SR AND ND ISOTOPIC EVIDENCE FOR PUNCTUATED CLAY DIAGENESIS, TEXAS GULF-COAST

We report Rb-Sr, Sm-Nd and mineralogical data for leached whole rock, < 2-mu-m and < 0.1-mu-m clay size fractions of lower- to mid-Tertiary shales spanning the smectite/illite transition, from a single deep well in the Texas Gulf Coast. The abrupt transition at 2400 m, from smectite- to illite-dominated assemblages is accompanied by a marked increase in Sr-87/Sr-86 from 0.708 to 0.711 in exchangeable sites. Leached authigenic illite from the < 0.1-mu-m fraction, sampled from below the smectite/illite transition, defines a Rb-Sr isochron age of 34.8 +/- 2.0 Ma. These data are consistent with a single episode of authigenic illite precipitation over a depth range of > 1200 m, independent of the duration of burial. The exchangeable Sr in the clay has remained an open system and has undergone isotopic exchange since that time. Minimum fluid/rock ratios of 0.2 to 2 are modeled from Sr isotopic data, consistent with rock-dominated, open system diagenesis. Leachates and residues of the < 0.1-mu-m fraction, below the smectite/illite transition appear to have achieved Nd isotopic equilibrium during clay diagenesis, and show no signs of subsequent exchange. The clay residues have low Sm-147/Nd-144 (< 0.085), indicating that authigenic illite has a lower Sm/Nd ratio than its bulk rock protolith. Conversely the coexisting exchangeable REE inventory has elevated Sm/Nd, as reflected by leachate Sm-147/Nd-144 as high as 0.188. This Sm/Nd fractionation, in conjunction with the preserved isotopic equilibrium may be useful for dating diagenesis and associated fluid-rock interaction in older rocks.