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argon geochronology laboratory
USGS argon geochronology laboratory in Denver, CO. Photo by M. Cosca, USGS.

This project supports the USGS argon geochronology laboratory in Denver. The USGS 40Ar/39Ar geochronology laboratory is a state-of-the-art research facility for determining absolute ages of minerals and rocks. The 40Ar/39Ar laboratory contributes critical geochronology to individual USGS research projects and to partners in academia and other Federal agencies. This facility houses necessary equipment for sample preparation and analysis, including high-sensitivity noble gas mass spectrometers and ultraviolet (UV) and infrared (IR) lasers. The versatility of the 40Ar/39Ar method permits determining the timing of processes and events such as igneous intrusions and extrusions, ore mineralization and hydrothermal fluid circulation, metamorphic cooling and exhumation, mineral formation and recrystallization, and shallow crustal faulting. Scientists are dependent on the geochronologist for data and interpretations to determine these parameters. This laboratory develops methodology for small and difficult sample analysis often at the limits of existing mass spectrometer technology.


Argon Geochronology Laboratory - Facilities

The 40Ar/39Ar Method

40Ar/39Ar geochronology is an experimentally robust and versatile method for constraining the age and thermal history of rocks. Such information is extremely valuable for understanding a variety of geological processes including the formation of ore deposits, mountain building and history of volcanic events, paleo-seismic events, and paleo-climate. The 40Ar/39Ar isotopic dating method has evolved into the most commonly applied geochronological method, and can be applied to many geological problems that require precise and accurate time and temperature control.

The 40Ar/39Ar method is used to date terrestrial, lunar, and meteoritic rocks and minerals ranging in age from approximately 10,000 years to the age of the solar system (~4.56 billion years). The method is derived from the natural and widely occurring radioactive isotope of potassium, 40K, which has a dual decay to 40Ca and 40Ar and a half-life of 1250 million years). Radiogenic 40Ar therefore accumulates in a mineral over geologic time. The time since accumulation of 40Ar began (age of the mineral or rock) can be determined by knowing the half-life of 40K, and measuring the abundance of potassium (and therefore 40K) and radiogenic 40Ar. The 40Ar/39Ar method takes advantage of another isotope of argon, 39Ar, which can be mesured as a proxy for potassium concentration by neutron irradiation to produce 39Ar from 39K. By irradiating a sample of unknown age with a standard of known age and then comparing the 40Ar/39Ar ratios of the unknown to the standard we can calculate the 40Ar/39Ar age for the unknown. The isotopic measurements on modern noble gas mass spectrometers are highly sensitive and precise. Very small amounts of sample material, ranging in size from a single mineral grain to a few milligrams of sample are commonly analyzed with small associated analytical errors (< 0.2%).

All samples and standards are irradiated in the USGS TRIGA reactor located at the Denver Federal Center. For information about the reactor and its capabilities go to Geological Survey TRIGA Reactor (GSTR) Services.

Disk preparation for geochronology analysis.
Purified minerals and rocks are loaded into aluminum disks, stacked,
Samples sealed in a tube for neutron irradiation.
and sealed in a quartz tube, followed by neutron irradiation.
Irradiated samples
Irradiated samples seen through viewports on ultra high vacuum sample chambers. Left chamber is for infrared (CO2) heating and right chamber is for in situ ultraviolet laser ablation. Signal sizes <1 x 10-20 moles are routinely measured by ion counting. Photo by M. Cosca, USGS.

USGS Denver Facilities

The 40Ar/39Ar laboratory consists of single- and multiple-collector mass spectrometers each attached to custom-built, stainless steel extraction lines that operate at pressures roughly one billionth of an atmosphere. The argon extraction lines are designed to incrementally heat, fuse, or ablate any potassium-bearing rock or mineral using either infrared or ultraviolet lasers. Research is focused on understanding the geochronology of ore deposits, mountain building, landscape formation, near-surface faulting, and mechanistic aspects of argon distribution in minerals and rocks.

