A precise 40Ar-39Ar reference age for the Durango apatite (U-Th)/He and fission-track dating standard

The distinctive Durango apatite is widely distributed as a reference material for mineralogy, geochemistry and thermochronology. The apatite occurs within the Cerro de Mercado open-pit iron mine, located in the northern portion of Durango City, Mexico. It is largely confined to vugs and open fractures within the lower portion of the ore deposit. Apatite and iron ore formed in situ and concurrently along with emplacement of small felsic intrusions along the southern margin of the Chupaderos caldera complex. This activity occurred between eruptions of two major ignimbrites from the caldera complex. Four single-crystal sanidine-anorthoclase 40Ar-39Ar ages from these ignimbrites and from a conformable older unit have been used to compute a reference age of 31.44±0.18 Ma (2σ) for the apatite itself. Four additional multi-crystal measurements are in general agreement with this age. Also in agreement are a set of 24 (U-Th-Sm)/He ages recently determined directly on the apatite, which have a mean of 31.02±1.01 Ma 1σ, with a standard error (S.E.) of ±0.22 Ma.