Implications of the carbonaceous chondrite Mn Cr isochron for the formation of early refractory planetesimals and chondrules

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1 Available online at Geochimica et Cosmochimica Acta 73 (2009) Implications of the carbonaceous chondrite Mn Cr isochron for the formation of early refractory planetesimals and chondrules Edward R.D. Scott a, *, Ian S. Sanders b a Hawaii Institute of Geophysics and Planetology, University of Hawaii at Manoa, Honolulu, HI 96822, USA b Department of Geology, Trinity College, Dublin 2, Ireland Received 10 March 2008; accepted in revised form 16 February 2009; available online 21 May 2009 Abstract An excellent 53 Mn 53 Cr isochron for bulk CI, CM, CO, CV, CB, and ungrouped C3 chondrites seems to suggest that each carbonaceous chondrite group acquired its Mn/Cr ratio 4568 ± 1 Myr ago. This age is indistinguishable from the age of Ca Al-rich inclusions (CAIs), which is considered to be the start of the solar system (t 0 ). However, carbonaceous chondrites were not assembled until at least Myr after t 0, to judge by the 207 Pb 206 Pb and 26 Al 26 Mg ages of the chondrules within them, and by the fact that they were not melted by heat from the decay of 26 Al. Presumably, therefore, these meteorites inherited their bulk Mn Cr isochron from precursor materials which experienced Mn Cr fractionation at t 0. As a possible physical mechanism for how the isochron was established initially, and later inherited by the carbonaceous chondrites, we propose the rapid formation at t 0 of planetesimals that were variably depleted in moderately volatile elements, and hence had variably low Mn/Cr. The planetesimals and the undepleted (high Mn/Cr) primitive dust from which they were made shared the same initial e 53 Cr, and therefore evolved on an isochron. We suggest that later impact-disruption of the planetesimals produced dusty debris, which became mixed, in various proportions, with unprocessed (high Mn/Cr) dust before accreting to the carbonaceous chondrite parent bodies. With mixing in a closed system, the isochron was unchanged. We infer that some debrisrich material was converted to chondrules prior to accretion. The chondrules could have been formed by flash melting of the mixed dust, or could instead have been made directly by the impact splashing of molten planetesimals, or by condensation from impact-generated vapor plumes. Ó 2009 Elsevier Ltd. All rights reserved. 1. INTRODUCTION The decay of the nuclide 53 Mn to 53 Cr with a half-life of 3.7 ± 0.4 Myr has been used to date events in the formation of meteorites. On the one hand Mn Cr mineral isochrons have been used to date the last isotopic exchange between minerals in meteorites from the eucrite-diogenite and angrite parent bodies (e.g., Lugmair and Shukolyukov, 1998; Nyquist et al., 2003a; Glavin et al., 2004; Sugiura et al., 2005). For rapidly cooled basaltic meteorites, these isochrons probably date igneous crystallization. On the other * Corresponding author. Fax: address: escott@higp.hawaii.edu (E.R.D. Scott). hand, bulk rock isochrons for eucrites and angrites appear to date early global mantle differentiation in the respective parent bodies (Lugmair and Shukolyukov, 1998; Shukolyukov and Lugmair, 2007). These two kinds of isochron are straightforward to interpret because they relate to wellunderstood igneous processes. A third kind of Mn Cr isochron was discovered by Shukolyukov and Lugmair (2006). They found that whole-rock samples of CI, CM, CO, CV and CB carbonaceous chondrites define an excellent correlation line (Fig. 1). Moynier et al. (2007) produced an almost identical isochron with additional data for CI, CO, and CV chondrites and an ungrouped C3 chondrite. Trinquier et al. (2008) also produced a bulk-rock Mn Cr isochron whose slope is within error of that in Fig. 1 by including data points for average ordinary chondrites and enstatite chondrites, and for the /$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi: /j.gca

2 5138 E.R.D. Scott, I.S. Sanders / Geochimica et Cosmochimica Acta 73 (2009) eucrite parent body, bulk Mars and bulk Earth, as well as for carbonaceous chondrites. Since carbonaceous chondrites are sedimentary rocks composed of grains with a variety of different origins, and they accreted on different parent bodies, there is no obvious reason why they should plot on an isochron, like bulk igneous meteorites from a single parent body. That they do so is quite remarkable, and has fundamental implications for our understanding of events in the early solar system. Assuming that this line is an isochron, Shukolyukov and Lugmair (2006) and Moynier et al. (2007) infer that the carbonaceous chondrites (or their precursor materials) were all formed from a single nebular reservoir with uniform 53 Cr/ 52 Cr. Volatility-driven Mn/Cr fractionation then led to separate reservoirs with different Mn/Cr ratios. The slope of the isochron implies a 53 Mn/ 55 Mn ratio of 8.5 ± at the time of Mn Cr fractionation. (This compares with a ratio of 6.53 ± for the data in Fig. 6 of Trinquier et al., 2008). Mapping this ratio onto the absolute timescale using LEW as an anchor for the Mn Cr chronometer, Shukolyukov and Lugmair (2006) inferred that the fractionation of Mn and Cr occurred at / 1.1 Myr ago. Moynier et al. (2007) inferred an identical age ( / 1.2 Myr ago). This is within error of the 207 Pb 206 Pb age of Myr for CAIs, which are the oldest dated objects and provide our best estimate for the age of the solar system (e.g., Amelin et al., 2002b, 2006; Jacobsen et al., 2008). By what physical process did evidence for early Mn Cr fractionation come to be preserved in carbonaceous chondrites? Moynier et al. (2007) suggest that volatility-driven fractionation of Mn and Cr in the nebula was accompanied by chondrule formation and planetesimal accretion in one brief time interval 4568 Myr ago. Shukolyukov and Lugmair (2006) also infer global high-temperature Mn/Cr fractionation in a nebular setting at that time but, acknowledging that chondrules may have formed a few million years after the beginning (see Kita et al., 2005a; Russell et al., 2006), they are non-committal about the details of how and when the fractionated materials came to reside in the carbonaceous chondrites. It is these details that we attempt to elucidate here Is the carbonaceous chondrite Mn Cr line an isochron? Fig Mn 53 Cr isochron diagram from Shukolyukov and Lugmair (2006) for seven whole rock samples from six carbonaceous chondrites showing e 53 Cr, the deviation of the 53 Cr/ 52 Cr ratio from the terrestrial standard in parts per 10 4, as a function of 55 Mn/ 52 Cr. Shukolyukov and Lugmair (2006), and Moynier et al. (2007) who obtained a very similar isochron line, inferred from the slope that Mn Cr fractionation occurred 4568±1 Myr ago. The eighth data point is for Tagish Lake (TL, an ungrouped C2 chondrite) by Yamashita et al. (2005) and was not included in calculating the isochron line. An alternative explanation for the carbonaceous chondrite line is that it is a