K-Ar and Rb-Sr dating of nanometer-sized smectite-rich layers from bentonite beds of the Campos Basin (Rio de Janeiro state, Brazil)

K-Ar isotopic dating has been applied to alkali-rich nanometer-sized illite separates of bentonites since the late 1990s. In the present study, K-Ar and Rb-Sr isotopic analyses were based on mineralogical determinations and morphological observations of similarly nm-sized separates (<0.02, 0.02-0.05, and 0.05-0.1 mu m) depleted in alkalis and recovered from Santonian (85.8-83.5 Ma) bentonites of the Campos Basin located offshore the southeastern Atlantic coast (Rio de Janeiro State, Brazil). On the basis of XRD analyses and geochemical/mineralogical simulations, the size fractions consist essentially of the smectite-rich end-member of the smectite-to-illite trend with no more than 9% authigenic illite layers. High K-Ar values from 42.6+ or -3.2 to 70.2+ or -2.1 Ma confirm the occurrence of detrital illite in one sample at least, the age data being meaningless. A second group of K-Ar values ranges from 15.5+ or -10.7 to 41.3+ or -10.8 Ma, while the smallest (<0.02 and 0.02-0.05 mu m) fractions with <0.42% K (sub 2) O lack detectable radiogenic (super 40) Ar and yield analytically 0 Ma ages. Two samples including that with the detrital illite were leached with dilute acid and the Rb-Sr method was applied to the untreated, leachate, and residual fractions of the different separates. The combined isotopic data suggest that illitization started at approximately 44+ or -4 Ma when the bentonites were subjected to a temperature of approximately 60 degrees C. The leachable Sr yielded (super 87) Sr/ (super 86) Sr ratios of 0.7106-0.7108, which is greater than those of seawater either during deposition or recently, and of the initial ashes. They do not correspond to the chemical signature of pore fluids, but more likely to removals from fragile edges of the illite-smectite layers probably impacted by the rough initial chemical treatment applied to empty the smectite interlayers. Illitization was either a side effect of a major contemporaneous smectitization or an independent process that occurred later, in the progressively buried bentonite beds. Of variable duration, it was probably dependent on burial-induced temperature increase in the bentonites, which monitored the fixation of K in the smectite layering with or without a changing fluid chemistry. On the basis of the combined K-Ar and Rb-Sr isotopic data, illitization lasted either until approximately 15 Ma or even 0 Ma for some of the finest size fractions.