Titanium isotopes and the radial heterogeneity of the solar system

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Earth and Planetary Science Letters 266 (2008) 233 – 244 www.elsevier.com/locate/epsl

Titanium isotopes and the radial heterogeneity of the solar system Ingo Leya a,⁎, Maria Schönbächler b , Uwe Wiechert c , Urs Krähenbühl d , Alex N. Halliday e a

Physikalisches Institut, University of Bern, CH-3012 Bern, Switzerland Department of Earth Science & Engineering, Imperial College London, UK c Institut für Geologische Wissenschaften, Freie Universität Berlin, Germany d Laboratory for Radiochemistry, University of Bern, Switzerland e Department of Earth Sciences, University of Oxford, Oxford, UK

b

Received 31 July 2007; received in revised form 11 October 2007; accepted 15 October 2007 Available online 22 October 2007 Editor: R.W. Carlson

Abstract The solar system is assumed to be uniform on a large scale in terms of the isotope composition of refractory elements. Here we show that the titanium (Ti) isotope compositions of carbonaceous chondrites differ from those of ordinary chondrites, eucrites, mesosiderites, ureilites, the Earth, Moon, and Mars, all of which are indistinguishable. Leachates and mineral separates demonstrate that this feature is homogeneously distributed within a range of phases in the carbonaceous chondrites Allende and Renazzo. The data therefore indicate that the solar nebula that fed planetesimals between ∼ 1 AU and ∼ 2.4 AU, e.g. Earth, Moon, Mars and the parent bodies of ordinary chondrites, eucrites, ureilites, and mesosiderites, was homogeneous for Ti (and Sr, Ba, Nd, Sm) isotopes. In contrast, carbonaceous chondrites, which probably formed beyond ∼ 2.7 AU appear to have acquired a distinct mix of primitive components, which is consistent with their lack of depletion in volatile elements and late formation. © 2007 Elsevier B.V. All rights reserved. Keywords: Ti-isotopes; early solar system; solar nebula; isotope heterogeneities

1. Introduction The solar system as sampled by meteorites is relatively homogeneous isotopically, indicating efficient mixing of the material from the precursor molecular cloud. The preservation of isotopic heterogeneities is usually attributed to lack of equilibration of pre-solar grains and/or some refractory condensates with gas and dust of average solar composition. Titanium is well suited for studies of ⁎ Corresponding author. E-mail address: [email protected] (I. Leya). 0012-821X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2007.10.017

solar system isotopic heterogeneities in dust and high temperature condensates because it forms one of the most refractory oxides and is concentrated in calcium aluminium refractory inclusions (CAIs) either by condensation or evaporation (Grossman, 1972). Titanium isotope variations could in principle result from three very different processes: i) nucleosynthetic heterogeneity resulting from incomplete mixing in the circumstellar disk from which the planets formed, e.g. Halliday (2003); Dauphas (2004), ii) irradiation early in the solar system close to the young stellar object, e.g. Feigelson et al. (2002a, 2002b); Goswami et al. (2001#),

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Gounelle et al. (2001#), and/or iii) irradiation by galactic cosmic-rays (GCR) since the samples were exposed near the surface of their parent body or as smaller isolated fragments in space, e.g. Leya et al. (2000#, 2001#). To evaluate these possible effects we have measured Tiisotope compositions to high precision in bulk chondrites, lunar samples, and a variety of achondrites. Titanium has 5 stable isotopes (46, 47, 48, 49, 50) which provide a rich opportunity distinguishing the differing production mechanisms. The effects of GCR exposure (point 3 above) can be readily evaluated by studying lunar samples. Some of the lunar rocks studied by us have exposure ages of more than ∼100 Ma, i.e., much longer than typical exposure ages for stony meteorites, which are in the range of a few tens of Ma. Therefore, GCR induced effects, if of importance, should be well detectable in the lunar samples. We have conducted a thorough study of these to ensure that there are no such effects. Considering irradiation effects in the early solar system (point 2 above) model calculations predict enrichments of 47Ti compared to the other Ti-isotopes. Considering production and burnout for 50 Ti/ 47 Ti, 49 Ti/ 47 Ti (used for normalisation), 48 Ti/ 47 Ti, and 46 Ti/47Ti results in negative anomalies for 50Ti/47Ti and 48 Ti/47Ti, whereas the effect for ɛ(48Ti/47Ti) would be much higher than for ɛ(50Ti/47Ti). For ɛ(46Ti/47Ti) any shift would only be very minor, because the effect on 46 Ti/47Ti (without normalisation) almost completely cancels with the effect on the normalising ratio 49Ti/47Ti. Consequently, such process would rather produce negative anomalies for iTi/47Ti ratios (e.g., Leya et al., 2005). In contrast, nucleosynthetic effects (point 1 above) can produce a variety of isotopic anomalies. Titanium-46 and 47 Ti are both produced primarily in oxygen-burning and silicon-burning processes in massive stars, both hydrostatic and explosive. Titanium-48, the daughter of radioactive 48Cr, is produced in explosive Si-burning and during fusion of He into heavy elements. The latter can occur either in the alpha-rich freeze-out near the core of a type II core-collapse supernova, or during explosive Heburning. Titanium-49, the daughter of 49Cr, forms in the explosive fusion of He into heavy elements either during alpha-rich freeze-out near the core of a core-collapse type II supernova of during explosive He-burning. Titanium-50 is created primarily in Type Ia supernovae having masses close to the Chandrasekhar mass limit. Therefore, 50Ti is correlated with 48Ca, 54Cr, 58Fe, 62,64Ni, and 66,70Zn. In addition, some 50Ti is also made by the s-process in presupernova massive stars and in AGB stars. For further information and references see Clayton (2003). Isotope anomalies in bulk chondrites have been identified for oxygen (Clayton, 1993), sulphur (Thiemens