mixing line between reservoirs that did not share the same initial 53 Cr/ 52 Cr. Given the 54 Cr anomalies in chondrite matrix, whole chondrites, and igneous meteorites (see Trinquier et al., 2007) and the 53 Cr and 54 Cr anomalies in CAIs (Papanastassiou, 1986; Birck and Allegre, 1988), how do we know that the solar nebula was isotopically homogeneous in 53 Cr/ 52 Cr so that we can infer age differences from the 53 Cr anomaly? Reliable values of the initial solar system 53 Mn/ 55 Mn and 53 Cr/ 52 Cr ratios have not been derived from CAIs. This is partly because of the existence of 53 Cr heterogeneities in spinel (Nyquist et al., 2003a; Kita et al., 2005a), but also because the Mn Cr systematics were disturbed by alteration that caused Mn diffusion (Papanastassiou et al., 2005). Alteration is especially prevalent in the Allende chondrite, the only one in which CAIs have been analyzed, causing extensive formation of alkali-rich and iron-rich secondary minerals (MacPherson, 2003). Ubiquitous 54 Cr anomalies are present in CAIs with smaller anomalies in bulk carbonaceous chondrites, and these are characteristic of the neutron-rich isotopes of elements in the Fe group such as 48 Ca and 50 Ti (Birck, 2004; Trinquier et al., 2007). However, CAIs and bulk chondrites do not display clear-cut anomalies in the lighter isotopes of these elements. Even if many whole CAIs do exhibit small presolar 53 Cr anomalies, the scarcity of CAIs (<1 9 vol.%), their low Cr concentrations which are 0.1 those of bulk chondrites (Birck and Allegre, 1988) and the lack of anomalies in non neutron-rich isotopes in bulk chondrites suggest that we should not expect to detect 53 Cr anomalies in bulk chondrites besides those due to 53 Mn decay. Since the e 53 Cr effects in bulk carbonaceous chondrites probably reflect 53 Mn decay, it is somewhat surprising that e 53 Cr and e 54 Cr effects in bulk chondrite compositions are correlated. The preferred explanation of Shukolyukov and Lugmair (2006) is that this correlation arises because chondrite matrices, which are enriched in Mn, also contain presolar grains enriched in 54 Cr. Given the ubiquitous evidence for live 53 Mn in the early solar system, it would be rather surprising if the carbonaceous chondrite line represented mixing between two reservoirs generated by nebular isotopic heterogeneity and had nothing to do with 53 Mn decay. Most importantly, the general consistency between 26 Al and 53 Mn ages, which we discuss in detail below, strongly supports the assumption of initial isotopic homogeneity, and the interpretation of the line as a true isochron. Clearly Mn Cr isotopic measurements on unaltered CAIs would be invaluable in testing this assumption.

3 Cabonaceous chondrite Mn Cr isochron DOES THE MN CR ISOCHRON DATE ACCRETION? 2.1. Chondrule ages If Moynier et al. (2007) are correct in their inference from the Mn Cr isochron that chondrites accreted immediately after Mn Cr fractionation, chondrules should be 4568 ± 1 Myr old, i.e. the same age as CAIs. However, published ages for chondrules seem to imply that chondrules were formed between about 1.5 and 5 Myr after the formation of CAIs. To assess the age of the formation of chondrules relative to the age of Mn Cr fractionation, we have compiled published 207 Pb 206 Pb, 26 Al 26 Mg, and 53 Mn 53 Cr ages, for chondrules, CAIs, and angrites (Tables 1 and 2, and references therein). The angrites are especially useful in exploring early solar system chronologies because they are unshocked, largely unmetamorphosed and very ancient, and because they have been dated with the same techniques that have been used to date chondrules and CAIs. To link the relative ages provided by the 53 Mn and 26 Al chronometers to the absolute 207 Pb 206 Pb timescale it is customary to use specific samples as anchors: CAIs for 26 Al (re-affirmed by Jacobsen et al., 2008), and the equilibrated angrite LEW for 53 Mn. For both chronometers, there is some disagreement about the best data for these anchors, but the differences are not critical for our purposes (see footnotes to Tables). Fig. 2 shows how data for these two anchors can be used to align the three chronometers. The errors shown for the 26 Al 26 Mg and 53 Mn 53 Cr ages in Fig. 2 and Table 2 do not include the errors in the ages of the anchors and the half lives of 26 Al and 53 Mn (see Table 2 footnotes). The inferred initial 26 Al/ 27 Al ratios in Table 2 give ages for chondrules in LL, CO, and CR chondrites that are 1.5 to >3 Myr younger than CAIs. Similar Al Mg ages were reported for chondrules in LL3.0 Semarkona ( Myr; Villeneuve et al., 2008), and can be inferred from the initial 26 Al/ 27 Al ratios reported by Rudraswami and Goswami (2007) for chondrules in L/LL chondrites. Altogether around 80 Al Mg ages for individual chondrules have now been reported, and the vast majority of them cluster between 1.5 and 2.5 Myr after CAIs. Al Mg ages for chondrules are based on so-called internal isochrons, where the high Al/Mg phase that controls the slope of the isochron is often interstitial glass and plagioclase. The possibility of diffusion during parent body heating means that the ages could record isotopic re-setting of chondrules, and not their formation by igneous crystallization. However, Kurahashi et al. (2008) showed that plagioclase grains in individual dated chondrules had both homogeneous and heterogeneous Mg distributions suggesting that the Al/Mg ratios and chondrule ages were not established during metamorphism. In addition, it would perhaps be fortuitous if so many chondrules experienced thermal resetting at almost exactly the same time on diverse parent bodies, when virtually no chondrules preserve ages between 0 and 1.5 Myr. Moreover, nearly all the selected chondrules come from pristine, unmetamorphosed meteorites of type in which resetting by diffusion would be least expected. Table Pb 206 Pb ages of CAIs, chondrules, and angrites a. Meteorite/sample Group Age (Myr) Reference CAIs Allende CV ±0.65 Amelin et al. (2002a) Efremovka E49 CV ±0.7 Amelin et al. (2002b) Efremovka E60 CV ±0.16 Amelin et al. (2006) Allende & E60 CV3 (4568.5±0.5) Bouvier et al. (2007) Allende CV ±0.9 Connelly et al. (2008a) Allende AJEF CV ±0.36 Jacobsen et al. (2008) Chondrules Gujba CB ±0.5 Krot et al. (2005) Acfer 059 CR ±0.6 Amelin et al. (2002b) Allende CV ±0.45 Connelly et al. (2008a) Angrites LEW Equil ±0.52 Lugmair and Galer (1992) LEW Equil ±0.15 Amelin (2008) Angra dos Reis Equil ±0.42 Lugmair and Galer (1992) D Orbigny Q.T ±0.12 Amelin (2008) D Orbigny Q.T ±0.6 Zartman et al. (2006) SAH b Q.T ±0.1 Connelly et al. (2008b) SAH Q.T ±0.4 Connelly et al. (2008b) Asuka Q.T ±1.6 Zartman et al. (2006) a Italicized data are used to anchor short-lived chronometers to the Pb-Pb timescale. Use of the LEW Pb Pb age of Amelin (2008) would increase absolute ages based on the Mn Cr system by 0.8 Myr. Values in parentheses are not plotted in Fig. 2. Abbreviations: Equil., equilibrated; Q.T., quench-textured. b Age determination by Baker et al. (2005) superseded by Connelly et al. (2008b).

