Radioisotope Dating of Meteorites: III. The Eucrites (Basaltic Achondrites)

Keywords: meteorites, classification, achondrites, eucrites, asteroids, 4-Vesta, radioisotope dating, Bereba, Cachari, Caldera, Camel Donga, Ibitira, Juvinas, Moama, Moore County, Pasamonte, Serra de Magé, Stannern, Yamato 75011, K-Ar, Ar-Ar, Rb-Sr, Sm-Nd, U-Th-Pb, Lu-Hf, Mn-Cr, Hf-W, Al-Mg, I-Xe, Pu-Xe, isochron ages, model ages, discordant radioisotope ages, accelerated radioactive decay, thermal disturbance, resetting, “primordial material,” geochemical signature, mixing, inheritance

Introduction

In 1956 Claire Patterson at the California Institute of Technology in Pasadena reported a Pb-Pb isochron age of 4.55 ± 0.07 Ga for three stony and two iron meteorites, which since then has been declared the age of the earth (Patterson 1956). Adding weight to that claim is the fact that many meteorites appear to consistently date to around the same “age” (Dalrymple 1991, 2004), thus bolstering the evolutionary community’s confidence that they have successfully dated the age of the earth and the solar system at around 4.56 Ga. These apparent successes have also strengthened their case for the supposed reliability of the increasingly sophisticated radioisotope dating methods.

Creationists have commented little on the radioisotope dating of meteorites, apart from acknowledging the use of Patterson’s geochron to establish the age of the earth, and that many meteorites give a similar old age. Morris (2007) did focus on the Allende carbonaceous chondrite as an example of a well-studied meteorite analyzed by many radioisotope dating methods, but he only discussed the radioisotope dating results from one, older paper (Tatsumoto, Unruh, and Desborough 1976).

In order to rectify this lack of engagement by the creationist community with the meteorite radioisotope dating data, Snelling (2014a) obtained as much radioisotope dating data as possible for the Allende CV3 carbonaceous chondrite meteorite (due to its claimed status as the most studied meteorite), displayed the data, and attempted to analyze them. He found that both isochron and model ages for the total rock, separated components, or combinations of these strongly clustered around a Pb-Pb age of 4.56–4.57 Ga, the earliest (Tatsumoto, Unruh, and Desborough 1976) and the latest (Amelin et al. 2010) determined Pb-Pb isochron ages at 4.553 ± 0.004 Ga and 4.56718 ± 0.0002 Ga respectively being essentially the same. Apart from scatter of the U-Pb, Th-Pb, Rb-Sr, and Ar-Ar ages, no systematic pattern was found in the Allende isochron and model ages similar to the systematic pattern of isochron ages found in Precambrian rock units during the RATE project that was interpreted as produced by an episode of past accelerated radioisotope decay (Snelling 2005c; Vardiman, Snelling, and Chaffin 2005).

Snelling (2014b) subsequently gathered together all the radioisotope ages obtained for 10 ordinary (H, L, and LL) and five enstatite (E) chondrites and similarly displayed the data. They generally clustered, strongly in the Richardton (H5), St. Marguerite (H4), Bardwell (L5), Bjurbole (L4), and St. Séverin (LL6) ordinary chondrite meteorites, at 4.55–4.57 Ga, dominated by Pb-Pb and U-Pb isochron and model ages, but confirmed by Ar-Ar, Rb-Sr, Re-Os, and Sm-Nd isochron ages. There was also scatter of the U-Pb, Th-Pb, Rb-Sr, and Ar-Ar model ages, in some cases possibly due to thermal disturbance. Again, no pattern was found in these meteorites’ isochron ages indicative of past accelerated radioisotope decay.

Snelling (2014a, b) then sought to discuss the possible significance of this clustering in terms of various potential creationist models for the history of radioisotopes and their decay. He favored the idea that asteroids and the meteorites derived from them are “primordial material” left over from the formation of the solar system, which is compatible with the Hebrew text of Genesis that could suggest God made “primordial material” on Day One of the Creation Week, from which He made the non-earth portion of the solar system on Day Four. Thus he argued that today’s measured radioisotope compositions of all these chondrites may reflect a geochemical signature of that “primordial material,” which included atoms of all elemental isotopes. So if some of the daughter isotopes were already in these chondrites when they were formed, then the 4.55–4.57 Ga “ages” for them obtained by Pb-Pb and U-Pb isochron and model age dating are likely not their true real-time ages, which according to the biblical paradigm is only about 6000 real-time years.

