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 elt 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 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 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 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 1), 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. with a composition distinct from this) yields a Although much valuable chronological in- 207Pb–206Pb age of 4,56678Ma (Tera and formation is now being obtained from chrono- Carlson, 1999)(Figure 1). Following this meters based on the decay of short-lived work, U–Pb analyses of several other radionuclides that were present in the early Allende CAIs gave a consistent, but more presolar system (see Chapter 1.16), long-lived cise, 207Pb–206Pb age of 4,56672Ma (Go¨pel chronometers (particularly those based on the et al., 1991; Alle` gre et al., 1995). In more recent 235,238U–207,206Pb systems) provide the only years, several studies have demonstrated the means of anchoring the relative ages provided importance of the removal of common lead for by the extinct chronometers to an absolute obtaining high-precision 207Pb–206Pb ages for timescale. In this review, an overview is pre- meteorites and their components (e.g., Lugmair sented of the chronological constraints that and Galer, 1992; Amelin et al., 2002, 2005). In have been obtained so far for events occurring particular, using extensive acid leaching to in the early history of the solar system based on remove the common lead component, Amelin long-lived radionuclides. Although results from et al. (2002) obtained a precise 207Pb–206Pb age earlier studies are briefly summarized, the focus of 4,567.270.6 Ma for two CAIs (E49 and of this review will be on more recent reports E60) from the Efremovka carbonaceous (re- (i.e., those published within the last decade duced CV3) chondrite (Figure 2). Additional or so) and their implications. For additional analyses of the E60 CAI using step-leaching details on previous studies of early solar system and 202Pb–205Pb double-spike in combination chronology based on both long- and short-lived with the results reported by Amelin et al. (2002) radionuclides, the reader is referred to several have yielded the most precise 207Pb–206Pb age excellent reviews (e.g., Wasserburg, 1985; of 4,567.170.2 Ma (Amelin et al., 2006). 4 Long-Lived Chronometers However, E60 is a relatively rare type of CAI 1.12 (forsterite-bearing Type B; Amelin et al., 2002), PAT and it is unclear whether its age (the most precisely defined though it is) is indeed represen- tative of that of the more common CAI types. Nevertheless, at present, this represents the best Age = 4.566 Ga estimate for time of formation of the earliest +_ 0.008 solids in the solar nebula and, therefore, the 0.9333 best estimate of the minimum age of the Pb C-1

206 solar system. 87 86

Pb/ The Sr/ Sr ratio has also been used as a 3529-40B 3529-40A 207 Egg-5A tracer for the formation time of CAIs. In this approach, a formation time interval is esti- Terr Pb mated based on the measured initial 87Sr/86Sr ratio of a particular sample with a low Rb/Sr 0.7467 Egg-5B ratio (such as a CAI) and the time that is Egg-3B taken to evolve to this composition from a less radiogenic strontium-isotope composition Egg-4 (such as the starting composition inferred for Egg-1 & Egg-6 the solar nebula) in an environment with a WA-5 given Rb/Sr ratio. The high Rb/Sr ratio in the 0 0.04 0.08 0.12 solar nebula (Anders and Grevesse, 1989; 204Pb/206Pb Chapter 1.03) implies that the 87Sr/86Sr ratio Figure 1 207Pb–206Pb isochron diagram of the would increase rapidly in material evolving in data of Chen and Wasserburg (1981) for Allende such an environment until a major Rb/Sr frac- CAIs. The data deﬁne a single array that corre- tionation event (such as CAI formation) deﬁnes 207 206 sponds to a Pb– Pb age of 4.566 Ga. PAT ¼ Pb the initial 87Sr/86Sr ratio of the object formed isotope composition of Canyon Diablo troilite. during this event. Comparison of the initial Reproduced by permission of Elsevier from Tera 87Sr/86Sr ratios for solar system materials can and Carlson (1999). potentially resolve time differences of the order of a million years or so. The antiquity of CAIs is indicated by their extremely unradiogenic

0.648 Efremovka CAI E49 Age = 4,567.17 ± 0.70 Ma MSWD = 0.88 0.644

Efremovka CAI E60 0.640 Age = 4,567.4 ± 1.1 Ma MSWD = 1.09

Pb 0.636 206

Pb/ 0.632 207

0.628 Acfer 059 chondrules Age = 4,564.66 ± 0.63 Ma 0.624 MSWD = 0.51

0.620 0.000 0.001 0.002 0.003 0.004 0.005 204Pb/206Pb Figure 2 207Pb–206Pb isochron diagram for acid-washed fractions from two Efremovka CAIs (E49 and E60); the weighted average of the 207Pb–206Pb ages obtained for these two CAIs is 4,567.270.6 Ma. Also shown are the Pb-isotope data for the six most radiogenic analyses of acid-washed chondrules From the CR chondrite Acfer 059. Reproduced by permission of American Association for the Advancement of Science from Amelin et al. (2002). Chondrites and their Components 5 strontium isotopic compositions. A CAI (D7) another 2–3 Myr afterwards. Recently, Amelin from Allende has the lowest reported 87Sr/86Sr et al. (2005) reported a 207Pb–206Pb age of ratio of any solar system material (Gray et al., 4,562.771.7 Ma from pyroxene-rich chondrules 1973). However, there is some complexity in and chondrule fragments from the Richardton the strontium isotopic composition of CAIs H5 equilibrated ordinary chondrite. Given the since other Allende inclusions analyzed by equilibrated nature of this chondrite, these Gray et al. (1973) and Podosek et al. (1991) authors argued that this age was the minimum have 87Sr/86Sr ratios that are slightly higher age for the formation of chondrules in this than in D7 (translating to a time span of up to sample (possibly corresponding to the time of B3 Myr between these and D7). Furthermore, cessation of lead loss). more recent analyses of the strontium-isotope Chondrules from the metal-rich CB chondrites composition of the D7 inclusion are also Gujba and Hammadah al Hamarah 237 gave slightly higher than the initial 87Sr/86Sr ratio 207Pb–206Pb ages of 4,562.770.5 Ma (Figure 3) that was reported for this CAI by Gray et al. and 4,562.870.9 Ma, respectively (Krot et al., (1973). Potential factors affecting this issue 2005). It had previously been suggested that may be the presence of nucleosynthetic anom- chondrules in the CB chondrites originated as a alies in the strontium-isotope composition or result of an energetic impact between large plan- the disturbance of the strontium isotopic com- etesimals (Rubin et al., 2003) and these relatively position by secondary events or by contamina- young ages have been used in support of this tion. In this regard, nucleosynthetic anomalies hypothesis (Krot et al., 2005). in strontium isotopes have been reported in CAIs that have been shown to record other fractionation and unknown nuclear (FUN) ef- 1.27.2.2 Ages of Secondary Events Recorded fects (Papanastassiou and Wasserburg, 1978; in Chondrites Loss et al., 1994). Chondritic meteorites record a variety of secondary alteration processes, including aqueous 1.27.2.1.2 Chondrules alteration, thermal and shock metamorphism, Chondrules are sub-millimeter to centimeter- and brecciation. These events spanned a long sized ferromagnesian silicate spherules found time interval, beginning almost close to the for- in chondrites. Although, in detail, there are mation of the solar system at B4.56 Ga and several hypotheses for the exact mechanism extending over a period of several millions of involved in chondrule formation (currently, the years thereafter. The following is a discussion of two leading ones being the X-wind and the the timescales for these different secondary shock-wave models; see Ciesla, 2005 and refer- alteration processes inferred from various ences therein): they are generally considered to long-lived chronometers, although the bounda- have resulted from transient heating events in ries between these processes are not always well the solar nebula. The earliest lead-isotope study deﬁned (e.g., an energetic impact event on a of chondrules was performed on those sepa- planetesimal may result in shock metamorphism rated from the Allende chondrite and gave an as well as thermal processing). age of 4,560767 Ma (Chen and Tilton, 1976). The low precision of this date was due to the relatively large unradiogenic Pb component in 0.638 these chondrules and small spread in the 207 206 Gujba Pb/ Pb ratios. More recently, using ex- 0.634 tensive leaching procedures to remove the com- Chondrules 3, 4, 5 mon lead component, the chondrules from a 0.630 carbonaceous chondrite belonging to the CR Pb group were precisely dated at 4,564.770.6 206

