Ar-Ar Geo-/Thermochronology

78 Pages·2016·7.57 MB·English

Ar-Ar Geo-/Thermochronology
((aann iinnttrroodduuccttiioonn))
Jörg A. Pfänder
TU Freiberg
V20160616
Ar-Ar Geo-/Thermochronology
Introduction: K-Ar dating
Principle: K-Ar Method:
Electron capture decay
of 40K to 40Ar:
fifi
4400 4400
KK AArr
(half life = 11.9 Ga)
19 18
Aldrich & Nier (1948): K-rich minerals have elevated 40Ar/36Ar ratios
when compared to atmospheric argon – this suggests that 40Ar is a
decay product of 40K
(Aldrich L.T. & Nier, A.O., 1948: Argon 40 in Potassium Minerals, Phys. Rev. 74, 876–877 –
Nachweis des Zerfalls von 40K zu 40Ca und 40Ar!)
Ar-Ar Geo-/Thermochronology
Introduction: K-Ar dating
!
Ar-Ar Geo-/Thermochronology
Introduction: K-Ar dating
Radioactive decay scheme of natural 40K
Half life of the 40K –
40K ln(2) 40Ar decay:
10.32 % (ec1) l =
T 11.93 Ga
1/2
l
= 0.581x10-10 1/a
ec
~0.001 %
Half life of the 40K –
4400CCaa ddeeccaayy::
1.397 Ga
0.16 % (ec2) 89.52 %
l
= 4.962x10-10 1/a
b
Half life (T ) of 40K:
1/2
1.25 Ga
40Ca
l
= 5.543x10-10 1/a
tot
40Ar
+ + - fi fi + + -
p e n n p e
Ar-Ar Geo-/Thermochronology
Introduction: K-Ar dating
40K branched decay to 40Ca (89.52%) and 40Ar (10.48%)
in the chart of nuclides:
Isotopes
+ + - fi
p e n
fi + + -
n p e
Ar-Ar Geo-/Thermochronology
Introduction: K-Ar dating
Abundances of naturally occuring Ar isotopes:
36Ar = 0.3364 ± 0.0006 %
Atmosphere: ~1% Argon
38Ar = 0.0632 ± 0.0001 %
with 40Ar/36Ar ~ 298.56 ± 0.31
(Lee et al., 2006)
40Ar = 99.600 %
Abundances of naturally occuring K isotopes:
39K = 93.2581 %
Only ~0.011% of K consists of 40K,
40K = 0.01167 % and only ~10% of 40K decay to 40Ar
41K = 6.7302 % BUT
K is a major element in numerous
rock forming minerals!
Ar-Ar Geo-/Thermochronology
Introduction: K-Ar dating
What can be dated by K-Ar and Ar-Ar?
Basically all K-bearing rocks and minerals !
Commonly used minerals:
K-feldspar, biotite, muscovite, hornblende, plagioclase
Whole rocks:
Dateable age range:
Basalte, rhyolite, tuffs, meteorites, ….
~~11000000 aa uupp ttoo 44..66 GGaa
Sanidine, anorthoclase
Plagioclase
Leucite
Biotite, Muscovite
Amphibole
Whole rock
1000 a (!) Range of applicability [Ma]
Ar-Ar Geo-/Thermochronology
Introduction: K-Ar dating
Amount of radiogenic 40Ar in a sample/mineral is
proportional to:
K-concentration in sample/mineral
Age of sample/mineral
AAmmoouunntt ooff rraaddiiooggeenniicc 4400AArr ((4400AArr**)) iinn aa ssaammppllee oorr
mineral as a function of time and remaining K-concentration:
l
40 Ar*= ec 40K (el tot t - 1) l = l + l + l = 5.543· 10- 10
l tot ec1 ec2 b
tot
l 0.581· 10- 10
= =
ec 0.1048
= +
40 Ar 40Ar 40Ar* l 5.543· 10- 10
tot
tot i
Ar-Ar Geo-/Thermochronology
Introduction: K-Ar dating
Leading to the classical K-Ar isochron diagram:
one sample = one age !
l
slope = ec (el t - 1)
tot
l
tot
slope ~ age
best fit
iissoocchhrroonn
40Ar  40Ar  40K l
=   + ec (eltott - 1)
36Ar  36Ar  36Ar l
i tot
= +
y b x m
datapoints
(40Ar/36Ar)
i
K-Ar whole-rock isochron of a tuff sample – each datapoint
represents a split of the same sample
Ar-Ar Geo-/Thermochronology
Introduction: K-Ar dating
Requirements to obtain a K-Ar age:
Decay constant‘s are constant over Earth‘s history
40Ar in a sample is only radiogenic (40Ar*)
oorr
non-radiogenic Ar can be determined and corrected for
Samples/minerals remain a closed system after
crystallisation/cooling