However, Snelling (2014a, b) admitted that drawing firm conclusions from the radioisotope dating data for just these 16 chondrite meteorites was premature, and recommended further studies of more meteorites. This present contribution is therefore designed to further document the radioisotope dating data for more meteorites, the basaltic achondrites or eucrites, so as to continue the discussion of the potential significance of these data.

The Classification of Achondrite Meteorites

The most recent classification scheme for the meteorites is that of Weisberg, McCoy, and Krot (2006), which is reproduced in Fig. 1. Based on their bulk compositions and textures, Krot et al. (2005) divided meteorites into two major categories, chondrites (meteorites containing chondrules) and achondrites (meteorites not containing chondrules or non-chondritic meteorites). They further subdivided the achondrites into primitive achondrites and igneously differentiated achondrites. However, Weisberg, McCoy, and Krot (2006) simply subdivided all meteorites into three categories—chondrites, primitive achondrites and achondrites (fig. 1).

The non-chondritic meteorites contain virtually none of the components found in chondrites. It is conventionally claimed that they were derived from chondritic materials by planetary melting, and that fractionation caused their bulk compositions to deviate to various degrees from chondritic materials (Krot et al. 2005). The degrees of melting that these rocks experienced are highly variable, and thus, these meteorites have been divided into the two major categories—primitive and differentiated. However, there is no clear cut boundary between these categories.

The differentiated non-chondritic meteorites, or achondrites (fig. 1), are conventionally regarded as having been derived from parent bodies that experienced large-scale partial melting, isotopic homogenization (ureilites are the only exception), and subsequent differentiation. Based on abundance of FeNi-metal, these meteorites are commonly divided into three types—achondrites, stony-irons, and irons. Each of these types contains several meteorite groups and ungrouped members (fig. 1). Several groups of achondrites and iron meteorites are likely to be genetically related and were possibly derived from single asteroids or planetary bodies.

The achondrites account for about 8% of meteorites overall, and the majority of them (about two-thirds) are HED meteorites (howardites, eucrites, and diogenites), believed to have originated from the crust of asteroid 4-Vesta (Norton 2002) (fig. 1). Other types include martian, lunar, and several types thought to originate from as-yet unidentified asteroids. These groups have been determined on the basis of, for example, their bulk Fe/Mn and ¹⁷O/¹⁸O ratios, which are thought to be characteristic “fingerprints” for each parent body (Mittlefehldt et al. 1998).

The achondrites represent the products of classical igneous processes acting on the silicate-oxide system of asteroidal bodies—partial to complete melting, differentiation, and magmatic crystallization (Mittlefehldt 2005). Iron meteorites represent the complimentary metal-sulfide system products of this process. Thus the achondrites consist of materials similar to terrestrial basalts and plutonic rocks, so they exhibit igneous textures, or igneous textures modified by impact and/or thermal metamorphism, and distinctive mineralogies indicative of igneous processes.

The HED meteorites are sometimes grouped with the angrites and aubrites (fig. 1) and termed the asteroidal achondrites, because of all having been differentiated on parent asteroidal bodies. The howardite-eucrite-diogenite (HED) meteorites have been traditionally classified into the one clan, because there is strong evidence they originated on the same parent body, the asteroid 4-Vesta (Binzel and Xu 1993; Consolmagno and Drake 1977; Drake 2001; Mandler and Elkins-Tanton 2013; McCord, Adams, and Johnson 1970; McSween et al. 2011; McSween et al. 2013, 2014; Righter and Drake 1997). This was one of the first links made between meteorites and an asteroid (Cloutis, Binzel, and Gaffey 2014; McCord, Adams, and Johnson 1970). Initially their spectroscopic similarity, which suggested the HED achondrites are impact ejecta off 4-Vesta, was deemed dynamically dubious owing to the apparent lack of a plausible pathway from Vesta to the earth. The discovery of the Vesta family of asteroids or “Vestoids” (Binzel and Xu 1993) extending from Vesta to resonance delivery zones solidified the link. This link has stood the test of time and has been confirmed by the in situ results provided by the Dawn mission to this asteroid (McSween et al. 2014). Thus the HED clan allows the confident association of specific types of igneous processes with an asteroid body of known size.

Fig. 1. The classification system for meteorites (after Weisberg, McCoy, and Krot 2006). (Click image for larger view.)

