Elsevier

Chemical Geology

Volume 214, Issues 3–4, 25 January 2005, Pages 249-263
Chemical Geology

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

https://doi.org/10.1016/j.chemgeo.2004.10.002Get rights and content

Abstract

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.

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

References (24)

  • J.J. Aranda-Gómez et al.

    Cenozoic volcanism and tectonics in NW Mexico—a transect across the Sierra Madre Occidental volcanic field and observations on extension related magmatism in the southern Basin and Range and Gulf of California tectonic provinces

  • Boyce, J.W., and Hodges, K.V., 2001, Chemical variations in the Cerro de Mercado (Durango, Mexico) fluorapatite:...
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