The unified screening protocol
One protocol runs on both clocks: screen → classify → correct-where-characterisable → cross-check → report honest uncertainty. The decisive move is Step 2 for zircon — grains are classified on age-independent grounds (crystal position, texture, common-Pb f₂₀₆), never by picking the youngest age — which keeps the procedure non-circular. Radiocarbon implements the same steps via an external ΔR offset.
One protocol runs on both clocks: screen → classify → correct-where-characterisable → cross-check → report honest uncertainty. The decisive move is Step 2 for zircon—grains are classified on age-independent grounds (crystal position, texture, common-Pb f₂₀₆), never by picking the youngest age—which is what keeps the procedure non-circular. Radiocarbon implements the same steps via matrix screening and an external ΔR offset.
The protocol applies to both chronometers; only the implementation of each step differs, following the detectability split of §3.
| Step | Radiocarbon | Zircon U–Pb |
|---|---|---|
| 1 Screen | matrix / diffusion screening[1] | common-Pb (²⁰⁴Pb / f₂₀₆); concordance; CL |
| 2 Classify | clean / correctable / exclude | autocryst / antecryst / Pb-loss (exclude) |
| 3 Correct | reservoir offset ΔR | ²⁰⁴Pb common-Pb; ²³⁰Th-diseq. correction |
| 4 Cross-check | paired dating; known-age horizons | sanidine ⁴⁰Ar/³⁹Ar; (U–Th)/He; stratigraphy |
| 5 Report | offset transferability stated | classification basis stated; ambiguity logged |
The decisive design choice is in Step 2 for zircon: classification is made on age-independent grounds—crystal position, texture, common-Pb content—not by selecting the youngest age. This is what keeps the procedure non-circular, and §7 demonstrates it on real grains.