Constraints on 10Be and 41Ca distribution in the early solar system from 26Al and 10Be studies of Efremovka CAIs

Earth and Planetary Science Letters 374 (2013) 11–23

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Constraints on 10Be and 41Ca distribution in the early solar system from 26Al and 10Be studies of Efremovka CAIs Gopalan Srinivasan a,n, Marc Chaussidon b a

Centre for Earth Science, Indian Institute of Science, CV Raman Avenue, Bangalore 560012, India Centre de Recherches Pétrographiques et Géochimiques, CRPG, UMR 7358, CNRS-Université de Lorraine, BP 20, 15 rue Notre Dame des Pauvres, 54501 Vandoeuvre-lès-Nancy, France

b

art ic l e i nf o

a b s t r a c t

Article history: Received 12 May 2012 Received in revised form 26 March 2013 Accepted 28 March 2013 Editor:T.M. Harrison Available online 6 June 2013

Three refractory coarse grained CAIs from the Efremovka CV3 chondrite, one (E65) previously shown to have formed with live 41Ca, were studied by ion microprobe for their 26Al–26Mg and 10Be–10B systematic in order to better understand the origin of 10Be. The high precision Al–Mg data and the inferred 26Al/27Al values attest that the precursors of the three CAIs evolved in the solar nebula over a period of few hundred thousand years before last melting–crystallization events. The initial 10Be/9Be ratios and δ10B values defined by the 10Be isochrons for the three Efremovka CAIs are similar within errors. The CAI 10Be abundance in published data underscores the large range for initial 10Be/9Be ratios. This is contrary to the relatively small range of 26Al/27Al variations in CAIs around the canonical ratio. Two models that could explain the origin of this large 10Be/9Be range are assessed from the collateral variations predicted for the initial δ10B values: (i) closed system decay of 10Be from a “canonical” 10Be/9Be ratio and (ii) formation of CAIs from a mixture of solid precursors and nebula gas irradiated during up to a few hundred thousand years. The second scenario is shown to be the most consistent with the data. This shows that the major fraction of 10Be in CAIs was produced by irradiation of refractory grains, while contributions of galactic cosmic rays trapping and early solar wind irradiation are less dominant. The case for 10Be production by solar cosmic rays irradiation of solid refractory precursors poses a conundrum for 41Ca because the latter is easily produced by irradiation and should be more abundant than what is observed in CAIs. 10Be production by irradiation from solar energetic particles requires high 41Ca abundance in early solar system, however, this is not observed in CAIs. & 2013 Elsevier B.V. All rights reserved.

Keywords: short-lived radionuclide irradiation 26Al 10Be 41Ca Ca-Al-RichInclusions.

1. Introduction Since its discovery (McKeegan et al., 2000) short-lived 10Be, which decays to stable 10B with a half-life of 1.39 Myr (Chmeleff et al., 2010; Korschinek et al., 2010), has served as an unambiguous evidence for the role of energetic particle induced reactions as a plausible mechanism and constraint for build-up of cosmochemical inventory of short-lived radioactive nuclides (SLRs) in the early solar system. The presence of 10Be in various CAIs (McKeegan et al., 2000; Sugiura et al., 2001; MacPherson et al., 2003; Chaussidon et al., 2006; Wielandt et al., 2012) and in refractory hibonites (Marhas et al., 2002; Liu et al., 2009, 2010) is considered as a proof for irradiation processes that took place around the young Sun and contributed significantly to the inventory of SLRs (see reviews by Chaussidon and Gounelle, 2006; Dauphas and Chaussidon, 2011). In contrast to 26Al, no

n

Corresponding author. Tel.: +91 80 22932557; fax: +91 80 22933405. E-mail address: [email protected] (G. Srinivasan).

0012-821X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.epsl.2013.03.048

clear canonical ratio has been defined for 10Be, in part because of the much lower number of CAIs studied but also because of the 10 Be/9Be variability of unclear origin observed in CAIs. A 10Be/9Be of 1
10−3 is often considered as representative of the solar system initial though there is no real demonstration for this assertion. The best defined 10Be/9Be ratio is for Allende type B CAI 3529-41 and is of (8.8 7 0.6)
10−4 (McKeegan et al., 2000; Chaussidon et al., 2006) but this CAI, though it contains large Li isotopic variations hinting to the in situ decay of 7Be (half_life 53 d), cannot be considered to be more representative of the solar system initial than other ones. Irradiation calculations considering different scenarios with variations in fluence of particles, compositions of targets, distances from the Sun, and gradual and impulsive flares from the Sun, all agree that (i) short-lived 41Ca, 36Cl, 53Mn can be co-produced with 10 Be at their canonical abundances, (ii) short-lived 60Fe cannot be of irradiation origin, (iii) only ≈10–20% of the measured 26Al abundance can be produced, and (iv) increase in 26Al production can be achieved only for very specific target compositions (e.g., Gounelle et al., 2001, 2006; Marhas et al., 2002; Leya et al., 2003).

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G. Srinivasan, M. Chaussidon / Earth and Planetary Science Letters 374 (2013) 11–23

Furthermore, the energy released by the early Sun in the accretion disk (maximum of 4.3
1043 erg in protons of energy higher than 10 MeV) can account for an irradiation production of 26Al at canonical abundance in a rocky reservoir of a few terrestrial masses only (Duprat and Tatischeff, 2008). In addition to helping to constrain the origin and distribution of SLRs, irradiation processes (protons, alpha particles and photons) might provide clues for models of chemical and isotopic fractionations, massdependent and non mass-dependent, in solids formed early in the accretion disk (Chaussidon and Gounelle, 2006). In addition to irradiation of gas and or solids by solar cosmic rays from a young and active Sun, other potential sources of 10Be include trapping of galactic cosmic rays (GCRs) in the presolar cloud, which are the astrophysical site of Li–Be–B nucleosynthesis by spallation reactions (Reeves et al., 1970), and solar wind implantation in CAIs. The trapping of GCRs in the solar system parent molecular cloud could contribute up to ≈80% of the 10Be observed in CAIs, i.e. 10 Be/9Be  1
10−3 depending on trapping efficiency and life time of molecular clouds (Desch et al., 2004). Because the production rate of 10Be by spallation reactions was probably ≈105 higher in the atmosphere of the active young Sun compared to the present day Sun, Bricker and Caffee (2010) have modeled by analogy with present day implantation of solar wind (SW) 10Be in lunar soils, that all the 10Be observed in CAIs could be proto-solar implanted SW. These different scenarios can be tested by the collateral effects of these production mechanisms on Li and B isotope compositions. A major unanswered question is to quantify the amounts of 10Be contributed by these various sources to the early solar system inventory. Whereas 10Be production cannot be avoided around the young active Sun in refractory solids directly exposed to solar cosmic rays, shielding takes place rapidly in the gas at progressive depths from the inner edge of the accretion disk. By obtaining independent constraints on the timing of the production and incorporation of 10Be into CAIs, it might be possible to assess whether the 10Be was produced prior to CAI formation or whether it was incorporated during the lifetime of the CAIs as objects floating within nebular gas. This would help to assess whether the production of 10Be predated CAIs or 10Be was incorporated early or late during the extended evolution of CAIs and/or its precursors. Recent developments of high precision bulk and in situ Mg isotopic analysis by MC-ICPMS and MC-SIMS have allowed to better define solar system initials for 26Al/27Al (noted hereafter 26Al/27Al(0)) and Mg isotopic composition (noted hereafter in delta notation δ26Mg(0)) at 5.2370.13
10−5 and −0.04070.004‰, respectively (Thrane et al., 2006; Jacobsen et al., 2008; Villeneuve et al., 2009; MacPherson et al., 2010). These initials correspond to the composition of the accretion disk where and when the condensation and first crystallization of CAIs took place. In such a view the inner solar system is considered isotopically homogeneous at 710% relative for Mg and Al isotopes so that 26Al relative ages and model ages can be calculated for chondrules and CAIs (Villeneuve et al., 2009; MacPherson et al., 2012). This view is disputed with very high precision Mg isotopic analysis (ppm level of precision, Bizzarro et al., 2011) and strong heterogeneities in 26Al/27Al(0) and/or δ26Mg(0) have been proposed (Larsen et al., 2011). One major question is to determine the rate and the level at which the Mg and Al isotopic heterogeneities that were present in the protosolar nebula, as demonstrated by the composition of the hibonites from CM chondrites (Liu et al., 2012a), were homogenized or partly homogenized before the formation of CAIs (Wasserburg et al., 2012). To better understand the origin of 10Be/9Be variations in CAIs, and by consequence the potential irradiation processes in the early solar system, high precision 10Be/9Be and 26Al/27Al data are required on the same CAIs. Heterogeneities in 26Al distribution have been proposed for a long time although this is inferred from a minority of refractory phases, such as hibonite grains, FUN CAIs

(e.g., Fahey et al., 1987a; Ireland, 1988; Liu et al., 2012a), and thus can complicate the interpretation of abundance variations in 10Be vis-à-vis 26Al. We report the results from a study of three CAIs from the CV3 chondrite Efremovka. These CAIs have been chosen because they have suffered minor low temperature alteration but might have had complex high temperature histories as shown by previous 26Al studies (Goswami et al., 1994). In addition one of them (E65) is one of the discovery CAIs for short-lived 41Ca (Srinivasan et al., 1994, 1996). Because of its very short half_life of 0.1 Myr and efficient production both in supernova and by irradiation, 41Ca in E65, which has been recently re-studied (Liu et al., 2012b), is an important parameter to further constrain the timing of addition and the sources of SLRs in these CAIs.