The HED clan is the most extensive suite of differentiated crustal rocks from an asteroid (Mittlefehldt 2005). Evidence that these achondrites belong in the same clan includes their identical oxygen isotopic compositions (Clayton and Mayeda 1996), similarities in Fe/Mn ratios in pyroxenes, the occurrence of polymict breccias consisting of materials of eucritic and diogenitic parentage (for example, the howardites), and the existence of rocks intermediate between diogenites and cumulate eucrites (Krot et al. 2005). The suite of meteorites comprising the HED clan is composed of mafic and ultramafic igneous rocks, most of which are breccias. The parent lithologies were mostly metamorphosed, which has obscured original igneous zoning in most cases (Mittlefehldt 2005). The suite contains four main igneous lithologies—basalt and cumulate gabbro (eucrites), and orthopyroxenite and harzburgite (diogenites). When both eucrite and diogenite clasts are present in a meteorite that is a polymict breccia, then it is a howardite. These lithologies are consistent with a postulated layered crust model for the HED parent body, 4-Vesta (Mandler and Elkins-Tanton 2013; McSween et al. 2013, 2014; Righter and Drake 1997; Takeda 1997).

The Eucrites

The eucrites are the most common of the achondrites. About 3% of the witnessed falls of all meteorite types are eucrites, which makes them the fourth most common meteorite to fall (Norton 2002). Of the HED meteorites, eucrites are by far the most common, about 52%. Until the meteorite finds in Antarctica became available with their large cache of eucrites, eucrites were defined as monomict breccias. However, the large number of eucrites recovered that show a wide variation of lithic fragments, unlike the fragments in howardites, has prompted the acceptance of eucrites as either monomict or polymict.

The most obvious external characteristic of a freshly fallen eucrite is its very black and lustrous fusion crust compared to the dull black crust of a chondrite, due to the intense heating of the outer surface during passage through the earth’s atmosphere (Norton 2002). Eucrites are Ca-rich and this combined with the usually present small amount of Fe gives these meteorites a “wet” look (fig. 2). Fusion crusts form in the final second or two of the ablation process as meteorites pass rapidly through the earth’s atmosphere during the fireball stage. The fusion crusts then rapidly cool, so contraction cracks often form, leaving the outer surface of the meteorites looking much like the crazing on pottery (figs. 2 and 3).

Fig. 2. The shiny black crust on this eucrite from Camel Donga, Western Australia, is typical of calcium-rich eucrites. Note the contraction cracks through the crust. The specimen measures 5 cm (about 2 in) in its longest dimension (after Norton 2002). (Click image for larger view.)

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Fig. 3. Contraction cracks in the crust of the Pasamonte eucrite. The specimen is about 6.2 cm (about 2.5 in) long (after Norton 2002). (Click image for larger view.)

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Fig. 4. This unbrecciated eucrite called Ibitira fell in the village of that name near Martinho Campos, Minas Gerais, Brazil, in 1957. It is the only eucrite known to have a vesicular texture. The millimeter-sized gas holes cover 5–7 vol. % of the rock. The scale rule is in cm so the specimen is about 10 cm (about 4 in) wide (after Norton 2002). (Click image for larger view.)

The similarities of eucrites chemically and petrographically to terrestrial basalts is frequently noted, but a broken face of a eucrite exposes a light gray interior, which is unlike the dark gray to black interiors of terrestrial basalts. Eucrite textures are also fine-grained, and often glomeroporphyritic due to clumps of phenocrysts set in the groundmass. This is typical of terrestrial volcanic rocks that have cooled more slowly, producing glomerocrysts of interlocking plagioclase and pyroxene crystals. If basaltic lava contains dissolved gases when it suddenly erupts onto the earth’s surface, the sudden reduction in pressure releases the gases which quickly form bubbles that make their way to the top of the flow. The eucrite, Ibitira, one of the few unbrecciated eucrites known, shows a remarkable vesicular texture (fig. 4), similar to that seen in a terrestrial basalt lava flow as that just described. Microscopically, the resemblance of most eucrites to terrestrial basalts is also most striking (fig. 5a and b).

Fig. 5. Thin section photomicrographs in transmitted light under crossed polars of four typical eucrites (basaltic achondrites) (after Krot et al. 2005; McSween et al. 2011).

(a) The unequilibrated noncumulate eucrite Pasamonte, showing the typical basaltic texture of plagioclase (light) and pyroxene (colored). (Click image for larger view.)

