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 40 K is one of 3 isotopes of Potassium ( 39 K, 40 K and 41 K) and is about 0.01% of the natural potassium found in rocks 40 K is radioactively unstable and decays with a half life T ½ = 1.25 x 10 9 years (λ = 1.76 x 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 K f is the amount of 40-Potassium left in the rock and Ar f the amount of 40-Ar created in the mineral then Note that the factor 1 / accounts for the fact that only 10.9% of the 40 K that decays created 40 Ar (the rest creates 40 Ca)

Cosmo Isotope production versus depth Gosse and Phillips, 2001

TCN Accumulation Stable TCN – linear increase Radioactive TCN – initial increase to steady state Concentration (atoms/g) Time 3 He, 21 Ne

The case of glacial erosion

N=concentration P=production rate =decay constant T=time Exposure dating requires:

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.

(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: (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. - 3 He, 21 Ne: 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.

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-206Pb-210 U-234 Rn-222 Th-230Ra-226 (stable) 4.5 x 10 9 years days years days 2.5 x x x 10 3 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-235Pa-231 Pb-207 Th-227 Ra-223 (stable) 7.1 x 10 8 years 3.2 x days 11

Blisniuk and Sharp (2003)