2. Sample description The present sample set consists of 3 CAIs labeled as E65, E66 and E36 from the Efremovka CV3 chondrite. CAI E65 is a type B1 CAI previously analyzed for 26Al (Goswami et al., 1994), 41Ca (Srinivasan et al., 1994) and 10Be (Srinivasan, 2001). The CAI E66 is an irregularly shaped coarse-grained type A CAI which has melilite, spinel, and hibonite as major mineral phases (its one end is sliced off because of sample preparation). Both CAIs E65 and E66 are on the same polished thick section separated by about 5 mm. E36 is a droplet shaped type B2 CAI which is primarily dominated by spinel, melilite, pyroxene and minor plagioclase, and is similar in morphology to type B2 CAI E60 (Goswami et al., 1994). The E65 CAI sample exposed in the thick section is irregularly shaped and appears to have zones of pyroxene-rich cores and melilite mantle. E65 section has a higher concentration of metal grains compared to E66 and E36. Melilite in all three inclusions shows a compositional zoning with gehlenitic content decreasing from rim to core. The pure spinel crystals have very minor abundance of Ti and Fe (TiO2 and FeOo1%). The spinels in E65, E66 and E36 are primarily enclosed in and/or associated with melilite and pyroxene; in E66 a few spinels are also present in association with hibonite. In E66 hibonite crystals skirt the rim. CAI E36 has uniform and high density of spinel distribution. The mineral phases and crystal habits suggest that all three CAIs are products of high temperature crystallization (e.g., Davis and Richter, 2005). The spinel crystals are mostly isometric, however, in E65 several resorbed crystals are also present (Fig. S1, Online Material). Resorbed spinels are signature of incomplete melting (Sheng et al., 1991) where the temperature of the melt did not exceed the liquidus of spinel. The plagioclase feldspar in E65 has incorporated sodium resulting in formation of low temperature alteration products.

3. Ion microprobe analytical techniques B and Mg isotopic compositions, Be/B and Al/Mg concentrations ratios were measured with the CRPG-CNRS ion microprobes Cameca ims 1270 and ims 1280HR2 using procedures and standards previously validated and described elsewhere (Chaussidon et al., 1997, 2006; Villeneuve et al., 2009, 2011; Luu et al., 2013), and few specific details are given below. B isotopic ratios and 9 Be/11B ratios were measured in mono-collection mode using the central electron multiplier. Mg isotopic ratios and 27Al/24Mg ratios were measured in multi-collection mode using four Faraday cups. Spinel, pyroxene, melilite, hibonite, and plagioclase feldspar were analyzed for 26Al, and melilite for 10Be. Efforts were made to analyze pure phases in each CAI. In some cases signals from neighboring phases were unavoidable because dimensions of the individual mineral phases were comparable to the primary beam size diameter of 20–25 μm. These analyses with mixed phases are indicated in Tables 1 and 2.

G. Srinivasan, M. Chaussidon / Earth and Planetary Science Letters 374 (2013) 11–23

13

Table 1 Al–Mg Composition of Efremovka CAIs. CAI E66 Sample spot

24

δ25Mg±2σ

δ26Mg±2σ

27

Al/24Mg±2σ

δ26Mgn±2σ

Mel+Hb #13 Mel+Hb #14 Mel+Hb #15 Mel+Hb #16 Mel+Hb #17 Mel+Hb #18 Mel+Hb #20 Mel+Hb #1 Mel+Hb #2 Mel+Hb #3 Mel+Hb #4 Mel #5 Mel+Hb #6 Mel+Hb #7 Mel+Hb #8 Mel #19 Mel #20 Sp #1 Sp #2 Sp #3 Sp #4 Sp #5 Pyx+Mel#1

1.18×107 7.53×106 8.19×106 1.07×107 4.34×106 2.93×107 1.33×107 2.09×106 1.94×107 1.50×107 7.89×106 5.94×107 1.01×107 5.30×106 1.35×107 4.95×107 9.10×107 1.66×108 1.91×108 1.78×108 1.54×108 1.41×108 5.56×107

−1.24±0.65 −1.88±0.66 −2.03±0.63 −1.10±0.58 −2.69±0.98 −0.24±0.49 −0.19±0.50 −5.08±1.22 −0.07±0.49 −0.23±0.49 −0.49±0.53 2.39±0.47 −0.48±0.51 −1.56±0.65 0.97±0.49 2.99±0.10 5.95±0.09 2.79±0.09 2.71±0.09 2.67±0.09 3.02±0.09 2.35±0.09 3.50±0.09

11.72±1.33 16.95±1.25 17.36±1.37 12.68±1.23 36.59±1.43 4.71±1.17 10.95±1.15 45.02±1.42 9.68±1.22 8.74±1.15 18.47±1.23 6.81±1.10 14.56±1.21 25.59±1.29 8.82±1.15 6.55±0.52 12.15±0.46 6.44±0.44 6.23±0.42 6.18±0.43 6.82±0.44 5.66±0.43 7.79±0.48

46.18±4.43 70.10±4.83 73.97±5.77 47.26±7.77 139.66±9.97 16.26±1.24 39.97±2.83 194.60±13.07 32.04±2.69 31.63±2.29 67.87±4.79 6.60±0.43 52.52±3.75 97.77±6.87 21.74±1.55 1.62±0.024 1.31±0.02 2.50±0.13 2.41±0.12 2.52±0.13 2.38±0.12 2.51±0.13 1.52±0.02

14.18±1.19 20.67±1.04 21.39±1.30 14.86±0.80 42.04±2.13 5.19±0.42 11.33±0.43 55.42±2.53 9.82±0.49 9.20±0.40 19.43±0.66 2.14±0.18 15.50±0.56 28.72±1.08 6.91±0.38 0.72±0.12 0.52±0.08 0.99±0.06 0.95±0.05 0.97±0.05 0.94±0.06 1.08±0.05 0.96±0.09

26 26

Mg (cps)

Al/27Al(i)=4.04±0.18×10−5 (Model 1, 95% confidence, MSWD=2.7) and δ26Mg(i)=0.28±0.05‰. Al/27Al(i)=4.05±0.12/0.1×10−5 (Robust Regression 95% confidence) and δ26Mg(i)=0.26±0.06/0.04‰.

CAI E65 Sample spot

24

δ25Mg±2σ

δ26Mg±2σ

Mel#1 Mel#2 Mel#3 Mel#4 Mel#5 Mel#6 Mel#7 Mel#8 Mel#9 Mel#10 Mel#11 Mel#12 Mel#13 Mel#14 Mel#15 Mel#16 Mel#17 Mel#18 Mel#25 Mel#26 Mel#27 Mel#28 Mel#29 Mel#30 Mel#31 Mel#32

5.97×107 4.06×107 5.67×107 1.33×108 3.11×108 3.39×108 3.05×108 6.26×107 2.40×107 3.71×107 4.23×107 4.40×107 3.91×107 2.68×107 4.92×107 5.18×107 6.67×107 1.83×107 4.49×107 4.10×107 5.58×107 4.85×107 3.63×107 2.57×107 4.11×107 3.81×107

3.16±0.46 3.65±0.47 2.35±0.46 6.67±0.46 4.21±0.46 2.21±0.46 3.03±0.46 3.13±0.46 1.29±0.48 2.95±0.47 3.69±0.47 3.66±0.47 1.57±0.47 2.21±0.47 1.50±0.47 3.47±0.47 2.67±0.46 3.21±0.22 3.41±0.10 2.62±0.10 3.20±0.10 3.63±0.51 4.37±0.51 4.08±0.52 4.51±0.51 4.54±0.52

6.72±1.05 8.22±1.08 5.37±1.05 13.21±1.03 8.67±1.02 4.80±1.02 6.44±1.01 6.67±1.05 5.46±1.10 7.30±1.08 9.08±1.08 8.70±1.08 5.16±1.08 7.21±1.11 4.93±1.06 8.23±1.06 6.67±1.07 8.15±0.68 7.39±0.52 6.29±0.52 6.93±0.52 7.70±0.96 9.05±0.96 9.24±0.96 9.44±0.96 9.19±0.96

Mg (cps)

Al/24Mg±2σ

δ26Mgn±2σ

2.86±0.19 4.69±0.30 3.94±0.26 2.12±0.14 2.30±0.15 2.27±0.15 2.36±0.15 2.32±0.15 9.29±0.60 4.87±0. 32 6.54±0.43 6.33±0.41 8.23±0.53 10.42±0.68 6.70±0.43 5.18±0.34 5.35±0.35 5.95±0.16 3.80±0.06 4.57±0.07 2.77±0.04 1.32±0.10 2.01±0.15 4.11±0.31 1.52±0.11 1.85±0.14

0.56±0.13 1.11±0.17 0.79±0.13 0.19±0.09 0.47±0.07 0.50±0.07 0.54±0.07 0.56±0.12 2.94±0.25 1.55±0.18 1.88±0.18 1.57±0.18 2.11±0.20 2.89±0.25 2.02±0.17 1.45±0.14 1.46±0.14 1.88±0.25 0.74±0.13 1.19±0.11 0.68±0.13 0.64±0.18 0.54±0.20 1.30±0.27 0.67±0.16 0.35±0.25

27

CAI E65 Sample spot Mel#33 Mel#34 Mel#35 Mel#36 Mel#37 Mel#38 Mel#39 Mel#40 Mel#41

24

Mg (cps) 6.44×107 2.33×107 4.01×107 2.50×107 3.77×107 1.00×107 1.16×107 6.41×107 4.73×107

δ25Mg±2σ 7.51±0.50 4.72±0.52 6.07±0.51 4.59±0.52 5.03±0.51 2.59±0.58 2.49±0.56 1.52±0.50 1.20±0.51

δ26Mg±2σ

27

Al/24Mg±2σ

15.36±0.96 10.57±0.97 12.38±0.96 10.31±0.96 10.39±0.96 10.06±1.00 9.73±1.00 3.51±0.96 2.96±0.96

2.11±0.16 4.37±0.33 2.21±0.17 3.75±0.28 1.62±0.12 16.13±1.22 14.88±1.12 1.57±0.12 2.59±0.20

δ26Mgn±2σ 0.74±0.13 1.38±0.27 0.57±0.19 1.39±0.30 0.60±0.20 5.02±0.63 4.88±0.56 0.56±0.13 0.64±0.18

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G. Srinivasan, M. Chaussidon / Earth and Planetary Science Letters 374 (2013) 11–23