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(b) The metamorphosed (equilibrated) noncumulate eucrite Ibitira, showing a recrystallized texture with plagioclase (white) and pyroxene (colored), with the round, dark areas in the center, bottom, and left being vesicles. (Click image for larger view.)

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(c) The cumulate eucrite Serra de Magé, consisting of large crystals of plagioclase (lighter material with straight twin lamellae) and mostly orthopyroxene with complex augite exsolution lamellae (darker material with irregular, sometimes worm-like exsolution lamellae). (Click image for larger view.)

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(d) The cumulate eucrite Moore County, consisting of large crystals of plagioclase (lighter material with straight twin lamellae) and colorful abundant orthopyroxene (with occasional exsolution lamellae) (scale bar is 2.5mm [0.09in]).

Mineralogically, the eucrites are quite simple. They consist almost entirely of plagioclase (30–50%) and clinopyroxene (40–60%), the clinopyroxene usually dominating by 10–20%. The plagioclase in eucrites is calcic, being primarily anorthite with some bytownite, that is, within the range An_75-95, and igneous zoning is commonly preserved. The clinopyroxene is low-Ca pigeonite, with a composition that varies widely from specimen to specimen, and even within a given specimen. A typical pyroxene composition (wollastonite-enstatite-ferrosilite) in mole percent might be Wo_1-25 En_42-48 Fs_43-52. Minor minerals include chromite (FeCr₂O₄), Fe-Ni metal, ilmenite (FeTiO₃) and troilite (FeS) as opaque minerals, orthopyroxene, and polymorphs of silica—quartz, tridymite, and cristobalite.

Eucrites are subdivided into three major subclasses—the noncumulate eucrites (basaltic eucrites), the cumulate eucrites (cumulate gabbros), and the polymict eucrites (polymict breccias of basaltic and cumulate eucrites) (Krot et al. 2005; Mittlefehldt 2005).

Noncumulate (basaltic) eucrites are mostly fragmental breccias of fine to medium grained, subophitic to ophitic basalts that are postulated to have formed originally as quickly cooled surface lava flows. They are known as unequilibrated, unmetamorphosed or least-metamorphosed, noncumulate eucrites (such as Pasamonte—see fig. 5a), and are composed of pigeonite and plagioclase, with minor silica, ilmenite, and chromite, and accessory phosphates, troilite, Fe-Ni metal, fayalitic olivine, zircon, and baddeleyite. As a result of their apparent fast cooling their pyroxenes (pigeonite of Mg# ~70–20) are zoned, and exsolution lamellae are only visible by TEM. However, most noncumulate eucrites appear to have been subsequently metamorphosed, and are thus known as metamorphosed or equilibrated noncumulate eucrites (such as Juvinas, Stannern, and Ibitira—see fig. 5b). They are highly abundant and so are also collectively referred to as the ordinary eucrites. They are unbrecciated or monomict-brecciated, metamorphosed basalts and contain homogeneous low-Ca pigeonite (Mg# ~42–30) with fine exsolution lamellae of high-Ca pyroxene. The pyroxenes were originally ferroan pigeonite (~Wo_7-15 En_29-43 Fs_48-58) which exsolved augite during metamorphism. In most eucrites, pyroxene Fe/Mg is uniform as a result of metamorphism, but original igneous zoning is preserved in very few. Plagioclase is calcic, with most in the range An_75-93, and igneous zoning is commonly preserved.

Cumulate eucrites are coarse-grained gabbros, many unbrecciated (such as Serra de Magé—see fig. 5c, and Moore County—see fig. 5d), composed of pigeonite, plagioclase, and minor chromite with silica, ilmenite, Fe-Ni metal, troilite, and phosphate as trace accessory phases. The original igneous pyroxene was pigeonite (~Wo_7-16 En_38-61 Fs_32-46) which exsolved augite and, in some, inverted to orthopyroxene. They contain orthopyroxene inverted from low-Ca clinopyroxene (Mg# ~67–58) and orthopyroxene inverted from pigeonite (Mg# ~57–45). Plagioclase is generally more calcic than that typical for basaltic eucrites, with most in the range An_91-95.

Polymict eucrites are polymict breccias consisting of fragmental and melt-matrix breccias mostly of eucritic material, but they also contain <10 vol.% of diogenitic component in the form of orthopyroxenite.

The Radioisotope Dating of the Eucrites

To thoroughly investigate the radioisotope dating of the eucrite achondrites all the relevant literature was searched. The objective was to find eucrite achondrites that have been dated by more than one radioisotope method, and a convenient place to start was Dalrymple (1991, 2004), who compiled lists of such data. The 12 eucrite achondrite meteorites that were found to have been dated multiple times by more than one radioisotope method—Bereba, Cachari, Caldera, Camel Donga (fig. 2), Ibitira (figs. 4 and 5b), Juvinas, Moama, Moore County (fig. 5d), Pasamonte (figs. 3 and 5a), Serra de Magé (fig. 5c), Stannern, and Yamato 75011—thus became the focus of this study. When papers containing radioisotope dating results for these eucrites were found, the reference lists were also scanned to find further relevant papers. In this way a comprehensive set of papers, articles, and abstracts on radioisotope dating of these basaltic achondrite meteorites was collected. While it cannot be claimed that all the papers, articles, and abstracts which have ever been published containing radioisotope dating results for these eucrites have thus been obtained, the cross-checking undertaken between these publications does indicate the data set obtained is very comprehensive.

All the radioisotope dating results of these 12 eucrites were then compiled and tabulated. For ease of viewing and comparing the radioisotope dating data, the isochron and model ages for some or all components of each of these 12 eucrites were tabulated separately—the isochron ages in Table 1 and the model ages in Table 2. The data in these tables were then plotted on frequency versus age histogram diagrams, with the same color coding being used to show the ages obtained by the different radioisotope dating methods—the isochron ages for whole-rock samples and some or all components of each of these 12 eucrites (fig. 6), and the model ages for whole-rock samples and components of each of these 12 eucrites (fig. 7).

Discussion

In contrast to the Allende CV3 carbonaceous chondrite meteorite (Snelling 2014a), there have been fewer radioisotope ages obtained for these eucrites (basaltic achondrites), even though all the radioisotope dating methods have been used on some of them, and a few of the methods on others. Yet the outcome is similar to that found for the ordinary and enstatite chondrites (Snelling 2014b).

Isochron Ages

A 4.55–4.57 Ga isochron age for Bereba, Ibitira, Juvinas, Pasamonte, Serra de Magé, and Yamato 75011 is clearly defined by a strong clustering of their isochron radioisotope data, via the Pb-Pb, U-Pb, Sm-Nd, Rb-Sr, and Lu-Hf methods, though not all these methods cluster for each of these eucrites (fig. 6). As expected, the Mn-Cr, Hf-W, Al-Mg, and I-Xe methods also yield isochron ages that coincide with the Pb-Pb and/or U-Pb isochron ages, simply because the Mn-Cr method is calibrated against the Pb-Pb isochron age of the Lewis Cliff 86010 angrite achondrite (Lugmair and Galer 1992; Lugmair and Shukolyukov 1998), the Hf-W method is calibrated against the Hf-W and Mn-Cr isochron ages of the St. Marguerite H4 chondrite anchored to the weighted average of its U-Pb whole-rock ages (Göpel, Manhès, and Allègre 1994; Kleine et al. 2002, 2004, 2005; Polnau and Lugmair 2001), the Al-Mg method is calibrated against the Pb-Pb isochron age of the CAIs in the CR chondrite Acfer 059 (Amelin et al. 2002; Amelin, Wadhwa and Lugmair 2006; Wadhwa et al. 2004), and the I-Xe method is calibrated against the I-Xe isochron age of the Shallowater aubrite achondrite (Claydon, Crowther, and Gilmour 2013), which is calibrated against the I-Xe isochron and Pb-Pb model ages of phosphate grains from the Acapulco primitive achondrite (Brazzle et al. 1999; Göpel, Manhès, and Allègre 1992, 1994; Nichols et al. 1994).

There is also considerable scattering of the isochron radioisotope age data for these and the other eucrites studied (fig. 6). Many Rb-Sr isochron ages are younger than the 4.55–4.57 Ga clustering (for Bereba, Juvinas, Moore County, Pasamonte, Stannern, and Yamato 75011), though some are older (for Juvinas and Yamato 75011). Similarly many Sm-Nd isochron ages are younger than the 4.55–4.57 Ga clustering (for Bereba, Cacheri, Ibitira, Juvinas, Moama, Moore County, Serra de Magé, and Stannern), while a few are older (for Bereba, Ibitira, and Moore County). Somewhat surprisingly, all the Pb-Pb isochron ages for Bereba, Cacheri, Moama, Moore County, Serra de Magé, and Stannern, and one for Juvinas, are younger than the 4.55–4.57 Ga clustering, much younger in the case of Stannern. In contrast, the U-Pb isochron (concordia) ages are always in or close to that clustering.

No consistent pattern is evident of Rb-Sr and Sm-Nd isochron ages always being younger than the Lu-Hf and U-Th-Pb isochron ages respectively in the order of the parents’ atomic weights or their decay rates (half-lives), according to their β and α decay mode respectively. Such a pattern would be potentially indicative of a past episode of accelerated radioisotope decay, as suggested by Snelling (2005c) and Vardiman, Snelling, and Chaffin (2005) from their radioisotope investigations of earth rocks and minerals.

However, it also has to be taken into account that there is still disagreement over the values of the decay constants and half-lives of, for example, ⁸⁷Rb and ¹⁷⁶Lu (Snelling 2014c, d), because of there being discrepancies between determinations based on comparisons of the Rb-Sr and Lu-Hf ages of meteorites (primarily eucrites), lunar rocks, and earth minerals and rocks with their U-Pb, K-Ar, and Ar-Ar ages. Indeed, the comparisons of ages involving meteorites (primarily eucrites) and lunar rocks yield a slightly higher decay constant and a slightly faster decay rate (half-life) for both ⁸⁷Rb and ⁷⁶Lu than for age comparisons involving earth rocks and minerals.

Thus if the different decay constants are used in calculating the Rb-Sr and Lu-Hf ages of eucrites, then the affect would be only small (0.6–4.0%) and would still result in Rb-Sr ages that are apparently too young and Lu-Hf ages that are apparently too old. This would not change the conclusion that there are no systematic isochron age differences based on the different atomic weights of the parent radioisotopes that would be due to a past accelerated decay event. Since in most instances the old decay constants have been used for calculating the isochron Rb-Sr ages but the meteorite decay constant has been used for calculating the isochron Lu-Hf ages, recalculating the isochron Rb-Sr ages using the meteorite decay constant would only bring them closer to agreement with the isochron Lu-Hf ages. This only serves to reinforce the lack of any consistent pattern in the isochron ages obtained by the different radioisotope systems, and thus there is no evidence of a past accelerated decay event.

Model ages

In contrast to the isochron ages for these eucrites (basaltic achondrites), there are many more model ages for them (fig. 7). A 4.55–4.57 Ga model age for Bereba, Cacheri, Camel Donga, Ibitira, and Juvinas is very clearly defined by a strong clustering of Pb-Pb and U-Pb model ages, and for Yamato 75011 supported by Sm-Nd and Rb-Sr model ages. As again expected, the Pu-Xe, Al-Mg, and Mn-Cr model ages always plot in, or closely adjacent to, the strong clustering of Pb-Pb and U-Pb model ages. This is because the Pu-Xe model ages are calibrated against the Pb-Pb isochron age of the Angra dos Reis (ADOR) angrite achondrite (Lugmair and Galer 1992; Lugmair and Marti 1977; Miura et al. 1998; Shukolyukov and Begemann 1996a, b), the Al-Mg model ages are calibrated against the Pb-Pb isochron age of the CAIs in the Allende CV3 chondrite (Jacobsen et al. 2008; Schiller, Baker, and Bizzarro 2010), and the Mn-Cr model age for Caldera is calibrated against its Sm-Nd and Pb-Pb isochron ages (Galer and Lugmair 1996; Wadhwa and Lugmair 1996).

Scattering of the model ages is prolific, with K-Ar and Ar-Ar model ages generally being younger than the 4.55–4.57 Ga clustering for all these eucrite meteorites, except for a few that are older for Ibitira, Moore County, Pasamonte, Serra de Magé, and Stannern (fig. 7). Similarly the U-Pb model ages are either younger or older than the clustering, or in most instances both, whereas for Camel Donga they are usually younger, but for Juvinas and Yamato 75011 they are invariably older. Some of the Pb-Pb model ages for Bereba, Cacheri, Camel Donga, Ibitira, Juvinas, and Stannern are younger than the 4.55–4.57 Ga clustering, sometimes much younger, and a few for Ibitira, Juvinas, and Pasamonte are much older. For Yamato 75011 the Rb-Sr model ages are scattered either side of the strong clustering. Where available for Ibitira and Pasamonte, the Th-Pb model ages are nearly all older.