(Amelin et al., 2002)(Figure 2), which indi- Pb/ 0.626 cates that these were formed 2.470.6 Myr after 207 CAIs. Lead-isotope compositions of chondrules 0.622 Age = 4,562.68 ± 0.49 Ma from Allende have also been recently analyzed MSWD = 1.3 and yield an older age of 4,566.771.0 Ma (Amelin et al., 2004), which overlaps within 0.618 the uncertainties with the 207Pb–206Pb age of 0.0000 0.0008 0.0016 0.0024 CAIs. As such, chondrules from these primitive 204Pb/206Pb chondrite groups define ages that suggest that Figure 3 207Pb–206Pb isochron diagram for three they began forming almost contemporaneously Gujba chondrules. Reproduced by permission of with CAIs and continued to form for at least Nature Publishing Group from Krot et al. (2005). 6 Long-Lived Chronometers 1.27.2.2.1 Aqueous alteration For the H chondrites, these authors noted a correlation between the 207Pb–206Pb ages of the Aqueous alteration of chondrites and their phosphates and their degree of metamorphism. components is thought to have occurred in a Specifically, 207Pb–206Pb model ages (with a variety of settings, including the solar nebula typical precision of 71 Ma) ranged from and on accreted parent bodies (Zolensky and 4,563 Ma for the Ste. Marguerite H4 chondrite McSween, 1988; Brearley, 2006; Chapter 1.09). to 4,504 Ma for the Guarenã H6 chondrite, The CI carbonaceous chondrites contain sev- thereby indicating that thermal processing of eral secondary mineral phases (particularly car- the H chondrite parent body(ies) extended bonates and sulfates) that provide a record of over a period of B60 Myr. The 207Pb–206Pb aqueous alteration on their parent body. The age of phosphate fractions from the Richard- carbonates are a good candidate for dating 7 87 86 ton H5 chondrite (4,550.7 2.6 Ma; Amelin using the initial Sr/ Sr approach since et al., 2005) falls within this time span. In this mineral is typically characterized by low the case of the L chondrites, 207Pb–206Pb model Rb/Sr ratios. Macdougall et al. (1984) reported ages for their phosphates ranged from 4,543 to the strontium isotopic compositions of 4,511 Ma, while for the LL chondrites these several carbonate separates (dolomite and ages ranged from 4,557 to 4,536 Ma. These ages breunnerite) from the Orgueil CI chondrite. also suggest that thermal metamorphism of the These authors showed that Orgueil carbonates 87 86 L and LL parent bodies had extended for tens have a range of Sr/ Sr ratios, with the of millions of years in the early history of the lowest one being similar to the most primitive solar system. Sr-isotope composition measured in Allende Although less precise than the U–Pb chro- CAIs (Gray et al., 1973; Podosek et al., 1991). nometer, the 39Ar–40Ar technique has also been This implies that the onset of aqueous applied toward constraining the duration of alteration on the CI chondrite parent body oc- thermal processing of chondrite parent bodies. curred essentially contemporaneously with its Some equilibrated (but unshocked) ordinary formation. chondrites show a range of 39Ar–40Ar ages from B4.5 to B4.4 Ga (with a typical precision 7 1.27.2.2.2 Thermal metamorphism of 30 Ma) (Turner et al., 1978; Hohenberg et al., 1981) which, to first order at least, is Most chondrites have experienced some comparable to the duration indicated by the U– degree of thermal metamorphism, defined here Pb systematics in phosphates from the ordinary as alteration resulting from heating at temper- chondrites. The initial 87Sr/86Sr method atures in the range of 400–1,000 1C at low (described in the previous section) may addi- lithostatic pressure for extended time periods tionally be used to assess the duration of ther- (McSween et al., 1988; Huss et al., 2006). Cal- mal processing of chondrite parent bodies. Two cium phosphates in chondritic meteorites are examples of the application of this approach minor but uranium-rich secondary minerals (i.e., to phosphates from Beaver Creek and that were formed during this thermal process- Guarenã chondrites) toward obtaining chron- ing of the parent bodies of these meteorites, ological information regarding formation most likely by oxidation of phosphorus-rich of secondary phosphates are illustrated in metal (Perron et al., 1988). As such, U–Pb sys- Figure 4. Time intervals of tens of millions of tematics in secondary phosphates from chond- years are obtained based on the initial 87Sr/86Sr rite groups that have experienced different isotopic compositions reported for ordinary degrees of metamorphic equilibration can chondrite phosphates (Wasserburg et al., 1969; provide constraints on the timescales involved Manhe` s et al., 1978; Brannon et al., 1988; Po- in thermal processing of planetesimals follow- dosek and Brannon, 1991) and assuming ing their accretion. The first studies of U–Pb evolution from a primitive strontium isotopic systematics in phosphates from a chondrite value similar to the average value for Allende were performed on the LL6 equilibrated ordi- inclusions (Gray et al., 1973; Podosek et al., nary chondrite Saint-Se` verin (Manhe` s et al., 1991) and equilibration of strontium isotopes 1978; Chen and Wasserburg, 1981). The results on the whole-rock scale. This is again broadly from these studies are consistent with a later consistent with the duration of metamorphic investigation of phosphates from this same me- events as indicated by the 207Pb–206Pb and teorite (Go¨pel et al., 1994) and together yield a 39Ar–40Ar ages discussed above. However, even 207Pb–206Pb age of 4,55876 Ma. In addition to though all three of these chronometers seem to Saint-Se` verin, Go¨pel et al. (1994) reported be indicating generally similar timescales of U–Pb systematics in phosphates from 14 other tens of millions of years for the thermal proc- equilibrated ordinary chondrites belonging to essing on the chondrite parent bodies, when the H4, H5, H6, L5, L6, LL5, and LL6 groups. specific ages from these three chronometers are Chondrites and their Components 7

Age (Ga) in terms of alkali elements such as rubidium) 4.48 4.52 4.56 would best record this late processing. As 0.7005 such, consideration of only the mesostasis- rich chondrules from their study and from Nebular evolution 20 previous investigations (Gray et al., 1973; 0.7000 Tatsumoto et al., 1976)yieldsa87Rb–87Sr age 15 of 4.3670.08 Ga. Within errors, this age is Guarena

Sr similar to that obtained for the Richardton phosphate ALL 86 0.6995 10 chondrules. Sr/ Bulk chondrite 87 evolution

Beaver Creek BabI 5 0.6990 phosphate 1.27.2.2.3 Shock metamorphism ALL 0 Impacts between solar system bodies have played an important role in their evolutionary 0.6985 100 80 60 40 20 0 histories and many chondritic meteorites pre- Interval (Ma) serve a record of these events (Sto¨ffler et al., 1988). The timing of impact events affecting Figure 4 Two examples (phosphates from the chondrite parent bodies have been determined Beaver Creek and Guarenã chondrites) illustrating 39 40 the application of the initial 87Sr/86Sr approach to- predominantly with the Ar– Ar dating ward obtaining chronological constraints. ALL ¼ method (e.g., Turner, 1969; Bogard et al., initial 87Sr/86Sr ratio for the D7 Allende CAI with 1976; Bogard and Hirsch, 1980; Kaneoka, the most primitive composition (Gray et al., 1973). 1981; Stephan and Jessberger, 1988; Kring Steeper solid line shows evolution of 87Sr/86Sr in a et al., 1996; Grier et al., 2004), although in nebular environment (87Rb/86SrB1.5); shallower some cases other isotope systems such as Rb– solid line shows evolution with bulk chondritic Sr, Pb–Pb, and Sm–Nd (roughly in that order 87 86 B Rb/ Sr ( 0.75). Shaded gray bands are extra- of susceptibility to disturbance) are also af- polations based on the measured Rb–Sr systematics fected by such events. Most experimental work in phosphates from the Beaver Creek and Guarenã chondrites (the width of the bands indicating the on the effects of shock alone on various iso- uncertainties). As shown here, the time interval be- topic chronometers indicates that shock pres- tween formation of Allende CAIs and formation of sures up to B60 GPa are usually insufficient to secondary phosphates in these two chondrites (i.e., reset these chronometers (Deutsch and Scha¨rer, indicated on the x-axis by the points of the intersec- 1994). However, if the shock event is associated tion of the gray bands with the bulk chondrite with thermal annealing, it can easily affect some evolution line) is tens of millions of years. Repro- isotopic systems (particularly the K–Ar system, duced by permission of Meteoritics and Planetary but also Rb–Sr) (e.g., Nyquist et al., 1991). In Science from Podosek and Brannon (1991). the particular case of the 39Ar–40Ar dating method, if the shocked sample is insufficiently heated or if it cooled rapidly, there may be only considered in detail (as was done by Go¨pel a small amount of the radiogenic 40Ar lost from et al., 1994), there does not appear to be any the sample and thus the K–Ar system would correlation between them. This could be indi- only be partially reset. On the other hand, if the cative of complex histories of ordinary chond- shock is accompanied by extended thermal an- rite parent bodies following their initial nealing (and also if the sample had small grain accretion, which might affect these three sizes facilitating diffusional loss of argon), then chronometers differently. the K–Ar system may be almost totally reset by Finally, 87Rb–87Sr investigations of chond- this event (at least in some minerals of a rules additionally indicate that thermal process- shocked sample that may be more susceptible ing of chondrite parent bodies extented to to being reset). In any case, the 39Ar–40Ar age B4.4 Ga. Chondrules from the Richardton of the shock event is determined by using a ordinary equilibrated (H5) chondrite yield stepwise temperature release of argon, which a 87Rb–87Sr age of 4.4570.03 Ga (Evensen helps to separate the K–Ar chronologies of et al., 1979). Studies of chondrules from the Al- different minerals of the shocked sample. lende carbonaceous chondrite also indicate that Bogard (1995) summarized the impact ages the 87Rb–87Sr system was significantly affected of various chondrite classes, that are mostly by late thermal processing (Gray et al., 1973; based on the 39Ar–40Ar method. Most chondri- Tatsumoto et al., 1976; Shimoda et al., 2005). In tic meteorites have 39Ar–40Ar ages that are particular, Shimoda et al.(2005)suggest that younger than B1.3 Ga (Figure 5). These young the mesostasis-rich chondrules (considered to ages are considered to be reflecting relatively be most susceptible to disturbance, particularly few impact events and, in fact, the peak at 8 Long-Lived Chronometers

