but does not eliminate, the possibility that these trends in Tdm are due to detrital variability. Close correlation of these trends with diagenetic changes in the rock and evidence of rare earth element mobility preserved in the diagenetic products suggests that diagenesis has modified Tdm.
Diagenetic modification of Tdm in Frio sandstone and Wilcox shale is due primarily to loss of radiogenic Nd, whereas increased 147Sm/144Nd produces elevated Tdm in deeper Wilcox sandstones. The Tdm of Frio shales does not change appreciably with depth, owing to the opposite effects on Tdm of decreased 147Sm/144Nd and decreased ϵNd(0). Loss of radiogenic Nd is due primarily to reactions involving unstable volcanic detritus, which is more abundant in the Frio than in the Wilcox. Decreased 147Sm/144Nd in authigenic illite, and consequent increased 147SM/144Nd in pore fluids, results in elevated Tdm in Wilcox and Frio sandstones containing late-diagenetic cement, and in decreased 147Sm/144Nd in deeper Frio shale. The similar Tdm trends observed in both units, which have different detrital characteristics, suggest that modification of Sm-Nd model ages in siliciclastic rocks may be commonplace during burial diagenesis.