Table 1 (continued ) CAI E65 Sample spot

24

δ25Mg±2σ

1.85×107 4.05×107 5.61×107 2.52×107 3.25×107 3.64×107 1.82×108 1.63×108 2.45×107 1.84×107 1.87×107 5.20×108 4.01×108 3.93×108 3.83×108 3.67×108 2.06×108 1.35×108

0.87±0.52 −0.43±0.51 1.31±0.51 1.20±0.52 1.32±0.51 0.93±0.51 4.03±0.50 3.73±0.50 3.31±0.52 2.23±0.52 2.27±0.52 0.79±0.19 1.05±0.19 1.99±0.24 0.94±0.19 0.55±0.19 2.59±0.50 3.27±0.50

4.03±0.97 0.78±0.96 3.41±0.96 3.57±0.96 4.44±0.96 3.10±0.96 8.66±0.95 8.01±0.95 7.58±0.96 5.78±0.97 6.35±0.97 1.49±0.54 2.08±0.60 3.70±0.76 1.61±0.56 0.97±0.56 5.16±0.95 6.42±0.95

7.56±0.57 4.85±0.35 2.08±0.16 4.21±0.32 4.59±0.35 3.41±0.26 2.03±0.15 2.37±0.18 3.79±0.29 5.80±0.44 5.67±0.43 0.093±0.003 0.125±0.005 0.115±0.003 0.100±0.003 0.103±0.003 0.44±0.03 0.86±0.07

2.34±0.33 1.63±0.17 0.86±0.14 1.24±0.28 1.86±0.22 1.29±0.18 0.81±0.06 0.75±0.05 1.14±0.30 1.45±0.34 1.94±0.34 −0.04±0.05 0.04±0.07 −0.18±0.33 −0.23±0.05 −0.10±0.05 0.11±0.07 0.07±0.08

Mg (cps)

δ25Mg±2σ

δ26Mg±2σ

27

δ26Mgn±2σ

8.21×106 1.55×107 4.12×106 2.64×108 2.00×108 2.00×108 2.40×108 2.25×108 2.32×108 1.39×108 1.41×108 1.41×108 1.40×108 1.40×108 1.39×108 1.24×108 1.22×108 2.72×108 1.98×108 1.83×108 1.97×108 1.83×108 1.67×108

−0.71±1.09 −0.42±0.72 −0.87±1.99 1.87±0.06 2.44±0.03 3.38±0.04 1.93±0.05 1.94±0.04 1.66±0.04 1.72±0.04 1.57±0.03 2.18±0.03 2.35±0.04 2.38±0.03 1.95±0.04 1.40±0.04 1.08±0.03 1.68±0.10 1.83±0.09 1.74±0.09 2.86±0.09 2.84±0.09 5.07±0.50

−0.15±1.34 0.45±1.25 5.70±1.48 4.50±0.56 5.59±0.39 7.43±0.44 4.69±0.50 4.60±0.48 4.09±0.48 3.92±0.40 3.51±0.43 4.60±0.43 5.30±0.44 5.27±0.41 4.25±0.49 3.51±0.40 2.67±0.41 3.88±0.60 4.22±0.43 4.11±0.44 6.41±0.44 6.55±0.45 10.66±0.95

Mel#42 Mel#43 Mel#44 Mel#45 Mel#46 Mel#47 Mel#48 Mel#49 Mel#50 Mel#51 Mel#52 Pyx#1 Pyx#2 Pyx#3 Pyx#4 Pyx#5 Pyx#6 (+Mel) Pyx#7 (+Mel)

δ26Mg±2σ

27

Al/24Mg±2σ

δ26Mgn±2σ

Mg (cps)

CAI E65 Sample spot An#1 An#2 An#3 Sp#2 Sp#3 Sp#4 Sp#5 Sp#6 Sp#7 Sp#8 Sp#9 Sp#10 Sp#11 Sp#12 Sp#13 Sp#14 Sp#15 Sp#17 Sp#18 Sp#19 Sp#20 Sp#21 Sp#22 26

Al/27Al(i) Al/27Al(i) 26 Al/27Al(i) 26 Al/27Al(i) 26

24

Al/24Mg±2σ

61.1±4.0 47.9±3.2 164.2±10.7 2.30±0.14 2.27±0.14 2.22±0.13 2.35±0.14 2.36±0.14 2.34±0.14 2.39±0.15 2.35±0.14 2.32±0.14 2.35±0.14 2.34±0.15 2.37±0.15 2.48±0.15 2.44±0.15 2.51±0.15 2.46±0.15 2.44±0.15 2.43±0.15 2.57±0.16 2.40±0.22

1.22±2.13 1.27±1.20 7.40±4.18 0.86±0.15 0.85±0.05 0.85±0.07 0.92±0.10 0.82±0.09 0.85±0.08 0.57±0.06 0.46±0.06 0.36±0.06 0.72±0.08 0.63±0.06 0.45±0.09 0.78±0.07 0.57±0.06 0.61±0.16 0.65±0.06 0.71±0.06 0.84±0.06 1.02±0.07 0.79±0.07

(mel+pyx+sp)=4.42±0.38×10−5 and δ26Mg(i)=−0.10±0.07‰ (Model 1, MSWD=9.6, 95% confidence). (mel+pyx+sp)=4.28±0.28/0.33×10−5 and δ26Mg(i)=−0.06±0.07/0.07‰ (Robust Regression 95% confidence). (plag+pyx)=5.6±8.4×10−6 and δ26Mg(i)=−0.07±0.09‰ (Model 1, MSWD=10.7, 95% confidence). (plag+pyx)=6.3±22.3/−2.9 ×10−6 and δ26Mg(i)=−0.10+0.13/−0.12‰ (Robust Regression, 95% confidence).

CAI E36 Sample spot #1n #2n #3n Mel#1 Mel#2 Mel#3 Mel#4 Mel#5 Mel#6 Mel#7 Mel#8 #4n #5n Mel#9 26 26

24

Mg (cps)

2.06×108 2.08×108 1.57×108 2.17×107 3.39×107 3.30×107 3.30×107 2.65×107 3.99×107 3.43×107 1.19×108 6.05×107 7.14×107 2.45×107

δ25Mg±2σ

δ26Mg±2σ

0.28±0.24 1.46±0.24 0.98±0.26 0.58±0.27 1.43±0.25 1.38±0.24 −1.95±0.26 −2.60±0.25 −2.47±0.25 −1.32±0.25 0.33±0.24 1.41±0.24 0.31±0.24 −3.54±0.25

1.17±0.61 3.53±0.62 2.40±0.84 8.74±0.79 4.62±0.69 4.98±0.74 −1.79±0.84 −2.73±0.74 −3.26±0.72 −0.29±0.75 2.41±0.62 3.70±0.71 1.61±0.65 −3.12±0.75

Al/27Al(i)=(4.56±0.31)×10−5 and δ26Mg(i)=0.04±0.09‰ (Model 1, 95% confidence, MSWD=3.1). Al/27Al(i)=(4.35±0.18)×10−5 and δ26Mg(i)=0.07±0.11‰ (Robust Regression, 95% confidence). n

Spots are a mixture of Melilite, spinel and pyroxene.

27

Al/24Mg±2σ

1.95±0.05 1.94±0.05 2.01±0.05 24.52±1.01 5.57±0.14 6.87±0.17 6.21±0.15 7.59±0.19 5.28±0.13 7.09±0.17 4.94±0.13 2.81±0.07 2.83±0.07 11.11±0.28

δ26Mgn±2σ 0.62±0.04 0.68±0.05 0.50±0.32 7.60±0.29 1.84±0.14 2.29±0.15 2.00±0.30 2.34±0.18 1.55±0.15 2.28±0.19 1.77±0.05 0.96±0.11 1.00±0.07 3.78±0.19

G. Srinivasan, M. Chaussidon / Earth and Planetary Science Letters 374 (2013) 11–23

Table 2 Be–B Composition of Efremovka CAIs. CAI E66 9

9

2.23×105 2.41×105 1.33×104 1.76×105 9.95×104 1.28×105 1.27×105 2.15×105 1.35×105 2.40×104 7.63×104 7.16×104 3.88×104 1.81×104 8.86×103 4.73×105 2.01×105 1.51×105 7.30×105 5.31×102

7.42±0.11 4.66±0.05 0.20±0.003 10.80±0.20 7.01±0.15 1.39±0.01 42.66±1.68 21.14±0.47 15.71±0.40 0.24±0.003 2.76±0.05 2.10±0.03 1.32±0.02 0.11±0.002 0.05±0.001 22.94±0.51 17.88±0.54 17.46±0.60 17.90±0.28 0.11±0.01

Be (cps)

#1 #2 #3 #4 #5 #6 #8 #9 #10 #11 #12 #13 #14 #15 #16 #1-1 #1-2 #1-3 #1-4 #1-5

Be/11B±2σ

10

B/11B±2σ

0.2547±0.0051 0.2511±0.0038 0.2524±0.0033 0.2595±0.0070 0.2514±0.0073 0.2541±0.0029 0.2707±0.0168 0.2685±0.0090 0.2603±0.0096 0.2459±0.0027 0.2536±0.0053 0.2509±0.0047 0.2513±0.0051 0.2469±0.0021 0.2529±0.0021 0.2736±0.0103 0.2623±0.0136 0.2657±0.0156 0.2642±0.0071 0.2601±0.0206

Be/9Be(i)=(7.6±2.9)×10−4 ; δ10B(i)=10.6±6.9 ‰ (Model 1, MSWD=2.3, 95% confidence).

