Since the enigma of contrasting Rb-Sr mineral and WR ages of metamporphic rocks had been solved more than four decades ago, it is known that geochronological systems are scale-dependent. For the Rb-Sr examples, metamorphism resulted in the redistribution of parent and daughter elements and concomitantly the resetting of the isotopic system at the mineral scale. As this redistribution occurred at scales smaller than the size of the WR samples used for dating, the WR samples did not record this redistribution and remained apparently closed systems. The same general interpretation also applies to other geochronologic systems, in particular systems involving a-decay (and, thus, also a-recoil). Recoil of the daughter nucleus implies an open system, which eventually results in the decoupling of the parent-daughter system if the daughter (or intermediate daughters in the U and Th decay series) is mobile. As this process works at a scale of some tens of nanometer, the effect is irrelevant for dating, unless for microcrystalline material, ion sieves (e.g., zeolites), and metamict material. The key parameter is not the physical size of the investigated sample, but the distance to the next free surface that allows rapid transport of daughter isotopes out of the system.

Romer, R.L. and Rocholl, A. (2004) Activity disequilibrium of ²³⁰Th, ²³⁴U, and ²³⁸U in old stilbite: effects of young U mobility and a-recoil. Geochim. Cosmochim. Acta., 68: 4705-4719.

Romer, R.L. (2003) Alpha-recoil in U-Pb geochronology: effective sample size matters. Contrib. Mineral. Petrol., 145: 481-491.

Romer, R.L. (2001) Isotopically heterogeneous initial Pb and continuous ²²²Rn loss in fossils: The U-Pb systematics of Brachiosaurus brancai. Geochim. Cosmochim. Acta, 65: 4201-4213.

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