Rubidium-strontium dating of shales by the total-rock method

Measurements of Rb87, Sr88, and Sr87/Sr88 are reported for total-rock samples of three shale formations. These data allow calculation of the time of deposition for one formation known to be of Lower Silurian age and of either the deposition or metamorphism for another. The calculation is based on the model that each formation was once uniform in its ratio of Sr87/Sr88 and that the observed distribution of Rb/Sr through each formation was established at that time. By considering the sources of error in our measurements, we believe that a small but definite range in Sr87/Sr88 actually occurred, rather than complete uniformity. Nevertheless, excellent agreement is found between our calculated value and the current estimates for the absolute age of Lower Silurian. The data for the third formation indicate that its ratio of Sr87/Sr86 was always highly nonuniform. Measurements of Sr87/Sr88 for sea water from various localities are also reported. The variation in this ratio is very small, and the absolute values are significantly lower than those calculated for Sr87/Sr88 in the shale formations at the times of their deposition. These observations show that the strontium fixed in the shales during deposition was not entirely precipitated from sea water: some must already have been enriched in radiogenic strontium relative to sea water. We therefore believe that a small amount of detrital mica or feldspar distributed irregularly through the shales must be responsible for the nonuniformity of their initial ratios of Sr87/Sr88.