14 14

12 L-chon 12 10 H-chon 8 10 LL-chon Number 6

8 4

2 6

Number 0 0.1 0.3 0.5 0.7 0.9 1.1 1.3 4 Ar−Ar Age in Ga

2 Plateau Intercept

0 0 0.4 0.8 1.2 1.6 2 2.4 2.8 3.2 3.6 4.0 4.2 Ar−Ar Age in Ga Figure 5 Histograms of impact-reset 39Ar–40Ar ages of ordinary chondrites, with plotted age interval of 0.1 Ga. Inset shows an expanded scale figure with data for chondrites having impact ages of o1.3 Ga. In main figure, ages defined by a significant age plateau (‘‘plateau’’) are in black while those defined primarily from the age intercept of a diffusion profile (‘‘intercept’’) are in gray. Reproduced by permission of Meteoritics and Planetary Science from Bogard (1995).

B0.5 Ga for the L class of ordinary chondrites stages of melting and igneous processing on in the histogram shown in Figure 5 is thought planetesimals (see Mittlefehldt et al., 1998 to be the result of a single large impact that and references therein). The acapulcoites and catastrophically disrupted the parent body of lodranites are thought to be the residual these meteorites (Haack et al., 1996a). At least products of partial melting of chondritic pre- B4 distinct impact events are required to cursors (Mittlefehldt et al., 1996; McCoy et al., account for the Ar–Ar ages of most chond- 1997a, b). 147Sm–143Nd systematics determined rites. The B0.3 and B0.5 Ga events are the by Prinzhofer et al. (1992) for Acapulco gave a best defined and affected the L and H ordinary very old age (4.6070.03 Ga), which the authors chondrite parent bodies. Additional events at interpreted as the time of recrystallization im- B0.9 Ga (affecting the L and H parent bodies) mediately following its formation event. Ura- and at B1.2 Ga (affecting the LL parent body) nium–lead systematics in phosphates from are also indicated. Acapulco give a 207Pb–206Pb model age of The Ar–Ar and Rb–Sr ages of some chond- 4,55772Ma(Go¨pel et al., 1992, 1994), indica- rites, however, indicate significantly older im- ting that this achondrite formed approximately pact ages of B3.5–4.0 Ga (e.g., Keil et al., B10 Myr after the formation of CAIs (at 1980; Stephan and Jessberger, 1988; Nakamura 4,567.170.2 Ma; Amelin et al., 2002, 2006). et al., 1994). These ages most likely reflect the More recently, a 207Pb–206Pb isochron age for time of heavy bombardment experienced by Acapulco phosphates and mixed grain frac- bodies in the inner solar system. This event has tions of 4,556.5270.78 Ma (or 10.670.8 Myr also been recorded in the impact-reset ages of after CAI formation) has been reported (Am- many achondrites (see Section 1.27.3.2.3) and elin and Pravdivtseva, 2005; Amelin et al., lunar samples (see Section 1.27.4.1.2). 2006). This 207Pb–206Pb age for Acapulco is marginally younger, but much more precise, than the 147Sm–143Nd age. McCoy et al. (1996) 1.27.3 DIFFERENTIATED METEORITES have argued that the older 147Sm–143Nd age could be due to disturbance during extensive 1.27.3.1 Primitive Achondrites: Timing of later metamorphism experienced by this mete- Incipient Differentiation on orite. The Divnoe meteorite is an ultramafic Planetesimals primitive achondrite whose relationship with Primitive achondrites, such as acapulcoites, other primitive achondrite groups is as yet un- lodranites, winonaites, and brachinites, are clear (Petaev et al., 1994; Weigel et al., 1996). considered to be the products of the earliest This meteorite also has an old 147Sm–143Nd age Differentiated Meteorites 9 of 4.6270.07 Ga. Although uncertainties are 4,56371 Ma for D’Orbigny (G. W. Lugmair, large (Bogdanovski and Jagoutz, 1996), the personal communication). This revised age is in youngest formation time indicated by this agreement with the results from a more recent 147Sm–143Nd age is B17 Myr after CAI forma- study that reported a 207Pb–206Pb model age tion. The above discussion shows that the onset of 4,563.870.6 Ma for D’Orbigny (Zartman of melting on some planetesimals, as evidenced et al., 2006). Finally, Baker et al.(2005) by primitive achondrites such as Acapulco and reported a highly precise and extremely ancient Divnoe, occurred within B10–20 Myr of the 207Pb–206Pb isochron age of 4,566.1870.l4 Ma beginning of the solar system. for the Sahara 99555 angrite (Figure 6). Given the 207Pb–206Pb age for CAIs of 4,567.170.2 Ma (Amelin et al., 2002, 2006), this 1.27.3.2 Basaltic and Other Achondrites: suggests that basalts were forming on the sur- Timing of Asteroidal Differentiation face of the angrite parent body within B1Myr and Cataclysm of CAI formation. The above discussion shows that the 207Pb–206Pb ages of the angrites span a 1.27.3.2.1 Crust-formation timescales from B chronology of achondrites and their time interval of almost 8Myr, the youngest components (LEW and ADOR) having an age of 4,558 Ma and the oldest (Sahara 99555) being 4,566 Ma. Primary crystallization ages of individual Like the angrites, the noncumulate eucrites members of achondrite groups such as the are pyroxene–plagioclase rocks. However, angrites and noncumulate eucrites, which rep- there are significantly greater numbers of resent basaltic rocks that formed in asteroidal known noncumulate eucrites than there are near-surface environments, provide constraints angrites. Recent high-precision oxygen-isotope on the timing of silicate differentiation and data of Wiechert et al. (2004) demonstrate that crust formation on planetesimals during the most noncumulate eucrites along with the cu- early history of the solar system. mulate eucrites, diogenites, and howardites Angrites are a small group of mineralogically have uniform D17O (within 70.02%) and thus unique basalts composed mostly of Ca–Al–Ti- lie on a single mass fractionation line, consist- rich pyroxenes (fassaite), olivine and anorthitic ent with their origin on a single parent body. plagioclase (see Mittlefehldt et al., 1998 and ref- Therefore, these basalts are the most numerous erences therein; Chapters 1.05 and 1.11). crustal rocks available from any single solar 147Sm–143Nd systematics in Angra dos Reis system body other than the Earth and (ADOR) and LEW 86010 (LEW) are well- the Moon. A handful of the noncumulate euc- behaved and give old crystallization ages be- rites (in particular Ibitira, but possibly also tween 4.5370.04 and 4.5670.04 Ga (Lugmair Caldera, Pasamonte, and ALHA 78132) have and Marti, 1977; Wasserburg et al., 1977; Jacob- oxygen-isotope compositions distinct from the sen and Wasserburg, 1984; Lugmair and Galer, others, implying either that these samples 1992; Nyquist et al., 1994). 147Sm–143Nd sys- originated on different parent bodies or that tematics have also been determined in the more isotopic heterogeneity was preserved on the recently discovered angrite D’Orbigny, and, de- spite some disturbance evident in the plagioclase, possibly due to late metamorphism and/or 0.629 terrestrial weathering, are generally consistent with earlier results for ADOR and LEW 0.628 (Nyquist et al., 2003a; Tonui et al., 2003). The most precise estimate of the crystallization age 207 206 Pb 0.627

