dating methods - pages.upd.edu.ph

Dating methodsStragraphic or geologic methods

Thomas INGICCO

An electron can be represented by a negatively charged sphere animated ofa rotational movement on itself. This autorotation, called spin of theelectron, involves the production of a magnetic moment µ whoseorientation will depend on the direction of rotation.

Electro-spin resonance (ESR)

When an atom, an ion or a molecule has an even number of electronsand that each of them is paired in doublets associating two electrons ofopposed spin, its total magnetic moment is equal to zero.

Example : Neon


When an atom, an ion or a molecule has an even number of electronsand that each of them is paired in doublets associating two electrons ofopposed spin, its total magnetic moment is equal to zero.

This substance is diamagnetic.

Example : Neon


Example : Oxygen

When an atom, an ion or a molecule has an odd number of electrons, oran even number of electrons which are not paired in doublets, its totalmagnetic moment is not null because of the presence of lone electrons.


Example : Oxygen



oxygen has two not paired electrons

Example : Dioxygen



Such a substance is known as paramagnetic substance.

If a lone electron is placed in anexternal magnetic field H, itsmagnetic moment only can taketwo orientations, parallel orantiparallel with H. Theapplication of an externalmagnetic field thus divides thenot paired electrons of aparamagnetic sample into twogroups, to which two states ofenergy correspond, calledZeeman levels, whoseseparation is proportional to thevalue of the intensity of theexternal field applied:

DE = E2-E1 = g b H

Zeeman effect

It is possible to inducetransitions between theZeeman levels, i.e. to reversethe spin of the lone electrons.This coincidence of energy iscalled resonance.

If we applie perpendicular to Ha microwave of frequency usuch as:

(h is the Planck's constant)

At the time of this resonance, anenergy absorption of themicrowave is observable. Thesignal used in ESR is thederivative of the signal ofabsorption.

DE = h n = g b H

ESR dating is based on the accumulation of non-paired electrons in the cristalline structure ofminerals under natural radioactivity. The ionic radioactive rays break the pairs of electronsand transfer a part of the non-paired electrons then formed from the original electronic level(Valence band) in a higher energy level (Conduction band) creating in the same time a hole inthe valence band. The accumulated electrons can only be released by energy input.

TL dosimetry

Gamma spectrometry

Annual dose determination

aD = External dose + Internal dose + Cosmic doseE x te r n a l d o s e

The internal dose of a sample is not constant in time, cause of variousphenomena : U series desiquilibrium, water content,…

Gamma spectrometry Alpha spectrometry

+

Annual dose determination

aD = External dose + Internal dose + Cosmic dose

I n te r n a l d o s e

The determination of theequivalent dose by ESR is doneby using the method known asthe addition method or theaccumulated doses method.

Total dose determinationPalaeodose

Teeth Bones

QuartzCarbonates

Datable supports

Aluminium centre

Titanium centre

30 ka

1,8 Ma

Potential and limits of the method

TeethHeatedQuartzCarbonates

Sediments& Quartz

Cosmogenic methods

Earth is continually bombarded with cosmic rays from outer space.

This causes nuclear reactions on earth resulting in exotic isotopes such as 14C. These aremeteoritic cosmogenic nuclides.

Cosmogenic methods

Primitive UFO

The particles emitted during the spallation fall on Earthand accumulate in crystal lattice exposed to theatmosphere.

The dating of exposure by cosmogenic isotopes is a geochemical dating methodwhich uses the production of rare isotopes by the cosmic rays, then theiraccumulation in the minerals crystal lattice to determine an exposure age.

· In paleosismology, to date a surface shifted by a fault.

· In geomorphology, to calculate erosion rates :by using a couple of isotope and their respective half-life.

· In geomorphology, to obtain the age of an alluvial terrace, a moraineor any other formation.

· In paleoglaciology, to estimate a deglaciation :the exposure starts when the ice does not cover any more the rock.

Cosmogenic methods

The cosmogenic nuclides are:Helium (4He), Beryllium (10Be), Aluminium (26Al), Chlore (36Cl), Carbon (14C) andNeon (21Ne)

The isotope concentrations are dependent on weathering rates.When quartz grains that were exposed at the surface are buried, the burial age canbe derived from the differential decay of 26Al and 10Be. Dating is possible for severalmillions of years.

1- K is 7th most abundant element in the crust (1 wt%)

2- K is a major constituent of many common minerals in igneous and metamorphic rocks

3- Developed in the late 1940’s, Used widely from 50’s through the 80’s it is still used today to date young volcanic rocks

The Potassium (K) / Argon (Ar) method

•The technique is based on the production of 40Ar by the decay of the naturally occurring radioisotope 40K.

