USGS-GD-Scientific Capabilities -  40ARGON/39ARGON Technique

 

40ARGON/39ARGON

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°C). The capabilities described are available as both analytical and scientific/technical collaborative support. The facilities represent a state of the art argon geochronology laboratory equipped with three modern high sensitivity automated mass spectrometers, CO2 (infrared) and Nd:YAG (in UV) lasers (for total fusion and in-situ analyses of < milligram samples), and low blank furnaces (for age spectra and very young samples). Samples are irradiated in the Denver TRIGA reactor before mass spectrometer analysis of argon isotopes.

 

  • Material:
Hornblende
White micas
Sanidine
Microcline
Adularia
K-evaporites
Impactites
Whole rock slates
Muscovite
Biotite
Anorthoclase
Plagioclase
Orthoclase
Anhydrous glasses
Whole rock basalts
Metals
  • Form:
extract and clean, analyze for argon isotopic composition on mass spectrometer as 'cleaned' argon thermally released from irradiated sample material.
  • Precision:
Typically ±0.25% to 0.5% - 0.1% to 0.2% for sanidine and anorthoclase. With optimal sample conditions, an age # 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
timing of evaporite formation
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:
CO2 laser, frequency quadrupled Nd:YAG laser operating in the UV range, argon ion lasers, low blank furnaces, ultrahigh vacuum extraction lines, custom fabricated SAES getter pumps. Located in Denver.
  • Automation:
Yes - VG1200b (36-50 analyses/day), MAP 216 (40-72 analyses/day), with multiple analyses per age determination. MAP 215 currently being automated.
  • Sample Size:
MAP 215, min = 10-12
moles 40Ar, typical = 10-11
moles 40Ar
MAP 216, min = 10-16
moles 40Ar, typical = 10-14
moles 40Ar
VG1200b, min = 10-12 moles
40Ar, typical = 10-11
moles 40Ar
 


 | Capabilities |
| Geochronology, Geochemistry, and Tracer Studies  |
| Geochronology  |

 


URL http://geology.cr.usgs.gov/capabilities/gronemtrac/geochron/argon/tech.html