and Jackson, 1995; Farquhar et al., 2000), titanium (Niemeyer, 1988), chromium (Shukolyukov and Lugmair, 1999; Podosek et al., 1997a, 1999; Rotaru et al. 1992; Trinquier et al. 2007), molybdenum (Yin et al., 2000; Dauphas et al., 2002), ruthenium (Chen et al., 2003; Papanastassiou et al. 2004), barium (Ranen and Jacobsen, 2006; Carlson et al., 2007), samarium (Carlson et al., 2007; Andreasen and Sharma, 2006), neodymium (Carlson et al., 2007; Andreasen and Sharma, 2006), and some noble gases (Zinner et al., 1989). For some elements the effects are still subject of some controversy. In a study of carbonaceous chondrites Hidaka et al. (2003) found anomalies for 135Ba/136Ba and 137Ba/136Ba in bulk samples. Together with anomalies found in various acid leachates the data were interpreted as indicating the presence of independent nucleosynthetic components for s- and r-processes in the solar system. In another study Ranen and Jacobsen (2006) measured large excesses in 138 Ba/136Ba and smaller excesses in 137Ba/136Ba but close to normal levels of 135Ba. Very recently, Andreasen and Sharma (2007) found excesses in 135Ba and 137Ba and no anomalies in 130,132,138Ba. The same results were obtained by another group (Carlson et al. 2007), which argued that the small excesses are due to an incomplete dissolution of SiC grains. Their interpretation is supported by recent data from Wombacher and Becker (2007), which found no nucleosynthetic anomalies for Ba isotopes in chondrite samples. Contradicting results also exist for Mo isotopes. While Chen et al. (2004) found isotope anomalies for Mo in iron meteorites and carbonaceous chondrites they measured normal Mo isotope ratios for ordinary chondrites and pallasites. In another study Dauphas et al. (2004) demonstrated that the Mo anomalies measured in bulk meteorites follow the predicted mixing lines between an s-process endmember and solar composition. While anomalies for Mo were also reported by Yin et al. (2000) and Dauphas et al. (2002) they were not confirmed by two other studies (Becker and Walker, 2003a; Lee and Halliday, 2002). In a recent study Brandon et al. (2005) found Os isotope anomalies in unequilibrated chondrites. Since they measured Os of terrestrial composition in metamorphosed bulk chondrites, they concluded that the anomalies detected for primitive meteorites are due to incomplete digestion of pre-solar grains. The same conclusion has been drawn by Yokoyama et al. (2007) based on Os isotope measurements of carbonaceous, enstatite, and ordinary chondrites. Finally, inconsistencies also exist for the Ru data. Becker and Walker (2003b) have argued that the solar system has a homogeneous Ru isotope composition. Their conclusion is in striking contrast to the results published by Chen et al. (2003) and Papanastassiou et al. (2004), which both

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found a depletion of up to 1ɛ for the pure s-process isotope 100 Ru in IIAB, IIIAB, IVAB, and AN iron meteorites and possibly also in pallasites. The effect for bulk material from Allende was larger and they found hints of added effects in 96Ru, 98Ru, and 104Ru (dependent on the normalisation used). Apparent inconsistencies also exist in the published data for Ti-isotopes in bulk meteorites. While one group found no anomalies for bulk samples of the carbonaceous chondrite Allende (CV3) (Heydegger et al., 1977), others reported anomalous Ti in bulk samples of the CI1 chondrite Ivuna and in matrix separates from the CM2 chondrites Murchison and Murray (Niemeyer and Lugmair, 1984). However, the apparent discrepancy might (at least partly) be due to the fact that the amount of pre-solar grains, which are the carrier of nucleosynthetic anomalies, is higher in CM and CI than in CV chondrites (Huss et al., 2003). Some of the apparent discrepancy over Ti is merely a function of the analytical precision of the methods. Here we present new high precision data for the Ti-isotope composition of lunar rocks, chondrites, ureilites, eucrites, and mesosiderites obtained by high resolution MC-ICPMS. All results are given in ɛ units, defined as the deviation of the isotopic ratio in parts per 104 (after correction for mass fractionation using internal normalisation) from the composition of the terrestrial Ti standard (see Table 1). To test for any cosmogenic effects we studied lunar whole rock samples and mineral separates from five high-Ti-mare basalts, a lunar norite, a ferroan anorthosite, and an olivine-normative mare basalt. These have exposure ages ranging from 2 to 130 Ma. In addition we investigated bulk samples from four carbonaceous chondrites: Allende CV3, Orgueil CI1, Murchison CM2, Renazzo CR2; three ureilites: ALHA

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77257, EET 96042, PCA 82506; two eucrites: Juvinas, Cachari; one ordinary chondrite: Forest Vale H4; one silicate clast from the mesosiderite Vaca Muerta, and one Martian meteorite: ALHA 84001. Finally, we analyzed leachates from Allende and mineral separates from Renazzo (Table 1). 2. Experimental Samples were crushed in a boron carbide mortar under a laminar flow of filtered air to avoid contamination. For all lunar samples and whole rock samples of stony meteorites a Parr® acid digestion bomb was used for the complete dissolution of the samples. The mineral separates of Renazzo and leachates of Allende were obtained as described in Table 1. For leachates of Allende we started with 2 g, which was treated sequentially with a series of progressively stronger reagents. After each treatment the sample was centrifuged and the supernatant removed. For Renazzo separation started with a hand magnet to obtain a magnetic and a non-magnetic fraction. The magnetic fraction was subsequently leached for 5 min with 6 M HCl–0.01 M HF to produce a magnetic leachate, see also (Schönbächler et al., 2003). Titanium was separated via a two-stage anion exchange chromatography. The details are given elsewhere (Leya et al., 2007#; Schönbächler et al., 2004). The ion exchange procedure allowed us to separate Ti, Zr, and Te fractions from the same sample aliquot. The Zr and Te data are presented in (Schönbächler et al., 2003, 2005a, 2005b; Fehr et al., 2006). Total procedural chemistry blanks were typically below 200 pg. However, it could reach ∼2 ng when Teflon bombs were used. The blank is negligible for most

