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Leah Morgan, U.S. Geological Survey, [email protected] Sidney Hemming, Columbia University, [email protected] Mike DeLuca, Columbia University, [email protected] 1. Introduction and Application 40 Ar/ 39 Ar geochronology can provide precise and accurate age constraints on rocks and minerals containing potassium, with an application range extending from the Holocene to as old as the Earth. The method has been applied to questions including tectonics, stratigraphy, volcanism, planetary geology, archaeology, and paleoclimate. 40 Ar/ 39 Ar geochronology relies on the branched decay of 40 K to 40 Ar and 40 Ca, with a half-life of 1.25 Ga (the decay constant, l = ln(2)/t 1/2 ). For most igneous and metamorphic rocks, the 40 Ar is trapped in material once it has cooled to a temperature below which the diffusion of argon is negligible. This decay forms the basis of the K-Ar method, where: = % & )1 + ,- /0 ∗ 2 /0 & & 3 4 The 40 Ar/ 39 Ar method is a variant of the K-Ar method, which uses the transmutation of 39 K into 39 Ar via a neutron-proton reaction using a nuclear reactor. 39 Ar is then measured as a proxy for K in the 40 K– 40 Ar decay scheme. Other isotopes of Ar ( 36 Ar, 37 Ar, 38 Ar) are measured to make corrections for atmospheric Ar and the production of interfering Ar isotopes in the reactor. By co-irradiating a standard of a known age (often Fish Canyon sanidine, FCs), a neutron flux parameter J can be calculated: = 6 &7 − 19 ( ∗ <= 2 >? ⁄ ) BCD where t is the assumed age of FCs. The 40 Ar/ 39 Ar age equation uses the calculated J value for an irradiation, along with argon isotopic measurements of a sample to determine the age of a sample: = 1 F1 + <= ∗ 2 >? G 3. Field Supplies and Sampling Sampling for 40 Ar/ 39 Ar geochronology is similar to sampling for many geochemical or petrographical techniques, although more material may be required than for other methods. For basalts and other igneous rocks, take care to obtain the freshest, and least altered, samples possible. For tephra, identify the mineral of interest and adjust sample quantities accordingly. Lexie Millikin sampling shoshonite at Ralston Buttes, Golden, CO. Required supplies varies depending on rock type, but a rock hammer, sample bags, knife, and field book are important, along with a GPS to capture location information. For tephra without obvious abundant sanidine, it is common to fill a large sample bag to ensure sufficient material for dating. 2. Age Range and Suitable Geologic Material The potentially applicable age range for 40 Ar/ 39 Ar geochronology ranges from the Holocene to the Hadean. However, the accessibility of younger samples depends on the K content of the material (see graph on right), limits of mass spectrometry measurements (improving), and the geological events following formation. Suitable materials must contain K. Commonly dated materials include basalt (groundmass, plagioclase, or whole rock), sanidine and other feldspars, amphibole, biotite, and white mica. Less common materials include sulfates (alunite, jarosite), feldspathoids, pyroxene, obsidian, clays, and evaporites.

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Page 1: 1. Introduction and Application 3. Field Supplies and ...Leah Morgan, U.S. Geological Survey, lemorgan@usgs.gov Sidney Hemming, Columbia University, sidney@ldeo.columbia.edu Mike DeLuca,

LeahMorgan,U.S.GeologicalSurvey,[email protected]

SidneyHemming,ColumbiaUniversity,[email protected],ColumbiaUniversity,[email protected]

1.IntroductionandApplication 40Ar/39Argeochronologycanprovidepreciseandaccurateageconstraintsonrocksandmineralscontainingpotassium,withanapplicationrangeextendingfromtheHolocenetoasoldastheEarth.Themethodhasbeenappliedtoquestionsincludingtectonics, stratigraphy, volcanism, planetary geology,archaeology,andpaleoclimate.

