Isotope chronology of meteorites and oxygen isotopes Part I: Radiometric dating methods Esa Vilenius 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 D 2 is lower than H 2 - 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 ( 87 Rb -> 87 Sr) 87 Rb -> 87 Sr + e - + anti e 86 Sr 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 Sr 2+ typically replaces Ca 2+. 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 (KAlSi 3 O 8 ), anorthite (CaAl 2 Si 2 O 8 )

A schematic plot of the ratio 87 Sr/ 86 Sr vs. 87 Rb/ 86 Sr of four minerals, where 86 Sr is a stable, non-radiogenic nuclide. (Cowley 1995) The 87 Rb -> 87 Sr clock (2) Freshly formed rock The different minerals in a rock have the same 87 Sr/ 86 Sr ratio (same size of ions). 87 Rb/ 86 Sr ratio is different for different minerals (host mineral depends on ion size). Old rock ( 87 Rb/ 86 Sr) t = ( 87 Rb/ 86 Sr) o exp(- t), decay constant =ln(2)/  half-life  = 5*10 10 years. The amount of the daughter nuclide at time t is ( 87 Sr) t = ( 87 Sr) o + [ ( 87 Rb) o - ( 87 Rb) t ] = ( 87 Sr) o + ( 87 Rb) t [exp( t) -1] => ( 87 Sr/ 86 Sr ) t = ( 87 Sr/ 86 Sr ) o + ( 87 Rb/ 86 Sr) t [exp( t) -1] -> Measure ( 87 Sr/ 86 Sr ) t and ( 87 Rb/ 86 Sr) t for at least 2 minerals, then solve t and ( 87 Sr/ 86 Sr ) o

The 87 Rb -> 87 Sr 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 87 Sr/ 86 Sr 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: 235 U -> 207 Pb 0.7*10 9 years 238 U -> 206 Pb 4.5*10 9 years Nonradiogenic nuclide 204 Pb Slope of the isochron: R1 = 207 Pb/ 204 Pb R2 = 206 Pb/ 204 Pb k = 238 U/ 235 U 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 26 Al 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)

26 Al -> 26 Mg chronometer ( 26 Mg / 24 Mg) = ( 26 Mg / 24 Mg) o + ( 26 Al / 27 Al)*( 27 Al / 24 Mg) slope -> ( 26 Al / 27 Al) Half-life years Ratio ( 26 Al / 27 Al) at the formation time of rock A low ratio indicates that decay of 26 Al 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 ± 0.6 Ma (lead-lead isochron). Formation of chondrules ± 0.6 Ma (lead-lead isochron), lasting 1-2 Myears. CAIs join chondrules forming chondrites at Myears, melting and differentiation of meteorite parent bodies. Allende CV3, 200x zoom