of the angrites is offered by their Pb– Pb 206 207 206

systematics. A Pb– Pb isochron deﬁned by Pb/ 0.626

LEW minerals gives an age of 4,558.273.4 Ma, 207 concordant with the highly precise model age of 4,557.870.5 Ma obtained from the extremely 0.625 radiogenic lead compositions in the pyroxenes 4,566. 18 ± 0.14 Ma MSWD = 1.5 (n = 7) of ADOR and LEW (Lugmair and Galer, 0.624 1992). Preliminary model ages derived from 0.0000 0.0004 0.0008 0.0012 the lead-isotope compositions of the D’Orbigny 204Pb/206Pb pyroxenes appeared to be in agreement with Figure 6 207Pb–206Pb isochron diagram for frag- those derived from ADOR and LEW pyroxenes ments of Sahara 99555 and NWA 1296 angrites (Jagoutz et al., 2003). However, a recent reeval- and acid-washed pyroxene from Sahara 99555. uation of these data by Jagoutz and colleagues Reproduced by permission of Nature Publishing has resulted in a somewhat older age of Group from Baker et al. (2005). 10 Long-Lived Chronometers Howardite–Eucrite–Diogenite (HED) parent determination of the 207Pb–206Pb mineral is- body (Wiechert et al., 2004). Unlike angrites ochron for the Asuka 881394 eucrite yielded a (which did not undergo any significant degree precise and ancient age of 4,566.570.3 Ma of recrystallization or metamorphism), the (Amelin et al., 2006). This is only 0.670.4 Myr noncumulate eucrites appear to record a pro- younger than the time of CAI formation tracted history of extensive thermal processing (4,567.170.2 Ma; Amelin et al., 2002, 2006) on their parent body subsequent to their and, as in the case of the Sahara 99555 angrite original crystallization. As a result, many of discussed earlier, indicates that crust formation the chronometers investigated in these samples on differentiated planetesimals occurred within appear to record varying degrees of disturbance B1 Myr of the formation of CAIs. from secondary thermal events. Nevertheless, Basaltic noncumulate eucrites thus show there are several lines of evidence that suggest clear evidence of having formed close to that these basalts crystallized very early in the B4.56 Ga in the crust of their parent planet- history of the solar system. Although typically esimal. In contrast, cumulate eucrites, which characterized by large uncertainties, the formed in the crust of the same parent planet- 147Sm–143Nd ages of several of these samples esimal as the noncumulate eucrites (Clayton such as Chervony Kut (4,580730 Ma; Wad- and Mayeda, 1996; Wiechert et al., 2004), have hwa and Lugmair, 1995), Juvinas significantly younger concordant Sm–Nd and (4,560780 Ma; Lugmair, 1974), Pasamonte Pb–Pb ages, ranging from the oldest of (4,5807120 Ma; Unruh et al., 1977), Piplia 4,456725 Ma (Sm–Nd) and 4,484719 Ma Kalan (4,570723 Ma; Kumar et al., 1999), and (Pb–Pb) for Moore County (Tera et al., 1997) Yamato 792510 (4,570790 Ma; Nyquist et al., to the youngest of 4,410720 Ma (Sm–Nd; 1997) are close to B4.56 Ga. In some cases Lugmair et al., 1977) and 4,399735 Ma (Pb– where 147Sm–143Nd systematics appear to Pb; Tera et al., 1997) for Serra de Mage´. Thus, record ages younger than B4.56 Sm–Nd and Pb–Pb systematics in the cumulate Ga, the 146Sm–142Nd systematics hint at early eucrites indicate that isotopic closure occurred crystallization of basaltic eucrites such as Cal- up to B150 Myr after the noncumulate eucrites dera (146Sm/144Sm ¼ 0.007370.0011; Wadhwa (Lugmair et al., 1977; Jacobsen and Was- and Lugmair, 1996) and Ibitira (146Sm/144Sm serburg, 1984; Lugmair et al., 1991; Tera B0.00970.001; Prinzhofer et al., 1992). This et al., 1997), possibly implying that active may be explained by a model proposed by magmatism persisted on the eucrite parent Prinzhofer et al. (1992), according to which the body (EPB) for this extended period (Tera apparent discrepancy between the long-lived et al., 1997). 147Sm–143Nd and the short-lived 146Sm–142Nd Since, as discussed above, the oldest basaltic systems can be interpreted in terms of a short meteorites formed within B10 Myr of CAI for- episodic disturbance resulting in partial reequi- mation (and some, specifically the angrite Sa- libration of the rare earth elements (REEs), hara 99555 and the eucrite Asuka 881394, predominantly between plagioclase (which has within only B1 Myr), the decay of short- very low REE abundances) and phosphates lived radionuclides such as 26Al and 60Fe (which are the primary REE carriers). As is the likely heat source for the early and ex- shown by the modeling results of these au- tensive differentiation experienced by their par- thors, such a disturbance could partially reset ent planetesimal. Energy sources that can the 147Sm–144Nd isochron, without having a account for later igneous activity (i.e., tens of resolvable effect on the 146Sm–142Nd system. millions of years after CAI formation) on small The Rb–Sr chronometer also generally indi- planetesimals are not obvious unless the cumu- cates ancient formation ages for the noncumulate eucrites are the crystallization products of late eucrites (e.g., Alle` gre et al., 1975; Nyquist impact melting on the EPB. Alternatively (and et al., 1986). The most precise of the absolute perhaps more likely), since the cumulate euc- chronometer, that is, the U–Pb system, appears rites are slowly cooled rocks that possibly to have been affected by postcrystallization formed deeper within the crust of their parent events and terrestrial Pb contamination in most body than noncumulate eucrites, the long-lived noncumulate eucrites, recording mineral iso- chronometers could be recording the long cool- chron ages in the range of 4,128–4,530 Ma ing times required to achieve subsolidus tem- (Tatsumoto et al., 1973; Unruh et al., 1977; peratures. This is supported by the modeling Galer and Lugmair, 1996; Tera et al., 1997). results of Ghosh and McSween (1998), which However, Ibitira whole-rock samples with show that, assuming reasonable parameters, it radiogenic Pb isotopic compositions gave old is possible to maintain temperatures in excess 207Pb–206Pb model ages of 4,55676Ma(Chen of the solidus temperature for basalt at a depth and Wasserburg, 1985) and 4,56073Ma of B100 km for over B100 Myr in a Vesta- (Manhe` s et al., 1987). Furthermore, a recent sized planetesimal. Differentiated Meteorites 11 Besides the angrites and the eucrites, there 207Pb–206Pb ages and 147Sm–143Nd internal are other types of achondrites, such as aubrites isochron ages of the cumulate eucrites (see ear- and ureilites, whose origins are somewhat lier discussion in this section). Patchett and enigmatic but which were nevertheless formed Tatsumoto (1980) reported the first whole- in the crusts of extensively differentiated aster- rock 176Lu–176Hf isochron for the eucrites. At oidal bodies. There are very few chronological the time, the half-life of 176Lu was not well- investigations of the aubrites, which are essen- constrained and these authors assigned an tially monomineralic rocks composed of age of 4.55 Ga for this whole-rock isochron coarse-grained enstatite. U–Th–Pb and Sm– and thus estimated a half-life of 35.3 Gyr Nd isotope systematics in the ureilites generally (corresponding to a decay constant for 176Lu 11 1 indicate early formation, although there are or l176Lu of 1.96 10 yr ). A more recent apparent complications resulting from later study of 176Lu–176Hf systematics in whole disturbance during a metasomatic event on rocks of eucrites by Blichert-Toft et al. (2002) the ureilite parent body and/or by recent ter- reported an errorchron corresponding to restrial contamination (Goodrich and Lugmair, an age of 4,604739 Ma for the noncumu- 1995; Torigoye-Kita et al., 1995a, b). late eucrites (Figure 9). The cumulate eucrites, on the other hand, defined a 176Lu–176Hf isochron with an age of 4,470722 Ma (Figure 9), 147 143 1.27.3.2.2 Global differentiation timescales indistinguishable from their Sm– Nd whole-rock age. Based on these whole-rock based on whole-rock isochrons and 147 143 176 176 initial 87Sr/86Sr Sm– Nd and Lu– Hf systematics in the eucrites, these authors suggested that cu- While an internal mineral isochron can mulate eucrites were formed B100 Myr after provide constraints on the timing of formation the noncumulate eucrites, while the latter were of an individual achondrite in the crust of a formed close to the beginning of the solar sys- planetesimal, a whole-rock isochron of a par- tem. In their study, Blichert-Toft et al. (2002) 11 1 ticular achondrite group can provide limits on assumed a l176Lu value of B1.93 10 yr the timing of the major fractionation event that (similar to that suggested by Patchett and established the parent/daughter element ratio Tatsumoto, 1980). However, studies of terres- in the whole rocks (which could have pre-dated trial samples on the one hand and meteoritic the formation of an individual sample). De- samples on the other indicate that the half-life 176 pending on the geochemical behavior of the of Lu may be either 37.2 Gyr (l176Lu parent and daughter elements, this major par- B1.86 1011 yr1; Scherer et al., 2001)or ent/daughter fractionation recorded in the whole rocks may reflect a global silicate fractionation event (possibly associated with crystallization of a magma ocean) that established Noncumulate eucrites 0.7004 the source characteristics for these rocks, or it T = 4.548 ± 0.058 Ga could simply reflect crystal fractionation from a 0.7002 I(87Sr/86Sr) = 0.698925 ± 14 parental melt, which resulted in the formation of these rocks. Whole-rock Rb–Sr isochrons 0.7000 Jonzak for the basaltic eucrites established early on 0.6998 Bereba that Rb–Sr fractionation in the mantle source Sr Nuevo Laredo

86 Juvinas Px reservoir of these achondrites occurred close to 0.6996 B Sr/ 4.6 Ga (Papanastassiou and Wasserburg, 87 0.6994 Ibitira 1969; Birck and Alle` gre, 1978). Smoliar (1993) Sioux County evaluated all available Rb–Sr data for the euc- 0.6992 Juvinas Pomozdino rites and obtained a whole-rock isochron age of Plag 4.5570.06 Ga for the noncumulate eucrites 0.6990 Juvinas 147 143 (Figure 7). A whole-rock Sm– Nd iso- 0.6988 chron for 18 noncumulate and cumulate euc- 0.000 0.005 0.010 0.015 0.020 rites was reported by Blichert-Toft et al. (2002) 87Rb/86Sr and gave a relatively young age of 87 87 4,464775 Ma (Figure 8). Since the slope of Figure 7 Rb– Sr whole-rock isochron for noncumulate eucrites. All data points indicate whole- this isochron is controlled by the cumulate rocks samples or clasts, with the exception of two eucrites (which have the most fractionated mineral fractions from Juvinas (Px ¼ pyroxene; whole-rock Sm/Nd ratios), the authors inter- Plag ¼ plagioclase) that are also plotted here. Data preted this to imply that cumulate eucrites were are from various literature sources given in Smoliar formed B100 Myr after solar system forma- (1993). Adapted by permission of Meteoritics and tion. This is supported by the relatively young Planetary Science from Smoliar (1993). 12 Long-Lived Chronometers

0.5142

0.5140

0.5138 Moama

0.5136 Nagaria 0.5134 Nd 0.5132 144

Nd/ Moore county 0.5130 Basaltic eucrites 143 Talampaya Serra de Magè Cumulate eucrites 0.5128 Caldera Not regressed BSE T = 4,464 ± 75 Ma 0.5126 143 144 Pasamonte ( Nd/ Nd)i = 0.50680 ± 0.00010 0.5124 MSWD = 1.26

0.5122 0.185 0.195 0.205 0.215 0.225 0.235 0.245 0.255 0.265 147Sm/144Nd Figure 8 147Sm–143Nd whole-rock isochron for eucrites. Reproduced by permission of Elsevier from Blichert- Toft et al. (2002).

0.2910

0.2900 Basaltic eucrites Cumulate eucrites Moama 0.2890

0.2880

0.2870 Hf Nagaria 177 0.2860 Talampaya Hf/

176 0.2850 Serra de Magé 0.2840 Moore County Caldera 0.2830

176 177 0.2820 ( Hf/ Hf)i = 0.27966 ± 0.00002 0.2810 0.020 0.030 0.040 0.050 0.060 0.070 0.080 0.090 0.100 0.110 176Lu/177Hf Figure 9 176Lu–176Hf whole-rock isochron for the eucrites. A statistically signiﬁcant isochron cannot be obtained if all data are considered together. If only noncumulate eucrites are considered (with the exception of one sample, Palo Blanco Creek) an errorchron corresponding to an age of 4,604739 Ma is obtained. If only the three cumulate eucrites Moama, Moore County, and Serra de Mage´are regressed together, they yield an isochron corresponding to an age of 4,470722 Ma. Reproduced by permission of Elsevier from Blichert-Toft et al. (2002).

11 1 34.9 Gyr (l176LuB1.98 10 yr ; Bizzarro uncertain, thereby limiting the usefulness of the et al., 2003), respectively. Amelin and Davis 176Lu–176Hf chronometer at the present time. (2005) have shown that this apparent discrep- Finally, the ancient ages for the establish- ancy cannot be accounted for by a possible ment of the highly volatile-depleted mantle branched decay of 176Lu. Although other pos- source reservoirs of the angrites and the sible explanations were evaluated by these eucrites on their respective parent bodies can authors, none were considered to be plausible. be inferred from their initial 87Sr/86Sr ratios. As such, the half-life of 176Lu still remains Papanastassiou and Wasserburg (1969) first Differentiated Meteorites 13 estimated the initial 87Sr/86Sr of the EPB (ba- resulted in a slighter higher Rb/Sr ratio in the saltic achondrite best initial or BABI). At the source of the noncumulate eucrites compared time, this was the most primitive known stron- to that of the cumulate eucrites, thereby tium isotopic composition. Subsequently, how- resulting in the higher initial 87Sr/86Sr ratio in- ever, an even more primitive strontium isotopic dicated by the whole-rock isochron for the composition was reported for an Allende CAI noncumulate eucrites. (ALL) (Gray et al., 1973). Assuming that the initial 87Sr/86Sr ratio at the beginning of the solar system is represented by the average in- 87 86 1.27.3.2.3 Inner solar system bombardment itial Sr/ Sr ratio measured in Allende CAIs history based on reset ages (Gray et al., 1973; Podosek et al., 1991), and that subsequent evolution of radiogenic stron- As discussed previously for chondritic mete- tium occurred in an environment with solar orites, the 39Ar–40Ar technique has also been Rb/Sr ratios, the initial 87Sr/86Sr ratios of the widely applied toward dating the thermal his- angrites (Lugmair and Galer, 1992; Nyquist tories of achondritic meteorites, and particu- et al., 1994, 2003a; Tonui et al., 2003) translate larly for determining the ages of impact events to an age difference of B4 Myr between CAI on their parent bodies (e.g., Podosek and Hun- formation and the timing of Rb/Sr depletion eke, 1973; Bogard et al., 1990; Takeda et al., event that established the angrite source char- 1994; Bogard and Garrison, 2003). As is the acteristics. The very low initial 87Sr/86Sr ratios case for severely shocked chondritic samples, of the eucrites similarly indicate that the vola- other isotopic chronometers such as Rb–Sr, tile depletion characterizing the EPB may have Pb–Pb, and Sm–Nd in some differentiated ac- occurred early in the history of the solar system hondrites that have experienced postshock (Carlson and Lugmair, 2000, and references thermal annealing record varying degrees of therein). A re-evaluation of the strontium-iso- disturbance (e.g., Unruh et al., 1977; Nyquist tope data for the eucrites by Smoliar (1993) et al., 1990). The impact ages of most achond- shows that whole-rock Rb–Sr isochrons for the rites (particularly the HED meteorites) fall noncumulate and the cumulate eucrites define within a relatively narrow time interval of slightly, but resolvably, different initial B3.4–4.1 Ga (Bogard, 1995; Bogard and Gar- 87Sr/86Sr ratios (that are both distinctly lower rison, 2003)(Figure 10). As discussed in the than the eucrite initial, BABI, previously de- earlier sections, these achondrites have crystal- fined by Papanastassiou and Wasserburg, lization ages that are close to B4.56 Ga and so 1969). In fact, the initial 87Sr/86Sr ratio for the age distribution shown in Figure 10 may be the cumulate eucrites proposed by Smoliar reasonably interpreted to reflect the timing of (1993) is, within errors, similar to that for the thermal metamorphism on the HED parent angrites (e.g., Lugmair and Galer, 1992), body resulting from large impacts (which are suggesting that the volatile depletion in their considered to be the most plausible heat source sources was established at similar times (possi- for these late events). Most of the HEDs are bly B4 Myr after CAI formation; see above). brecciated rocks that preserve clear textural, However, the slighter higher initial 87Sr/86Sr mineralogical, and chemical evidence of shock ratio defined by the noncumulate eucrites is metamorphism (e.g., Sto¨ffler et al., 1988; Met- potentially problematic since the simplest in- zler et al., 1995), further supporting the above terpretation would be that their source evolved interpretation. with a chondritic Rb/Sr ratio for B4 Based on the age distribution shown in Figure Myr longer (and is thus younger) than that of 10, it has been suggested that there was a the cumulate eucrites, further implying peak in the impactor flux (i.e., a cataclysm) on that these two types of eucrites originated on the HED parent body at B3.7 Ga, which distinct parent bodies (Smoliar, 1993). This is decreased sharply and tailed down to ages inconsistent with recent high-precision oxygen- slightly younger than B3.4 Ga (Bogard, 1995; isotope data (Wiechert et al., 2004) that suggest Bogard and Garrison, 2003). Therefore, the that the cumulate and noncumulate eucrites 39Ar–40Ar ages of the HED meteorites suggest (with the possible exception of Ibitira) origin- that the region where their parent body resided ated on a common parent planetesimal. As in the inner solar system experienced a period of discussed by Carlson and Lugmair (2000),a heavy bombardment during the time interval possible explanation could be that the severe extending from B4.1 Ga until at least B3.4 Ga. volatile depletion on the EPB did not occur in a The peak at B4.48 Ga in Figure 10 has been single-step process, but rather took place explained by Bogard and Garrison (2003) over the course of its accretionary and early in terms of a single large impact that excavated differentiation history. Subsequently, the proc- these eucritic meteorites from their parent ess of magma ocean crystallization may have body. 14 Long-Lived Chronometers

13 under the assumption that the Re–Os system at- Rb−Sr & Pb−Pb tained closure in the IIIAB irons almost con- 12 Eucrite − temporaneously with the formation of the ages Approx. Ar Ar 11 angrites ADOR and LEW at 4,557.870.5 Ma Precise Ar−Ar (Lugmair and Galer, 1992). Indeed, the Mn–Cr 10 model ages for the IIIAB irons (Hutcheon and 9 Olsen, 1991; Hutcheon et al., 1992; Sugiura and Hoshino, 2003) and these angrites (Nyquist 8 et al., 1994; Lugmair and Shukolyukov, 1998) B7 7 are approximately coincident within 5Myr and, therefore, this assumption may be valid at