40Ar/39Argeochronologyreliesonthebrancheddecayof40Kto40Arand40Ca,withahalf-lifeof1.25Ga(thedecayconstant,l=ln(2)/t1/2).Formostigneousandmetamorphicrocks,the40Aris trapped in material once it has cooled to a temperaturebelowwhich the diffusion of argon is negligible. This decayformsthebasisoftheK-Armethod,where:

𝐴𝑔𝑒 = %&𝑙𝑛 )1 + ,-/0 ∗

2/0&&34

The40Ar/39Armethod isavariantoftheK-Armethod,whichusesthetransmutationof39K into39Arviaaneutron-protonreactionusinganuclear reactor. 39Ar is thenmeasuredasaproxyforKinthe40K–40Ardecayscheme.OtherisotopesofAr(36Ar, 37Ar, 38Ar) are measured to make corrections foratmosphericArandtheproductionof interferingArisotopesin the reactor. By co-irradiating a standard of a known age(oftenFishCanyonsanidine,FCs),aneutronfluxparameterJcanbecalculated:

𝐽 =6𝑒&7 − 19

( 𝐴𝑟∗<= 𝐴𝑟2

>?⁄ )BCD

wheretistheassumedageofFCs.The40Ar/39ArageequationusesthecalculatedJvalueforanirradiation,alongwithargonisotopicmeasurementsofasampletodeterminetheageofasample:

𝐴𝑔𝑒 =1𝜆𝑙𝑛 F1 + 𝐽

𝐴𝑟<= ∗

𝐴𝑟2>? G

3.FieldSuppliesandSamplingMethods Sampling for 40Ar/39Ar geochronology is similar to samplingformanygeochemicalorpetrographicaltechniques,althoughmorematerialmayberequiredthanforothermethods.Forbasalts and other igneous rocks, take care to obtain thefreshest, and least altered, samples possible. For tephra,identifythemineralofinterestandadjustsamplequantitiesaccordingly.

LexieMillikinsamplingshoshoniteatRalstonButtes,Golden,CO.

Requiredsuppliesvariesdependingonrocktype,butarockhammer,samplebags,knife,andfieldbookareimportant,alongwithaGPStocapturelocationinformation.Fortephrawithoutobviousabundantsanidine,itiscommontofillalargesamplebagtoensuresufficientmaterialfordating.

2.AgeRangeandSuitableGeologicMaterial Thepotentiallyapplicableagerangefor40Ar/39ArgeochronologyrangesfromtheHolocenetotheHadean.However,theaccessibilityofyoungersamplesdependsontheKcontentofthematerial(seegraphonright),limitsofmassspectrometrymeasurements(improving),andthegeologicaleventsfollowingformation.SuitablematerialsmustcontainK.Commonlydatedmaterialsincludebasalt(groundmass,plagioclase,orwholerock),sanidineandotherfeldspars,amphibole,biotite,andwhitemica.Lesscommonmaterialsincludesulfates(alunite,jarosite),feldspathoids,pyroxene,obsidian,clays,andevaporites.

Page 2: 1. Introduction and Application 3. Field Supplies and ...Leah Morgan, U.S. Geological Survey, lemorgan@usgs.gov Sidney Hemming, Columbia University, sidney@ldeo.columbia.edu Mike DeLuca,

LeahMorgan,U.S.GeologicalSurvey,[email protected]

SidneyHemming,ColumbiaUniversity,[email protected],ColumbiaUniversity,[email protected]

5.Laboratories

ArizonaStateUniversity,KipHodges,http://tectonics.asu.edu/page4/index.html

AuburnUniversity,BillHames,http://www.auburn.edu/cosam/centers/animal/index.htm

BerkeleyGeochronologyCenter,PaulRenneandAlDeino,http://bgc.org/

CopenhagenUniversity,MichaelStorey,http://quadlab.ku.dk/

CurtinUniversity,FredJourdan,https://sites.google.com/site/fredjourdancv/the-40ar-39ar-laboratory

Lamont-DohertyEarthObservatory,SidneyHemming,http://www.ldeo.columbia.edu/res/fac/argon/

NASAMarshall,BarbaraCohen,https://planetary.msfc.nasa.gov/MNGRL.html

NewMexicoBureauofGeology,BillMcIntoshandMattHeizler,https://geoinfo.nmt.edu/labs/argon/

OregonStateUniversity,AnthonyKoppers,http://geochronology.coas.oregonstate.edu/

RutgersUniversity,BrentTurrin,http://geology.rutgers.edu/people-directory/26-research/436-noble-gas-lab

SUERC,DarrenMarkandDanBarfod,https://www.gla.ac.uk/research/az/suerc/nercfacilities/argonisotopefacility/

UniversityofMelbourne,DavePhillipsandErinMatchan,http://earthsci.unimelb.edu.au/research/research-themes/geochronology#research

UniversityofWisconsin,BradSingerandBrianJicha,https://geochronology.geoscience.wisc.edu/

USGSDenver,LeahMorganandMikeCosca,https://minerals.usgs.gov/science/argon-geochronology/

USGSMenloPark,AndyCalvertandBobFleck

VrijeUniversiteitAmsterdam,KlaudiaKuiperandJanWijbrans,http://www.geo.vu.nl/~wijj/Jan_Wijbrans/Argon_Geochronology_Laboratory.html

4.SampleIntegrityandConsiderations

Keyassumptions:

• Materialbeingsampledbeganretainingradiogenic40Aratthetimeofinterest(eruption,metamorphiccooling,etc.)