4 5140 E.R.D. Scott, I.S. Sanders / Geochimica et Cosmochimica Acta 73 (2009) Table 2 26 Al 26 Mg and 53 Mn 53 Cr isochron ages of CAIs, chondrules, carbonaceous chondrites, and angrites relative to CAIs. Meteorite Group Initial 53 Mn/ 55 Mn (M) or 26 Al/ 27 Al (A) Age rel. CAI a (Myr) ±2r References Carbonaceous chondrites CI, CM, CO, CV 8.5 ± M 0.9 (+1.0/ 0.9) Shukolyukov and Lugmair (2006) CB, ungr. 8.5 ± M Moynier et al. (2007) CAIs A 0 Canonical value Chondrules Chainpur LL ± M 1.8 (+2.0/-1.5) Yin et al. (2007) Yamato CO ± A 2.7 ± 0.2 Kunihiro et al. (2004) A Kurahashi et al. (2008) EET 92042, 92147, GRA 95229, CR2 < A >3.0 Nagashima et al. (2007) NWA 721 Acfer 094 C3 ungr. 12 ± A 1.5 ± 0.4 Hutcheon et al. (2000) Semarkona LL A Kita et al. (2000) Bishunpur, Krymka LL3.15, LL ± A 1.77 (+0.29/ 0.21) Kita et al. (2005b) Angrites Asuka Q.T ± M 5.4 ± 0.7 Sugiura et al. (2005) D Orbigny Q.T ± M 4.9 (+0.4/ 0.5) Nyquist et al. (2003b) Sugiura et al. (2005) D Orbigny Q.T ± M Glavin et al. (2004) D Orbigny Q.T. 5.1 ± A 4.83 (+0.07/ 0.06) Spivak-Birndorf et al. (2005) LEW Equil ± M 9.3 ± 0.3 Lugmair and Shukolyukov (1998) SAH Q.T. 4.1 ± A 5.06 (+0.36/ 0.25) Spivak-Birndorf et al. (2005) SAH Q.T ± M 5.0 ± 0.7 Sugiura et al. (2005) a Italicized data are used to anchor short-lived chronometers to the Pb-Pb timescale. Al-Mg ages are calculated using the CAI value of for 26 Al/ 27 Al (canonical value). Use of as favored by McKeegan and Davis (2007) would increase Al-Mg ages relative to CAIs by 0.25 Myr. Mn Cr ages are calculated using the inferred initial 53 Mn/ 55 Mn ratio for LEW (Lugmair and Shukolyukov, 1998) and the Pb-Pb ages for LEW by Lugmair and Galer (1992) and CAIs by Amelin et al. (2006). Errors in the ages reflect uncertainty in the initial ratios of the samples and do not include errors in the data for the anchors and half-lives of 26 Al and 53 Mn (0.73 ± 0.04 and 3.7 ± 0.4 Myr). The inference that chondrule ages date crystallization rather than resetting is strengthened by 207 Pb 206 Pb ages for chondrules in CV and CR chondrites. These are also about 2 Myr younger than CAIs (Table 1 and references therein). Those in CB chondrites are younger still, at nearly 5 Myr after CAIs. In particular, the Pb Pb ages for chondrules in the CV chondrite, Allende, are 2.3 ± 1.0 Myr younger than CAIs from the same meteorite, measured at the same time and in the same laboratory, and 1.7 ± 0.5 Myr younger than the best estimate for CAIs in CV3 chondrites (Connelly et al., 2008a). Note that the Pb-Pb ages of Allende chondrules by Amelin and Krot (2007) are not listed separately in Table 1 as they were included by Connelly et al. (2008a) in their results. Mn Cr ages, though less precise, support the chronology based on Pb Pb and Al-Mg ages; they show that chondrules in LL chondrites formed 2.7 ± 1.9 Myr after CAIs (Yin et al., 2007). Hf W ages for chondrules have not so far been determined, but Kleine et al. (2008) argue that the internal Hf W mineral isochron they obtained from the rapidly cooled H4 chondrite, Ste Marguerite, dates chondrule formation because this meteorite has not been heated above the Hf W closure temperature. The Ste Marguerite isochron gives an age of 1.7 ± 0.7 Myr after CAIs, when compared with the precise Hf-W isochron for CAIs determined by Burkhardt et al. (2008). The time of Mn Cr fractionation among carbonaceous chondrites is 0.9 ± 1.0 Myr before CAIs using the LEW anchor. If the uncertainty in the anchors is included, this error is increased to about 1.5 Myr, or to about 1.8 Myr if, in addition, the half-life uncertainty is included. Thus, even with all errors at their limit, both Pb Pb and the most precise short-lived isotope ages, viz., Al Mg ages (Kurahashi et al., 2008), suggest that the Mn Cr fractionation among carbonaceous chondrites predates the formation of their chondrules, and hence predates their accretion, by at least 0.4 Myr Implications of 26 Al heating for accretion times A consideration of the internal heating effect of 26 Al in planetesimals also argues for accretion of the carbonaceous chondrite parent bodies >1.5 Myr after CAIs. At the start of the solar system, 26 Al was quite uniformly distributed in the nebula (at least, within the inner nebula) with the canonical 26 Al/ 27 Al ratio of about This inference is based on the concordance of ages based on the Al-Mg chronometer with those based on other high-resolution chronometers (e.g. Table 2). It is also based on the strikingly uniform isotopic abundances of magnesium in Earth, Moon, Mars and asteroids (Thrane et al., 2006). Canonical 26 Al stores enough radiogenic energy to melt bodies made from cold primitive dust four-times over, and planetesimals that accreted within the first 1.5 Myr and large enough (e.g. >20 km radius; Hevey and Sanders, 2006) to insulate their own interiors would almost certainly have overheated and

5 Cabonaceous chondrite Mn Cr isochron 5141 Fig. 2. Ages of chondrules, CAIs, and angrites derived from 53 Mn 53 Cr, 26 Al 26 Mg, and 207 Pb 206 Pb isotope systematics, and the Mn Cr isochron age for carbonaceous chondrites (CC) from Fig. 1. Vertical lines show how the relative Mn Cr and Al Mg ages are anchored to the absolute Pb Pb ages. Good agreement between the Al Mg and Mn Cr ages of the quench-textured angrites, D Orbigny (D O) and Sahara (SAH), at 4562 Myr suggests that the Al Mg and Mn Cr timescales are well aligned. Short and long-lived chronometers both indicate that chondrules formed Myr after CAI formation. The Mn Cr carbonaceous chondrite age of 4568 Myr is within error of CAI formation. For data see Tables 1 and 2. 2r error bars for Al Mg and Mn Cr ages exclude additional errors in the anchors. Meteorite abbreviations: As, Asuka ; LEW, LEW 86010; Ef, Efremovka; All, Allende; AR, Angra dos Reis. melted in all but a thin outer carapace. That many planetesimals did, in fact, melt internally within the first million years is now borne out by the antiquity of iron meteorites, based on their e 182 W values (Burkhardt et al., 2008; Qin et al., 2008). When Moynier et al. (2007) argued for the coeval formation and accretion of chondrules in carbonaceous chondrites at 4568 ± 1 Myr, they correctly noted that such early accretion, without melting, requires that the parent bodies were smaller than 20 km across. However, evidence suggests that carbonaceous chondrite parent bodies were probably much more than 20 km across, so could not have accreted so soon after CAI formation. First, CO chondrites appear to have cooled slowly over 100 Myr or longer, like many ordinary chondrites, which are derived from parent bodies 200 km or more across (Wood, 1967; Trieloff et al., 2003). Second, there is an inverse correlation among chondrite groups between chondrule age and the maximum metamorphic temperature experienced by the group (Scott, 2007). This relationship indicates that chondrite parent bodies were large enough to retain heat from 26 Al decay and that they accreted more than 1.5 Myr after CAIs formed, by which time there was insufficient 26 Al remaining to cause melting (Sanders and Taylor, 2005). Finally, we note that if each carbonaceous chondrite group is derived from a single parent body, as is commonly supposed, and if these bodies were <20 km across, then the total volume of carbonaceous chondrite material would be an unrealistically tiny fraction of the material in the asteroid belt. These inferences about size and thermal history, combined with the chronological data, convince us that the carbonaceous chondrite parent bodies accreted at least 0.4 Myr, and probably more than Myr after the Mn Cr fractionation event recorded by the isochron. Accretion took place more than about 5 Myr after CAIs in the case of the CB chondrites. Thus, contemporaneous chondrule formation and accretion 4568 ± 1 Myr ago, as proposed by Moynier et al. (2007), seems unlikely. It follows that the isotopic evidence for Mn/Cr fractionation at the beginning, implied by the isochron, was somehow preserved for more than 1.5 Myr in chondrule precursor materials, and only after this time lapse were chondrules made and the parent bodies accreted. So how was the isochron first established, and how did it eventually become inherited by the carbonaceous chondrites? 3. ESTABLISHING AND PRESERVING THE ISOTOPIC RECORD OF EARLY MN CR FRACTIONATION The Mn Cr isochron implies that Mn and Cr were strongly fractionated at the start of the solar system. Shukolyukov and Lugmair (2006), Moynier et al. (2007), and Trinquier et al. (2008) attribute the fractionation to a difference in the volatility of these two elements. During hightemperature thermal processesing of dust, the more refractory element, Cr, would have become concentrated in early condensates from very hot gas, or in evaporative residues. In contrast, manganese, which is more volatile, would have fractionated preferentially into the hot vapor phase and ended up in late, lower-temperature condensates. Thus, for example, one may envisage refractory components with low Mn/Cr concentrated in hot regions, perhaps close to the infant sun, while low-temperature condensates from the vapor, with high Mn/Cr were, perhaps, concentrated in cooler regions further from the sun. In this way it is possible that chemically distinct reservoirs of nebular dust came to be generated. Because the starting material prior to fractionation was, presumably, uniform primitive dust with a CI-like chemistry, the different reservoirs would have had the same initial 53 Cr/ 52 Cr, and so would have evolved on an isochron. The issue is how the materials in these reservoirs managed to retain a spread of Mn/Cr ratios throughout the period (>1.5 Myr) before and during the process of chondrule formation, such that the isochron survives in the bulk carbonaceous chondrites today Reservoirs of nebular dust If, as in the example above, the original reservoirs were isolated and chemically distinct volumes of nebular dust in space, then survival of the isochron requires preservation of these reservoirs, which would appear unlikely. We believe such reservoirs would have become homogenized by nebula-wide mixing long before they were partly converted to chondrules. Theoretical predictions of turbulence in the disk suggest rapid radial as well as vertical mixing (Ciesla,

6 5142 E.R.D. Scott, I.S. Sanders / Geochimica et Cosmochimica Acta 73 (2009) ). Also, the carbonaceous chondrite parent bodies are generally assumed to have accreted within a narrow range of heliocentric distance, making difficult the notion that the bodies were fed from reservoirs of dust at very different distances from the sun Planetesimals as low Mn/Cr reservoirs Our preferred way of preserving the Mn/Cr isochron involves making planetesimals. These are well-sealed reservoirs, incapable of undergoing isotopic exchange with nebular dust. We propose that refractory grains made during the early Mn/Cr fractionation were quickly accreted to, and became stored in, a suite of planetesimals with diverse subsolar Mn/Cr ratios. The contents of these first-generation planetesimals would remain locked up and isolated from the primitive CI-like dust of the nebula; planetesimals and CI-like dust, having started with the same 53 Cr/ 52 Cr, would subsequently have evolved on an isochron (Fig. 3), as described above. We suggest that at various times between 1.5 and 5 Myr after the beginning, some of the planetesimals were broken up by impacts, and that the resulting debris and dust, mixed with varying amounts of unprocessed primitive dust, accreted to make a new generation of planetesimals the carbonaceous chondrite parent bodies. Of course, some of the dust would have become converted into chondrules during the time interval between impact disruption and parent body accretion. Provided there was no net gain or loss of Mn or Cr during breakup, mixing, chondrule formation, and re-assembly of material, i.e. provided that mixing was in a closed system, the new parent bodies would have had compositions plotting on the original isochron (Fig. 3). Thus, the carbonaceous chondrites would have inherited the 4568 ± 1 Myr isochron thanks to refractory precursor materials formed at that time becoming temporarily stored in an early generation of planetesimals. The bulk Mn/Cr ratios of carbonaceous chondrites are correlated with the abundance of matrix, showing that chondrules tend to have lower Mn/Cr ratios than matrices (Shukolyukov and Lugmair, 2006; Fig. 4). In our mechanism, therefore, chondrules are made preferentially from the low Mn/Cr refractory precursors carried in the early planetesimals, and the matrix is a mixture of unprocessed CI-like nebular dust, plus dust that may have condensed during chondrule formation, plus fragments of chondrules and other dusty impact debris. Although we infer that each group of carbonaceous chondrites formed from a mixture of two kinds of component, i.e. from planetesimal debris and fine-grained primitive dust, we emphasize that there is not a one-to-one relationship between planetesimal debris and chondrules, nor between primitive nebular dust and matrix. Chondrules formed from both components and matrix formed from processed and unprocessed dust. The likely existence of planetesimals at the very start of the solar system is demonstrated by the marked deficit of 182 W in many iron meteorites, mentioned above. 182 Wis the daughter of the short-lived radioactive isotope 182 Hf. Tungsten is siderophile and partitions into metal, while hafnium is lithophile and resides mainly in silicates. The 182 W deficit is therefore attributed to the early segregation of molten metal cores, and their isolation from the hafnium parent isotope that remained in the silicate mantles. 182 W/ 184 W in many iron meteorites, after corrections for changes due to cosmic ray exposure, is identical within error to the 182 W/ 184 W ratio at the time CAIs were formed (Kleine et al., 2005; Markowski et al., 2006a,b; Qin et al., 2008; Burkhardt et al., 2008). These data imply that the iron meteorite parent bodies melted within 1 Myr of the time of CAI formation. On a similar note, the accretion Fig. 3. Schematic 53 Mn 53 Cr diagram showing how CB, CV, and CR chondrites could have inherited a record of Mn Cr fractionation that occurred prior to chondrule formation. At the beginning Mn Cr fractionation led to refractory materials that were quickly aggregated into low Mn/Cr planetesimals, so at t = 0 the nebula contained volatile-rich dust with CI-like composition and refractory planetesimals with the same e 53 Cr value. At 1.6, 3 and 4.4 Myr later, debris from impact disruption of the planetesimals became mixed with CI-like nebular dust and underwent closed system processing to make chondrules and matrix which accreted to form the CV, CR, and CB chondrites. The bulk compositions of the chondrites remained on the isochron that the array of planetesimals and CI-like dust would have defined, even though mixing occurred at three different times. Fig. 4. Volume percentage of matrix vs. Mn/Cr ratio for 8 groups of carbonaceous chondrites. The good correlation shows that Mn (like other moderately volatile elements) is enriched in the matrix and depleted in chondrules. Data from Shukolyukov and Lugmair (2006), Scott and Krot (2007) and sources therein.

7 Cabonaceous chondrite Mn Cr isochron 5143 of differentiated asteroids less than 1 Myr after CAI formation and prior to the accretion of chondrite parent bodies was inferred from 26 Al 26 Mg model ages for eucrites and diogenites and thermal modeling (Bizzarro et al., 2005). Thus our appeal to planetesimals to capture and store the newly formed precursor materials with low Mn/Cr is supported by good evidence. It is further supported by the marked depletions of volatile siderophile elements like Ga and Ge in most magmatic groups of iron meteorites (e.g., Mittlefehldt et al., 1998). 4. DISCUSSION 4.1. Fractionation of Mn and Cr due to volatility An understanding of Mn Cr fractionation among carbonaceous chondrites has important implications for the origin of the diverse abundances of Mn and other moderately volatile elements in chondrites. Moderately volatile elements are defined as those that have 50% equilibrium condensation temperatures between 1230 K and 660 K in a gas of solar system composition at 10 4 atm (Lodders, 2003; Palme and Jones, 2004). These elements are depleted in chondrites (except CI chondrites) relative to Mg, Si, and Cr, which are more refractory. The nature and origin of the volatile element depletions in chondrites have been vigorously debated. Larimer and Anders (1967) advocated a two-component model in which matrix with essentially CI elemental abundances was mixed with chondrules that lost volatiles when they formed from matrix material. However, Wasson and Chou (1974) and Wasson (1977) argued that the volatile element depletions were correlated inversely with condensation temperature, predated chondrule formation, and reflected incomplete condensation in a cooling solar nebula as a result of gas loss, settling of solids or orbital migration. This model has generally been favored as the compositions of chondrules and matrices do not closely match predictions of the two-component model (Palme et al., 1988; Bland et al., 2005). The detailed mechanism of volatility-driven fractionation of Mn and Cr is not fully understood. According to the equilibrium nebula condensation calculations of Palme and Jones (2004) and Wasson (1985), Cr and Mn have 50% condensation temperatures of 1277 and 1190 K, respectively. Why should the carbonaceous chondrite groups show significant fractionations of two elements that have such similar condensation temperatures? In an attempt to answer this question we make the following points. First, we note that the temperature interval of 87 K quoted above between the 50% condensation temperatures of Cr and Mn is much smaller than the 138 K value ( K) obtained by Lodders (2003). Second, it seems plausible that the earliest accreting solids in a cooling gas had low Mn/Cr ratios as Cr would have condensed into Fe Ni soon after Mg, Si, and Fe had begun condensing to form forsterite and metallic Fe Ni and well before Mn condensed to form (Mg,Mn) 2 SiO 4 and (Mg,Mn)SiO 3 (Wai and Wasson, 1977; Lodders, 2003). Third, the 50% condensation temperatures for Cr, Mg, Si, and Fe all lie in the narrow range of K. Since Mg, Si and Fe are all abundant condensable elements, the equilibrium model predicts that the concentration of condensed solids would have increased 10-fold as temperatures dropped by 100 K. It is therefore plausible that the forsterite and Fe Ni into which these four elements are predicted to have condensed would have formed grains that were large enough to preclude significant formation of (Mg,Mn) 2 SiO 4 and (Mg,Mn)SiO 3 by gas-solid reaction. It is also possible that the forsterite and Fe-Ni were removed by rapid accretion into planetesimals while the temperature was still above the condensation temperature for Mn. Although there are many occurrences of Cr-bearing metal in relatively pristine chondrites (e.g., Campbell et al., 2005), other phases may also have played a role. For example, Petaev and Wood (2005) note that Cr-spinel appears as a stable phase in several equilibrium condensation models. In addition, other kinetic factors would have been increasingly important as temperatures decreased. For example, Mn condensation may have been hindered because formation of enstatite by reaction of forsterite and Si-rich gas was slow enough to prevent significant formation of enstatite in the nebula (Imae et al., 1993) Cosmic setting for the depletion of moderately volatile elements According to Cassen (1996, 2001) moderately volatile elements were depleted in the hot inner solar system. Ciesla (2007, 2008) agreed that this model could account for the observed elemental trends, but he rejected it as the conditions needed to generate the required high temperatures in the inner solar system to vaporize silicates appeared inconsistent with other constraints. In particular, planetesimals would have to accrete within 10 5 yr to retain the volatile depletion signature of the hot nebula. Ciesla was aware that chondrites accreted more than 1.5 Myr after CAI formation (after chondrules formed), and by this time the dust at 2 4 AU would have been volatile-rich. However, we argue here for extremely early accretion of planetesimals, and we infer, therefore, that the Cassen Ciesla model for the depletion of volatiles is still viable. An alternative setting to the hot inner nebula for depleting volatiles is the aftermath of large impacts, as invoked for some differentiated bodies like Vesta (e.g., Halliday and Porcelli, 2001). If large collisions played any role in establishing the Mn Cr fractionations among carbonaceous chondrites, the collisions must have occurred early in solar system history (<1 Myr after CAIs) to account for the Mn Cr correlation in Fig. 1. Also in the context of large impacts, re-accretion of condensates and other debris from the hot impact plume must presumably have occurred rapidly, before the more volatile elements had cooled enough to condense. A third suggestion for the origin of volatile element depletions comes from Yin (2005) who argues that they were inherited from the interstellar medium. He notes that widespread isotopic anomalies are observed in meteorites, e.g., 54 Cr in carbonaceous chondrites and in differentiated materials (Shukolyukov and Lugmair, 2006; Trinquier et al., 2007), showing that some fraction of solar system

8 5144 E.R.D. Scott, I.S. Sanders / Geochimica et Cosmochimica Acta 73 (2009) dust was not completely vaporized. Yin argues that memories of dust-gas fractionation in the diffuse interstellar medium could have been preserved by refractory grains with icy mantles in the parental molecular cloud and inherited by rocky bodies. However, the age of 4568±1 Myr requires that Mn/Cr variations were developed from an isotopically and chemically homogenized source <2 Myr before the solar nebula formed. Since the lifetime of dust grains in the interstellar medium is at least a few million years and that of the molecular cloud was of comparable duration (see Yin, 2005), it is unlikely that the Mn Cr fractionation in chondrites was inherited from interstellar grains. An additional problem for this model is that the refractory grains in the interstellar medium, whose compositions are inferred from that of the interstellar gas, are not sufficiently depleted in Mn to generate the Mn deficiencies observed in chondrites. Refractory interstellar grains are inferred to have Mn/Cr ratios >0.9 the cosmic value (see Fig. 1 of Yin, 2005), much higher than CB chondrites, which have a Mn/Cr ratio of 0.1 CI. This appears to preclude the interstellar medium as a source for the Mn Cr isochron, even if the 53 Mn was added after, rather than before, the formation of the refractory interstellar grains. Whether the depletion of moderately volatile elements occurred in the hot inner nebula, or in impact plumes following major collisions, most if not all of the resulting refractory precursors with low Mn/Cr were, we believe, accreted rapidly to planetesimals at 4568 ± 1 Myr to preserve separate reservoirs and prevent homogenization by nebula-wide mixing Chondrule precursor material: nature and source of refractory components Given the above constraints from the equilibrium condensation model, we infer that the refractory precursor material for chondrules was largely composed of forsterite with minor metallic Fe Ni. Some refractory materials that seem to match the mineralogical requirements survive in carbonaceous chondrites, and these may be representative of the materials from which chondrules were made. They can be divided into two kinds: nebular products that probably drifted in space after being made, and debris from planetesimals that, according to our interpretation, would have accreted soon after CAI formation. Possible nebular precursors include amoeboid olivine aggregates (AOAs), which are porous polycrystalline particles composed largely of forsterite, metallic Fe Ni and nuggets composed of the CAI minerals, spinel, anorthite and Al Ti pyroxene (e.g., Scott and Krot, 2007). AOAs appear to have formed contemporaneously with CAIs. In addition there are refractory forsterites in the matrix and in chondrules, which have appropriately low Mn/ Cr ratios and appear to predate chondrules (Pack et al., 2004, 2005). AOAs occupy up to 5 vol.% of chondrites, compared to <0.5 vol.% for refractory forsterites. CAIs are themselves nebular refractory products preserved in chondrites, but they are so highly refractory that they are chemically inappropriate as an abundant precursor for chondrules. If coarse-grained refractory particles existed in the nebula, they could have been mixed with volatile-rich nebular dust and later concentrated to various degrees by size sorting to produce batches of material that were spread out along the isochron, and accreted to make the different parent bodies. However, a major problem with nebular precursors is that we need roughly equal amounts of refractory material and CI-like dust to make typical chondrules and it seems unlikely that such large amounts of refractory materials could be preserved in the nebula for 2 3 Myr, though they could, of course, survive if accreted to small planetesimals. Possible evidence for refractory precursors with a planetesimal origin comes from studies of type I chondrules in the CV chondrite, Vigarano, by Libourel and Krot (2007). These authors have identified Fe-Ni metal-bearing forsterite aggregates with granoblastic textures as an important constituent of type I chondrules, which constitute >95% of the chondrules in CR, CO, and CV chondrites (see Scott and Krot, 2007). Although, a nebular origin has not been excluded for these forsterite aggregates (Whattam et al., 2008), Libourel and Krot argue that they were fragments of bodies that had been heated, as their textures require prolonged high-temperature annealing. Equally important, they argue that the forsterite aggregates are the source of the large euhedral forsterite crystals (more than 50 lm across) in type I chondrules. These aggregates might be the refractory precursor materials of chondrules that we require provided they had an appropriate broadly chondritic composition. However, detailed analyses are needed to determine whether they originally had appropriate Mn/Cr ratios and O isotopic compositions, and whether they are related to the refractory forsterites studied by Pack et al. (2005). Bearing in mind the likelihood of planetesimal heating due to the decay of 26 Al (and to a lesser extent of 60 Fe), we infer that the forsterite aggregates could have been derived from metamorphosed planetesimals smaller than 20 km across (i.e. too small to melt), or from the outer sub-surface zones of larger planetesimals that did melt at depth (Hevey and Sanders, 2006). Another possibility is that the host planetesimals were broken up by impacts and re-accreted, perhaps several times, without ever reaching melting temperatures until, eventually, the forsterite aggregates ended up inside chondrules. Planetesimals substantially more than 20 km in diameter would have become almost entirely molten. We re-visit this idea when discussing the origin of chondrules in the next section How did chondrules form? We are not prescriptive regarding the mechanism of formation of chondrules. We merely infer that much of the material from which chondrules were made had been stored for between about 1.5 and 5 Myr in refractory planetesimals before being returned to the nebula in small pieces as a result of impacts. While conventional thinking supposes that clumps of nebular dust were heated rapidly and converted to droplets by such heat sources as solar flares or nebular shocks, impacts between planetesimals,

9 Cabonaceous chondrite Mn Cr isochron 5145 both molten and solid, have also been invoked (Zook, 1981; Lugmair and Shukolyukov, 2001; Sanders and Taylor, 2005; Libourel and Krot, 2007). Provided that the planetesimals had appropriately low Mn/Cr ratios, and chondrules and matrix formed by closed-system processing of material from these planetesimals and nebular dust, the Mn Cr isotopic systematics for the isochron will be satisfied irrespective of whether the chondrules were made from preexisting melt, by flash melting of dust plus annealed solids (for example, by shock waves), or by condensation of impact vapor. Since we infer that most of the chondrule precursor material was carried in planetesimals, it is perhaps simpler to imagine a direct link between planetesimals and chondrules, than to argue that the planetesimals were first pulverized by impact and that the resulting dust later aggregated into clumps that were melted by rapid heating. A link between chondrule formation and planetesimals has recently been advocated by Alexander et al. (2008). They claim that the presence of Na in olivine phenocrysts in Semarkona (LL3.0) chondrules implies a high vapor pressure of Na during chondrule crystallization, and an extreme density of matter (dust-to-gas ratio) in the chondrule-forming region. In the unique case of the CB chondrites, Krot et al. (2005) propose that chondrules may have condensed from a hot impact plume following a large collision nearly 5 Myr after CAIs were formed. More generally, if the planetesimals were internally totally molten (beneath a thin brittle crust) as a result of 26 Al decay, then impacts would have released copious volumes of molten droplets which quickly would have cooled to form chondrules in the manner first postulated by Zook (1981), and reviewed by Sanders and Taylor (2005). Such a high degree of melting requires accretion of larger bodies (e.g. >20 km in radius) very early (e.g. <1 Myr after CAI formation), which is compatible with the model we propose here. Irrespective of how chondrules were made, why should their ages cluster close to 2 Myr after CAIs? A possible explanation is that chondrules were, in fact, being produced throughout the first 2 Myr period of solar system history, and that as fast as they were made they accreted to planetesimals which themselves became molten. The concentration of 26 Al declined rapidly (its half life is 0.73 Myr) and by about 1.5 Myr after CAIs it had lost its capacity to induce total melting of newly accreted bodies, and thereafter chondrules would have survived in large numbers (Sanders and Taylor, 2005). In a similar vein, Connelly et al. (2008a) noted that the paucity of old chondrules may reflect their early incorporation into the parent bodies of differentiated meteorites, which are now commonly believed to predate the accretion of chondrite parent bodies. The above explanations are not entirely satisfactory because older CAIs and AOAs were not destroyed by melting. Perhaps these last objects were mostly lost, with only a few surviving to record their formation at the beginning, whereas chondrules were made over a protracted period and, statistically, the tiny fraction of old survivors became swamped by huge numbers of younger siblings. If so, careful searching should eventually find the old chondrules. Hitherto, two chondrules with ages of 0.8 ± 0.4 Myr after CAIs have been reported (a type II chondrule in the LL3 chondrite Bishunpur (Mostefaoui et al., 2002), and a metal-rich porphyritic olivine-pyroxene chondrule with anorthite from the CV3 chondrite Efremovka (Srinivasan et al., 2000)). The idea of chondrule cannibalization through planetesimal heating suggests that during the first 1.5 to 2 Myr after CAI formation, planetesimals were continually accreting, becoming heated, breaking apart by impact, and the debris re-accreting onto new planetesimals, with the possibility that previously unprocessed dust was being added at all stages. This scenario would account for the fragments of sintered forsterite-rich rock within chondrules, as discussed in Section 4.3. It would also appear to be consistent with chondrule production by impacts between molten or solid bodies. Such multiple recycling of materials through early generations of planetesimals prior to the eventual accretion of the carbonaceous chondrite parent bodies would have involved closed-system mixing and, as noted earlier, would not have re-set the original 4568 ± 1 Myr isochron. This suggests that major Mn Cr fractionation happened only once, at the beginning, and raises the question of whether any Mn Cr fractionation accompanied chondrule formation Were Mn and Cr fractionated during chondrule formation? If the composition of each chondrule simply reflects the composition of its mixture of primordial components, the bulk chondrule compositions would lie on the carbonaceous chondrite line defining the same 4568 Myr age as the whole rocks. However, type I chondrules in carbonaceous chondrites show some evidence for exchange of Mn and Cr between gas and liquid during their