The USGS 40Ar/39Ar geochronology laboratory in Denver is designed for determining absolute ages of minerals and rocks via high-precision isotope measurements using state-of-the-are gas source mass spectrometers. The 40Ar/39Ar laboratory currently operates two mass spectrometers, a single collector Mass Analyser Products 215-50 mass spectrometer and a multiple collector Thermo Fisher Scientific ARGUSVI mass spectrometer; a Thermo Scientific HELIX-MC mass spectrometer is scheduled for installation in 2016. Each mass spectrometer is attached to low-blank, stainless steel, noble gas extraction line that operates at ultra-high vacuum (UHV). Samples are loaded into special holders and placed into infrared (IR) or ultraviolet laser chambers (UVLC) such that argon isotopes can be released from single grains and whole-rock samples by incremental heating and fusion or by spatially controlled ablation when mineral separation is impossible or in cases when we wish to analyze mineral inclusions, overgrowths, or preserve textural relationships.

Facilities include workshop space with a disk mill, multiple rock saws, jaw crusher, and a sample storage room. We also have a chemical workspace for heavy liquid separation and Frantz magnetic separation.

The sample preparation facilities include a series of microscopes to allow the inspection of mineral separates, sample picking, and loading of the irradiation packets. A separate and secure microscope room is used for the unloading of irradiated samples and radioactive storage.

Components of the argon lab instruments.
Primary components of the USGS Argon Geochronology Laboratory instrumentation: 1) CO2 Laser, 2) Extraction Line, 3) UV Laser, 4) Mass Spectrometer, and 5) Sample Chamber. Photo by M. Cosca, USGS.





External Collaborators




Page, W.R., Menges, C.M., Gray, Floyd, Berry, M.E., Bultman, M.W., Cosca, M.A., and VanSistine, D.P., 2016, Geologic map of the Rio Rico and Nogales 7.5’ quadrangles, Santa Cruz County, Arizona: U.S. Geological Survey Scientific Investigations Map 3354, 32 p. pamphlet, 2 sheets, scale 1:24,000,


Bradley, D.C., O’Sullivan, Paul, Cosca, M.A., Motts, H.A., Horton, J.D., Taylor, C.D., Beaudoin, Georges, Lee, G.K., Ramezani, Jahan, Bradley, D.B., Jones, J.V., and Bowring, Samuel, 2015, Synthesis of geological, structural, and geochronologic data (phase V, deliverable 53), chap. A of Taylor, C.D., ed., Second projet de renforcement institutionnel du secteur minier de la République Islamique de Mauritanie (PRISM-II): U.S. Geological Survey Open-File Report 2013–1280-A, 328 p.,

Cosca M.A., 2015, Potassium–Argon (Argon–Argon), Structural Fabrics, in Rink W.J., and Thompson J.W., eds., Encyclopedia of Scientific Dating Methods: SpringerReference Encyclopedia of Earth Science Series, Springer-Verlag Berlin Heidelberg, p. 642-647. doi: 10.1007/978-94-007-6304-3_124

Cosca, M., 2015, Book Review: Advances in 40Ar/39Ar Dating: From Archaeology to Planetary Sciences. Edited by F. Jourdan, D.F. Mark, and C. Verati. Geological Society of London Special Publications: American Mineralogist, v.100, p. 664. View Cosca Review [PDF file, 196 KB].

Fleck, R.J., du Bray, E.A., John, D.A., Vikre, P.G., Cosca, M.A., Snee, L.W., and Box, S.E., 2015, Geochronology of Cenozoic rocks in the Bodie Hills, California and Nevada: U.S. Geological Survey Data Series 916, 26 p.,

Horton, F., Lee, J., Hacker, B., Bowman-Kamaha’o, M., and Cosca, M., 2015, Himalayan gneiss dome formation in the middle crust and exhumation by normal faulting: New geochronology of Gianbul dome, northwestern India: Geological Society of America Bulletin, 127 (1-2), p. 162-180. doi: 10.1130/B31005.1

McFadden, R.R., Teyssier, C., Siddoway, C.S., Cosca, M.A., and Fanning, C.M., 2015, Mid-Cretaceous oblique rifting of West Antarctica: Emplacement and rapid cooling of the Fosdick Mountains migmatite-cored gneiss dome: Lithos, 232, p. 306-318. doi: 10.1016/j.lithos.2015.07.005

Mercer, C.N., Hofstra, A.H., Todorov, T.I., Roberge, J., Burgisser, A., Adams, D.T., and Cosca, M., 2015, Pre-Eruptive Conditions of the Hideaway Park Topaz Rhyolite: Insights into Metal Source and Evolution of Magma Parental to the Henderson Porphyry Molybdenum Deposit, Colorado: Journal of Petrology, 56 (4), p. 645-679. doi: 10.1093/petrology/egv010

Morgan, L.E., 2015, Noble Gas Mass Spectrometry, in W.J., and J.W. Thompson (eds.), Encyclopedia of Scientific Dating Methods: Springer Reference Encyclopedia of Earth Science Series, Springer-Verlag Berlin Heidelberg, p. 608. doi: 10.1007/978-94-007-6304-3_118

Morgan, L.E., and Davidheiser-Kroll, B., 2015, Pressure disequilibria induced by rapid valve closure in noble gas extraction lines: Geochemistry, Geophysics, Geosystems, 16, p. 1923-1931. doi: 10.1002/2015GC005823

Mulch, A., Chamberlain, C.P., Cosca, M.A., Teyssier, C., Methner, K., Hren, M.T., and Graham, S.A., 2015, Rapid change in high-elevation precipitation patterns of western North America during the Middle Eocene Climatic Optimum (MECO): American Journal of Science, 315 (4), p. 317-336. doi: 10.2475/04.2015.02

Ramalho, R.S., Brum da Silveira, A., Fonseca, P.E., Madeira, J.,. Cosca, M., Cachão, M, Fonseca, M.M., and Prada, S.N., 2015, The emergence of volcanic oceanic islands on a slow-moving plate: The example of Madeira Island, NE Atlantic: Geochemistry, Geophysics, Geosystems (G3), 16 (2), p. 522-537. doi: 10.1002/2014GC005657

Taylor, R.D., Goldfarb, R.J., Monecke, T., Fletcher, I.R., Cosca, M.A., and Kelly, N.M., 2015, Application of U-Th-Pb Phosphate Geochronology to Young Orogenic Gold Deposits: New Age Constraints on the Formation of the Grass Valley Gold District, Sierra Nevada Foothills Province, California: Economic Geology, 110 (5), p. 1313-1337. doi: 10.2113/econgeo.110.5.1313

Thompson, R.A., Shroba, R.R., Machette, M.N., Fridrich, C.J., Brandt, T.R., and Cosca, M.A., 2015, Geologic map of the Alamosa 30’ × 60’ quadrangle, south-central Colorado: U.S. Geological Survey Scientific Investigations Map 3342, 23 p., scale 1:100,000, (Supersedes Open-File Report 2005–1392, and Open-File Report 2008–1124.)


Carrapa, B., Mustapha, F.S., Cosca, M., Gehrels, G., Schoenbohm, L.M., Sobel, E.R., DeCelles, P.G., Russell, J., and Goodman, P., 2014, Multisystem dating of modern river detritus from Tajikistan and China: Implications for crustal evolution and exhumation of the Pamir: Lithosphere, 6 (6), p. 443-445. doi: 10.1130/L360.1

Cosca, M.A., Thompson, R.A., Lee, J.P., Turner, K.J., Neymark, L.A., and Premo, W.R., 2014, 40Ar/39Ar geochronology, isotope geochemistry (Sr, Nd, Pb), and petrology of alkaline lavas near Yampa, Colorado: Migration of alkaline volcanism and evolution of the northern Rio Grande rift: Geosphere, 10 (2), p. 374-400. doi:10.1130/GES00921.1

Idleman, L., Cosca, M.A., Heizler, M.T., Thomson, S.N., Teyssier, C., and Whitney, D.L., 2014, Tectonic burial and exhumation cycles tracked by muscovite and K-feldspar 40Ar/39Ar thermochronology in a strike-slip fault zone, central Turkey: Tectonophysics, 612-613, 4 February 2014, p. 134-146. doi:10.1016/j.tecto.2013.12.003

McQuarrie, N., Tobgay, T., Long, S.P., Reiners, P.W., and Cosca, M.A., 2014, Variable exhumation rates and variable displacement rates: Documenting recent slowing of Himalayan shortening in western Bhutan: Earth and Planetary Science Letters, 386, pp. 161-174. doi: 10.1016/j.epsl.2013.10.045

Thompson, R.A., Turner, K.J., Shroba, R.R., Cosca, M.A., Ruleman, C.A., Lee, J.P., and Brandt, T.R., 2014, Geologic map of the Sunshine 7.5' quadrangle, Taos County, New Mexico: U.S. Geological Survey Scientific Investigations Map 3283, scale 1:24,000,

Thompson, R.A., Turner, K.J., Shroba, R.R., Cosca, M.A., Ruleman, C.A., Lee, J.P., and Brandt, T.R., 2014, Geologic map of the Ute Mountain 7.5' quadrangle, Taos County, New Mexico, and Conejos and Costilla Counties, Colorado: U.S. Geological Survey Scientific Investigations Map 3284, scale 1:24,000,


Buchs, D.M., Pilet, S., Cosca, M., Flores, K.E., Bandini, A.N., and Baumgartner, P., 2013, Low-volume intraplate volcanism in the Early/Middle Jurassic Pacific basin documented by accreted sequences in Costa Rica: Geology, Geophysics, Geosystems (G3), 14 (5), p. 1552-1568. doi: 10.1002/ggge.20084

Farmer, G.L., Glazner, A.F., Kortmeier, W.T., Cosca, M.A., Jones, C.H., Moore, J.E., and Schweickert, R.A., 2013, Mantle lithosphere as a source of postsubduction magmatism, northern Sierra Nevada, California: Geosphere, 9 (5), p. 1102-1124. doi: 10.1130/GES00885.1

Kellogg, K.S., Lee, Keenan, Premo, W.R., and Cosca, M.A., 2013, Geologic Map of the Harvard Lakes 7.5' Quadrangle, Park and Chaffee Counties, Colorado: U.S. Geological Survey Scientific Investigations Map 3267, 22 p., scale 1:24,000,

Lüdecke, T., Mikes, T., Rojay, F.B., Cosca, M.A., and Mulch, A., 2013, Stable isotope-based reconstruction of Oligo-Miocene paleoenvironment and paleohydrology of Central Anatolian lake basins (Turkey): Turkish Journal of Earth Sciences, 22 (5), p. 793-819. doi: 10.3906/yer-1207-11 [PDF file, 13.4 MB]

Zhou, R., Schoenbohm, L.M., and Cosca, M.A., 2013, Recent, slow normal and strike-slip faulting in the Pasto Ventura region of the southern Puna Plateau, NW Argentina: Tectonics, 32 (1), p. 19-33. doi: 10.1029/2012TC003189


Tollo, R.P., Aleinikoff, J.N., Mundil, R., Southworth, C.S., Cosca, M.A., Rankin, D.W., Rubin, A.E., Kentner, A.E., Parendo, C.A., and Ray, M.S., 2012, Igneous activity, metamorphism, and deformation in the Mount Rogers area of SW Virginia and NW North Carolina: A geologic record of Precambrian tectonic evolution of the southern Blue Ridge Province, in Eppes, M.C., and Bartholomew, M.J., eds., From the Blue Ridge to the Coastal Plain: Field Excursions in the Southeastern United States: Geological Society of America Field Guide 29, p. 1–66,

Trexler, J., Cashman, P., and Cosca, M., 2012, Constraints on the history and topography of the Northeastern Sierra Nevada from a Neogene sedimentary basin in the Reno-Verdi area, Western Nevada: Geosphere, 8 (3), p. 548-561. doi: 10.1130/GES00735.1


