HOW RELIABLE ARE "HIGH-RESOLUTION" RADIO-ISOTOPIC AGES?

Radio-isotopic ages with both precision and accuracy at the permil level are desirable to examine the synchronism of sequence boundaries. The occurrence of isotopically datable minerals in primary volcanic deposits intercalated within sedimentary sequences potentially permits high-resolution age determinations with a precise stratigraphic significance.

Due to their high analytical precision (< 0.2% of the age in many cases) U/Pb and ⁴⁰Ar/³⁹Ar dating methods are the most important techniques applied to Phanerozoic rocks. The potential accuracy of the U/Pb method is comparable to its achievable precision (though even this potential has been overstated), due to the feasibility of gravimetrically calibrated spikes and to the precisely determined uranium decay constants. However, recent studies have shown that the application of the U/Pb method to Phanerozoic zircons in volcanic layers is often (but not always) limited - even with state-of-the-art single-crystal or ion microprobe techniques - by the now-familiar problems of subtle Pb loss combined with varying amounts and sources of inheritance and/or pre-eruptive magma residence time, so that the resulting age-scatter prohibits extraction of a statistically robust age. In contrast, despite its excellent potential age resolution, the ⁴⁰Ar/³⁹Ar method is limited in absolute accuracy to no better than ±2% due mainly to uncertainties in the ⁴⁰K decay constants and the true ⁴⁰K-⁴⁰Ar of standards.

Acknowledgment of the above-described limitations (well-understood by most geochronologists, but less so by the stratigraphic community) is particularly important for questions requiring time-scale accuracy, such as studying sequence boundaries. In particular it is critical not to confuse reliability with stated analytical precision, because the latter seldom include all sources of error and in any case are not generally comparable. We present recently acquired U/Pb data showing that inferences from U/Pb-zircon dating must be based on sufficiently large suites of single-crystal (or smaller), high-precision analyses which are subjected to a stringent and realistic interpretation. Also, we present results from ongoing studies designated to intercalibrate U/Pb and K/Ar isotopic systems.