Table 1 Experimental procedure and Ti-isotope data for the separates of Allende and Renazzo Allende Step

Reagent

Procedure

ɛ(50Ti/47Ti) ± 2σ

ɛ(48Ti/47Ti) ± 2σ

ɛ(46Ti/47Ti) ± 2σ

1 2 3 4

50% HAc a 4 M HNO3 6 M HCl 13.5 M HF + 3 M HCl

2 days, RT b 5 days, RT b 1 day, 80 °C 4 days, 100 °C

−1.2 ± 8.1 1.4 ± 2.1 3.3 ± 1.4 3.3 ± 1.2

2.3 ± 0.3 0.5 ± 0.2 − 0.2 ± 0.2 − 0.6 ± 0.2

3.7 ± 2.0 0.9 ± 1.4 0.8 ± 1.9 0.5 ± 1.5

Non-magnetic fraction Magnetic leachate

Metal removed with a hand magnet Metal cleaned up twice with a hand magnet and leached with 6 M HCl + 0.01 M HF

3.0 ± 1.5 1.2 ± 3.9

− 1.7 ± 0.5 − 2.2 ± 0.2

1.3 ± 0.9 0.5 ± 0.8

Renazzo Fraction n.m. m.l

For leachates from Allende we started with 2 g, which was treated sequentially with a series of progressively stronger reagents. After each treatment, the sample was centrifuged and the supernatant was removed. Renazzo was first separated mechanically with a hand magnet to obtain a magnetic and a non-magnetic fraction. The magnetic fraction was subsequently leached for 5 min with 6 M HCl–0.01 M HF to produce a magnetic leachate. a Room Temperature. b HAc = CH3COOH.

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of the data analyzed in this study, which usually contain more then a few tens of nanograms of Ti; some samples even contain a few milligrams of Ti. Samples compromised by large blank corrections are labelled in the tables, for most of them no results are given. Measurements of isotopic ratios were performed with the high resolution MC-ICPMS (NU 1700) at the ETH Zürich as outlined in (Leya et al., 2007). A brief summary is given in the following. The isotopes 46Ti, 47Ti, 48Ti, 49 Ti, and 50Ti were measured for each sample in two data collection protocols. Two protocols were necessary because the detector array of the Nu 1700 does not allow the simultaneous measurement of all Ti-isotopes including selected Ca-, V-, and Cr-isotopes, which are needed to correct for isobaric interferences. For both data collection protocols the mass resolution was about 2000, which allows interferences on mass 50 from, for example, 14 36 N Ar+, to be fully resolved. Isobaric interferences from 46 Ca, 48Ca, 50V, and 50Cr were corrected using signals on mass 44 (44Ca), mass 51 (51V), and mass 53 (53Cr), respectively. The reliability of the corrections was investigated using synthetic standard solutions. It is possible to

correct shifts up to 60ɛ and 30ɛ (deviation from the correct ratio) in 48Ti/47Ti and 46Ti/47Ti caused by 46Ca and 48Ca, respectively. Correcting V- and Cr-interferences were reliable up to about 1.6% and 16%, respectively. Measurements of 44Ca, which were used to correct for Ca interferences, are very sensitive to baseline variations caused by the large 40Ar beam, which was probably the reason for the limited range of reliable Ca corrections (compared to the other isobaric interferences). Interferences of doubly charged Zr were corrected using Zr/Ti ratios obtained from solution aliquots by ICPMS and experimentally determined Zr2+/Ti+ ratios. The corrections are reliable up to 0.3%. For most of the samples studied here the corrections are low and well within the reliable range. For some samples, however, corrections are too high and further chemical purification is needed. Such samples will not be discussed further. For additional information see (Leya et al., 2007). Instrumental mass fractionation is internally corrected via 49Ti/47Ti = 0.749766 (Niederer et al., 1981) using the exponential law. The long-term reproducibilities (2σ) for 50 Ti/47Ti, 48Ti/47Ti, and 46Ti/47Ti are 0.28ɛ, 0.34ɛ, and

Fig. 1. Titanium isotope composition for lunar whole rock samples and mineral separates. Shown are deviations in ɛ-units relative to terrestrial Ti (ɛ = 0) (standard solution Alfa Aesar). Mass discrimination was corrected using the exponential law (Leya et al., 2007) and a value of 49 Ti/47Ti = 0.749766 (Niederer et al., 1981). The 49Ti/47Ti ratio was chosen to correct for mass fractionation because it can be reliably measured and has only minor interferences. All samples except one are normal within uncertainties. The data therefore clearly indicate that Earth and Moon share the same Ti-isotope composition (see text).

