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So when a mineral grain forms (specifically, when it first cools below its trapping temperature), it effectively sets the uranium-lead "clock" to zero. Lead atoms created by uranium decay are trapped in the crystal and build up in concentration with time. If nothing disturbs the grain to release any of this radiogenic lead, dating it is straightforward in concept. In a 704-million-year-old rock, 235U is at its half-life and there will be an equal number of 235U and 207Pb atoms (the Pb/U ratio is 1). In a rock twice as old there will be one 235U atom left for every three 207Pb atoms (Pb/U = 3), and so forth.
U-Pb dating is commonly carried out on a limited set of minerals, namely zircon, quartz and apatite. Zircon is a particularly valuable mineral for this purpose for the following reasons:
“While zircon is one of the best isotopic clocks for dating many geological processes,” Cavosie says, “our results show that it is very challenging to use ex situ zircon to date a large impact of known age.”
Assuming that the changes Δ C and Δ µ' both occur within roughly the same small distance δ x , the width of the interface, the helium flux J in equation (A2) becomes: