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1.27 Long-Lived Chronometers M 1.27 Long-Lived Chronometers M. Wadhwa Arizona State University,Tempe, AZ, USA 1.27.1 INTRODUCTION 1 1.27.1.1 Basic Principles 1 1.27.1.2 Application to Meteorites and Planetary Materials: A Historical Perspective 2 1.27.2 CHONDRITES AND THEIR COMPONENTS 3 1.27.2.1 Formation Ages of Chondritic Components 3 1.27.2.1.1 Calcium-, aluminum-rich inclusions 3 1.27.2.1.2 Chondrules 5 1.27.2.2 Ages of Secondary Events Recorded in Chondrites 5 1.27.2.2.1 Aqueous alteration 6 1.27.2.2.2 Thermal metamorphism 6 1.27.2.2.3 Shock metamorphism 7 1.27.3 DIFFERENTIATED METEORITES 8 1.27.3.1 Primitive Achondrites: Timing of Incipient Differentiation on Planetesimals 8 1.27.3.2 Basaltic and Other Achondrites: Timing of Asteroidal Differentiation and Cataclysm 9 1.27.3.2.1 Crust-formation timescales from chronology of achondrites and their components 9 1.27.3.2.2 Global differentiation timescales based on whole-rock isochrons and initial 87Sr/86Sr 11 1.27.3.2.3 Inner solar system bombardment history based on reset ages 13 1.27.3.3 Iron Meteorites and Pallasites: Timescales of Core Crystallization on Planetesimals 14 1.27.4 PLANETARY MATERIALS 16 1.27.4.1 Timing of Lunar Differentiation and Cataclysm from Chronology of Lunar Samples 16 1.27.4.1.1 Lunar differentiation history 16 1.27.4.1.2 Lunar bombardment history 17 1.27.4.2 Timescales for the Evolution of Mars from Chronology of Martian Meteorites 18 1.27.5 CONCLUSIONS 19 1.27.5.1 A Timeline for Solar System Events 19 1.27.5.2 Outlook and Future Prospects 20 REFERENCES 21 1.27.1 INTRODUCTION radioisotope at any given time, such that 1.27.1.1 Basic Principles P ¼ P eÀlt ð1Þ Long-lived radioactive isotopes, defined 0 here as those that have half-lives in excess of a few hundred million years, have been utilized for where P is the number of atoms of the parent chronology since the early part of the twentieth isotope remaining at present, P0 the initial century. The decay of a radioactive (‘‘parent’’) abundance of the parent isotope at the time isotope involves its spontaneous transformation, of isotopic closure, t the time elapsed since iso- sometimes through other intermediate radioiso- topic closure (e.g., crystallization age for a rock) topes, into a stable (‘‘daughter’’) isotope at a rate and l the decay constant. Equation (1) may proportional to the number of atoms of the be rewritten in terms of the abundance of the 1 2 Long-Lived Chronometers radiogenic daughter isotope (D*) as follows: 1955, 1956) heralded the modern age of isotope 207 206 Dn ¼ Pðelt À 1Þð2Þ chronology. He obtained a Pb/ Pb age from three stony meteorites of 4.5570.07 Ga and However, since the total number of atoms of suggested that this represented the time of the daughter isotope (D) is the sum of the ra- formation of the solar system and the Earth. diogenic (D*) and the nonradiogenic (D0) com- Since that time, (1) advances in analytical in- ponents, strumentation (allowing more precise isotopic ratio measurements), (2) more accurately deter- D ¼ D þ Pðelt À 1Þð3Þ 0 mined decay constants, and (3) more appro- Normalizing to a stable isotope of the daughter priate sample selection have led to increasingly element (Ds), refined and precise estimates of this age. By lt chance, changes in these three factors have com- D=Ds ¼ D0=Ds þ P=Ds ðe À 1Þð4Þ pensated one another in such a way that half a As such, the slope in an isochron plot for a century later, Patterson’s initial estimate of the age of the solar system still agrees with the cur- long-lived chronometer (i.e., where D/Ds is lt rent best estimate of this age. The 207Pb/206Pb plotted versus P/Ds) is given by (e À1), from which the age (t) may be determined. systematics in the refractory calcium-, alumi- The past several decades have seen significant num-rich inclusions (CAIs), believed to be improvements in the precision and accuracy of among the first solids formed in the early his- chronological information based on the decay tory of the solar system, have been utilized to of long-lived radioisotopes. These have resulted provideanestimateofthe(minimum)ageofthe particularly from advances in the mass spectro- solar system. As will be discussed in more detail metric techniques for measurement of isotope in the section below, the most recent analyses of ratios and better constriants on the relevant de- lead-isotope systematics in CAIs from the cay constants. Chronometers based on the de- Efremovka carbonaceous (CV3) chondrite yield 7 cay of radioisotopes essentially date the time of a highly precise age of 4,567.1 0.2 Ma (Amelin isotopic closure following a chemical event that et al., 2002, 2006). fractionated the parent element from the The various long-lived radioisotopes that daughter element. Assuming that parent/daugh- have thus far been used for chronological ter isotope ratios can be determined accurately