Albion, J., Ovtcharova, M., Bussy, F., Cosca, M., Schaltegger, U., Bussien, D., and Lewin, E., 2011, Lifetime of an ocean island volcano feeder zone: constraints from U–Pb dating on coexisting zircon and baddeleyite, and 40Ar/39Ar age determinations, Fuerteventura, Canary Islands: Canadian Journal Earth Sciences, 43 (10), p. 1511-1532. doi: 10.1139/E10-032

Cosca, M., Stunitz, H., Bourgeix, A-L., and Lee, J.P., 2011, 40Ar* loss in experimentally deformed muscovite and biotite with implications for 40Ar/39Ar geochronology of naturally deformed rocks: Geochimica et Cosmochimica Acta, 75 (24), p. 7759-7778. doi: 10.1016/j.gca.2011.10.012

Drenth, B.J., Turner, K.J., Thompson, R.A., Grauch, V.J.S., Cosca, M.A., and Lee, J.P., 2011, Geophysical expression of elements of the Rio Grande Rift in the northeast Tusas Mountains - Preliminary interpretations, in Koning, D.J. and others, eds., Geology of the Tusas Mountains and Ojo Caliente area Guidebook: New Mexico Geological Society Fall Field Conference Guidebook - 62, p. 165-175. View Drenth paper [PDF file, 3.59 MB].

Schlup, M., Steck, A., Carter, A., Cosca, M., Epard, J-L., and Hunziker, J., 2011, Exhumation history of the NW Indian Himalaya revealed by fission track and 40Ar/39Ar ages: Journal of Asian Earth Sciences, 40 (1), p. 334-350. doi: 10.1016/j.jseaes.2010.06.008


Cole, J.C., Trexler Jr., J.H., Cashman, P.H., Miller, I.M., Shroba, R.R, Cosca M.A., and Workman, J.B., 2010, Beyond Colorado's Front Range—A new look at Laramide basin subsidence, sedimentation, and deformation in north-central Colorado, in Morgan, L.A., and Quane, S.l., eds., Through the Generations: Geologic and Anthropogenic Field Excursions in the Rocky Mountains from Modern to Ancient: Geological Society of America Field Guide 18, p. 55-76. doi: 10.1130/2010.0018(03)

Ramalho, R.S., Helffrich, G., Cosca, M., Vance, D., Hoffmann, D., and Schmidt, D.N., 2010, Vertical movements of ocean island volcanoes: Insights from a stationary plate environment: Marine Geology, 275 (1-4), p. 84-95. doi: 10.1016/j.margeo.2010.04.009

Ramalho, R., Helffrich, G., Cosca, M., Vance, D., Hoffmann, D., and Schmidt, D.N., 2010, Episodic swell growth inferred from variable uplift of the Cape Verde hotspot islands: Nature Geoscience, 3, p. 774-777. doi:10.1038/ngeo982

van der Lelij, R., Spikings, R.A., Kerr, A.C., Kounov, A., Cosca, M., Chew, D., and Villagomez, Dl, 2010, Thermochronology and tectonics of the Leeward Antilles: Evolution of the southern Caribbean Plate boundary zone: Tectonics, 29, TC6003. doi: 10.1029/2009TC002654


Bröcker, M., Klemd, R., Cosca, M., Brock, W., Larionov, A.N., and Rodionov, N., 2009, The timing of eclogite facies metamorphism and migmatization in the Orlica–Śnieżnik complex, Bohemian Massif: constraints from a multimethod geochronological study: Journal of Metamorphic Geology, 27 (5), p. 385-403. doi: 10.1111/j.1525-1314.2009.00823.x

Larson, P.B., Phillips, A., John, D., Cosca, M., Pritchard, C., Andersen, A., and Manion, J., 2009, A preliminary study of older hot spring alteration in Sevenmile Hole, Grand Canyon of the Yellowstone River, Yellowstone Caldera, Wyoming: Journal of Volcanology and Geothermal Research, 188 (1-3), p. 225-236. doi: 10.1016/j.jvolgeores.2009.07.017


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