formation (Krot et al., 2004). Since chondrules and matrices probably formed in the same nebular region (Bland et al., 2005; Scott and Krot, 2005), volatiles that were preferentially lost during chondrule formation would probably have been scavenged by matrix grains. Thus Mn-poor chondrules and Mn-rich matrix would, together, retain unchanged chemistry. A mechanism of this sort may explain the remarkable complementarity of compositions of chondrules and matrix reported in some meteorites by Palme and Klerner (2000). Neither composition alone is close to solar in its chemistry, but together they combine to give a primitive solar composition. If chondrules and matrix acquired a uniform Cr isotopic composition but diverse Mn/Cr ratios when they formed, then their compositions will define an isochron with a gradient that records the time of chondrule formation, assuming no subsequent processing (Fig. 5). Although we lack Mn Cr isotopic data for chondrules and matrix samples from carbonaceous chondrites, the bulk chondrite data from Moynier et al. (2007) provide some clues to their compositions. These data are quite consistent with the isochron determined by Shukolyukov and Lugmair (2006), yet the Mn/Cr ratios for CO and CV chondrites determined by Moynier et al. deviate significantly from the values of Kallemeyn and Wasson (1981). For example, the

10 5146 E.R.D. Scott, I.S. Sanders / Geochimica et Cosmochimica Acta 73 (2009) Fig. 5. Schematic 53 Mn 53 Cr diagram illustrating how chondrules and matrix in chondrites would retain a record of their formation age if Mn and Cr were fractionated between chondrules and matrix during chondrule formation. We assume that chondrules and matrix formed from a mixture of CI-like nebular dust and refractory materials from planetesimals 3 Myr after CAIs formed, as in Fig. 3. The bulk chondrite would evolve on the original isochron, but if the e 53 Cr values of the chondrules and matrix were homogenized when they formed (e.g. due to evaporation and condensation), individual chondrules and matrix would plot today on a more gently sloping isochron that records formation at 3 Myr after CAIs (as shown). This model is probably more appropriate for ordinary chondrites than for carbonaceous chondrites, as the text explains. latter authors found that the Mn/Cr ratios for CO chondrites had a standard deviation of 2.1%, whereas Moynier et al. (2007) reported values for two CO chondrites that deviated by 6% and 56% from the mean value of Kallemeyn and Wasson, with one value plotting close to the CI value. Shukolyukov and Lugmair s value deviated by only 2% from this mean. The samples analyzed by Moynier et al. were smaller and less representative than those of the other authors (Qing-zhu Yin, personal communication), so it is plausible that compositions of both separated matrix and of chondrules in carbonaceous chondrites plot relatively close to the bulk Mn Cr line, and do not have the fractionation postulated in Fig. 5. Assuming that the bulk compositions of enstatite and ordinary chondrites plot off the carbonaceous chondrite Mn Cr line (Lugmair and Shukolyukov, 1998; Nyquist et al., 2001; Shukolyukov and Lugmair, 2004; Shukolyukov, personal communication 2008), either their chondrules and matrices did not form from the refractory and CI-like components we identified for carbonaceous chondrites or else conditions during chondrule and matrix formation were very different. For example, Mn and Cr may have been fractionated during chondrule formation because Mn-rich matrices or chondrules or Cr-bearing metallic Fe-Ni grains were preferentially lost or gained. Chondrules in the Chainpur LL chondrite have bulk compositions that define a line on the Mn Cr diagram with a slope indicating an age of 2.7 ± 1.9 Myr after CAIs, taking the latter to be the Mn Cr fractionation age of carbonaceous chondrites (Yin et al., 2007). Thus, chondrules in LL chondrites appear to have had a relatively uniform Cr isotopic composition when they formed, and their whole chondrule formation age is consistent with the Al-Mg mineral isochron ages of LL chondrules (Table 2; Kita et al., 2000, 2005b). The intercept of the whole chondrule line on the Mn Cr diagram suggests that they could have formed from CI-like precursor material (Moynier et al., 2007) and that refractory precursor materials were not involved. However, the uncertainties in the Mn Cr data are such that a two-component model like that in Fig. 3 with net Mn loss cannot be excluded. While chondrules in LL chondrites may have formed from a CI-like precursor, we can demonstrate that it was not the rule for chondrules in carbonaceous chondrites. In Fig. 6 we show a hypothetical situation in which chondrules and matrices are made from CI-like material at 1.6 Myr, 2 Myr and 4.4 Myr after CAI formation (the ages of chondrules in CO, CV and CB chondrites). In this model the chondrules (and the matrices) inherit the e 53 Cr values of the CI-like dust at the time they form, and lose Mn by evaporation at the same time so the bulk Mn/Cr ratio of the chondrules, and also, therefore, of the bulk meteorite, becomes sub-solar. A further three fictitious chondrite groups, CX, CY and CZ, are included for good measure. While this model generates a correlation line, the scatter is much larger than in Fig. 1. In addition, the slope is shallower, and the intercept on the vertical (e 53 Cr) axis much higher, although we note that the initial e 53 Cr value of the solar system is not very well defined. Clearly, the isochron in Fig. 1 could not have been produced by volatile loss during chondrule formation with no refractory precursor component. Fig. 6. Hypothetical 53 Mn 53 Cr diagram showing how carbonaceous chondrites might plot assuming that they were formed solely from CI-like material with Mn (and other moderately volatile elements) lost by evaporation during chondrule formation. In this model, the isotopic composition of the CI-like material is assumed to have evolved from A to B during decay of 53 Mn (consistent with Fig. 1). e 53 Cr values for the CB, CV, and CO chondrites are calculated assuming their bulk chemical compositions (chondrules and matrices) were established by volatile loss from CI-like material that had the appropriate initial e 53 Cr, at 4.4, 1.6, and 2.0 Myr after CAI formation (as in Fig. 2). Three fictitious groups, CX, CY, and CZ, which formed 2.5, 1.5, and 3.0 Myr after CAIs are also shown. This model generates a correlation line, but the scatter is much larger, the slope is gentler, and the intersection on the e 53 Cr axis is higher, than in Fig. 1.