A precise 40Ar–39Ar reference age for the Durango apatite (U–Th)/He and fission-track dating standard
Introduction
The Durango apatite is a distinctive lemon-yellow fluorapatite that is found in coarse crystals of exceptional form and clarity within the open pit iron mine at Cerro de Mercado, on the northern outskirts of Durango City, Durango, Mexico (Fig. 1). Young et al. (1969) provide an extensive description of the mineralogy and geochemistry of the apatite. This widely available mineral has found increasing use as a standard for spectroscopic and microprobe analyses, as well as for fission-track and (U–Th)/He dating. The aim of this paper is to summarize the geologic setting of the apatite and to present a precise and independent determination of its age.
Detailed geologic studies in the Durango City area began during the early 1970s (Keizer, 1973, Swanson, 1974, Lyons, 1975). McDowell and Keizer (1977) included K–Ar ages for the entire volcanic section near Durango. Swanson et al. (1978) described the Chupaderos caldera complex, which is the source for many of the older units in the Durango volcanic section. Lyons (1988) focused upon the detailed relationships present within the Cerro de Mercado mine. The following geologic description has been derived primarily from these references.
Section snippets
Geologic setting
Detailed mapping near Durango City documented a felsic volcanic section of approximately 800 m composite thickness dominated by ash-flow tuffs (Keizer, 1973, Swanson, 1974, Lyons, 1975). These rocks are part of the Sierra Madre Occidental volcanic province of western Mexico, which is a major remnant of the mid-Tertiary ignimbrite flare-up of western North America. The upper portion of the Durango section is a simple succession of tabular and conformable units derived from unknown sources
Previous results
Based upon 14 K–Ar ages, McDowell and Keizer (1977) published a range of from 33 to 29 Ma for rocks that cover the full section of the Durango volcanic sequence. [All ages quoted or presented in this paper have been computed with the decay constants of Steiger and Jaeger (1977). Decay constants for the K–Ar and U–Pb isotopic systems and their uncertainties have been the subject of a number of recent papers (e.g. Renne et al., 1998, Min et al., 2000, Begemann et al., 2001), which suggest that
Conclusions
The Cerro de Mercado iron deposit and its associated Durango apatite is tightly bracketed stratigraphically by two ignimbrites erupted from the Chupaderos caldera complex. The apatite crystallized from a vapor phase and does not represent early crystallization from a long-lived magma chamber. The apatite has experienced minimal burial since its crystallization. These characteristics, along with the coarseness of the crystals, make the Durango apatite a promising geochronology standard for which
Acknowledgements
It is obvious that this contribution would not have been possible without heavy reliance upon the research projects of three Masters students. Although the essence of those studies has been published previously, Richard Keizer, James Lyons and Eric Swanson are deserving of renewed recognition for their fine work. Keizer and Paul Guenther also provided help with the K–Ar dating. The (U–Th)/He age determinations were supported by NSF EAR 0105981. Comments by Lyons, Swanson, Mark Cloos, Todd Housh
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