• Since the timeof interest,material has not beenheatedorotherwisealteredinawaythataffectedargonretentionordistribution.

Approaches:

• Single-crystal total fusion (SCTF). Argon fromindividualgrainsisfullyreleasedinasingleheatingstep.Appropriatefortephraandotherrocks thatmaycontainxenocrysticcontamination.

• Step-heating (or incremental heating). Argon isreleased in steps of increasing temperature (orlaserpower). Appropriate for lavas,metamorphicphases,andothermaterialswithoutxenocrystsbutwiththepotentialforcomplexthermalhistories.

Mineralseparation:

• Careful application of crushing, sieving, andseparation techniques allow for the isolation ofmineralsofinterest.

• Coarsegrainedmaterials(frequently250–500µm)areideal.

Exampleagespectrumfromawholerockbasaltchip.X-axisshowsthepercentageof39Arreleased ineachstep;Y-axisshowstheageofeachstepinthereleaseofArfromthesample.


Sidney Centennial

Mark Nardi, Supervisory Geographer [email protected] MD-DE ... · 9/17/2020  · MD-DE-DC Water Science Center, Dover DE [email protected] Geospatial Analysis Soil Water Balance Modeling

USGS Pathways Programs For Managers Presented by: Susan Fong-Young, [email protected] [email protected] Alicia Gomez, [email protected] [email protected]

USGS National Hydrography Dataset Newsletter · Microsoft Login Option for Markup App and WBD Web Edit Applications Contacts: Marcelle Caturia ([email protected]), Lily Niknami ([email protected])

Interferons, 1986, Sidney P. Colowick, Sidney Pestka ... · Interferons, 1986, Sidney P. Colowick, Sidney Pestka, 012182019X, 9780121820190, Academic Press, 1986 ... guides readers

Sidney crosby

Talk by: Peter J. Haeussler [email protected] 907-786-7447

Abstract 1 - New Mexico State University, Christina L. Ferguson. 2 . [email protected]. 1, [email protected]. 2 . U.S. Geological Survey, New Mexico Water Science Center . Abstract 1

SIDNEY SAMUELS

From: GS-D-EI Freedom of Information Act

Sidney Chalhoub

Abstract 1 - animas.nmwrri.nmsu.edu, Christina L. Ferguson. 2 . [email protected]. 1, [email protected]. 2 . U.S. Geological Survey, New Mexico Water Science Center . Abstract 1 . The

Metropolitan sidney

Sidney nolan

Trent Hare, USGS, [email protected]

Colorization Tool for Isis Sponsor: Jeff Anderson USGS Astrogeology, Computer Scientist [email protected] Presented by Deborah Lee Soltesz [email protected]

Shorebird Subsistence Harvest and Indigenous Knowledge in [email protected], [email protected], 267-2269. [email protected], [email protected],

Analyzing Spectral Signatures in Imagine D. Meyer [email protected] E. Wood [email protected]

Sidney Tarrow

Monografia Sidney

Sidney Smith

EDIFICIO SIDNEY

Supervised Classification in Imagine D. Meyer [email protected] E. Wood [email protected]

Sidney lewis

Sidney Chalhouub

WaterSmart, Reston, VA, August 1-2, 2011 [email protected]

General Circulation and Climate Zones Martin Visbeck DEES, Lamont-Doherty Earth Observatory [email protected]

2010-2011 Unit Report - Amazon Web Services€¦ · [email protected] [email protected] University of Vermont Mary C. Watzin, Dean Rubenstein School of Environment and

Sidney Form

USGS Sentinel Lake Project with MN DNR Fisheries – Feb 2011 Richard L. Kiesling ([email protected]) Emily Resseger ([email protected])[email protected]

Atmospheric Properties II Martin Visbeck DEES, Lamont-Doherty Earth Observatory [email protected]

Pregoeiro Sidney

Geoethics - ldeo.columbia.edu

Sidney Kiosk

Continuous Water-Quality Field Methods Micelis Doyle & Joe Rinella U.S. Geological Survey 503-251-3226 & 503-251-3278 [email protected] [email protected]