Concurrent U–Pb age and seawater 87Sr/86Sr value of a marine cement

The purpose of this contribution is to show that it is possible to obtain accurate depositional U–Pb ages and original marine 87Sr/86Sr values from the same sample of marine cement. Altered marine aragonite cements from algal mounds of the Late Carboniferous (Bursum Stage) Laborcita Formation yield a U–Pb age of 300.2±3.8 Ma, consistent with the existing chronostratigraphic framework. We use fluid rock interaction models to extrapolate to original seawater 87Sr/86Sr values based on Sr isotope and trace element analyses of multiple petrographically identified components that should represent mixes of primary and diagenetic fluids. Such modeling provides a range of 0.70807–0.70820 for the 87Sr/86Sr value of the marine end member. These data, combined with a 87Sr/86Sr value of 0.70849±0.00001 for unaltered aragonite from algal plates and bivalves in the underlying Holder Formation, show a decreasing trend in 87Sr/86Sr toward the end of the Carboniferous. Whereas the range in 87Sr/86Sr values from the Laborcita Formation is consistent with existing Sr curves, the 87Sr/86Sr values of the aragonite are appreciably higher than these curves (at least 0.0001). The aragonite from the Holder Formation has greater than 6000 ppm Sr, and thus these components should reflect the original marine value. Data from marine cements and biogenic aragonite from other sections are necessary to evaluate whether the 87Sr/86Sr values from the Sacramento Mountains truly record the global value.