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0.28ɛ, respectively, as demonstrated with replicate measurements of synthetic standard solutions, terrestrial rocks, and the carbonaceous chondrite Allende. The uncertainties given below are either the external reproducibilities, i.e. the standard deviation of the weighted means, or the internal error, whatever is larger (Leya et al., 2007). 3. Results 3.1. Ti-isotopes in lunar whole rock samples and mineral separates The Ti-isotope data for lunar whole rock samples and mineral separates are shown in Fig. 1 and are summarized in Table 2. All data, except one, are normal within the uncertainties, indicating that Earth and Moon have the same Ti-isotope composition. The one exception is the High-Ti-mare basalt 71596, which is anomalous for 50 Ti/47Ti, 48Ti/47Ti, and 46Ti/47Ti at the 2.7σ, 6.5σ, and 2.5σ level, respectively. We tested to see whether matrix effects might play a role in high-Ti samples by analysing a terrestrial ilmenite but we found no anomalies (Table 2). Furthermore, (preliminary) model calculations indicate that effects due to galactic cosmic-ray (GCR) interactions during the 115 Ma of exposure should be negligible. The model predictions are confirmed by the experimental data for, e.g., High-Ti-mare basalt 77516, which has an exposure age of about 130 Ma, i.e. slightly longer than 71596, but has a normal Ti-isotope composition. Therefore, we have no explanation for this apparently discrepant result. However, the remainder of our data clearly demonstrates that Earth and Moon have the same Tiisotope composition. 3.2. Ti-isotopes in Martian meteorites, mesosiderites, ureilites, eucrites, and ordinary chondrites Identical Ti-isotope compositions are also observed for the Martian meteorite ALHA 84001, the mesosiderite Vaca Muerta (silicate clast), and the ordinary chondrite Forest Vale (H4). Also the eucrites Juvinas and Cachari, and the ureilites ALHA 77257, EET 77257, and PCA 82506 have Ti-isotope compositions indistinguishable from terrestrial (see Table 3 and Fig. 2). The new data therefore indicate that Earth, Moon, Mars, eucrites, ureilites, mesosiderites, and ordinary chondrites appear to have formed from material with the same Ti-isotope composition. This finding is consistent with data for Zr isotopes, which also do not display isotopic differences between these bodies (Schönbächler et al., 2003). In addition, a recent study of Nd and Sm isotopes shows

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Table 2 Ti isotopic data and Ti concentrations for lunar samples Sample

Ti [ppm]

ɛ ɛ ɛ (50Ti/47Ti) ± (48Ti/47Ti) ± (46Ti/47Ti) ± 2σ 2σ 2σ

71566 — high- 8.2 0.3 ± 0.9 Ti-mare basalt 1.5 ± 1.1 71596 — high- 6.2 Ti-mare basalt a 77516 — high-Ti-mare basalt Whole rock 16 0.3 ± 0.8 Pyroxene n.d. − 0.2 ± 0.9 Ilmenite n.d. − 0.1 ± 0.8 75075 — high-Ti-mare basalt Whole rock 10 0.4 ± 2.5 Feldspar 3.0 − 0.2 ± 0.9 Pyroxene 5.0 − 0.9 ± 1.9 Ilmenite 15 × 104 − 1.1 ± 1.2 70035 — high-Ti-mare basalt Feldspar 1.3 × 103 − 0.4 ± 0.7 Pyroxene 53 × 103 − 0.0 ± 0.8 Ilmenite 92 × 103 − 0.7 ± 0.9 77215 — norite Whole rock 26 × 103 0.1 ± 0.9 250 − 0.6 ± 0.8 Feldspar b Feldspar b n.d. − 0.1 ± 0.9 Pyroxene 700 − 0.1 ± 0.5 Heavy 41 × 103 n.d. mineral residue a 60025 — ferroan anorthosite Whole rock 150 0.2 ± 0.9 Feldspar 110 3.0 ± 2.7 Pyroxene s2.1 × 103 0.7 ± 3.6 Ilmenite 94 × 103 1.7 ± 2.3 15555 — olivine-norm mare basalt Whole rock 6.8 × 103 0.1 ± 1.0 Feldspar 640 − 0.6 ± 1.3 Pyroxene 6.6 × 103 − 0.3 ± 0.5 Ilmenite n.d. 2.4 ± 2.1 Terrestrial n.d. 0.2 ± 0.9 ilmenite

0.6 ± 0.3

− 0.3 ± 1.8

− 1.3 ± 0.4

− 1.9 ± 1.5

0.3 ± 0.3 0.3 ± 0.3 0.2 ± 0.2

− 0.4 ± 0.9 0.0 ± 1.2 − 0.7 ± 0.6

0.4 ± 0.4 0.1 ± 0.21 − 0.2 ± 0.3 0.0 ± 0.2

− 0.4 ± 1.4 0.2 ± 0.7 − 0.5 ± 1.1 0.4 ± 0.7

0.3 ± 0.5 0.0 ± 0.1 0.1 ± 0.3

0.1 ± 1.5 − 0.2 ± 1.4 − 0.8 ± 0.7

0.3 ± 0.3 n.d. n.d. 0.1 ± 0.3 − 0.1 ± 0.2

0.1 ± 0.9 − 0.1 ± 1.6 − 0.6 ± 1.5 0.1 ± 1.4 − 0.1 ± 0.6

0.0 ± 0.2 − 0.8 ± 0.8 − 0.1 ± 0.4 − 0.2 ± 0.3

0.8 ± 0.7 0.4 ± 1.7 0.2 ± 1.1 0.4 ± 1.4

0.3 ± 0.2 0.5 ± 0.2 0.2 ± 0.3 0.5 ± 0.6 0.5 ± 0.2

− 0.5 ± 0.9 0.2 ± 0.6 0.4 ± 1.2 0.7 ± 0.6 0.6 ± 0.6

Given are ɛ-values for 50Ti/47Ti, 49Ti/47Ti, and 46Ti/47Ti for lunar whole rock samples and mineral separates. The ɛ-values are defined as parts per 10,000 deviation from a terrestrial standard (ɛ = 0). Mass discrimination has been corrected using the exponential law and a value of 49Ti/47Ti = 0.749766 (Niederer et al., 1981). All samples, except one, are normal within the uncertainties, indicating that Earth and Moon have the same Ti-isotope composition. a The data are compromised by high Cr concentrations. b The data are compromised by high Ca concentrations.

