isotope chronology of meteorites and oxygen isotopes part i: radiometric dating methods esa vilenius...

12
Isotope chronology of meteorites and oxygen isotopes Part I: Radiometric dating methods Esa Vilenius 13.2.2006 Outline • Introduction • Rubidium-Strontium chronometer • Problems of radiometric chronometers • Lead-lead method • Short-lived isotopes • Chronology of early Solar System

Upload: clare-amelia-roberts

Post on 05-Jan-2016

256 views

Category:

Documents


4 download

TRANSCRIPT

Page 1: Isotope chronology of meteorites and oxygen isotopes Part I: Radiometric dating methods Esa Vilenius 13.2.2006 Outline Introduction Rubidium-Strontium

Isotope chronology of meteorites and oxygen isotopesPart I: Radiometric dating methods

Esa Vilenius 13.2.2006

Outline

• Introduction

• Rubidium-Strontium chronometer

• Problems of radiometric chronometers

• Lead-lead method

• Short-lived isotopes

• Chronology of early Solar System

Page 2: Isotope chronology of meteorites and oxygen isotopes Part I: Radiometric dating methods Esa Vilenius 13.2.2006 Outline Introduction Rubidium-Strontium

What can be dated?

- Formation age of solid material

- Formation intervals (relative to other meteorites)

- Reheating events (metamorphic ages)

- Cosmic ray exposure age (meter-sized objects)

- Terrestrial age

Page 3: Isotope chronology of meteorites and oxygen isotopes Part I: Radiometric dating methods Esa Vilenius 13.2.2006 Outline Introduction Rubidium-Strontium

What changes isotopic abundances?

• radioactive decay and its effects on neighboring nuclides

• bombardment by high-energy particles (cosmic rays)

• fractionation (= differentiation between isotopes)

- example 1: binding energy of D2 is lower than H2

- example2: evaporation of water favors lighter isotopes of H and O in the

gas phase, and heavier in the liquid phase

Page 4: Isotope chronology of meteorites and oxygen isotopes Part I: Radiometric dating methods Esa Vilenius 13.2.2006 Outline Introduction Rubidium-Strontium

Conditions and assumptions

- Decay constant of parent nuclide accurately known.

- Several samples of the rock are available, with variation in parent/daughter ratios.

- Material has been a closed system w. r. t. parent and daughter nuclides.

- Initial isotopic composition of the daughter element was homogeneous in all samples.

- Radiogenic component of the daughter nuclide can be distinguished from the initial,

nonradiogenic component.

radiogenic nuclide = product of radioactive decay

Page 5: Isotope chronology of meteorites and oxygen isotopes Part I: Radiometric dating methods Esa Vilenius 13.2.2006 Outline Introduction Rubidium-Strontium

The Rubidium-Strontium clock (87Rb -> 87Sr)

• 87Rb -> 87Sr + e- + anti e

• 86Sr is the nonradiogenic nuclide.

• CASE 1: Caused by melting, Rb and Sr ions

floated freely in a homogeneous liquid.

• At the time of crystallization Rb and Sr ions

are squeezed into minerals, where they

occur as impurities. Rb+ typically replaces K+

and Sr2+ typically replaces Ca2+.

• CASE 2: In the primordial solar system Rb

and Sr were well-mixed in the gas. The ratio

Rb/Sr is different in the gas and solid

phases, because Rb+ has a tendency for

substitution in minerals with low melting

temperatures.

Examples of K- and Ca-bearing minerals:

orthoclase (KAlSi3O8), anorthite (CaAl2Si2O8)

Page 6: Isotope chronology of meteorites and oxygen isotopes Part I: Radiometric dating methods Esa Vilenius 13.2.2006 Outline Introduction Rubidium-Strontium

A schematic plot of the ratio 87Sr/86Sr vs. 87Rb/86Sr of four minerals, where 86Sr is a stable, non-radiogenic nuclide. (Cowley 1995)

The 87Rb -> 87Sr clock (2)

Freshly formed rock

The different minerals in a rock have the same 87Sr/86Sr ratio (same size of ions).

87Rb/86Sr ratio is different for different minerals (host mineral depends on ion size).

