Transcript Dating Quaternary alluvial fans with morphologic methods

Intro to Geomorphology (Geos 450/550)
Lecture 4: dating methods
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More on field trip #2
Radiometric techniques
Cosmogenic techniques
Additional detail on luminesence, U-series
Scarp diffusion methods:
1) full-fit method (Pelletier et al., 2006)
Radio-isotope chronometers
Potassium-Argon (K-Ar) Dating
• The isotope 40K is one of 3 isotopes of Potassium (39K, 40K
and 41K) and is about 0.01% of the natural potassium found in
rocks
• 40K is radioactively unstable and decays with a half life T½ =
1.25 x 109 years (λ = 1.76 x 10-17 s-1) to a mixture of 40Calcium (89.1%) and 40-Argon (10.9%).
• Because Argon is a gas it escapes from molten lavas. Minerals
containing potassium that solidify from the lava will initially
contain no argon.
• Radioactive decay of 40K within creates 40Ar which is
trapped in the mineral grains.
• If the ratio of 40Ar/40K can be measured in a rock sample via
mass spectrometry the age of lava can be calculated.
K-Ar Dating Formula
N0
ln
= lT
NT
If Kf is the amount of 40-Potassium left in the rock and Arf
the amount of 40-Ar created in the mineral then
NT = K f
N 0 = K f + Arf / 0.109
æ K f + Arf / 0.109 ö
T = ln ç
÷
l è
Kf
ø
1
Note that the factor
1 / 0.109 accounts
for the fact that
only 10.9% of the
40K that decays
created 40Ar (the
rest creates 40Ca)
Cosmo
Isotope
production
versus
depth
Gosse
and Phillips,
2001
TCN Accumulation
Concentration (atoms/g)
Stable TCN – linear increase Radioactive TCN –
initial increase to steady state
Time
The case of glacial erosion
Exposure dating requires:
N
N=concentration
P=production rate
=decay constant
T=time
P

(1  e
t
)
 N 
ln 1 

P 
.
t

With constant exposure
ratio of isotope production eventually decreases
Upon burial or shielding
ratio decreases below the constant exposure line
(1) TCN Production
increases with
latitude.
(2) TCN production
increases/decreases
with changes in
geomagnetic field.
50,000 m
(3) TCN
Production
increases with
elevation.
Sea Level
Shielding of cosmic rays by surrounding topography
Production (and accumulation) of TCN affected by:
(1) self-shielding
(2) Topographic shielding
(3) Erosion
(4) Burial
Uncertainties in TCN dating:
(1) Calibration/measurement of production rates.
(1) Changes in geomagnetic field over time, particularly Holocene.
(2) Previous exposure.
Sampling Strategies :
surface stability (i.e., desert pavements, desert varnish).
Highest, flattest surface on deposit.
Largest, flattest boulder on deposit.
Sample Preparation
crush rocks
Physical and chemical mineral-separation processes.
3He, 21Ne: melt mineral at 1400 C under vacuum, measure gas on
mass spectrometer.
Radioactive TCN: chemical processes to extract element of
interest. Isotopic ratios measured on AMS.
Applications of TCN: dating surfaces, estimate rates
of geomorphic processes.
(1) Estimating Fault Displacement Rates.
Fault Scarp Profile of 75 ka Alluvial Surface
near the Bar Ten flow
30
(Whitmore 98529-3)
25
4.2
20
vertical displacement
15
= 7m
21.75
10
3.5
5
0
300
250
200
150
Distance (m)
100
50
0
Displacement Rates on the Toroweap and Hurricane faults
Thermoluminescence /
Optically stimulated luminescence
Background
TL/OSL measurement
TL ‘saturation’
Uranium-series dating I
U-238
4.5 x 109
years
2.5 x 105
7.5 x 104
U-234
years Th-230 years Ra-226
1.6 x 103
years
Pb-206
(stable)
138
days
Po-210
22
years
Pb-210
3.8
days
U = uranium; Th = thorium; Ra = radium;
Rn = radon; Pb = lead; Po = polonium
Rn-222
Uranium-series dating II
U-235
7.1 x 108
years
Pa-231
3.2 x 104
years Th-227
19
days
Pb-207
11
days
Ra-223
(stable)
U = uranium; Pa = protactinium; Th = thorium;
Ra = radium; Pb = lead;
Blisniuk and Sharp (2003)