Geology Reference
In-Depth Information
(a)
(b)
K-Ar
Ar-Ar
Typical 'excess-
argon' spectrum
Excess Ar?
1050° 1075°
1100°
1150°
Age plateau
1000°
Typical argon-
loss spectrum
Ar loss?
900°
800°
700°
0
10
20
30
40
50
% of 39 Ar released
To tal fusion gives
only one age result
Stepped heating gives a
spectrum of age results
Figure 10.2.1 A cartoon illustrating key differences between K-Ar and Ar-Ar dating techniques. (a) K-Ar geochronology
involves total fusion of the sample, which yields only one age result, providing no indication of argon loss or
excess argon. (b) Ar-Ar dating entails step-heating of the sample, providing a spectrum of age measurements
that can reveal either argon loss during the sample's history (dark shaded spectrum - numbers show illustrative
temperatures in °C for each step), or an 'excess argon' spectrum (light shading) indicating exchange of 40 Ar with
an external reservoir. The vertical thickness of the shaded rectangles in (b) represents precision (±1 σ ). For clarity,
only the first half of the heating profile is shown in (b).
50
Amphibole
Excess
argon
45
40
Excess
argon
35
Plateau age = 30.44 ± 0.20 Ma, 75.9% 39 Ar
30
25
20
40
60
80
100
0
% 39 Ar released
Figure 10.2.2 A typical Ar-Ar age spectrum for a hornblende separate from a volcanic rock. The vertical depth of each
shaded rectangle indicates the precision of the age determination for that step (±1 σ ). (Source: Baker et al . (1996).
Reproduced with permission of Elsevier.)
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