Number 6 this level of uncertainty. 5 Assuming then that the IIIAB magmatic irons attained closure of the Re–Os system at 4 4,558 Ma, most iron meteorite groups give relatively old Re–Os ages (i.e., older than 3 B4.5 Ga) (Shen et al., 1996; Smoliar et al., 2 1996; Horan et al., 1998; Cook et al., 2004). There is an apparent discrepancy, however, in 1 the Re–Os systematics reported in the IVA 0 magmatic irons. While Shen et al. (1996) report 3 3.2 3.4 3.6 3.8 4 4.2 4.4 a Re–Os age for the IVA irons that is Age in Ga 60745 Myr older than for IIAB irons, the data Figure 10 Histogram of impact-reset ages of the of Smoliar et al. (1996) give an age of eucrites, with plotted age interval of 0.1 Ga. In the 7 39 40 4,456 25 Ma, signiﬁcantly younger than other case of Ar– Ar ages, those reported with uncer- iron meteorite groups, which these authors at- tainties (‘‘precise Ar–Ar’’) are shown in black while tributed to later disturbance of the Re–Os sys- less precise ones reported without uncertainties (‘‘approximate Ar–Ar’’) are shown in gray. Rb–Sr tem. As Horan et al. (1998) have pointed out, and Pb–Pb ages o4.3 Ga are shown as the stipled while the Re–Os isotopic compositions of most areas. Reproduced by permission of Elsevier from of the IVA irons analyzed by Smoliar et al. Bogard and Garrison (2003). (1996) do lie on the 4,456725 Ma isochron, some (specifically, Duel Hill and Bushman Land) are consistent with the older age re- 1.27.3.3 Iron Meteorites and Pallasites: ported by Shen et al. (1996). It is possible that Timescales of Core Crystallization on the Re–Os isotope systematics in different IVA Planetesimals irons are variably disturbed, perhaps by processes such as breakup and reassembly of the Limits on the timescales involved in metallic parent planetesimal (a process that has been core formation and crystallization on planet- invoked to explain the range of metallographic esimals may be obtained from chronological cooling rates of the IVA irons, e.g., Haack investigations of iron-rich meteorites, such as et al., 1996b). This may have resulted in the magmatic irons (that represent the cores of dif- resetting of the Re–Os system in some IVA ferentiated asteroidal bodies) and pallasites (con- irons but not in others. sidered to have formed near the core–mantle As noted earlier, Re–Os ages reported so boundary). Once the metal has segregated into far use a 187Re decay constant that was deter- the core of a planetesimal, the timescales mined by assuming that the IIIAB isochron involved in the crystallization of this metal may should give the same age as the U–Pb age of the be constrained by isochrons based on bulk sam- angrites, so the accuracy of these ages is only as ples and mineral phases of magmatic iron me- good as the validity of this assumption. Never- teorites and pallasites. In recent years, theless, the age range indicated by Re–Os precise Re–Os isochrons have been obtained isochrons is largely independent of the precise for various groups of the iron meteorites (Figure value of half-life, so the results for various 11). Nevertheless, one of the main problems af- magmatic iron meteorite groups suggest that fecting the ability to obtain accurate and precise core crystallization (or more specifically, ages based on such isochrons has been the large Re–Os isotopic closure) in planetesimals (B73%) uncertainty in the measured decay spanned a period of B30 Myr (although the constant of 187Re (Lindner et al., 1989). Recent relatively large errors certainly allow this time Re–Os studies (e.g., Smoliar et al., 1996; Horan interval to be signiﬁcantly narrower than this). et al., 1998; Chen et al., 2002) have assumed a This time interval for core crystallization (the more precise value for the 187Re decay constant process that is most likely to have produced Differentiated Meteorites 15

0.4 0.5 0.6 0.7 0.4 0.6 0.8 1.0 0.15 0.18 IIIA Irons IIA Irons

0.14 0.16

4 4

2 2 0.13 0.14 0 0 − − Os 2 2

188 −4 0.12 −4

Os/ 0.4 0.5 0.6 0.7 0.4 0.6 0.8 1.0

187 0.4 0.5 0.6 0.7 0.30 0.35 0.40 0.15 IVA Irons 0.126 IVB Irons

0.14 0.124 8 4 4 2 0 0.122 0 0.13 −4 −2 −8 0.120 −4 0.4 0.5 0.6 0.7 0.30 0.35 0.40 (a) 187Re/188Os

187Re/188Os 0.3 0.4 0.5 0.6

0.140 IA isochron: T = 4,537 ± 21 Ma 0.136 Io = 0.09556 ± 16 MSWD = 1.0

0.132 IA irons - isochron set Other IAB-IIICD irons 0.128 IA isochron

0.124 +2

187Os 0 188 IIA isochron Os −2

−4 Sh CD Se

0.3 0.4 0.5 0.6 (b) 187Re/188Os Figure 11 187Re–187Os isochrons for meteorites from (a) the magmatic IIIA (4,557712 Ma), IIA (4,53778Ma), IVA (4,456726 Ma; open diamonds indicate three samples that were omitted from the regression), and IVB (4,527729 Ma) groups and (b) the nonmagmatic IAB-IIICD (4,537721 Ma) groups (labeled data points are: CD—Canyon Diablo, Se—Seela¨sgen, and Sh—Shrewsbury). Ages are calculated assuming a decay constant of 1.666 1011 yr1. Insets show the deviation in parts per 104 (i.e., in e units) of the data points from the best-ﬁt line; these deviations are calculated relative to the IIA isochron (shown as the horizontal line in each of the insets). (a) Reproduced by permission of American Association for the Advancement of Science from Smoliar et al. (1996). (b) Reproduced by permission of Elsevier from Horan et al. (1998).

Re–Os fractionation among the different mem- inferred (predominantly from the short-lived bers of a particular group of the magmatic 182Hf–182W chronometer) to have occurred irons) is distinct from that for core formation over a relatively short time period of only a (or metal-silicate segregation) on the iron few Myr after CAI formation (e.g., Markowski meteorite parent bodies. The latter has been et al., 2006). 16 Long-Lived Chronometers

0.16 Pallasites Glorieta Mtn Slope = 0.0775 ± 0.0008 −11 −1 0.15 T = 4.50 ± 0.04 Ga ( = 1.66 x 10 a ) I = 0.09599 ± 0.00046 Brenham Springwater 0.14 Chen et al. (2002) Os Newport Shen et al. (1998) 188 Os/

187 10 S Eagle station Thiel Mtns 0.13 ES B Otinapa 0 PAL TM − GM 10 M F 0.12 Marjalahti −20 Finmarken 0.3 0.4 0.5 0.6 0.7 0.8

0.4 0.6 0.8 187Re/188Os Figure 12 187Re–187Os isochron diagram for pallasite meteorites. Inset shows deviations from the best-ﬁt line 187 188 (dPAL) versus Re/ Os ratios. Reproduced by permission of Elsevier from Chen et al. (2002).