that Earth, ordinary chondrites, the eucrite parent body, and possibly the Moon, share the same p-process contribution of 144Sm and 146Sm, indicating that the part of the solar nebula from which these objects were

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Table 3 Titanium isotope data and Ti concentrations for bulk samples of carbonaceous chondrites, ureilites, eucrites, an ordinary chondrite, a mesosiderite, and a Martian meteorite Type ɛ (50Ti/47Ti) ± 2σ

ɛ (48Ti/47Ti) ± 2σ

ɛ (46Ti/47Ti) ± 2σ

CV3 CV3 CV3 CV3 CV3 CV3 CV3 CV3 CV3 CI1 CI1 CR2

4.3 ± 0.9 2.5 ± 1.3 3.5 ± 1.0 n.d. 2.6 ± 1.0 4.1 ± 2.6 b 3.1 ± 2.7 n.d. 3.4 ± 0.5 n.d. b − 0.1 ± 1.6 1.4 ± 2.5 3.0 ± 0.5

− 0.5 ± 0.8 − 0.3 ± 0.2 − 1.2 ± 0.4 − 0.7 ± 1.1 0.2 ± 0.2 n.d. c − 0.5 ± 0.3 − 3.4 ± 0.2 − 1.2 ± 0.6 − 1.7 ± 0.2 1.8 ± 0.5 − 1.4 ± 0.2 − 1.3 ± 0.4

0.3 ± 2.1 0.6 ± 1.6 − 0.5 ± 1.6 0.7 ± 2.2 0.8 ± 1.5 0.5 ± 2.2 1.2 ± 1.5 1.3 ± 0.6 1.0 ± 0.44 3.2 ± 2.0 3.4 ± 1.9 1.1 ± 0.6 1.2 ± 0.4

Ureilites ALHA 77257 EET 96042 PCA 82506 bUreilitesN4

− 0.9 ± 0.8 0.6 ± 1.5 − 1.2 ± 1.6 − 0.7 ± 0.6

0.0 ± 0.5 2.6 ± 2.4 0.1 ± 0.9 0.1 ± 0.4

Eucrites Juvinas b Juvinas Cachari bEucritesN

− 1.1 ± 2.1 − 0.9 ± 1.3 − 1.2 ± 1.3 − 1.1 ± 0.8

− 0.5 ± 0.5 − 0.2 ± 0.5 0.2 ± 0.2 − 0.5 ± 0.3

− 0.3 ± 2.5 − 0.7 ± 3.4 0.0 ± 0.5 0.8 ± 1.3

− 0.5 ± 1.5

− 0.5 ± 0.3

0.8 ± 1.3

0.2 ± 1.1

0.4 ± 0.4

0.5 ± 1.3

Sample

Carbonaceous Allende a Allende a Allende a Allende a Allende a Allende Allende Allende bAllendeN d Orgueil Orgueil Renazzo bCarbonaceousN

d

d

Ordinary Forest Vale

H4

Mesosiderite Vaca Muerta Martian ALHA 84001

− 0.5 ± 1.5

c

0.2 ± 0.9 0.1 ± 6.6 0.6 ± 2.3 0.3 ± 0.9

0.4 ± 0.7

0.6 ± 3.1

Given are ɛ-units for Ti/ Ti, Ti/ Ti, and Ti/ Ti for carbonaceous chondrites, ureilites, eucrites, an ordinary chondrite, a mesosiderites, and a Martian meteorite. a Data are taken from Leya et al. (2007). b The data are compromised by large Cr corrections. c The data are most probably compromised by a large blank caused by the Teflon bombs used for sample digestion. Note that the blank was not of terrestrial composition, therefore slightly shifting the isotope ratios. d Weighted mean values. 50

47

49

47

46

47

formed was homogeneous (Andreasen and Sharma, 2006). In contrast to the evidence for isotopic homogeneity in (this part of) the solar nebula, various authors have reported variations in Cr (Shukolyukov and Lugmair, 1999; Podosek et al., 1997a, 1999; Rotaru et al., 1992; Trinquier et al., 2007), Mo (Chen et al.,

