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Intro to Geomorphology (Geos 450/550)Lecture 4: dating methods • More on field trip #2• Radiometric techniques• Cosmogenic techniques• Additional detail on luminesence, U-series
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 40-Calcium (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
If Kf is the amount of 40-Potassium left in the rock and Arf the amount of 40-Ar created in the mineral then
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)
TCN Accumulation
Stable TCN – linear increase Radioactive TCN – initial increase to steady state
Con
cent
rati
on (
atom
s/g)
Time
3 He, 21 Ne
N P
(1 e t )
t ln 1 N
P
.
N=concentrationP=production rate=decay constantT=time
Exposure dating requires:
(1) TCN Production increases with latitude.
(2) TCN production increases/decreases with changes in geomagnetic field.
Production (and accumulation) of TCN affected by:(1) self-shielding(2) Topographic shielding(3) Erosion(4) Burial
Uncertainties in TCN dating:(5) Calibration/measurement of production rates.
(1) Changes in geomagnetic field over time, particularly Holocene.
(6) 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.
0
5
10
15
20
25
30
050100150200250300
Fault Scarp Profile of 75 ka Alluvial Surfacenear the Bar Ten flow
(Whitmore 98529-3)
Distance (m)
3.5
21.75
4.2
vertical displacement = 7 m
Uranium-series dating I
U-238
Po-210Pb-206 Pb-210
U-234
Rn-222
Th-230 Ra-226
(stable)
4.5 x 109
years years
days
years
years days
2.5 x 105 7.5 x 104
22 3.8138
1.6 x 103
years
U = uranium; Th = thorium; Ra = radium; Rn = radon; Pb = lead; Po = polonium
Uranium-series dating II
U = uranium; Pa = protactinium; Th = thorium; Ra = radium; Pb = lead;
U-235 Pa-231
Pb-207
Th-227
Ra-223
(stable)
7.1 x 108
years years
3.2 x 104
19days
days
11