Fundamentals of the 40Ar39Ar method

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Fundamentals of the 40Ar/39Ar method or (Why
argon is so useful in studies of
petrology/tectonics) Mike Cosca University of
Lausanne Switzerland
2008EURISPET Seminar, Canberra Isotopes Applied
to Petrological Problems
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Argon and potential for geochronology and
thermochronology Ar is a noble gas, is
chemically inert, and mobile at elevated
temperatures (prone to diffusive loss or gain in
many geological environments). The
relative mobility of argon in minerals may
provide information on thermal histories and in
some cases may be used as a trace element.
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Argon and potential for geochronology and
thermochronology (cont.) Primary factors
controlling argon mobility and transport Extrinsi
c variables --Temperature, Pressure,
Fluids Intrinsic variables --Defects
(dislocations), composition and
structure Recrystallization --Dissolution
and re-precipitation --Annealing of
defects/dislocations by chemical- or
deformation-induced grain boundary migration
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Argon and potential for geochronology and
thermochronology (cont.) As a noble gas, argon
transport within a mineral lattice is probably
different from chemically bonded atoms within a
lattice. Argon transport is controlled by both
extrinsic and intrinsic variables -- petrology
and mineralogy needed to asses the relative
contributions of variables.
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A very common approach, but there is so much more
to learn.
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40Ar/39Ar Geochronology
The 40Ar/39Ar method is a variant of the K-Ar
dating method and largely, but not completely,
replaces the the K-Ar method. This is a
relative method requiring the use of
well-calibrated standards. Developed at
UC Berkeley by Craig Merrihue and Grenville
Turner (1969). Standards and samples are
prepared and analyzed under identical conditions.
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40Ar/39Ar Geochronology
Demonstrated age range
4.6 Ga
2 ka
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Some applications of 40Ar/39Ar geochronology
Conventional (furnace and infrared laser)
Volcanic stratigraphy Time scale
calibrations Hominid evolution Igneous intrusions
Igneous and metamorphic cooling
(thermochronology) Ore mineralization Ductile and
brittle faulting/fluid flow Exhumation rates
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Some applications of 40Ar/39Ar geochronology
(cont.)
Less conventional (excimer laser)
Mineral growth events and overgrowths related
to --prograde metamorphism --mineralization
--shearing/faulting --fluid flow Intragrain
zoning related to --shearing and thermal
events --incorporation of inherited argon
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40Ar/39Ar Method
Applicable to any K-bearing mineral or rock, such
as Hornblende , muscovite, biotite,
K-feldspar Plagioclase, whole rocks Glass,
feldspathoids, alunite--jarosite,
smectite-illite, Mn-oxides
(very common)
(less common)
(even less common)
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Of the three isotopes of K, only 40K is
radioactive (atomic abundance of only 117 ppm).
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What happens to those 0.01167 of radioactive
40K atoms??
40K undergoes branched decay 40K 40Ca
(89.52)
b-decay
electron capture
40K 40Ar (10.48)
lb 4.962 x 10-10 a-1 lec 0.581 x 10-10 a-1 l
5.543 x 10-10a-1 t1/2 1.25 Ga
(Value from Steiger and Jäger (1977), but work by
others in progress to measure with greater
precision and accuracy)
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Note only 40Ar is radiogenic 38Ar and 36Ar are
present in amounts closely reflecting those
retained in Earth's atmosphere following solar
system formation, planetary accretion, and
degassing.
37Ar (35.1 days)
40Ar/36Ar in present day Earth atmosphere 295.5
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K-Ar Age Equation
(both K and Ar are measured on splits of the same
sample)
40Ar radiogenic 40Ar (generally assumed
40Ar 40Artotal 36Ar x 295.5)
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40Ar/39Ar Age Equation 39K is transformed
into 39Ar by neutron irradiation. The 39Ar is
proportional to 39K, therefore total K. Ar-Ar
age eq
where tu sample age l decay constant
known J neutron flux known 40Ar/39Ar
measured ratio in sample
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The J factor (The neutron fluence needs to be
monitored, so we use standard minerals of known
age) For example standard mineral age Taylo
r Creek sanidine 28.34 /- 0.16 Ma Fish
Canyon sanidine 28.02 /- 0.16 Ma GA1550
biotite 98.8 /- 0.5 Ma MMhb hornblende 520
.4 /- 1.7 Ma
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High-purity samples ready to be sent for
irradiation
Standards
Samples
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Samples irradiated from minutes to several hours
and rotated to minimize neutron flux gradients
Sample irradiated utilizing fast (gt1 Mev)
neutrons 39K (n,p)? 39Ar Atomic abundance
of K istopes is constant, thus 39Ar is a proxy
for 40K
Oregon State Triga reactor
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U-Zr hydride fuel
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"J"-curve
FCTs 28.02 Ma
TCs 28.34 Ma
Height in package (mm)
J value (single crystals of FCT and TCR sanidines)
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Practical aspects of 40Ar/39Ar analysis
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Primary methods of liberating argon isotopes in
Lausanne lab 1) Incremental heating by
furnace 2) Incremental heating by CO2 infrared
laser (l 10, 600 nm) 3) Ablation by excimer
(KrF) ultraviolet laser (l 248 nm)
1)
2)
3)
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Gas purification system
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High temperature furnace (25C- 1800C)
Typically used for fine- grained materials,
low-K minerals/rocks, and samples where
controlled heating is required.
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CO2 laser (10600 nm) typically used for
incremental heating of single grains or groups of
grains
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Single grain of sanidine
ca. 100 samples of unknowns and standards
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Ultraviolet laser (192-248 nm) typically used for
in situ analysis preserving textural relationships
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White mica
Quartz
50 mm
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Nier ion source
500kg magnet
Quad lenses
Detectors
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Quad lenses
Detector array
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F Faraday, A Axial mult, L Low mass mult
F 40, 39
F 40
A 38, 37
A 38, 39, 37
L 36
L 36, (37)
Setup dictated by sample size K-content Age
Q1
Q2
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Multicollecton (40 Faraday, 38 Axial, 36 Low mass
ion multiplier)
Mean 321.8 /- 0.4 (0.1)
40Ar/36Ar
128 hours of consecutive measurements (Fri-Wed)
n 337
Hour of day
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Corrections that need to be applied