10

CAI E65 9

9

10

2.77×105 3.29×105 9.83×103 1.09×105 1.85×105 1.09×105 6.19×104 3.07×105 1.76×105 2.17×105 2.03×105 2.09×105 8.51×104 1.59×105 8.63×104 8.81×104 6.45×104

15.34±0.26 2.80±0.02 0.05±0.001 7.16±0.14 17.80±0.40 4.33±0.07 3.45±0.07 22.66±0.43 13.01±0.26 50.12±1.59 6.19±0.09 5.25±0.07 3.86±0.07 6.94±0.11 15.73±0.50 40.85±1.85 4.23±0.089

0.2571±0.0066 0.2529±0.0025 0.2480±0.0020 0.2560±0.0071 0.2599±0.0087 0.2544±0.0055 0.2556±0.0066 0.2627±0.0077 0.2602±0.0077 0.2824±0.0143 0.2538±0.0048 0.2567±0.0044 0.2524±0.0059 0.2541±0.0058 0.2671±0.0122 0.28167±0.0202 0.2542±0.0071

Be (cps)

#1 #2 #3 #4 #5 #6 #7 #8 #9 #10 #11 #12 #13 #14 #15 #16 #17

Be/11B±2σ

B/11B±2σ

Be/9Be(i)=(7.0±1.7)×10−4 ; δ10B(i)=10.1±5.7 ‰ (Model 1, MSWD=0.76, 2σ).

10

CAI E36 9

#1 #2 #3 #4 #5 #6 #7 #8 #9 #10 #11 #12 #13 #14 #15 #16 #17 #18 #19 #20

Be (cps)

2.26×104 1.55×105 1.46×105 1.75×105 1.39×105 1.90×105 3.65×105 3.33×105 2.69×105 3.51×105 4.84×105 1.66×105 1.19×105 3.36×105 2.59×105 1.16×105 3.24×105 3.13×105 3.36×105 2.48×105

9

Be/11B±2σ

10

B/11B±2σ

1.79±0.05 0.97±0.01 2.63±0.03 10.42±0.19 5.20±0.08 47.94±1.59 56.93±1.47 3.94±0.04 0.61±0.003 6.24±0.07 34.76±0.62 8.61±0.15 3.89±0.06 48.68±1.22 13.25±0.22 42.76±1.74 6.81±0.08 1.33±0.01 6.58±0.07 2.04±0.01

0.2537±0.0079 0.2485±0.0022 0.2515±0.0037 0.2621±0.0070 0.2531±0.0054 0.2840±0.0152 0.2864±0.0120 0.2542±0.0030 0.2523±0.0013 0.2552±0.0037 0.2789±0.0080 0.2591±0.0065 0.2511±0.0050 0.2833±0.0115 0.2625±0.0065 0.2762±0.0180 0.2588±0.0041 0.2533±0.0018 0.2527±0.0040 0.2503±0.0025

Be/9Be(i)=(7.0±1.4)×10−4, δ10B(i)=15.1±4.1 ‰ (Model 1, MSWD=1.4, 95% confidence).

10

15

Matrix effects on ion yields (Be relative to B and Al relative to Mg) and Mg isotopic instrumental mass fractionation (αinst) were determined using a set of in house and international standard silicate minerals (olivine, pyroxene, hibonite, spinel, anorthite) and glasses (of basaltic, pyroxene and melilite composition, BHV0 and NBS glasses). The only significant matrix effects were observed for ion yields of hibonite and spinel which show relative Al/Mg ion yields lower by 20% and higher by 16%, respectively, relative to that of all silicates. In E66, most of the spots with high 27Al/24Mg ratios are not pure hibonite but mixtures of melilite and hibonite: for these spots a bulk 27Al/24Mg ion yield was calculated from the relative fractions of hibonite and melilite determined from the measured 27Al/24Mg ratios assuming average compositions for melilite (28.76 wt% Al2O3, 3.14 wt% MgO, Fahey et al., 1987b) and hibonite (91 wt% Al2O3 and 0.01 wt% MgO). The additional error introduced by this correction was estimated to 72% relative by varying, within reasonable values, the average composition used for melilite. Matrix effects on αinst for Mg isotopes show variations of several ‰ among all mineral analyzed (for instance ≈2‰/amu difference between pyroxene and spinel). A 1.9‰/amu effect on αinst was observed between our two standard melilite glasses (having respectively Al2O3 ¼35 wt%, MgO¼3.5 wt%, 27Al/24Mg ¼ 12.8 and Al2O3 ¼11 wt%, MgO¼7 wt%, 27Al/24Mg ¼2.0). All the melilite data were corrected with an average αinst, without precisely correcting δ25Mg for small matrix effects due to variations of the 27Al/24Mg ratios because (i) it has no significant effect on the calculation of the 26Mg excesses when these effects are higher than a few tenths of permil (Luu et al., 2013) and (ii) the melilite 27Al/24Mg in the three CAIs studied did not show large variations (except in E66 when the beam was overlapping with hibonite). The 26Mg excesses (or δ26Mg* values) were calculated using 26 Mg/24Mg ¼0.13932 and 25Mg/24Mg ¼0.12663 as normalization ratios (Catanzaro et al., 1966) and using an exponential mass fractionation law with an exponent β¼ 0.514 (Davis et al., 2005). Because of the low count rates on 10B and 11B in some spots in melilite, the 10B/11B and 9Be/11B ratios were systematically calculated from the ratio of total counts (after removing the 20 first cycles which was necessary to get rid of surface contamination). The B isotopic compositions are given and discussed below either as 10B/11B ratios or expressed in the classical delta notation relative to 11B/10B ¼ 4.04558 for NBS951 (Spivack and Edmond, 1986). Either δ10B (δ10B ¼(10B/11Bsample/10B/11BNBS951−1)
1000) or δ11B (δ11B ¼(11B/10Bsample/11B/10BNBS951−1)
1000) are used. This is because δ11B values are classically used in the geochemical literature (and were used in the cosmochemical literature before the discovery of 10Be) to discuss B isotopic fractionations. δ10B are more straightforward to use when discussing effects due to 10Be decay (as a first approximation valid within 72.5% for δ11B from −50‰ to 50‰, δ10B ¼−δ11B). All errors reported in Tables 1 and 2 are two sigma errors calculated by quadratic addition of errors due to counting statistic, external reproducibility determined for 27 Al/24Mg, 9Be/11B, 10Β⧸11Β, δ25Mg, δ26Mg, and δ26Mg* from repeated analyses of the standards, and other sources of errors (see above for mixtures of hibonite and melilite in E66, and below for spinels). The samples and the standards were measured in several different sessions. The external reproducibility of standards was different from session to session. This is reflected in the errors of the results obtained from CAIs in different analytical sessions. The 27Al/24Mg ratios of large spinels in E65 were treated as the best representative of the external reproducibility for E65 spinels. In the case of B, the two sigma errors on 10Β⧸11Β ratios calculated from statistical uncertainty on total counts are similar (higher by 6 715‰ in average) than the 2 sigma standard errors of ratios calculated from individual measurement cycles, implying that the errors are dominated by counting statistics.

16

G. Srinivasan, M. Chaussidon / Earth and Planetary Science Letters 374 (2013) 11–23

The Al–Mg and Be–B data were regressed using Isoplot version 2.6 (Ludwig, 1999) to infer the initial 26Al/27Al and 10Be/9Be from the respective slopes and the initial δ26Mg and δ10B from the respective intercepts. The initial 26Al/27Al and 10Be/9Be are hereafter noted as 26Al/27Al(i) and 10Be/9Be(i) and initial δ26Mg and δ10B are hereafter noted as δ26Mg(i) and δ10B(i). We report results using Model 1 and Model 3 (Robust Regression) schemes for regression analyses from Isoplot for Al–Mg and only using Model 1 for Be–B system. For Model 1 isochron data 26Al/27Al(i) and 10Be/9Be(i) and δ26Mg(i) and δ11B(i) the errors are 95% confidence limit which is more than 2 sigma errors when MSWD 41 but in case of isochrons with an MSWD  1 the two are comparable. In all isochron figures and their captions we indicate the regression model used for obtaining the slope and intercept, MSWD, and the 2s or the 95% confidence limit (errors) as applicable. Interpretation using alternative model for regression other than our preferred choice will nominally change calculated time intervals and the evolution based on the δ26Mg(i) vis-à-vis 26Al/27Al(i) but will not substantively alter the final conclusions.

4. Results 4.1.

26

Al/27Al systematic

Results obtained from in-situ measurements of mineral phases in Ca–Al-rich inclusions E65, E66 and E36 for Al–Mg and Be–B compositions are given in Tables 1 and 2. In CAI E66, all the mineral phases were measured (spinel– melilite–pyroxene–hibonite), and the best fit isochron using Isoplot Model 1 26Al/27Al(i) ¼ 4.04 70.18
10−5 and δ26Mg(i) ¼0.28 7 0.05‰ (95% confidence, MSWD¼2.7) and the Model 3 26Al/27Al(i) ¼ 4.0570.12/0.10
10−5 and δ26Mg(i) ¼0.2670.06/0.04‰ (95% confidence). The Models 1 and 3 values are identical within errors, and Model 1 values are used for further interpretation. The scatter in the high Al/Mg phases, hibonite and melilite, (Fig. 1a) is reflected in the high value of MSWD. From the mineral assemblage and its isochron for E66 we infer that the measured 26Al abundance is indicative of the last melting and crystallization at a time when 26Al/27Al 4.04
10−5, and subsequently the CAI experienced minor thermal or alteration event resulting in exchange of Al–Mg between mineral phases. In CAI E65 two isochrons are defined by spinel, pyroxene and melilite, and plagioclase and pyroxenes respectively. For spinel– pyroxene–melilite data using Isoplot Model 1 26Al/27Al(i) ¼4.42 7 0.38
10−5 and δ26Mg(i) ¼−0.10 7 0.07‰ (MSWD ¼9.6, 95% confidence) and Model 3 26Al/27Al(i) ¼4.28 70.28/0.33
10−5 and δ26Mg(i) ¼ −0.06 70.07‰ (95% confidence). For plagioclase–pyroxene data using Model 1 26Al/27Al(i) ¼5.6 7 8.4
10−6 (MSWD ¼ 10.7, 95% confidence) and Model 3 26Al/27Al(i) ¼6.3 722.3/2.9
10−6 (95% confidence). The melilite–pyroxene–spinel regression using the Models 1 and 3 overlap within errors, we use the Model 1 data for further interpretation. The melilite, pyroxene and spinel data show significant scatter from the isochron (Fig. 1b) suggesting exchange of Mg after the crystallization perhaps through low temperature alteration process. From the 26Al/27Al values obtained from Model 1 analyses plagioclase–pyroxene data, the lower limit is not resolved from zero with in errors and the upper limit of 1.4
10−5 is less than the values determined from the spinel– pyroxene–melilite isochron data. The 26Al/27Al values determined from plagioclase–pyroxene is significantly lower than the value reported for spinel–pyroxene–melilite data albeit that MSWD for both isochrons is 10. In CAI E36 mixtures of melilite, spinel and pyroxene were analyzed because pure spinel and pyroxene phases are extremely small and scattered all over the CAI. The measured data define an

isochron with Isoplot Model 1 as 26Al/27Al(i) ¼(4.56 7 0.31)
10−5 and δ26Mg(i) ¼0.047 0.09‰ (95% confidence, MSWD¼ 3.1) and with robust regression 26Al/27Al(i) ¼(4.3570.18)
10−5 and δ26Mg(i) ¼ 0.0770.11‰ (95% confidence). The isochron shows evidence for some minor disturbance as shown by the scatter in melilite data (Fig. 1c), to that extent it is similar to CAI E66, and the 26Al/27Al(i) and δ26Mg(i) using Isoplot Models 1 and 3 (robust regression) overlap within errors. The 26Al/27Al(i) and δ26Mg(i) values obtained from Model 1 or 3 Al–Mg regression analyses of E66 data are resolved at three sigma level from the solar system initial 26Al/27Al(0) ¼5.2370.13
10−5 and δ26Mg(0) ¼−0.04070.004‰ respectively (Jacobsen et al., 2008; Villeneuve et al., 2009). In case of E36 the Model 1 26Al/27Al(i) values overlap with 26Al/27Al(0) values at 3 sigma level while Model 3 is fully resolved. The E36 δ26Mg(i) values obtained from both Model 1 and Model 3 overlap with δ26Mg(0) at 2 sigma level. 4.2.

10

Be/9Be systematic

Only melilite was analyzed in the three CAIs for Be–B composition and in all cases the Isoplot Model 1 is used for regression analyses. In E66 the highest value for 10Be/11B is  40 and the 10Be isochron yields 10Be/9Be(i) ¼ 7.6 72.9
10−4 and δ10B(i) ¼10.6 7 6.9‰ (MSWD ¼2.3, 95% confidence, Fig. 2a). The 10Be/11B ratios in E65 range up to 50 and the 10Be isochron yields 10Be/9Be(i) ¼ 7.0 71.7
10−4 (2s, MSWD ¼0.76) and δ10B(i) ¼10.1 75.7‰ (2s) (Fig. 2b). In E36 the 10Be isochron yields 10Be/9Be(i) ¼ 7.0 7 1.4
10−4 and δ10B(i) ¼15.1 74.1‰ (95% confidence, MSWD¼ 1.4, Fig. 2c). The 10Be isochrons for the three CAIs are well defined and indistinguishable within errors, and unlike Al–Mg system in these CAIs, the Be–B system is undisturbed (at least within the present analytical errors) because two CAIs can be fitted with MSWD of  1 and the third has an MSWD of 2.3 indicating minor disturbance. The inferred 10Be abundance in Efremovka CAIs overlaps within errors with the highest ratio of 10Be/9Be ¼8.8 7 0.6
10−4 reported for Allende CAI 3529-41 (McKeegan et al., 2000; Chaussidon et al., 2006). The ranges of variations of 26Al/27Al(i) and 10Be/9Be(i) are not large enough to determine any correlation between these two radionuclides

5. Discussion 5.1. Implications of

26

Al/27Al variations in Efremovka CAIs

The three coarse-grained Efremovka CAIs (E65, E66 and E36) like many CAIs studied earlier are products of high temperature crystallization as implied by experiments carried out over the last three decades (e.g., Davis and Richter, 2005) showing that mineral composition such as melilite, spinel, pyroxene, plagioclase feldspar, and hibonite are crystallization products of melts with bulk composition similar to CAIs. The 26Al mineral isochrons thus represent the 26Al/27Al ratio in the CAI melts at the time of the last melting–crystallization events, if perturbations of the isochrons due to low temperature alteration are minor. The 26Al/27Al(i) ratio of the two Efremovka CAIs (E65: 4.32 7 0.36
10−5 and E36: 4.56 7 0.31
10−5) overlap within errors while E66 value (4.04 7 0.18
10−5) is significantly less. Since E65 Al– Mg isochron using melilite–pyroxene–spinel data shows significant scatter from the isochron suggesting major disturbance, as also reflected in MSWD of  10, 26Al abundance in E65 cannot be used to infer reliable time intervals of formation. Assuming that the reservoir from which CAIs were derived was once homogenized for 26Al to 26Al/27Al  5.23
10−5 (Thrane et al., 2006; Jacobsen et al., 2008; MacPherson et al., 2010, 2012) and considering this time of homogenization as time zero, the

G. Srinivasan, M. Chaussidon / Earth and Planetary Science Letters 374 (2013) 11–23

60

E66 26Al/27Al

(i)=(4.04±0.18)

26Mg* = (i)

10-5

+0.28±0.05

MSWD=2.7

40 30

26

Mg* (

)

50

20

Mel Mel+Hib Pyx Sp

10 0 0 6

40

80

120

200

160

E65 26Al/27Al

5

(i)=(4.42±0.38)

26Mg* = (i)

4

10-5

-0.10±0.07

26

Mg* (

)

MSWD= 9.6

3 2 1

Mel Pyx Sp

0 -1

0

4

8

12

16

8

E36 26Al/27Al

(i)=(4.56±0.31)

26Mg* = (i)

6

10-5

+0.04±0.09

4

26

Mg* (

)

MSWD= 3.1

2

Mel Mix (Sp+Mel+Pyx) 0

0

4

8

12

16

20

24

27Al/24Mg Fig. 1. The three vertical panels show the 26Al–26Mg isochron diagrams for the three CAIs E66 (panel a), E65 (panel b) and E36 (panel c). The 27Al/24Mg and δ26Mg* (‰) are the measured quantities for all the panels and are in Table 1. The analyzed phases in all CAIs are indicated by different symbols: melilite (Mel), pyroxene (Pyx), spinel (Sp), hibonite (Hb), mixture of spinel +pyroxene+mel (Mix). In E66 the hibonite and pyroxene data may have some contribution from surrounding melilite. The 26Al/27Al(i) and δ26Mg(i) calculated by linear regression using Isoplot Model 1 and the errors are 95% confidence which is more than 2 sigma errors in case MSWD is 4 1 and comparable to 2 sigma when MSWD  1. The dashed line shown for reference is the CAI precursors solar system initial 26Al/27 Al(0) and δ26Mg(0) (see text for details and references). The 26Al/27 Al(i) and δ26Mg(i) using Isoplot Model 1 are shown in figures.

17

mineral isochrons of E66 and E36 indicate crystallization at 0.272 7 0.046/0.048 Ma (2s) and 0.144 7 0.069/0.074 (2s) Ma for the two CAIs respectively after time zero. The lower and upper limits for this time interval for E66 and E36 respectively are distinguishable at 2 sigma errors while they overlap at three sigma levels. Using Model 3 values the crystallization times are indistinguishable at two sigma level. E66 and E36 Al–Mg isochrons have MSWD 4 1 using Isoplot Model 1 for regression analyses, the data hint at the possibility of E66 and E36 crystallized at different times, with the caveat this time gap is not resolved using Model 3 values. Fractionation events in the nebula, condensation and/or evaporation which separated refractory from volatile elements leading to the formation of CAI precursors, assigned as time zero was followed by a minimum time gap of 144,000 yr prior to the crystallization of CAI E36 and up to at maximum 272,000 yr before crystallization of E66. The comparison of the δ26Mg(i) inferred from the intercept of the Al–Mg isochron with the value predicted by the growth curve of δ26Mg because of 26Al decay (MacPherson et al., 2012) is a diagnostic for closed or open system evolution of CAI precursors before final crystallization. Applied to chondrules (Villeneuve et al., 2009) this prediction implies a homogeneous distribution of Mg and Al isotopes in the CAI forming region with δ26Mg(0) ¼ −0.04‰ when 26Al/27Al(0) ¼5.23
10−5. If a CAI remained a closed system, 26Mg excess would build up due to 26Al decay at a rate depending on its bulk 27Al/24Mg. In a single step melting model when 26Al is still extant, the last melting/crystallization event, as recorded by 26Al/27Al(i) ratio, would homogenize the Mg isotopic composition to the δ26Mg(i) value as determined from the 26Al isochron. Considering closed system decay of 26Al from initial 26 Al/27Al(0)  5.23
10−5 and consequent growth in bulk δ26Mg values in CAI precursors with variable bulk 27Al/24Mg (Fig. 3), CAIs E66 and E36 with δ26Mg(i) ¼0.28 70.05‰ and 0.04 70.09‰ evolved as a closed system with 27Al/24Mg  4.5 and 1.5 respectively, although for E36 partial exchange cannot be ruled out. For the CAI E65 δ26Mg(i) ¼ −0.10 70.07‰ is within errors consistent with solar system initial δ26Mg(0) ¼−0.04‰, implying an open system evolution prior to final crystallization resulting in exchange of non-radiogenic Mg perhaps with the nebular gas (with solar 27 Al/24Mg  0.1011) to varying degrees, thus redistributing the Mg and erasing the radiogenic 26Mg due to 26Al decay prior to last crystallization event. X-ray mapping based accurate determination of bulk 27Al/24Mg in CAIs is beyond the scope of the present study. The Al–Mg system of the E66, E36 and E65 have suffered alteration to varying degrees as exhibited in the scatter of the Al–Mg data and MSWD values of the isochrons. Based on 26Al isochron we also infer that following crystallization Al–Mg system in E66 and E36 suffered less alteration compared to E65 (Fig. 1). Model 1 based regression analyses of plagioclase feldspar analyzed near the edge of CAI E65 together with pyroxene as anchor for the low Al/Mg phases gives 26Al/27Al(i) ¼5.6 7 8.4
10−6 (MSWD ¼ 10.7, 95% confidence), The lower limit is unresolved from zero and the upper limits of 1.4
10−5 is 3 times less than the 26 Al/27Al(i) for melilite–pyroxene–spinel isochron. The mineral assemblage of melilite, spinel, pyroxene and plagioclase feldspar is a product of igneous crystallization of type B1 CAI like E65. The presence of Na in plagioclase feldspar and the lower 26Al abundance in E65 plagioclase feldspar is indicative of a later alteration event. It is not obvious if the scatter in the spinel and melilite (Fig. 1b) is related to the alteration event that introduced Na, a volatile element into the CAI, or if it was a different event, since Na is not present in melilite. Our spinel–pyroxene–melilite isochron 26 Al/27Al ratios overlap within errors with 3.99 70.27
10−5 determined using melilite–spinel–pyroxene–plagioclase feldspar in E65 (Goswami et al., 1994). However, unlike the plagioclase analyzed in the same polished section of E65, which is altered, the

18

G. Srinivasan, M. Chaussidon / Earth and Planetary Science Letters 374 (2013) 11–23

E66 10Be/9 Be =(7.6±2.9) (i) 10B = +10.6±6.9 (i)

10-4

10B/11B

MSWD= 2.3

E65 10Be/9 Be =(7.0±1.7) (i) 10B = +10.1±5.7 (i)

10-4

5.2. Constraints on variations in CAIs

10B/11B

MSWD= 0.76

E36 10Be/9 Be =(7.0±1.4) (i) 10B = +15.1±4.1 (i)

are equivalent to model values since isochron is force fitted through origin using pyroxene data. Based on the upper limit of 26 Al abundance the alteration in E65 took place at least 1.4 Ma after the time zero defined by the separation of CAI precursors from the solar nebula, alternatively incorporation of Na and exchange of Mg isotopes from plagioclase because of an alteration event much after complete decay of 26Al cannot be ruled out; in the latter case there is no time constraint. The high quality of the Be–B isochron in all three CAIs suggests that 10Be in these CAIs was produced and/or incorporated prior to crystallization. By corollary 26Al which can also be produced by irradiation along with 10Be, albeit less efficiently, was not the product of irradiation of the solid CAI. The lower 26Al abundance observed in these CAIs compared to initial solar system abundance following separation from the nebula, is because of elapsed time since the separation of solid precursors. The CAI precursors evolved over varying time periods and E66 26Al data suggests a period of 272,000 yr before final crystallization. Some of the CAIs were subjected to secondary alteration, much later, as evidenced by introduction of volatile elements.

10-4

Be origins from coupled B and Be isotopic

The 10Be/9Be(i) in CAIs show a large range of variations from ≈3
10−4 to ≈1
10−3 (McKeegan et al., 2000; Sugiura et al., 2001; MacPherson et al., 2003; Chaussidon et al., 2001, 2006; Wielandt et al., 2012). A 10Be/9Be(i) higher by one order of magnitude (10.4 71.6
10−3) has even been found recently in one CAI from the CH/CB Isheyevo (Gounelle et al., 2013). The origin of this wide range in 10Be abundance at the time of isotopic closure for the Be–B system in CAIs is a key question. The three Efremovka CAIs in the current study have indistinguishable 10Be/9Be ratios within errors. The variations in 26Al/27Al(i) in CAIs suggest that following their early formation some of them further evolved at high temperature in the disk (MacPherson et al., 2010, 2012), extending for ≈0.27 Myr as in the case of Efremovka CAI (E66) of this study. Such an extended evolution cannot account for the wide range in 10 Be/9Be(i) in CAIs because half-life of 10Be is longer than the half-

10B/11B

MSWD= 1.4

10

27

24

Reservoir Al/ Mg

0.6

1.5 2 2.5 3 4 5 0.1011

δ26Mg* ‰

0.4

9Be/11B Fig. 2. The three vertical panels show the 10Be–10B isochron diagrams for the three CAIs E66 (a) top panel, E65 (b) middle panel, and E36 (c) bottom panel. The ratios 9 Be/11B and 10B/11B are the measured quantities (Table 2). Melilite is the only phase analyzed in the CAIs. The 10Be/9Be(i) and δ10B(i) calculated by linear regression for the three isochrons (solid lines) using Isoplot Model 1 and the 95% confidence limit or 2s values are shown for errors in each panel for each CAI.

plagioclase analyzed by Goswami et al. (1994) in E65 do not show any sign of alteration. Altered plagioclase in CAI E44 also has low 26 Al/27Al¼ 4.6 71.1
10−6 (Goswami et al., 1994). The Efremovka section was polished between the two analyses and it is not clear if the unaltered plagioclase was completely lost during the process. The inferred 26Al/27Al values based on plagioclase–pyroxene data

E66 0.2

E36 0.0

E65 -0.2 0

10-5

2x10-5

3x10-5

4x10-5

5x10-5

6x10-5

26Al/27Al Fig. 3. Growth curves of δ26Mg* (‰) values against initial 26Al/27Al ratios decaying as function of time are shown for different closed systems with variable 27Al/24Mg ratios. The curve for the bulk solar system (with 27Al/24Mg ¼ 0.1011) is shown in black. This is calculated with solar system initial, referred to with subscript (0), of δ26Mg*(0) ¼ −0.040 (‰); 26Al/27Al(0) ¼ 5.23
10−5, so that the Earth is plotting on the solar system growth curve (Villeneuve et al., 2009). Any closed system evolution of the CAI precursors or the CAI itself before last melting event will result in the δ26Mg (i) and 26Al/27Al(i) of the CAI plotting on the growth curve corresponding to the bulk 27Al/24Mg of the CAI. Only two of our three Efremovka CAI samples (E36 and E66) could be explained by such a simple evolution.

G. Srinivasan, M. Chaussidon / Earth and Planetary Science Letters 374 (2013) 11–23

life of 26Al by a factor of two. Low temperature alteration also cannot account for the 10Be/9Be variations as discussed previously (Sugiura et al., 2001). Further, the three Efremovka CAIs show no disturbance in the 10Be isochron and have the same inferred 10 Be/9Be ratio even though E65 shows significant signs of scatter in the 26Al–26Mg system in melilite and low temperature alteration which has reset the plagioclase after the last melting– crystallization event. Two scenarios can be invoked to explain the 10Be/9Be(i) range observed in CAIs: (i) radioactive decay of 10Be from a homogeneous solar system initial 10Be/9Be(0) of at least 1.4
10−3, and (ii) large heterogeneities in 10Be/9Be due to mixing in CAIs of Be produced by irradiation of the nebula gas, of dust precursors, or of the CAIs themselves. As discussed in the last section, the case for 10 Be production by solar cosmic ray irradiation of gas and dust poses a conundrum for 41Ca which is easily produced by irradiation and should be more vastly abundant in CAIs than observed. Because different collateral variations of the 10B/11B ratios can be predicted in these two scenarios, one way to decipher the origin of the 10Be/9Be range is to search for relationships between the 10 Be/9Be(i) and the 10B/11B(i) ratios inferred from the 10Be isochrons.

5.2.1. Available data set for 10Be/9Be and initial 10B/11B in CAIs and hibonites The above two scenarios can be tested with literature data for B and Be isotopic compositions in CAIs (McKeegan et al., 2000; Sugiura et al., 2001; MacPherson et al., 2003; Chaussidon et al., 2001, 2006; Wielandt et al., 2012; Gounelle et al., 2013). However, all these data, or all these CAIs, are not easily comparable for precision or quality of the 10Be isochron. 10B/11B(i) inferred with a precision better than 730‰ (2 sigma) is used as a criteria for the robustness (or quality) of the 10Be isochron. Three CAIs (Vig 16239, Vig 477-5, and Leo 355-5b) with 10B/11B(i) 2 sigma errors from 735‰ to 739‰ from MacPherson et al. (2003) and two CAIs (Efremovka E69 and Allende 001) with 7105‰ and 773‰ from Sugiura et al. (2001) are excluded from further analyses. A compact type A CAI from Efremovka, E104 is also excluded from the data set because the MSWD of the linear regression calculated between 10B/11B and 1/11B is slightly better than that of the linear regression calculated between 10B/11B and 9Be/11B (Wielandt et al., 2012). The reasons for poorly constrained 10B/11B(i) intercepts or 10 Be isochrons is an open question; it is not necessarily due to analytical uncertainties, but may reflect that the source of B isotope variations in CAIs is not limited to radioactive decay of 10 Be but could include mixing between B from several sources with different isotopic compositions. The 10B/11B(i) of CAIs with poorly defined 10Be isochrons seem systematically different from CAIs with better defined 10Be isochrons. The δ10B(i) intercepts of these CAIs are all (except All 001) significantly lower (δ10B(i) ranging from +1.7‰ to −40.4‰) than of CAIs showing well defined 10Be isochrons which have generally positive δ10B(i) values (i.e. negative δ11B(i) values, see Figs. 4 and 5). This seems to be also the case (δ10B(i) ¼ −18 725%) for the CAI from Wielandt et al. (2012) for which the existence of a 10Be isochron can be questioned. The presence of B enriched in 11B in meteorites is rare, but has previously been reported in chondrules with a few δ11B values up to +44‰ (13% of the chondrule data showing δ11B 4+10‰, Chaussidon and Robert, 1995, 1998). The origin of this 11B-rich component has been ascribed primarily to low-energy collisions between protons and oxygen nuclei which may result in 11B/10B ratios up to ≈8 (Cassé et al., 1995; Ramaty et al., 1996; Chaussidon and Gounelle, 2006), however, the process is not fully understood partly because the cross sections are poorly constrained. A total of 22 CAIs pass the above criteria and are considered further in the following discussion and shown in Figs. 4 and 5 (see