•40K has a very long half life and its decay over archaeological times is very small.

•For the K/Ar clock to work, a mineral has to be degassed of any previously present Ar.

•This applies to volcanic minerals only. •Conventionally, K (e.g. flame photometry) was measured separately from Ar (mass spectrometry).

•Ar/Ar dating utilises neutron activation to convert 39K to 39Ar.

•Both 39Ar (a measurement of K) and 40Ar are measured in a mass spectrometer with high accuracy.

•K/Ar and Ar/Ar have made a huge contribution in the understanding of human evolution in East Africa.


40Ar/36Ar atmosphic ratio is :

39K = 93.2581% 40K = 0.01167% 41K = 6.7302%

36Ar = 0.337% 38Ar = 0.063% 40Ar = 99.60%

295.5 (Nier 1950)


Potassium 40 is radioactive and decays with a period of 1.25 billions of year; so it presently remains less than 10% of the initial stock present during condensation of the matter 4.5 billion years ago.

Photon

Excited Stable

40K has a half life of 1.25 Ga (λ = 5.5545x10-10a-1) (Renne et al., 2010) 40K decays into two different isotopes:

40Ca by beta decay (β-): 89.52% (λβ = 4.884x10-10a-1) 40Ar by electron capture (β+): 10.48% (λe = 0.580x10-10a-1)

40K decay

Because we are interested in the amount of Ar produced from K, we use the ratio of λe to λ in calculating the age from the Ar/K ratio:

40K decay

Atmospheric Ar (Ar A): Ar with isotopic composition found in

present day atmosphere (40Ar/ 36Ar: 295.5)

Radiogenic Ar ( 40Ar*): Ar formed from in situ decay of 40K for K-Ar dating, 40Ar*= 40Ar

T-( 36Ar*295.5)

Trapped Ar: Ar of atmospheric composition trapped or

incorporated in a rock or mineral

Excess Ar ( 40ArE): component of 40Ar, apart from atmospheric,

incorporated into sample by process other than in situ decay of 40K

Nomenclature for Ar

Constantes

Ar onthogenesis

40Ar is a gas diffusing at HT. The isotopic clock starts with the eruption.

t=0 : 40Ar/36Arech : 295.5

With time, the ratio increases 40Ar/36Arech >295.5

Ar onthogenesis

The argon methods

Because K is solid and Ar is a noble gas we have to measure 40K and 40Ar separately and by different methods:

40K: flame photometry, atomic absorption spectrometry, isotope dilution, X-ray florescence, gravimetric chemistry, nutron activation

40Ar: Noble gas mass spectrometer (using spike of Ar of atmospheric composition 40Ar/36Ar: 295.5 to compare with measured samples)

Problems: Can only get “total gas” ages No indication of diffusive profiles No way to evaluate presence of excess 40Ar (older ages) Precision is ~1%

How method works ?

1: The field

2: hard work : Crushing, sieving

3: mineral Separation: Magnetic, density, hand picking

4: fusion, purification of gas

5: Argon isotopes analysis

Sample is subjected to neutron bombardment (few minutes to hours) in a nuclear reactor to transform small proportion of 39K atoms to 39Ar.

A standard of known K-Ar age is irradiated with unknown to monitor neutron flux and used to calculate the extent to which 39Ar was produced from 39K.

Relative abundances of Ar isotopes are measured in a gas source mass spectrometer for both standard (to calculate J) and unknown: 40Ar, 39Ar, 37Ar, 36Ar, 38Ar

Ratios are corrected for atmospheric argon (which may have been incorporated in sample and in the vacuum system) and Ar isotopes produced during irradiation.

Composition of atmospheric argon is known

Dispositif analytique 40Ar/39Ar

Silicon surface barrier detector Alpha-emitting thin sample

to vacuum pump counting chamber

planchet

signal output

Gas source mass spectrometer

Groundmass

0,1 à 5 % K

Sanidine

K-Feldspath

9 to 14% K

Micas

Biotite

5 to 8% K

Hornblende

Hamphibole

0.2 to 1.5% K

Leucite 22% K

Advantages of the 40Ar/39Ar method:

(1) K and Ar measured simultaneously by measurement of only Ar isotopic ratios. Isotope ratios can be measured more precisely than concentrations of K and Ar (as in the K-Ar method) aren’t analyzing different aliquots! - more precise age!

NB: However accuracy of 40Ar/39Ar age is limited by uncertainties in the age of standard and Decay Constants, i.e. ~0,5%

(2) Allows to evaluate excess Argon

(3) Can indicates an opening of the isotopic system

Ar and K application range

dating methods - pages.upd.edu.ph

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