2004; Dauphas et al., 2002, 2004; Yin et al., 2000; Becker and Walker, 2003a,b; Lee and Halliday, 2002), Ba (Hidaka et al., 2003; Ranen and Jacobsen, 2006; Andreasen and Sharma, 2007; Carlson et al., 2007; Wombacher and Becker, 2007), and Ru (Becker and Walker, 2003b; Chen et al., 2003; Papanastassiou et al., 2004) in bulk rock meteorites. As discussed in detail above, the effects for Mo, Ru, and Ba are not well constrained, because contradicting results were published by various groups. A recent study by Trinquier et al. (2007) demonstrates that all differentiated samples (basaltic achondrites, iron meteorites, angrites, SNCs) and ordinary chondrites show variable 54Cr deficits. However, most of the anomalies are small, e.g. ∼ 0.4ɛ for ordinary chondrites and ∼ 0.2ɛ for Martian meteorites. Anomalies of a similar magnitude would be hard to detect for Ti considering the external reproducibility of our measurements. Therefore, the possibility remains that very small Ti-isotope anomalies below our current detection limit exist. Taking into account the current analytical precision, our new data demonstrate that a large part of the inner solar system including the Earth, Moon, Mars, eucrites, ureilites, mesosiderites, and ordinary chondrites formed from material with an identical Ti-isotope composition at the sub-permil level. 3.3. Ti-isotopes in carbonaceous chondrites The Ti-isotope data for carbonaceous chondrites are shown in Table 3 and Fig. 3. Our data clearly demonstrate positive anomalies for 50Ti/47Ti compared to Earth, Mars, Moon, ordinary chondrites, eucrites, ureilites, and mesosiderites. Enrichments in 50Ti have also been reported in earlier studies of matrix material of Murchison (CM2) and Murray (CM2) (Niemeyer and Lugmair, 1984) and in whole rock samples from various CI (Orgueil, Ivuna, Tonk, Alais), CM (Cold Bokkeveld, Mighei, Essebi, Murchison), CO (Ornans, Kainsaz, Lancé, Felix), and CV (Vigarano, Leoville, Mokoia, Kaba) chondrites (Niemeyer, 1985), while another study found no anomalies for 50Ti in matrix material from Allende (Niederer et al., 1981). Our data show that 50Ti excesses are more pronounced in CV and CM chondrites compared to CI chondrites, confirming earlier work of Niemeyer (1985). The finding of this study that some carbonaceous chondrites display high 46Ti/47Ti (Fig. 3) is also in agreement with earlier studies in which excesses of more than 2ɛ have been observed for matrix material from Murchison and Murray and bulk material from Ivuna (Niemeyer and Lugmair, 1984). This, however, is in contrast with the results of another study, in which

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Fig. 2. Titanium isotope composition for eucrites, ureilites, an ordinary chondrite, a silicate clast from a mesosiderite, and a Martian meteorite. For further information see Fig. 1. The data indicate that Earth, Mars, ureilites, eucrites, ordinary chondrites, and mesosiderites share the same Ti-isotope composition. In contrast, carbonaceous chondrites show positive anomalies for 50Ti/47Ti and indicate slightly negative anomalies for 48Ti/47Ti.

terrestrial 46 Ti/ 47 Ti ratios were reported for bulk material from various CI, CM, CO, and CV chondrites (Niemeyer, 1985). There are also differing reports on 48 Ti/47Ti. We observe deficits of (on average) ∼ 1ɛ, in agreement with Niederer et al. (1981). The results are harder to reconcile with the data of Niemeyer and Lugmair (1984) and Niemeyer (1985). The scatter for 48 Ti/47Ti (Fig. 3) is larger than expected considering the external reproducibility, which may indicate possible heterogeneities not only between meteorite classes but also within individual meteorites. It is well established that CAIs are enriched in 50Ti relative to Earth by about 10ɛ (Niemeyer, 1988; Niemeyer and Lugmair, 1984; Heydegger et al., 1977; Niederer et al., 1981). Bulk CV-, CM-, CI-, and CR-chondrites contain about 10%, 5%, b 1%, and 0.5% CAIs, respectively (Scott and Krot, 2004). From mass balance calculations 50Ti/47Ti anomalies of about 1ɛ, 0.5ɛ, b0.1ɛ and 0.05ɛ are expected for CV-, CI-, CM- and CR-chondrites, respectively, if the anomalies in bulk material were only due to CAIs. Therefore, only about 1ɛ can be caused by dissolved CAIs. Since the average 50Ti/47Ti anomaly is 3ɛ for bulk Allende, the remaining carrier phases must be situated in chondrules and/or matrix.

3.4. Ti-isotopes in mineral separates from Renazzo and leachates from Allende To further explore the sitting of anomalous Ti in carbonaceous chondrites two further experiments were performed, a leach experiment for Allende and mineral separation for Renazzo. The results are summarized in Table 1. The uncertainties of the HAc fraction are large and not definitive because of a low Ti concentration and high blank. The last two leach steps of Allende display clear anomalies for 50Ti/47Ti and 48Ti/47Ti. The individual 46 Ti/47Ti ratios are indistinguishable from terrestrial within uncertainties. The finding that Ti-isotope anomalies are pervasively distributed within carbonaceous chondrites is also confirmed by the results obtained for mineral separates from Renazzo. Both fractions (non-magnetic and magnetic leachate) show positive anomalies for 50Ti/47Ti and 46 Ti/47Ti and negative anomalies for 48Ti/47Ti (Table 1). The results are in contrast to 96Zr data, for which isotope heterogeneities were found in aliquots of the same samples (Schönbächler et al. 2003). Isotope heterogeneities were also found for 54 Cr in leach fractions of various carbonaceous chondrites (Rotaru et al., 1992; Podosek et al., 1997a; Trinquier et al., 2007). However, in a

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Fig. 3. Titanium isotope composition for carbonaceous chondrites. For further information see Fig. 1. Our data are shown as black symbols. Also displayed are results from earlier studies (grey symbols), although they are not normalised to 47Ti like ours but to 46Ti (Niemeyer and Lugmair, 1984) or 48Ti (Heydegger et al., 1977). Note that data from (Niemeyer, 1985) are average values from at least 3 meteorites (for each class) each measured at least two times, which explains the rather small uncertainties. The data demonstrate that carbonaceous chondrites show positive anomalies for 50 Ti/47Ti and indicate slightly positive anomalies for 46Ti/47Ti. For 48Ti/47Ti there are no clearly resolved anomalies, whereas this ratio shows a larger than normal scatter, possibly indicating Ti-isotope heterogeneities.

