Dating for Anthropology

Radiocarbon dating is limited to the analysis of archaeological samples because of its relatively short half life T1/2 = 5730 y. For dating samples with older history - geological samples for exploring earth history, paleonthological samples for studying the origin and evolution of life, or anthropological samples forexploring the origin and early development of mankind – other methods are typically used involving longer-lived radionuclides.

Most frequently used radionuclide methods are: • potassium-argon method: >10 000 y• uranium series method: >10 000 – 1 000 000 y

3.7. Potassium-Argon Dating40K has unusual decay mode• e--capture to 1.excited state in 40Ar (11%)• β--decay to 40Ca (89%)

40K is very long-livedT1/2=1.28·109 y

40K is in body material and in rock content, 40Ar is noble gas with no chemical reactivity40Ca cannot be distinguished from natural Ca content

22401821

4019 AreK K ⇒+ −

40Ar/40K time-relation

build-up of 40Ar/40K abundance ratio in material

)e(.)K(N)Ar(N

y.)e()K(N.)Ar(N

t

t

1110

1045 :decayK for 1110

40

40

110-40

4040

−⋅=

⋅=

−⋅⋅=

⋅

−

⋅

λ

λ

λ

T1/2=1.28·109 y

anthropological time range

time-determination

teKNArN t ⋅⋅≈−⋅= −⋅ 11

40

40

1082.5)1(11.0)()( λ

chemical or physical analysis of this ratiogives age of sample.

minerals (bone material) contain ~2-10% of Kwith ~0.021% of 40K

40Ar must be releasedand can be counted bymass spectrometry

Problem: zero-point40K is in mineral material since the original formation• for rock material dating back to geological formation

since 40Ar remains trapped• for volcanic material dating back to last activity

since 40Ar is released in melting process

• for organic material dating back to mineralizationsince organic material is porous and releases 40Ar

Zero-point in 40Ar counting

lava sample

40Ar build-up by 40K decay

40Ar depletion by melting

Possibly reflected in two step 40Ar release yielding two different dates, one for the second melting and one for the first melting period of rock.

Volcanic dating

Potassium Argon method is often applied to material embedded in volcanic layers. dating the material above and below the sample gives age range information.

Release and analysis of 40ArTo count 40Ar nuclei the argon must be released from rock sample!

Typical analysis techniques require • melting the sample • catching the released gas• ionizing the argon gas• selecting the gas atoms by mass spectrometry

error sources:loss of 40Ar due to diffusion → simulates low ageinfusion of atmospheric 40Ar → simulates large age(measurement of 36Ar content in sample)

Efficiency of 40Ar countingSuppose you have a 10g sample which is 3.2 Million years old,

that sample contains ~ <1g K ⇒ ~0.00021g 40K

How many 40Ar atoms have been generated by decay?

atomsArArNeArN

eKNKNKNArN

atomsKg

gg

atomsKg

yy

tt

40151840

102.31028.12ln

1840

400

40400

40

401823

4023

1047.5)99827.01(1016.3)()1(1016.3)(

)1()()()()(

1016.340

00021.010023.600021.0

10023.640

69

⋅=−⋅⋅=

−⋅⋅=

−⋅=−=

⋅=⋅⋅

≡

⋅≡

⋅⋅⋅

−

⋅−λ

Number of 40K basically constant (within uncertainties)

Laser Heating TechniqueRelease by laser heating . Released 40Ar atoms can Also be counted using mass spectrometer techniques.

Main advantage of laser heating is the high spatial resolution ~μm!

Dating the Ashes by counting Atoms

40Ar/40K Method

⎟⎟⎠

⎞⎜⎜⎝

⎛+⋅= 1

)()(09.9ln1

40

40

ArNArNt

λ40Ar is easily released, 40K must be detected by

measuring characteristic γ-activity with Eγ=1.461 MeV

Age is determined by ratio of 40Ar and 40K abundance in sample.

