Isotope chronology of meteorites and oxygen isotopes Part I: Radiometric dating methods Esa Vilenius...
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Transcript of Isotope chronology of meteorites and oxygen isotopes Part I: Radiometric dating methods Esa Vilenius...
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
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
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
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
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)
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
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.
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
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)
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)
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
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.
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