Crustal Imaging and Characterization Team

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USGS ⁴⁰Ar/³⁹Ar Geochronology Laboratory

Facilities

Equipment

• » MAP 215 Rare-Gas Mass Spectrometer
• » MAP 215-50 Rare Gas Mass Spectrometer

Technique

⁴⁰Ar/³⁹Ar provides isotopic ages that are used to derive thermal histories of geologic materials. Samples of any age throughout Earth's history may be dated accurately and precisely by the ⁴⁰Ar/³⁹Ar method, but a practical minimum age is about 10,000 years. Before present. This technique, particularly when applied to various minerals within a single sample and used in conjunction with other geochronologic techniques, can provide not only the age of formation, but also its post-formation thermal and alteration history (based on argon "closure" temperatures in various minerals, the technique can delineate thermal history over a range of approximately 100-550^oC.)

Three mass spectrometers are routinely in operation. These instruments are dedicated to argon geochronology using high vacuum furnaces and lasers. A fourth mass spectrometer is soon to come on line. This new instrument will include a heating chamber that can analyze single crystals, as well as two lasers, one for heating and the other for ablation studies. Sample preparation and mineral separation laboratories reside in the same building as the argon laboratory. The world's best-characterized research nuclear reactor (USGS TRIGA reactor), in which argon geochronology samples from laboratories around the world are irradiated, also resides in this building.

Material:

Hornblende	Muscovite
White micas	Biotite
Sanidine	Anorthoclase
Microcline	Plagioclase
Adularia	Orthoclase
K-evaporites	Anhydrous
Impactites	Glasses
Whole rock	Slates
Basalts	Metals

Form:

Samples are prepared in our mineral separation laboratory. Samples and standards are irradiated in the USGS TRIGA reactor for a length of time necessary to produce ⁴⁰/³⁹Ar ratios beween 1 and 100. Irradiated samples are heated in a low-blank furnace or argon is released by a laser. The released gases are cleaned for analysis of the argon isotopic composition on a mass spectrometer. Generally, argon from a sample is released in a step-wise fashion.

Precision:

Typically +/- 0.11% to 0.25% (1 sigma). With optimal sample conditions, an age of 10,000 years is possible.

Applications:

Numerical dating of rock and mineral samples for studies of timing of:

• » mineralization
• » geologic mapping in igneous and metamorphic terrains
• » stratigraphic relationships
• » sediment provenance
• » pluton emplacement and uplift histories
• » timing of faulting
• » timing of metamorphism
• » cooling trajectories in metamorphic and igneous rocks
• » differential uplift and timing hazards assessment in recently active volcanic terrain
• » timing of bolide impacts
• » the geologic time scale
• » argon concentration and isotopic composition in metals and other man made inorganic materials

Special Peripherals:

Low-blank double vacuum furnaces, CO₂ laser, Excimer laser, ultrahigh vacuum extraction lines, automated extraction lines, automated sample chamber.

Sample Size:

From a single grain to 500 mg depending on age and sample composition

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U.S. Department of the Interior | U.S. Geological Survey
URL: http://crustal.usgs.gov/laboratories/geochronology/facilities.htm
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Page Last Modified: 08-May-2008@12:04