• Initial 40Ar trapped from atmosphere
• (or other trapped component)
• Neutron-induced interferences
• Radioactive decay subsequent to irradiation
• Laboratory specific corrections (blanks, mass
  discrimination,
• detector intercalibration, non linearity, gain,
  etc.)

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Initial trapped argon (atmospheric)
correction Nearly all samples have some
trapped initial argon that is generally assumed
to have an atmospheric composition (40Aratm) A
correction may be applied using observed atomic
abundances Atomic abundances (in ) in
atmosphere 36Ar (0.337) 38Ar (0.063) 40Ar
(99.600) Therefore (40Ar/36Ar)atm
295.5 Thus 40Ar 40Armeas
295.5(36Aratm)
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Interfering Isotopes Several Ar isotopes
created during neutron bombardment of various
elements need correcting (for this we irradiate
salts like KCl and CaF2) Isotope produced
Target Element Process 36Ar 40Ca (n,na) 37
Ar 40Ca (n,a) 38Ar 37Cl (n,g)38Cl?b-decay
39Ar 42Ca (n,a) 39Ar 39K (n,p) 40Ar
40K (n,p)
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• Radioactive Decay Corrections
• Radioactive Ar nuclides produced during
  irradiation
• need decay-corrections for age calculations
• 37ArCa used to correct 36Ar produced from 40Ca
• (t1/2 35.1 days) therefore Ca-rich samples
  (amphiboles,
• plagioclase, whole rocks) should be analyzed
  within
• several half lives after irradiation
• 39ArK primary isotope used for dating
  (relativey long
• half life, t1/2 269a, on a human time
  scale)

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• Systematic Corrections
• Corrections specific to individual extraction
  line and mass
• spectrometer system
• Blanksa significant correction when doing laser
  work because argon is present at the percent
  level in the atmosphere and is easily introduced
  into the high vacuum system. Blanks originate
  from the furnace (variable with temperature), the
  extraction line, and samples within the laser
  chamber. Mass spectrometer blanks are generally
  minor, as are other isobaric interferences.

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Systematic Corrections (cont.)
Mass spectrometer source and detector mass
discrimination Detectable when a reference gas
(air) does not yield the accepted isotope ratio.
May be due to stray magnetic fields, beam
scattering, variable ionization of argon
isotopes. Solution Calibrate system using
atmospheric pipette with a known argon ratio
(40Ar/36Ar)atm 295.5
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Data plotting Age frequency, age spectrum,
multi-isotope correlation (isochron) diagrams
Probability
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Incremental heating age spectrum
Data plotted on age spectrum diagrams like this
almost always include a correction for initial
trapped argon assuming an isotopic composition of
present day atmosphere
Age Plateau
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Implicit interpretation of a step-heating spectrum
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28.02 /- 0.04
Fish Canyon Tuff sanidine
Age (Ma)
FAAAL
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25
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28.31 /- 0.06 Ma
Taylor Creek sanidine
FAAAL
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Age (Ma)
25
2
Alder Creek sanidine
Age (Ma)
AAAAA
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1.18 /- 0.02 Ma
0
0
50
100
39Ar released
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Dating volcanic stratigraphy around Yellowstone
Feeley ad Cosca, 2003 GSA Bulletin
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Feeley ad Cosca, 2003 GSA Bulletin
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Cosca et al., 2005 Int. Geol. Rev.
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What might a spectrum from such a grain look like?
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Implicit interpretation of a step-heating spectrum
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Factors that may affect the shape of an age
spectrum include Thermal overprinting, mixtures
of grains with different ages, argon recoil,
extraneous argon, non atmospheric initials, etc
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Ternary Isotope Correlation Plot
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39ArK
37ArCa
38ArCl
Plots may be able to deconvolve ages from argon
sited in multiple reservoirs
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Isotope Correlation Plots (mixing diagrams)
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elt-1
40Ar/36Ari
40Ar/36Ari
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Isochron plots
Isochron plots make no assumptions about initial
trapped argon ---- the intercepts define the
isotopic compostion of the trapped argon
component directly
Ca from 37Ar K from 39Ar
Winer, Feeley, Cosca, (2004) JVGR
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Winer, Feeley, Cosca, 2004
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Complications, uncertainties, and interpretive
models
Loss of argon by diffusion, recrystallization,
microstructural defects, presence of fluids
Gain of extraneous argon (inherited or excess) by
fluids, xenocrystic contamination, or other
processes
Single and multi-crystal diffusion kinetics,
length scales of diffusion in minerals
Decay constant of 40K
A
A couple of petrological examples using age
spectrum techniques.
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Jaboyedoff and Cosca, (1999) CMP
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Illite/Smectite Mixed-layer Mica (detrital)
Cumulative 39Ar Released
Cumulative 39Ar Released
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Laboratory Degassing Results
40Ar Released
Temperature (C)
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Neoformed Illite (dioctahedral mica) in
Mixed-Layer I/S vs. Age
Illite Layer in Illite/Smectite
Diagenesis
Ar/Ar Total Fusion Age (Ma)
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