19

Supplementary Annex for Table with all data). In addition, the 10 Be/9Be(i) and 10B/11B(i) determined for hibonites from Murchison are included in the data set. The “isochron” for hibonites is calculated using different hibonite grains which may not be genetically related to each other. The ratios (10Be/9Be(i) ¼5.1 7 1.4
10−4 and 10B/11B(i) ¼0.253 7 0.002, or δ11B(i) ¼−21.8 7 8.8‰) determined by Liu et al. (2009) are preferred to the ratios (10Be/9Be(i) ¼5.2 7 2.5
10−4 and 10B/11B(i) ¼0.272 7 0.016, or δ11B(i) ¼ −92 7 58‰) from Marhas et al. (2002), Marhas and Goswami (2003), because the former data have better precision. One CAI (Vig 477-4b) from MacPherson et al. (2003), though it fulfills the criteria defined above, has a positive δ11B(i) of +11.0 7 5.7‰ and is not shown in Figs. 4 and 5. The two scenario discussed below (Figs. 4 and 5) cannot explain, by themselves only, the composition of Vig477-4b. 5.2.2. Scenario 1: homogeneous distribution of 10Be in the CAIs forming region By analogy with the distribution of 26Al, the simplest scenario to consider for 10 Be is that, early on, Be and B isotopes were homogenized in the CAI forming region. This is a direct outcome of presolar origin of 10 Be by trapping of galactic cosmic rays in the parent molecular cloud (Desch et al., 2004). On the other hand irradiation scenarios predict heterogenous distribution of 10 Be depending on the composition of the irradiated components and the fluence to which they were submitted (see Scenario 2); although homogenization through evaporation–condensation processes and turbulent mixing in the disk cannot be ruled out (Gounelle et al., 2001, 2006; Leya et al., 2003). In case of an initial homogenous distribution of Be and B isotopes, the increase of the 10 B/11B ratio with time due to the production of radiogenic 10 B in a closed system having a given 9Be/11 B ratios is simply expressed by the following mass balance equation:   10 B=11 BðtÞ ¼ 10 B=11 Bð0Þ þ9 Be=11 B
10 Be=9 Beð0Þ −10 Be=9 BeðtÞ where the subscripts (0) and (t) refer to isotopic ratios at time 0 taken as a reference and at time t after time 0, respectively (because 9 Be and 11B are both stable and non-radiogenic, their ratio is not changing with time). Fig. 4 shows that all existing data (CAIs and hibonites, except Vigarano 477-4b and Isheyevo 411, not shown in Fig. 4) can be explained by 10Be decay in reservoirs having 10Be/9 Be(0)≈1.4
10−3 and 9 Be/11 B ratios from 2 to 10. Because the 9Be/11B of the nebular gas of solar (or chondritic) composition is ≈0.029 (Lodders, 2003), putative reservoirs having 9Be/11B ratios from 2 to 10 can only be refractory solids. This is because Be is more refractory than B with temperatures of 50% condensation of 1452 and 908 K, respectively (Lodders, 2003). Such reservoirs could be CAI precursors or the CAIs themselves. The implications of such an interpretation would be that (i) there was a time 0 in the accretion disk sampled by CAIs and hibonites when the 10 Be/9Be ratio was homogenized to ≈1.4
10 −3 and (ii) the variations observed for the 10B/11B (i) ratios of each CAIs reflect isotopic homogenization of B at times indicated by the 10 Be/9Be(i) ratios, i.e. from ≈0.8 to ≈3 Myr after the time zero defined by 10 Be/9Be(0) ¼1.4
10−3 . Because in this interpretation, the time zero corresponds to the start of the evolution in closed system of refractory solids it must coincide with the age of the oldest refractory solids in the solar system, and could thus be assumed to be the U–Pb age of CAIs. Though appealing this interpretation contradicts the time– temperature histories that can be inferred for the CAIs from their 26 Al–26Mg systematics unless 26Al abundance in the accretion disk was heterogeneous by up to an order of magnitude. The closure

20

G. Srinivasan, M. Chaussidon / Earth and Planetary Science Letters 374 (2013) 11–23

Time (Myr) 4

+40

3

2

1

0 0.240 Solar system initial 0.245

0

0.255 Efremovka CAIs Allende CAIs 0.260 Axtell CAI NWA 779 CAI Isheyevo CAIs 0.265 Murchison hibonites

-40 -60

B/11B(i)

0.250 -20

10

δ11B(i) (‰)

+20

-80 0

2×10

-4

4×10

-4

6×10 10

-4

8×10

-4

1×10

-3

1.2×10

-3

1.4×10

-3

Be/9Be(i)

Fig. 4. Variations of the initial B and Be isotopic compositions derived from 10Be mineral isochrons in CAIs and hibonites compared to growth curves of B isotopic composition with decay of 10Be. The different symbols correspond to all the data existing for 10Be isochrons determined with high precision for CAIs and hibonites (see text for references and discussion and Electronic Annex for data Table). Three growth curves are calculated assuming (i) closed system decay of 10Be, (ii) three 9Be/11B ratios of 2, 5 and 10, likely to describe bulk composition of CAIs, and (iii) a solar system initial defined by 10Be/9Be(0) ¼ 1.4
10−3 and δ11B(0) ¼ −3.3‰. For this model to explain CAI compositions, CAIs must have been remelted to homogenize their B isotopic composition at the times indicated by their 10Be/9Be(i) ratios, i.e. ≈0.8–3 Myr after condensation, which seems in total contradiction with time inferred from 26Al/27Al systematic.

50

0.240 0.250

0

0.260 0.270 0.290

-150

0.300

B/11B(i)

0.280

-100

10

δ11B(i) (‰)

-50

0.310

-200

0.320 -250

0.330

-300 0

2×10-4

4×10-4

6×10-4 10

8×10-4

1×10-3

-3 1.2×10-3 1.4×10

Be/9Be(i)

Fig. 5. Variations of the initial B and Be isotopic compositions derived from 10Be mineral isochrons in CAIs and hibonites (same symbols as in Fig. 4) and modeled for solids condensed in a solar nebula undergoing irradiation from the young active Sun. The nebula is considered to be a mixture of a small fraction of refractory solids produced during the earliest steps of condensation and of un-fractionated solar gas (see text for details on the compositions used). Because the gas and the refractory solids have very different Be/B ratios, they develop during irradiation (the gas and the grains are exposed to the same proton fluence) very different B and Be isotopic ratios. In this model, CAI compositions should fall on the mixing curves between solids and gas, which are shown here for three solid/gas ratios (of 1
10−3, 5
10−3 and 1
10−2), the dots on the mixing curves corresponding to various fractions (0, 0.01, 0.02, 0.03, 0.04, 0.05, 0.1, 0.2,…,1) of Be in CAI condensed from the gas. The B and Be isotopic compositions of the CAIs can be explained by between ≈5 and ≈50% of Be condensed from the gas. In CAIs δ11B(i) o≈−20‰ imply solid/gas ratios of the parent reservoir 45
10−3 and a dominant fraction ( 490%) of Be originating from the irradiation of solids (for the irradiation parameters considered here).

temperature for B diffusion in melilite is similar or higher than that for Mg diffusion (Sugiura et al., 2001). The isotopic homogenization of B requires a high temperature process which would also homogenize Mg isotopes. A 0.8−3 Myr time interval would imply 26Al/27Al ratios in melilite from 2.5
10−5 to 3.4
10−6, which is at odds with CAI data in general. The 10Be/9Be ratio of 7.0– 7.8
10−4 for the three Efremovka CAIs would imply 26Al/27Al ratios of 2.0–2.5
10−5. The low 10Be/9Be of 5.1
10−4 for hibonites would imply that they formed ≈1.7 Myr after the time zero at variance with their 26Al/27Al ratios indicating very early formation in the disk before 26Al was homogenized in the CAI forming region (Liu et al., 2012a).

5.2.3. Scenario 2: heterogenous distribution of Be and B isotopes resulting from irradiation processes The second possible explanation for the range of 10Be/9Be(i) and 10B/11B(i) shown by CAIs and hibonites is to consider mixing in various proportions of two reservoirs characterized by different Be and B isotopic ratios. Obviously, in such a scenario chronological information cannot be retrieved from the 10Be/9Be variations in CAIs. A very simple way to produce these two reservoirs is irradiation by the young active Sun of the inner edge of the accretion disk, where collisions between accelerated solar protons (and alpha particles) and primarily oxygen atoms (in the gas or in solids) can produce the different isotopes of