systematic study Rotaru et al. (1992) demonstrate that the internal heterogeneity for Cr decreases with increasing petrographic grade, reflecting the effect of secondary metamorphism in the parent bodies. Consequently, heterogeneities are expected to be smaller in Allende (CV3) and Renazzo (CR2) than in, e.g., Orgueil (CI1) and Murchison (CM2). This consistently explains the occurrence of isotope heterogeneities in some carbonaceous chondrites, i.e. those having a low petrographic grade. Note, however, that Rotaru et al. (1992) found even for the least metamorphosed chondrites, i.e., Orgueil, Alais, Ivuna, Murchison, and Murray, no heterogeneities for Ti, Fe, Ni, and Zn isotopes. This finding is interpreted by a decoupling of Cr from Ti, Fe, Ni, and Zn. Such a decoupling also explains the finding of 54Cr heterogeneities in Allende leachates (Rotaru et al. 1992; Trinquier et al. 2007), whereas our data indicate that Allende is homogeneous for Ti-isotopes. The observation that Cr in carbonaceous chondrites is special in a way not yet recognized is further confirmed by Podosek et al. (1997b). These authors performed a leach experiment for Orgueil and found normal isotope composition for Fe, Rb, Sr, Ca, and (most probably K) in all leach

fractions. However, considering the finding of 96Zr heterogeneities in aliquots of our samples (Schönbächler et al. 2003), we propose that Ti is also decoupled from Zr. Note that the finding of normal Ti for Allende leachates by Rotaru et al. (1992) is simply due to their rather high detection limit of about 5ɛ. Therefore, the anomalies found by us were not detectable by Rotaru et al. (1992). Considering both arguments, i.e., lower heterogeneities in metamorphosed chondrites and a decoupling of Ti (and Fe, Ni, Zn) from Cr (and Zr), indicates that our new data, though more precise, supports the results from Rotaru et al. (1992) and Podosek et al. (1997b). Finally, a study of density separates from Murchison suggests that anomalous Ti is homogeneously distributed even in less metamorphosed carbonaceous chondrites (Niemeyer, 1985), which further confirms our data. 4. Discussion Our high precision data provide evidence that the Earth, Moon and Mars, as well as parent bodies of the eucrites, ureilites, mesosiderites, and ordinary chondrites are

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indistinguishable in terms of their Ti-isotope composition. It is of course possible that with analyses of further samples greater heterogeneity will be found among ordinary chondrites and perhaps ureilites. Nevertheless, there is no reason why this initial study should have uncovered such a reproducible feature of the inner solar system unless it represents a widespread and common feature. In contrast, carbonaceous chondrites in general are enriched in 50Ti/47Ti and 46Ti/47Ti and are slightly depleted in 48Ti/47Ti. The leach experiment for Allende and mineral separation for Renazzo indicate that anomalous Ti in carbonaceous chondrites is not confined to special phases in refractory inclusions. The Ti-isotope anomalies are very unlikely caused by cosmic-ray induced spallation and/or special irradiation scenarios close to the early sun. As discussed above, any hypothetical irradiation scenario would result in negative anomalies for 50Ti/47Ti and 48 Ti/47Ti, whereas the effect for ɛ(48Ti/47Ti) would be much higher than for ɛ(50Ti/47Ti). For ɛ(46Ti/47Ti) any shift would only be very minor, because the effect on 46 Ti/47Ti (without normalisation) almost completely cancels with the effect on the normalising ratio 49Ti/47Ti. Therefore, the excesses in 50Ti/47Ti must have a nucleosynthetic origin. This is less clear for 48Ti/47Ti and 46Ti/47Ti since irradiating material close to the early sun might be able to slightly alter both isotope ratios. From nuclear reaction systematics one would expect slightly negative anomalies for 46Ti/47Ti and 48Ti/47Ti, whereas the effect for the latter ratio should be much larger. There is no indication of negative anomalies for 46Ti/47Ti in our data (Fig. 3), but we cannot exclude the possibility that small negative anomalies in 48Ti/47Ti measured for carbonaceous chondrites are, at least partly, due to particle irradiation. However, this is limited to one data point for Orgueil. In mainstream SiC grains the largest Ti anomalies are observed for 50Ti with excesses of up to 20%. For the other Ti-isotopes the excesses and/or deficits are smaller (∼b10%) (Clayton, 2003; Amari et al., 2001a). Therefore, if the nucleosynthetic Ti anomalies are due to pre-solar SiC grains, the anomalies are expected to be more pronounced for mass 50 than for the other Ti-isotopes, in agreement with our data. Since pre-solar SiC grains are very acidresistant, they may not entirely dissolve during the procedure used by us and thus only small amounts of their anomalous Ti might be released during our bulk rock digestions. However, even such small amounts of highly anomalous Ti from (partly dissolved) SiC grains could result in the 50Ti enrichment observed by us for carbonaceous chondrites. As the abundance of SiC grains in Allende is much smaller than in Murchison (less than 1 ppm in CV chondrites, compared to ∼13 ppm in CM condrites (Huss et al., 2003)), the 50Ti anomalies in Allende

