Dating methods Stragraphic or geologic methods. Thomas INGICCO

Dating methods Stragraphic or geologic methods Thomas INGICCO

Electro-spin resonance (ESR) An electron can be represented by a negatively charged sphere animated of a rotational movement on itself. This autorotation, called spin of the electron, involves the production of a magnetic moment µ whose orientation will depend on the direction of rotation.

Electro-spin resonance (ESR) When an atom, an ion or a molecule has an even number of electrons and that each of them is paired in doublets associating two electrons of opposed spin, its total magnetic moment is equal to zero. Example : Neon

Electro-spin resonance (ESR) When an atom, an ion or a molecule has an odd number of electrons, or an even number of electrons which are not paired in doublets, its total magnetic moment is not null because ofthepresenceofloneelectrons. Example : Oxygen

Zeeman effect If a lone electron is placed in an external magnetic field H, its magnetic moment only can take two orientations, parallel or antiparallel with H. The application of an external magnetic field thus divides the not paired electrons of a paramagnetic sample into two groups, to which two states of energy correspond, called Zeeman levels, whose separation is proportional to the value of the intensity of the external field applied: DE=E2-E1=gbH

If we applie perpendicular to H a microwave of frequency u such as: DE = h n = g b H (h is the Planck's constant) It is possible to induce transitions between the Zeeman levels, i.e. to reverse the spin of the lone electrons. This coincidence of energy is called resonance. At the time of this resonance, an energy absorption of the microwave is observable. The signal used in ESR is the derivative of the signal of absorption.

ESR dating is based on the accumulation of non-paired electrons in the cristalline structure of minerals under natural radioactivity. The ionic radioactive rays break the pairs of electrons and 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 in the valence band. The accumulated electrons can only be released by energy input.

Annual dose determination ad=externaldose+internaldose+cosmicdose External dose Gamma spectrometry TL dosimetry

Annual dose determination ad=externaldose+internaldose+cosmicdose Internal dose Gamma spectrometry Alpha spectrometry + Theinternaldoseofasampleisnotconstantintime,causeofvarious phenomena : U series desiquilibrium, water content,

Total dose determination Palaeodose The determination of the equivalent dose by ESR is done by using the method known as the addition method or the accumulated doses method.

Datable supports Teeth Bones Carbonates Quartz

Potential and limits of the method Teeth 30 ka Carbonates Sediments &Quartz Heated Quartz 1,8 Ma Aluminium centre Titanium centre

Cosmogenic methods

Cosmogenic methods Earth is continually bombarded with cosmic rays from outer space. This causes nuclear reactions on earth resulting in exotic isotopes such as 14 C. These are meteoritic cosmogenic nuclides. The particles emitted during the spallation fall on Earth and accumulate in crystal lattice exposed to the atmosphere. Primitive UFO

Cosmogenic methods Thedatingofexposurebycosmogenic isotopes is a geochemical dating method which uses the production of rare isotopes by the cosmic rays, then their accumulation in the minerals crystal lattice to determine an exposure age. Inpaleosismology, to date a surface shifted by a fault. Ingeomorphology, to calculate erosion rates : by using a couple of isotope and their respective half-life. Ingeomorphology, to obtain the age of an alluvial terrace, a moraine or any other formation. Inpaleoglaciology, to estimate a deglaciation : the exposure starts when the ice does not cover any more the rock. The cosmogenic nuclides are: Helium ( 4 He), Beryllium ( 10 Be), Aluminium ( 26 Al), Chlore ( 36 Cl), Carbon ( 14 C) and Neon ( 21 Ne) The isotope concentrations are dependent on weathering rates. When quartz grains that were exposed at the surface are buried, the burial age can be derived from the differential decay of 26 Al and 10 Be. Dating is possible for several millions of years.

The Potassium (K) / Argon (Ar) method 1- K is 7 th 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 40 Ar by the decay of the naturally occurring radioisotope 40 K. 40 K 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 39 K to 39 Ar. Both 39 Ar (a measurement of K) and 40 Ar 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.

The Potassium (K) / Argon (Ar) method 39 K = 93.2581% 40 K = 0.01167% 41 K = 6.7302% 36 Ar = 0.337% 38 Ar = 0.063% 40 Ar = 99.60% 40 Ar/ 36 Ar atmosphic ratio is : 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

40 K decay 40 K has a half life of 1.25 Ga ( λ = 5.5545x10-10 a -1 ) (Renne et al., 2010) 40 K decays into two different isotopes: 40 Ca by beta decay ( β -): 89.52% ( λ β = 4.884x10-10 a -1 ) 40 Ar by electron capture ( β +): 10.48% ( λ e = 0.580x10-10 a -1 )

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

Nomenclature for Ar Atmospheric Ar (Ar A ): Ar with isotopic composition found in present day atmosphere ( 40 Ar/ 36 Ar: 295.5) Radiogenic Ar ( 40 Ar*): Ar formed from in situ decay of 40 K for K- Ar dating, 40 Ar*= 40Ar T -( 36 Ar*295.5) Trapped Ar: Ar of atmospheric composition trapped or incorporated in a rock or mineral Excess Ar ( 40 Ar E ): component of 40 Ar, apart from atmospheric, incorporated into sample by process other than in situ decay of 40 K

Ar onthogenesis Constantes

Ar onthogenesis 40 Ar is a gas diffusing at HT. The isotopic clock starts with the eruption. t=0 : 40 Ar/ 36 Ar ech : 295.5 With time, the ratio increases 40 Ar/ 36 Ar ech >295.5

The argon methods

How method works? Because K is solid and Ar is a noble gas we have to measure 40 K and 40 Ar separately and by different methods: 40 K: 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 40 Ar/ 36 Ar: 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 40 Ar (older ages) Precision is ~1%

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

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

signal output Gas source mass spectrometer Silicon surface barrier detector Alpha-emitting thin sample planchet counting chamber to vacuum pump

Groundmass 0,1 à 5 % K

K-Feldspath 9 to 14% K Sanidine

Micas 5 to 8% K Biotite

Hornblende 0.2 to 1.5% K Hamphibole

Leucite 22% K

Advantages of the 40 Ar/ 39 Ar 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 40 Ar/ 39 Ar 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