Old rock

(87Rb/86Sr)t = (87Rb/86Sr)o exp(-t),

decay constant =ln(2)/half-life = 5*1010 years.

The amount of the daughter nuclide at time t is

(87Sr)t = (87Sr)o + [ (87Rb)o - (87Rb)t ]

= (87Sr)o + (87Rb)t [exp(t) -1]

=> (87Sr/86Sr )t = (87Sr/86Sr )o + (87Rb/ 86Sr)t [exp(t) -1]

-> Measure (87Sr/86Sr )t and (87Rb/ 86Sr)t for at least 2 minerals, then solve t and (87Sr/86Sr )o

Page 7: Isotope chronology of meteorites and oxygen isotopes Part I: Radiometric dating methods Esa Vilenius 13.2.2006 Outline Introduction Rubidium-Strontium

The 87Rb -> 87Sr clock (3)

Kaushal and Wetherill (1969)

Example of results1: H-group chondrites

Whole-rock Rb-Sr isochron of 16 H-chondrite meteorites

=> Common formation age 4.69±0.07 Gyr.

Example of results2: formation intervals

Initial 87Sr/86Sr ratios from isochrons of 6 meteorites.

Page 8: Isotope chronology of meteorites and oxygen isotopes Part I: Radiometric dating methods Esa Vilenius 13.2.2006 Outline Introduction Rubidium-Strontium

Contamination and isochrons

Graphics from Stassen (1998)

System not closed w. r. t. parent nuclide -> loss of colinearity

System not closed w. r. t. daughter nuclide -> loss of colinearity

Daughter nuclide partially homogenized

-> partial reset of isochron

-> colinear, but wrong age

Page 9: Isotope chronology of meteorites and oxygen isotopes Part I: Radiometric dating methods Esa Vilenius 13.2.2006 Outline Introduction Rubidium-Strontium

The lead-lead double clock

• Two systems: 235U -> 207Pb 0.7*109 years

238U -> 206Pb 4.5*109 years

• Nonradiogenic nuclide 204Pb

• Slope of the isochron:

R1 = 207Pb/204Pb

R2 = 206Pb/204Pb

k = 238U/235U

CAIs are 2.5 Myears older than chondrules (Amelin et. al. 2002)

Page 10: Isotope chronology of meteorites and oxygen isotopes Part I: Radiometric dating methods Esa Vilenius 13.2.2006 Outline Introduction Rubidium-Strontium

Short-lived radioactive isotopes

• Parent nuclides extinct

• Excess amount of daughter nuclides

• A stable isotope of the parent is used in measurements

• Uniform initial concentration of parent nuclides

• Differences in concentration => relative crystallization ages

• Inclusions containing 26Al must have been cool enough to prevent isotopic exchange within Myears following the production in a supernova => samples of interstellar grains

McKeegan and Davis (2002)

Page 11: Isotope chronology of meteorites and oxygen isotopes Part I: Radiometric dating methods Esa Vilenius 13.2.2006 Outline Introduction Rubidium-Strontium

26Al -> 26Mg chronometer

(26Mg / 24Mg) = (26Mg / 24Mg)o + (26Al / 27Al)*(27Al / 24Mg)

slope -> (26Al / 27Al)

• Half-life 720 000 years

• Ratio (26Al / 27Al) at the formation time of rock

• A low ratio indicates that decay of 26Al predates solar-system formation

Page 12: Isotope chronology of meteorites and oxygen isotopes Part I: Radiometric dating methods Esa Vilenius 13.2.2006 Outline Introduction Rubidium-Strontium

Early Solar System chronology

• At 4568 Ma a supernova triggers gravitational collapse.

• CAIs are the first solid material (aluminium-26 relative ages)• Formation of CAIs 4567.2 ± 0.6 Ma (lead-lead isochron).

• Formation of chondrules 4564.7 ± 0.6 Ma (lead-lead isochron),• lasting 1-2 Myears.

• CAIs join chondrules forming chondrites at 4565 - 4564 Myears,• melting and differentiation of meteorite parent bodies.

www.spacedaily.com

www.spaceflightnow.com

Allende CV3,

200x zoom

www.zeiss.com