G. Srinivasan, M. Chaussidon / Earth and Planetary Science Letters 374 (2013) 11–23

Li–Be–B elements (Gounelle et al., 2001, 2006; Leya et al., 2003). This part of the disk can be considered to be a mixture of a small fraction of refractory solids produced during the earliest steps of condensation and of un-fractionated solar gas. For a given proton fluence, very different Be and B isotopic ratio will develop in the gas and the refractory high temperature condensates (i.e. the putative solid precursors of the CAIs) because of their strong difference in Be/B due to the fact that Be is much more refractory than B, as noted above. The progressive changes in B and Be isotopic compositions due to the addition of spallogenic B and Be can be calculated as mixing curves between (i) the non-irradiated reservoirs sharing the solar 10B/11B ratio (10B/11B ¼0.2480 or δ11B ¼−3.3‰; Zhai et al., 1996) but having different B/Be (28 for the solar gas, Lodders, 2003; 0.1 taken as a typical ratio in CAIs, Chaussidon et al., 2006) and the (ii) pure spallogenic component (10Be/9Be ¼ 0.1, 10B/11B ¼ 4, B/Be ¼17, Meneguzzi et al., 1971; Read and Viola, 1984; Reeves, 1994). Significant variations of the 10B/11B and B/Be spallogenic ratios depending on the energy of the protons are anticipated (Ramaty et al., 1996) but this is not considered further in the present calculation because (i) the major cause of the difference in B and Be isotopic compositions between the gas and the solids is their difference by two orders of magnitude in B/Be ratio and (ii) the objective of this calculation is to demonstrate the feasibility of this scenario, not to constrain precisely the irradiation conditions. A small fraction only of the two mixing curves (up to 10 Be/9Be¼1.4
10−3) is shown in Fig. 5. The relative amounts of 10 Be which will be produced by spallation reactions in the solids and in the gas depend on the relative amounts of oxygen in the two reservoirs, which in turn depend on the relative fractions of solids and gas. Three solid/gas mass ratios of 1
10−3, 5
10−3 and 1
10−2 corresponding to typical values predicted for the first steps of a condensation sequence (for T41500 K, Davis and Richter, 2005) are considered in the calculation. For a given solid/gas ratio, solid and gas that have very different B and Be isotope compositions will coexist in the disk. Partial isotopic exchange between the gas and the solids and continuous condensation of the gas onto the earliest solids will lead to solids having intermediate B and Be isotopic compositions, following the three mixing curves shown in Fig. 5 for the three solid/gas ratios. The 10Be/9Be(i) and 10B/11B(i) ratios of CAIs and hibonites, if interpreted in the framework of this scenario, would indicate that a major fraction of their Be is derived from solid precursors. For instance for a solid/gas ratio of 5
10−3, 97–98% of Be coming from the solids is required for the CAIs with a low δ11B(i) of −50‰, while this fraction can be ≈70–80% for a CAI with a δ11B(i) of −10‰ (Fig. 5). Such a conclusion for hibonites would be in agreement with the fact that they could be irradiated early condensates (Liu et al., 2012a). Because melting of a CAI or of its precursors is the only process that can homogenize the Be isotopic composition to the level shown by the 10Be isochrons in CAIs melilite (e.g., Fig. 2a–c for the present Efremovka CAIs), it seems logical to postulate that the spallogenic B and Be must be produced before the last melting events “dated” by the 26Al isochron. This would give a time window for irradiation processes of ≈3
105 yr for the present Efremovka CAIs, shorter or even longer windows (up to≈7
105 yr) being possible from 26Al in various CAIs (MacPherson et al., 2012). This seems compatible with previous studies (Gounelle et al., 2001, 2006; Leya et al., 2003) which have shown that 10Be/9Be ratios of 10−3 (even of 10−2, Gounelle et al., 2013) can easily be produced by irradiation of the accretion disk around the young active Sun for characteristic timescales of a few 104–106 yr. As pointed out by Leya et al. (2003) shorter timescales of ≈1 yr would be required to explain the presence of 7Be inferred for one Allende CAI (Chaussidon et al., 2006) but this would imply very high-energy (and rare) flaring events of the young Sun.

21

The amounts of spallogenic 10Be originating from trapping of GCR in the presolar molecular cloud parent to the solar system (Desch et al., 2004) and/or from solar wind implantation in CAIs precursors (Bricker and Caffee, 2010) can be constrained at first order from Fig. 5. Because by definition the Be/B ratios in the parent molecular cloud and in the solar photosphere are the same as the solar ratio, the addition of these spallogenic components should follow the curve labeled “irradiated gas” in Fig. 5. As discussed above, in order to explain the low initial δ11B values of CAIs and hibonites, a dominant contribution (80–90%) of 10Be from the irradiation of solids must be present. Thus the fraction of 10Be possibly of presolar origin (GCR), which must be homogeneously distributed, seems minor. However a significant fraction of SW implanted 10Be cannot be ruled out for CAIs with initial δ11B values close to 0‰. It is even conceivable that the positive δ11B values found in some CAIs (see discussion in Section 5.2.1) could reflect low energy spallation reactions in the solar atmosphere. 5.3. Collateral consequences of

10

Be origin on the origin of

41

Ca:

In addition to 26Al and 10Be another important SLR is 41Ca, whose presence was demonstrated in coarse grained CAI E65 (Srinivasan et al., 1996) and its abundance was recently revised 41 Ca/40Ca(i) ¼ 1.4 70.6
10−9 (MSWD ¼3.6) (Liu et al., 2012b). The extremely short half-life for 41Ca, 0.1 Myr, makes it valuable in constraining the time scales of processes that produced SLRs and formation time scales of first CAIs. 41Ca can be produced by both stellar nucleosynthesis (e.g., Wasserburg et al., 1995; Cameron et al., 1995) and particle irradiation scenarios (e.g., Leya et al., 2003). Several workers (e.g., Leya et al., 2003) have shown that 10Be production by SCR (at a level of 10Be/9Be≈10−3) would be accompanied by 41Ca production, with 41Ca/40Ca ratios as high as 2
10−7 or more, i.e., few orders of magnitude higher than the ratios observed in CAIs. This prediction seems robust unless calculations overestimate 41Ca production because of incorrect parameters. The correlated presence and/or absence of 26Al and 41Ca were shown in refractory hibonite grains (Sahijpal and Goswami, 1998, Sahijpal et al., 2000). Egg3 Allende CAI 26Al/27Al(i) ¼5.297 0.39
10−5 (Wasserburg et al., 2012) and 41Ca/40Ca(i) ¼1.177 0.24
10−8 (Sahijpal et al., 2000). In hibonite grain CH-B6 26Al/27 Al(i) ¼5.170.4
10−5 and 41Ca/40Ca(i) ¼ 1.0770.40
10−8 (Sahijpal et al., 2000). Using 26Al as a chronometer, [(26Al/27Al)0/(26Al/27Al)i]τ (26Al)/τ(41Ca) (Gounelle et al., 2006) gives the value of multiplicative factor for scaling the measured 41Ca abundance in sample to obtain the solar system initial 41Ca/40Ca(0) at time zero where τ(26Al) and τ(41Ca) are the mean-lifes of 26Al and 41Ca respectively. The solar system initial (26Al/27Al)0 value determined from whole-rock CAI samples (Jacobsen et al., 2008) anchors time zero, and (26Al/27Al)i is the measured value in a sample in which 41Ca abundance has also been determined. Using the 26Al/27Al(i) ¼4.4270.38
10−5 in E65 from spinel–proxene–melilite data, the solar system initial 26 Al/27Al(0) 5.23
10−5, the multiplicative factor for E65 is 4.0, and the 41Ca/40Ca(0) in early solar system is  5.670.74/1.31
10−9, albeit that MSWD for the E65 Al–Mg is very high rendering its 26Al signature less reliable for temporal calculations. Using the measured 26 Al abundance in Egg3 and CH-B6, the scaling factor is 0.9 and 1.2 and initial solar system 41Ca/40Ca(0) ¼1.170.4/−0.3
10−8 and 1.370.2
10−8 respectively. The solar system initial 41Ca abundance inferred from Egg3 and CH-B6 data overlap with in errors, however they do not converge with values determined using data E65. The estimate of 41Ca at time zero and its variability is not constrained to the same extent in all samples because of disparities in Al–Mg measurement qualities, it nevertheless underscores the importance of investigating solar system initial 41Ca/40Ca(0)

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G. Srinivasan, M. Chaussidon / Earth and Planetary Science Letters 374 (2013) 11–23

ratio more carefully through high quality measurements of both 26 Al and 41Ca. If 41Ca was co-produced with 10Be then 41Ca abundance can be estimated by using production functions (e.g., Leya et al., 2003). For the measured 10Be/9Be  7
10−4 in E65 the 41Ca abundance is expected at least two orders of magnitude higher than the observed 41Ca/40Ca  1.4
10−9. If some or whole of 10Be was produced by irradiation of refractory solids or nebular gas by solar cosmic rays then a component of 41Ca was also coproduced. The extremely low abundance of 41Ca in CAIs needs further investigation.

6. Conclusion The developments over the last years of high precision MCICPMS and MC-SIMS studies of the 26Al–26Mg system in chondrules and CAIs (e.g., Thrane et al., 2006; Jacobsen et al., 2008; Villeneuve et al., 2009, 2011; MacPherson et al., 2010, 2012; Larsen et al., 2011) have allowed to identify much more precisely than before several key steps in the history of these objects. The new constraints obtained by comparing whole-rock and mineral isochrons, and variations of 26Al/27Al(i) vis-a-vis δ26Mg(i) are (i) early separation from the nebula of the refractory precursors, (ii) protracted closed and open system high-temperature processing in the nebula, (iii) last high temperature melting (partial or total) and crystallization and, (iv) late (up to few Myrs) low-temperature perturbations. This sets a more precisely constrained framework for the understanding of the origin and distribution of SLRs like 10 Be and 41Ca than before. The high precision Al–Mg data for the three Efremovka CAIs show that following separation from nebula CAI precursors were continuously processed as closed and open system over a span of nearly several hundred thousand years before final crystallization. The low temperature alteration of the CAIs evidenced by the incorporation of Na perhaps took place much later. The large spread in the 10Be/9Be(i) and δ11B(i) of these CAIs and other published data for CAIs do not support a chronological interpretation of the 10Be/9Be(i) variations and a homogeneous distribution of 10Be in early solar nebula as it is for 26Al. Irradiation of refractory precursors and the nebula gas by solar cosmic rays during the few hundred thousand years window limited by 26Al decay is consistent with CAIs and hibonite compositions. The above poses new questions for the origin and distribution of 41Ca in the early solar system. Using the measured 26Al abundance in CAI E65 (this work) and 41Ca abundance (Srinivasan et al., 1994, Liu et al., 2012b) leads to a 41Ca/40Ca(0) ratio of  5.6
10−9 at time zero, defined by CAI precursor separation with 26Al/27Al(0)  5.2
10−5. The divergence in estimated 41Ca/40Ca(0) values for CAIs Egg3 and refractory hibonite grain CH-B6 from the value estimated from CAI E65 is not in keeping with origin of 26Al and 41Ca from a common stellar source. These estimated 41Ca/40Ca(0) are one to two orders of magnitude lower than those predicted by irradiation process as constrained by 10Be abundance. If low temperature remobilization of K isotopes in CAIs is not the reason of this discrepancy, then the “missing” 41Ca holds probably a key to the question regarding the nature of irradiation in the early solar system.

Acknowledgment This work was supported by Professional Development Fund provided to GS by Indian Institute of Science, Bangalore, India.

The author (GS) acknowledges the gracious support and hospitality provided by CRPG-CNRS to complete this work. This work was supported by grants from European Research Council (ERC grant FP7/2007–2013 Grant Agreement no. [226846] Cosmochemical exploration of the first two Million Years of the Solar System— CEMYSS), and ANR-08-BLAN-0260-CSD6. This is CRPG publication number 2245.

Appendix A. Supporting Material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.epsl.2013.03.048.

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