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

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 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)

CosmoIsotopeproductionversus depth

Gosseand Phillips,2001

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

The case of glacial erosion

N P

(1 e t )

t ln 1 N

P

.

N=concentrationP=production rate=decay constantT=time

Exposure dating requires:

With constant exposureratio of isotope production eventually decreases

Upon burial or shieldingratio decreases below the constant exposure line

(1) TCN Production increases with latitude.

(2) TCN production increases/decreases with changes in geomagnetic field.

(3) TCN Production increases with elevation.

Sea Level

50,000 m

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:(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

Displacement Rates on the Toroweap and Hurricane faults

Thermoluminescence /Optically stimulated luminescence

Background

TL/OSL measurement

TL ‘saturation’

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

Blisniuk and Sharp (2003)