Erg Chech 002 is a sample of the oldest known asteroidal crust. Its pyroxene and whole-rock lead-lead ages cluster tightly around 4,566 million years — the formation of the Solar System's first igneous rocks. But zirconolite crystals in the same meteorite yield 4,558 million years: eight million years younger.
The zirconolite didn't form with the crust. It formed when a later impact shock-metamorphosed the rock, resetting the uranium-lead clock in the newly crystallized mineral. The tiny needles — three micrometers wide, thirty long — are the remnant of that secondary event, faithfully recording the date of destruction, not creation.
The problem: acid-leaching methods, which dissolve minerals sequentially to extract radiogenic lead, can inadvertently dissolve even a trace of zirconolite alongside the pyroxene. A tiny contamination of 4,558 Ma lead in a 4,566 Ma measurement doesn't produce an obvious anomaly. It produces a slightly younger age that still looks plausible — and slightly wrong.
This is the measurement equivalent of a palimpsest. The original text (the crust's formation) and the overwritten text (the shock event) are both present in the sample. The high-precision dating method reads both as one. The dates are accurate for what they measure; the error is in assuming they measure the same event. The Solar System's oldest rock contains two clocks, running at different offsets, and the standard method reads their average.