Re–Os systematics in pallasites indicate that and Mars. More detailed discussions on they may be younger than iron meteorites by these topics may be found in the Chapters B60 Myr (Figure 12). However, this appar- 1.21 and 1.22 in this volume. ently young age may be due to later reequili- bration of the Re–Os system (Chen et al., 2002). Cook et al. (2004) recently reported the 1.27.4.1 Timing of Lunar Differentiation and first high-precision 190Pt–186Os isochrons for Cataclysm from Chronology of Lunar the IIAB and IIIAB magmatic irons, and esti- Samples mated ages for these meteorite groups of 1.27.4.1.1 Lunar differentiation history 4,323780 Ma and 4,325726 Ma, respectively. These ages are somewhat younger than Re–Os Lunar samples returned by the Apollo ages for iron meteorites, and these authors and Luna missions as well as the lunar mete- suggested that this discrepancy could reflect an orites broadly fall within two compositional error in the decay constant for 190Pt. categories: mare basalts and feldspathic (highlands) rocks. The mare basalts are a geochemically diverse group and are comprised of 1.27.4 PLANETARY MATERIALS high-titanium (9–14 wt.% TiO2), low-titanium (1–5 wt.% TiO2), and very low-titanium Besides the Earth, the Moon and Mars are (o1 wt.% TiO2) basaltic samples. The feld- the only other planetary bodies from which spathic rocks are also geochemically and there are samples currently available for petrologically diverse and are composed of a chronological investigations. In the case variety of pristine igneous rocks as well as poly- of the Moon, there are the samples that were mict breccias. Among the pristine igneous returned by the Apollo and Luna missions, as samples are the ferroan anorthosites and well as B40 distinct meteorites of basaltic and magnesium-rich rocks. The latter are composed feldspathic compositions that are thought to of subgroups of magnesian-suite, alkali-suite, have originated from a variety of terrains on granites/felsites, and KREEP basalt and quartz the Moon (Chapter 1.21). In the case of Mars, monzodiorite rocks. there are currently about three dozen or so Among known lunar samples, the ones that distinct meteorites of mafic and ultramafic have yielded the oldest crystallization ages are compositions thought to have originated from the ferroan anorthosites, which are thus in- this planet (Chapter 1.22). The following ferred to be remnants of the earliest-formed provides a brief summary of the chronology lunar crust (Hanan and Tilton, 1987; Carlson of these samples and the inferred differentia- and Lugmair, 1988; Borg et al., 1999a; Alibert tion and evolutionary histories of the Moon et al., 1994; Norman et al., 2003). These Planetary Materials 17 ages have been determined mostly using the earlier have been used to estimate a minimum Sm–Nd chronometer (in one case, with the age of B4.3 to B4.5 Ga for this event (e.g., U–Pb chronometer), and span a time interval Carlson and Lugmair, 1988; Borg et al., 1999a; from B4.29 Ga70.06 (Borg et al., 1999a)to Norman et al., 2003). Estimates for the forma- B4.56 Ga70.07 (Alibert et al., 1994). Norman tion of the KREEP component in the lunar et al. (2003) have argued that plagioclase in mantle (thought to represent the residuum these anorthosites may have been subject to from Moon-wide differentiation; e.g., Wood, later disturbance by impact metamorphism and 1972; Warren and Wasson, 1979) also place if only the mafic minerals are considered, these limits on the timing of this event. This was es- yield an Sm–Nd age for four ferroan anortho- timated to be at B4.6 Ga from the Rb–Sr sites of 4.4670.04 Ga. This is then likely to be model age of lunar soils (Papanastassiou et al., the best estimate of the crystallization age of 1970), B4.42 Ga from U–Pb systematics (Tera the oldest lunar crustal samples. and Wasserburg, 1974), and B4.36 Ga from The crystallization ages of the magnesium- Sm–Nd model ages of KREEP samples (Lug- rich highlands rocks (determined predomi- mair and Carlson, 1978). Nyquist and Shih nantly with Sm–Nd, but also with the Rb–Sr (1992) estimated an average value from vari- and U–Pb chronometers) are also generally an- ous Rb–Sr model ages for KREEP to be cient. Among these, the oldest (B4.1–4.5 Ga) 4.4270.14 Ga. They suggested this as the best are the magnesian-suite norites, troctolites, and value for the timing of lunar global differenti- dunites, some of which may have formed con- ation, with the uncertainty reflecting the temporaneously with the ferroan anorthosites. possibility that this event did not occur at a These were followed by the alkali-suite (B4.0– sharply defined time and that some Rb–Sr 4.3 Ga), granites/felsites (B3.8–4.3), KREEP fractionation may have occurred during the basalts (B3.8–4.0 Ga), and quartz monzodio- petrogenesis of KREEP basalts. Finally, com- rite rocks (B4.3 Ga) (see Nyquist and Shih, bined 147Sm–143Nd and 146Sm–142Nd systemat- 1992; Papike et al., 1998; Snyder et al., 2000; ics for lunar basaltic samples indicate that the and references therein). sources of these basalts were established in the The formation ages of the mare basalts and lunar mantle B200 Myr after the beginning of lunar pyroclastic glasses based on the Sm–Nd, the solar system (Nyquist et al., 1995b; Rb–Sr, and U–Pb chronometers are summa- Rankenburg et al., 2006), a timescale that is rized in table 4 of Chapter 1.21. Among the consistent with the others discussed above. oldest dated basaltic lunar material are the mare-like clasts in highland breccias from the Apollo 14 landing site that have ages as old 1.27.4.1.2 Lunar bombardment history as B4.2 Ga (Taylor et al., 1983). Most mare volcanism, however, postdated the period of The flux of impactors on the Moon as a heavy bombardment at B3.9 Ga (see discussion function of time is a topic that is highly debated of impact ages of lunar samples in the following and is of great interest since it has implications section). Thus far, one of the youngest mare for the bombardment history of the Earth, basalts to be dated is the KREEP-rich basaltic which may in turn have played a role in the lunar meteorite NWA 773 that has 39Ar–40Ar evolution of the Earth’s atmosphere and in the and Sm–Nd ages of B2.9 Ga (Fernandes et al., initiation and evolution of life on this planet. A 2003; Borg et al., 2004). Fernandes et al. (2003) distinct spike in the bombardment history of reported a similarly young 39Ar–40Ar age of the Moon or a ‘‘lunar cataclysm’’ at B3.9 Ga B2.8 Ga for another lunar meteorite, NWA was first explicitly hypothesized on the basis of 032, an unbrecciated mare basalt, and suggested 39Ar–40Ar, U–Pb, and Rb–Sr systematics in that this also represented the time of crystalli- rocks from the Apollo 15, 16, and 17 landing zation for this basalt. Figure 16 of Chapter 1.21 sites that appeared to have been reset or dis- summarizes the crystallization ages of the turbed at this time (Tera et al., 1974). Subse- variety of lunar samples discussed here. quent studies of 39Ar–40Ar ages of the Apollo The Moon is thought to have undergone and Luna impact melt rocks further demon- major global differentiation early in its history strated the lack of ages significantly older than that resulted in the formation of the earliest B4.0 Ga (e.g., Dalrymple and Ryder, 1991, crust (represented by the feldspathic highlands 1993, 1996; Swindle et al., 1991), thus support- rocks) as well as the mantle source reservoirs of ing this hypothesis. However, since the Apollo the basaltic lunar samples. The timing of this and Luna samples represent only a very small lunar global differentiation has been inferred proportion of the lunar crust, they may be from a variety of methods. The crystallization dominated by the effects of a few large impacts. ages of the feldspathic highlands rocks, par- Lunar meteorites provide a potential source of ticularly the ferroan anorthosites, discussed a more random, possibly less biased, selection 18 Long-Lived Chronometers of material from the Moon. Recent 39Ar–40Ar internal isochron for the ALH84001 or- investigations of impact melt clasts from thopyroxenite (Jagoutz et al., 1994; Nyquist several lunar meteorites have yielded ages that et al., 1995a). All other dated martian meteor- are typically o3.9 Ga (Fernandes et al., 2000; ites have internal isochron ages (based mostly Cohen et al., 2000, 2005; Gnos et al., 2004). on the Sm–Nd and Rb–Sr chronometers) that Although these studies support the cataclysm are younger than B1.3 Ga, with the youngest hypothesis in that there are no ages older than samples being only B170 Ma. Based on their B4.0 Ga, they also indicate that, rather than geochemical features (particularly the trace- dropping sharply to a nearly constant rate (as and minor-element zonations in their minerals; had been suggested by some studies of the e.g., Jones, 1986), these ages are generally in- Apollo samples; e.g., Guinness and Arvidson, terpreted to reflect the crystallization ages of 1977; Bogard et al., 1994), the impact flux was these samples, thus indicating that Mars may declining gradually in the B3 or so billion still be a geologically active planet. A recent years following the enhanced period of bom- study has suggested that the young ages of the bardment at B3.9 Ga. A recent 39Ar–40Ar shergottites may in fact reflect the timing of study of glass spherules from the Apollo 14 secondary alteration, and that their formation soils also suggests a gradual decline in the im- ages are close to B4.0 Ga (Bouvier et al., 2005). pact flux (by a factor of B2–3) between B3.5 This interpretation requires that the late-stage and B0.5 Ga (Culler et al., 2000), followed by a minerals (i.e., phosphates) in these shergottites marked enhancement in the impact flux (by a be affected by this secondary alteration event. factor of B3–5) within the last B400–500 Myr. However, studies of trace-element microdistri- Another similar study of glass spherules from butions have shown that these phosphates were Apollo 12 soils (Levine et al., 2005) was con- formed by closed system fractional crystalliza- sistent with, but did not require a recent in- tion of the shergottite parent melts and that crease in the bombardment rate. However, this they preserve their original igneous composi- interpretation has been questioned (Horz, tions (Wadhwa et al., 1994). As such, at the 2000), and remains one of the controversial is- present time, the interpretation of the Sm–Nd sues related to the lunar bombardment history and Rb–Sr internal isochrons ages of the that has yet to be resolved. A more thorough shergottites as their crystallization ages is the discussion of the impact ages of a variety of most likely. Therefore, as discussed by Borg lunar samples obtained with various chrono- and Drake (2005) and summarized in Figure meters based on long-lived radionuclides (par- 13, there is evidence from the martian meteor- ticularly the 39Ar–40Ar method), including the ites for igneous events on Mars at 4,3007130, implications for the bombardment history of 1,327729, 57577, 474711, 33279, and the Moon, has been provided in Chapter 1.21. 17472 Ma. Besides primary igneous events, the martian meteorites also provide evidence for secondary 39 40 1.27.4.2 Timescales for the Evolution of Mars events on Mars. A Ar– Ar age in the range of from Chronology of Martian B3.8–4.1 Ga for the ALH84001 orthopyroxenite Meteorites has been interpreted as possibly reflecting the time of heavy bombardment on Mars co- The martian meteorites represent a variety of incident with a similar event on the Moon (Ash volcanic to subvolcanic as well as plutonic et al., 1996; Turner et al., 1997). Moreover, car- igneous rocks that are broadly categorized, bonates in this rock, thought to be precipitated as based on their petrologic and geochemical a result of interaction with an aqueous fluid in a characteristics, into four groups: the basaltic near-surface environment, have also been dated and lherzolitic shergottites, the clinopyroxenitic at B3.9 Ga based on Rb–Sr and U–Pb system- nakhlites, the dunitic chassignites, and the atics (Borg et al., 1999b). In addition to the car- orthopyroxenite ALH84001 (Chapter 1.22). bonates in ALH84001, there are other products Chronological investigations of these martian of near-surface aqueous alteration in the other meteorites and their implications for the martian meteorites, in particular, a hydrous min- evolution of Mars have been thoroughly re- eral (iddingsite) in the nakhlites (e.g., Bunch and viewed by Nyquist et al. (2001) and Borg and Reid, 1975; Treiman et al., 1993) and a variety of Drake (2005). As such, only a brief summary is secondary minerals (including carbonates, sul- presented here. fates, and clays) in the shergottites (e.g., Gooding Figure 13 summarizes the absolute ages for et al., 1988, 1990).BasedonRb–SrandK–Ar various events in martian history based on the systematics in iddingsite-rich fractions (Shih radiometric dating of the martian meteorites et al., 1998, 2002; Swindle et al., 2000), it has and their components. The oldest formation been argued that this hydrous secondary phase age of B4.5 Ga is yielded by a 147Sm–143Nd was formed at B600–700 Ma. The only age Conclusions 19