)(11.0)( 4040 KNKA ⋅⋅= λγ

Sensitivity is limited by characteristic activity

Example: 40K countingminerals (bone material) contain ~2-10% of Kwith ~0.021% of 40K

You have 10 g of sample material⇒ <1g K ⇒ ~0.00021g 40K

;/6)(101

/109.11016.31028.12ln11.0)(

1016.340

00021.010023.600021.0

10023.640

40

7

8189

40

401823

4023

sKAsywith

yy

KA

atomsKg

gg

atomsKg

γ

π

γ

γ

γ

=

⋅≈

⋅=⋅⋅⋅

⋅=

⋅=⋅⋅

≡

⋅≡

Typical detection efficiency is ~1% ⇒ Nγ=0.06 γ/sTo achieve significant statistics of 10% (100±10)You need to count for t = 100/0.06=1666 s ≈ 1/2 h

Problems of 40K activity counting

• Significant 40K background in laboratory environment (concrete)Count rate ~ 1 event/s ⇒ heavy shielding required

• Good statistics requires long counting time and >10g sample size

• Uncertainties due to different counting methods for 40K and 40Ar (different efficiencies in producing 40Ar ionsand detecting 40K γ-radiation.)

Development of alternative method using identical techniques ⇒ 39Ar/40Ar counting

39Ar/40Ar MethodMethod has advantage of parallel counting of 39Ar and 40Ar atomsin mass analyzer ⇒ identical efficiencies, reduced uncertainties!

Problem of generating 39Ar and relating it to 40K

Fixed ratio of stable 39K and radioactive 40K isotopes

39K93.2581

40K0.0117

41K6.7302 Z=19

Conversion of 39K into 39Ar by neutron activation:

39K(n,p)39Ar

Standard Reactor TechniquesConventional TRIGA reactorNeutron flux of Fn ~ 1014 n s-1 cm2

http://geology.cr.usgs.gov/facilities/gstr/

Conversion rate 39K⇒39Ar

;1055.1)(10544.11010)(

10

10110544.1)(1

)()(

1939

22211422739

2114

227),(

223939

39),(

39

39

39

−

−−−

−−

−

⋅=

⋅⋅⋅=

≈

=≈

⋅=⇒

⋅⋅=

sArYcmsncmArY

cmsnF

cmmbKNKofg

KNFArY

n

pnK

npnK

σ

σ

Full conversion would take quite a long time, a 1 hour generation of measurable 39Ar content is sufficient

to produce the measurable amount of N(39Ar)=5.5·1012!

39Ar/40Ar Method

Advantages:Parallel release of 39Ar and 40Ar by heatingParallel separation of 39Ar and 40Ar in mass spectrometerParallel counting in identical detector systems

Neutron flux dependent parameters are typically determined byusing well known 39K standard sample ⇒ relative measurement

)()(

1)()(ln1

40

39

),(

39

40

39KNKNtFJ

JArNArNt

irrpnKnn

n

⋅⋅⋅=

⎟⎟⎠

⎞⎜⎜⎝

⎛−⋅⋅=

σ

λ Jn determined with astandard sample thatis exposed to parallel treatment

Hornblende (mineral) from the McClure Mountains, Colorado (a.k.a. MMhb-1).

Example 1: The cradle of mankind

Ex Africa Semper Aliquid Novi (Always something new out of Africa) Pliny the Elder

The Olduvai GorgeOlduvai Gorge is an archaeological site located in the eastern Serengeti Plains in northern Tanzania. The gorge is a very steep sided ravine roughly 30 miles long and 295 ft. deep. Exposed deposits show rich fossil fauna, many hominid remains belong to the one of the oldest stone tool technologies, called Olduwan. The time span of the objects recovered date from 2,100,000 to 15,000 years ago.

fossil samples from Olduvai site

post-cranial remainsof homo erectus: 0.5 million years

Australopithecus boisei1.8 million years

mineralized fossil

Danakil Rift Valley

Hadar, Ethiopia

Lucy

australopethicus afarensis3.5 Million years

The Discovery of Lucy

Chronology of Evolution

Documentation

Footprints of our Ancestors?

The Age of the Moment!3,500,000 years ago, our very remote ancient ancestors walked through a landscape very like that which we see today. On one particular day the volcano Sadiman puffed out a lot of gray ash, which covered the plains. A rain shower dampened and settled the ashes, so that the local animals left their crisp, clear tracks when they walked. Through this desolated grey landscape, traveled three hominids walking on two legs. A large, medium sized and small individual walked together. Some days later, a fresh ash fall buried the tracks, until they were excavated in 1978. How do we know the age of the footprints?