investigations of meteorites and their compo- and precisely and that the decay constant is nents are given in Table 1. Among these, the known, meaningful age information based on ones that have been most commonly applied are the 40K–40Ar, 87Rb–87Sr, 147Sm–143Nd, such chronometers may only be obtained if: 235,238 232 207,206,208 (1) there was complete equilibration of the iso- and U, Th– Pb chronometers. topic composition of the daughter element prior These have mostly been used for determining to fractionation of the parent element from the the crystallization and secondary alteration daughter element; and (2) there has been no (e.g., by shock metamorphism) ages of various disturbance of isotope systematics following the classes of meteorites. For the same meteorites, isotopic closure event that is to be dated. different chronometers may date different events in their histories, depending on the geo- 1.27.1.2 Application to Meteorites and chemical behaviors of the parent and daughter Planetary Materials: A Historical elements and their ease of equilibration. For 40 40 Perspective example, while the K– Ar system in most basaltic eucrites is partially or totally reset Clair Patterson’s analyses of terrestrial and as a result of shock metamorphism at 3.4– meteoritic lead isotopic compositions (Patterson, 4.1 Ga (Bogard, 1995), the 147Sm–143Nd ages Table 1 Long-lived radioisotopes used for chronological studies of meteorites. Radioisotope Daughter isotope Reference stable isotope Half-life (109 years) 40K 40Ar, 40Ca 36Ar 1.27 87Rb 87Sr 86Sr 48.8 147Sm 143Nd 144Nd 106 176Lu 176Hf 177Hf 35.7 187Re 187Os 188Os 41.6 190Pt 186Os 188Os 489 232Th 208Pb 204Pb 14.01 235U 207Pb 204Pb 0.704 238U 206Pb 204Pb 4.469 Chondrites and their Components 3 of several samples belonging to this class of Tilton, 1988; Podosek and Nichols, 1997; Carl- meteorites still reflect their crystallization at son and Lugmair, 2000; Kita et al., 2005; B4.5 Ga. Chapter 1.16). Of all the long-lived chronometers applied to meteorites so far, the combined 235,238U–207,206 Pb systems provide the highest time resolution. This is so because the combination of two chronom- 1.27.2 CHONDRITES AND THEIR eters that involve the same parent and daughter COMPONENTS elements effectively allows the determination of a 1.27.2.1 Formation Ages of Chondritic 207 206 time ‘‘t’’ (or a Pb– Pb age) without having Components to measure the parent/daughter elemental ratio 1.27.2.1.1 Calcium-, aluminum-rich inclusions and based only on the isotopic composition (207Pb/206Pb ratio) of the daughter element, which CAIs are refractory millimeter- to centime- can be very precisely measured. Moreover, the ter-sized objects found in primitive chondrite relatively short half-life of 235U compared to meteorite groups. They are thought to repre- the other radioisotopes in Table 1 implies that, sent some of the first solids that formed in the following a parent/daughter fractionation event, solar protoplanetary disk. The earliest lead-iso- the 207Pb/206Pb ratio evolves rapidly over geolo- tope studies of CAIs (Chen and Tilton, 1976; gic timescales, thereby allowing sub-Myr time Tatsumoto et al., 1976) indicated that these resolution. The 207Pb–206Pb age for a sample can were indeed ancient objects that formed in the either be a single-stage model age, which is deter- earliest history of the solar system, close to mined by subtracting an assumed isotopic com- 4.56 Ga. Subsequently, Chen and Wasserburg position for ‘‘common Pb’’ (which includes the (1981) reported the lead-isotope compositions initial Pb and any extraneous Pb of terrestrial of several CAIs from the Allende carbonaceous or extraterrestrial origin) from the measured (oxidized CV3) chondrite. Considering the composition, or an isochron age. The latter is most radiogenic of these samples and regres- obtained from a regression of the data for mul- sing these data through the Canyon Diablo tiple samples, or components of a sample, on a lead-isotope composition (assumed here as the Pb–Pb isochron plot (i.e., 207Pb/206Pb versus initial lead composition for the solar system), 204Pb/206Pb) to obtain the purely radiogenic these authors reported an age of 4.559 Ga for 207Pb/206Pb ratio (i.e., the intercept of this Allende CAIs. However, if all of the data for isochron plot) from which an age is calculated. CAIs from Chen and Wasserburg (1981) are As long as it is reasonable to assume that all taken together, they fall along a single linear samples plotted on a Pb–Pb isochron plot shared array in a Pb–Pb isochron plot that (although the same common lead component, the isochron it does not pass through the Canyon Diablo method of calculating the age is the preferable lead-isotope composition, implying that these one since no assumption of a common lead com- CAIs contain a common lead component position need be made.
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