Salts shergottites (0−175 Ma) Iddingsite nakhlites (633 ± 23 Ma) Carbonates ALH84001 (3,929 ± 37 Ma)

Shergotty (165 ± 11 Ma) Zagami (169 ± 7 Ma) LA1 (170 ± 7 Ma) NWA856 (170 ± 19 Ma) EET79001A (173 ± 10 Ma) 174 ± 2 Ma Y793605 (173 ± 14 Ma) EET79001B (173 ± 3 Ma) ALH77005 (177 ± 6 Ma) LEW88516 (178 ± 9 Ma) NWA1056 (185 ± 11 Ma) Y980459 (290 ± 40 Ma) QUE94201 (327 ± 10 Ma) 332 ± 9 Ma NWA1195 (348 ± 19 Ma) DaG 476 (474 ± 11 Ma) Dhofar 019 (575 ± 7 Ma) Nakhla (1,260 ± 70 Ma) NWA998 (1,290 ± 50 Ma) Y000593 (1,310 ± 30 Ma) 1,327 ± 39 Ma Lafayette (1,320 ± 50 Ma) Chassigny (1,362 ± 62) Gov. Valad. (1,370 ± 20 Ma) ALH84001 (4,500 ± 130 Ma) Silicate differentiation (4,526 ± 21 Ma) Core segregation (4,556 ± 1 Ma) LEW86010; silicate differentiation reference (4,558 ± 0.5 Ma) CAI (solar system formation reference) (4,567 ± 0.6 Ma)

0 1,000 2,000 3,000 4,000 4,657 Age (Ma) Figure 13 Absolute ages of events in Mars’ history based on radiometric dating of martian meteorites and their components. Crystallization ages of the shergottites (B170–575 Ma; filled circles, open squares, filled triangle, and open diamond), the nakhlites (B1.3 Ga; open circles), and the orthopyroxenite ALH84001 (B4.5 Ga) are shown. The ages of aqueous alteration events recorded by secondary products in the martian meteorites are shown as the open triangles. Also shown are the ages of global differentiation events (core formation and silicate differentiation) based on the 147,146Sm–143,142Nd and 182Hf–182W systematics of the martian meteorites. Reproduced with permission from Borg and Drake (2005). constraint on the secondary alteration products 147Sm–143Nd and 146Sm–142Nd systematics), a in the shergottites may be obtained from consid- more precise estimate of 4,525720 Ma for the eration of the crystallization ages of the partic- timing of major silicate differentiation on Mars ular samples (B175 Ma) in which these are that established the source reservoir found; as such, it is estimated that these second- of the shergottites has been made (Borg ary minerals were formed at some time et al., 2003; Foley et al., 2005). The short-lived o175 Ma. Based on the abundances of altera- 146Sm–142Nd and 182Hf–182W chronometers tion products in the martian meteorites and their further indicate that the nakhlite mantle source discrete formation time intervals, Borg and may have been established contemporaneously Drake (2005) argued that aqueous fluids were with that of the shergottites, or it may have pre- present episodically, and not continuously, in the dated this event by a few tens of millions of near-surface environment on Mars. years (Foley et al., 2005). Finally, whole-rock Rb–Sr and U–Pb systematics of the martian meteorites indicate that global silicate differentiation on Mars occurred 1.27.5 CONCLUSIONS close to B4.5 Ga (Shih et al., 1982; Chen and Wasserburg, 1986). Using an approach similar 1.27.5.1 A Timeline for Solar System Events to that applied by Nyquist et al. (1995b) to From the application of various chronome- the lunar basaltic samples (i.e., combined ters based on the long-lived radionuclides to 20 Long-Lived Chronometers meteoritic and planetary materials, the follow- solidification within B10 Myr of CAI for- ing may be inferred as the sequence of events mation, but this process on other differen- that occurred in the history of the solar system: tiated parent bodies may have extended for another tens of millions of years. 1. The earliest solids to form in the solar 8. Global silicate differentiation on the Moon protoplanetary disk were the refractory and Mars and establishment of mantle CAIs, which formed at 4,567.170.2 Ma. source reservoirs for lunar and martian This represents the minimum age of the basalts occurred B200 Myr and within solar system. B50 Myr after the beginning of the solar 2. Chondrules from CV and CR chondrites system, respectively. began forming more or less contempora- 9. The earliest crustal (highlands) rocks on neously with CAIs (4,566.771.0 Ma for the Moon was formed at B4.5 Ga. The Allende chondrules), and continued to oldest basaltic lunar materials have ages form for at least another 2–3 Myr. of B4.2 Ga, although most mare basalts 3. Accretion and differentiation of planet- crystallized after B3.9. Mare volcanism esimals also began almost contemporane- continued at least till B2.9 Ga. ously with CAIs, with basalts forming on 10. Crystallization ages of martian meteorites the surfaces of these bodies well within range from B4.5 Ga for the ALH84001 B1 Myr of CAI formation. The accretion orthopyroxenite to B170 Ma for some process for planetesimals is also likely to shergottites, suggesting that magmatic ac- have continued for a few million years tivity on Mars began early and may still (with at least some chondrite parent bodies continue. Aqueous alteration events re- possibly being assembled a few million corded in the martian meteorites are esti- years after CAIs). mated to be have occurred at B3.9 Ga, 4. Energetic collisions between the accreted 600–700 Ma, and o175 Ma. These discrete planetary embryos resulted in the forma- formation times suggest that aqueous fluids tion of impact-generated chondrules sev- were present episodically, and not contin- eral millions of years after CAI formation. uously, in the near-surface environment on Specifically, chondrules from the CB Mars. chondrites, which are thought to result 11. Members belonging to several groups of from such a process, were formed B5 Myr meteorites (eucrites and chondrites) and after CAIs. planetary materials (lunar samples and the 5. Thermal processing of accreted undifferen- martian meteorite ALH84001) record im- tiated planetesimals began early (e.g., at pact ages that are consistent with a period B4,563 Ma for the parent body of the Ste. of heavy bombardment in the inner solar Marguerite H4 chondrite), but this process system that peaked close to B4 Ga. Peaks continued for tens of millions of years after in the impact ages at o1.3 Ga for the the beginning of the solar system. Meta- chondrites and close to B4.5 Ga for the morphism on chondrite parent bodies eucrites may point toward a few large im- is likely to have extended for at least pact events that occurred on their parent B60 Myr after CAI formation. Aqueous bodies. alteration of the CI chondrite parent body is likely to have begun almost as soon as it was accreted. 1.27.5.2 Outlook and Future Prospects 6. Basaltic melts continued to be generated on differentiated planetesimals for a period of Among the long-lived chronometers, the B10 Myr after solar system formation. In one based on the 235,238U–207,206Pb systems the case of the EPB, these basalts were then provides the best precision. With extensive lea- subjected to a complex and protracted (las- ching procedures to effectively remove the com- ting tens of millions of years) history of mon lead component and use of a double spike thermal processing, most likely resulting to improve the analytical precision, it is now from large impacts on the surface of the possible to attain a precision better than EPB. Based on the ages of the cumulate 70.5 Myr on 207Pb–206Pb ages for radiogenic eucrites, it is inferred that either igneous samples. As such, more future studies of early activity may have lasted for B100– solar system chronology are likely to focus on 150 Myr on the EPB or isotopic systems the utilization of this chronometer as the abso- in these samples record slow cooling at lute anchor for other chronometers based on depth in the EPB. short-lived radionuclides, which also have the 7. The iron-nickel cores of some differentiated potential for providing sub-Myr time resolution planetesimals began crystallization and for events occurring in the earliest history of the References 21 solar system. Nevertheless, there are numerous Anders E. and Grevesse N. (1989) Abundances of the assumptions and complexities involved in the elements: meteoritic and solar. Geochim. Cosmochim. Acta 53, 197–214. utilization of chronometers based on the short- Ash R. D., Knott S. F., and Turner G. 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