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The closure of isotopic chronometers to loss or exchange of radiogenic daughters is best analysed in terms of phenomena that induce recrystallization, i.e. fluid circulation and strain. In the presence of these fast phenomena, temperature is not a rate‐controlling parameter for isotope transport. Only in those rare instances where no fast mechanism can be petrographically documented, do minerals survive very high temperatures without appreciable modification of their isotopic record. For these (admittedly unlikely) rocks, a self‐consistent upward recalibration of the ‘closure temperature’ is defined.

The lunar meteorite Sayh al Uhaymir 169 consists of an impact melt breccia extremely enriched with potassium, rare earth elements, and phosphorus [thorium, 32.7 parts per million (ppm); uranium, 8.6 ppm; potassium oxide, 0.54 weight percent], and adherent regolith. The isotope systematics of the meteorite record four lunar impact events at 3909 +/- 13 million years ago (Ma), approximately 2800 Ma, approximately 200 Ma, and <0.34 Ma, and collision with Earth sometime after 9.7 +/- 1.3 thousand years ago. With these data, we can link the impact-melt breccia to Imbrium and pinpoint the source region of the meteorite to the Lalande impact crater.

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