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should be smaller than in Murchison. However, this is not observed and thus may indicate that SiC grains are not the only carrier phase of anomalous 50Ti. Furthermore, we show below that simple mass balance calculations indicate that only a small fraction of the measured 50Ti anomalies could be due to SiC grains, even if they would have been completely dissolved. As shown before, carbonaceous chondrites have a different Ti-isotope composition than other planetary bodies such as the Earth. This could be caused by an inhomogeneous distribution of pre-solar SiC grains in the solar nebula (assuming that SiC grains are at least partly dissolved in our dissolution procedure). Mainstream SiC grains are enriched in 50Ti by about 20% (≡2000ɛ) and such grains have Ti concentrations close to that of CIchondrites (Clayton, 2003; Amari et al., 2001a, 2001b). Considering now that the SiC concentrations in CM (e.g., Murchison) and CV (e.g., Allende) chondrites is ∼13 ppm and b 1 ppm (Huss et al., 2003), respectively, demonstrates that any contribution from SiC grains, even if they would have been completely dissolved in our dissolution procedure, is marginal, indicating that pre-solar SiC grains are not the major carrier phase of anomalous 50Ti. As a consequence, incomplete dissolution of SiC grains, which often serves as an explanation for isotope anomalies, e.g. for Os (Brandon et al., 2007; Yokoyama et al., 2007), is not able to explain the Ti data, as SiC grains are not the major carrier phase of anomalous 50Ti. As an alternative scenario one could propose that pre-solar grains (other than SiC), originally incorporated in solar system material, could have been affected by aqueous alteration thereby transferring exotic Ti to other minerals. However, since the anomalies in CV3 are as pronounced as in CM2, despite the fact that the latter are less affected by aqueous alteration, such a scenario seems improbable. Another possibility would be that presolar grains have already reacted by melting and/or vaporization with other phases in the solar nebula, thereby transferring their exotic isotope signature to minerals formed in the solar system. In general, it can be concluded that since the same 50Ti anomalies are found in most leach fractions from Allende (only the HAc- and HNO3-fractions are normal within the (large) uncertainties) and all mineral separates from Renazzo, the exotic Ti must be a pervasive feature of the material that now makes up the carbonaceous chondrites. Independent of the actual carrier phase of anomalous Ti, the fact that anomalies have only been observed for carbonaceous chondrites but not for other solar system objects indicates that in the beginning presolar grains were inhomogeneously distributed in the solar nebula. The regions in which carbonaceous chondrites formed must have had a distinctly higher

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concentration of such grains. As such the data provide evidence of a large-scale heterogeneity in the circumstellar disk from which the planets were formed. Carbonaceous chondrite parent bodies probably exist between 2.5 and 3.5 AU in the outer asteroid belt whereas other objects come from regions between 1 and 2.4 AU (Andreasen and Sharma, 2006). Therefore, carbonaceous chondrites may have received a greater portion of material with a provenance in the outer solar system. This would be consistent with their smaller degree of volatile element depletion compared with many objects from the inner solar system. Furthermore, carbonaceous chondrites such as Allende contain chondrules whose ages typically postdate those of CAIs by ∼2 Ma (Russel et al., 2006). This being the case, the parent bodies must have taken longer to accrete than the b 1 Ma timescales inferred by theories based on a runaway growth for planetary embryos in the inner solar system. Accretion in outer parts of the circumstellar disk could therefore also explain the fact that chondrites have not been heated and melted greatly by short-lived nuclides. However, the ultimate cause of this disk-scale heterogeneity is unclear. It probably reflects a feature of the molecular cloud of gas and dust from which the solar system was made. Note that large-scale heterogeneities have also been observed for, e.g., 54Cr (Rotaru et al., 1992, Shukolyukov and Lugmair, 1999; Trinquier et al., 2007) and the p-nuclides 144Sm and 146Sm (Andreasen and Sharma, 2006). The latter authors argued that the variation could be explained by the grain-size model (Meyer and Clayton, 2000), in which nuclides are adsorbed onto the surfaces of gas and/or dust grains, whose particle size increases away from the core of the solar nebula. However, further work is needed to elucidate this in more detail. The data obtained here for Ti also indicate that ordinary chondrites (as well and eucrites, mesosiderites, ureilites) represent, at least for Ti-isotopes, the best match to the terrestrial composition. The same genetic relationship has been obtained by Andreasen and Sharma (2007) based on Sr, Ba, Nd, and Sm data. In addition, based on Cr-isotopes also Shukolyukov and Lugmair (2006) arrived at the conclusion that the precursor material of the ureilite parent body was different from known carbonaceous chondrites classes. 5. Conclusions The Earth, Moon, Mars, eucrites, mesosiderites, ureilites, and ordinary chondrites have identical Tiisotope compositions defining a homogeneous inner solar system composition. In contrast, carbonaceous chondrites are enriched in 50Ti/47Ti and 46Ti/47Ti and

are slightly depleted in 48Ti/47Ti. A leach experiment for Allende and mineral separation for Renazzo indicate that anomalous Ti in carbonaceous chondrites is not confined to special phases in refractory inclusions but is distributed within a range of phases (at least in Allende and Renazzo). Since effects due to galactic cosmic-ray (GCR) irradiation and/or special irradiation scenarios close to the early sun are unable to produce positive anomalies for 50Ti/47Ti, the observed excess must have a nucleosynthetic origin. For 46Ti/47Ti and 48Ti/47Ti early irradiation effects cannot be ruled out completely but are at best small. The lunar data, which are normal within uncertainties despite exposure ages of more than 100 Ma, suggest that recent GCR irradiation effects are negligible. Carbonaceous chondrites therefore appear to have acquired a distinct mix of primitive components, which is different from the Earth and this, combined with their lack of depletion in volatile elements and late formation would be consistent with accretion further out in the disk from which the terrestrial planets formed. Acknowledgements IL acknowledges the support from the Swiss National Science Foundation (SNF). ANH acknowledges the support of ETH, the SNF, the UK's Science and Technology Facilities Council, and the Royal Society. We thank F. Oberli and colleagues at the Institute for Isotope Geochemistry and Mineral Resources for their support of the MC-ICPMS facility at ETH. The work benefited from constructive comments from R.W Carlson and two anonymous reviewers. References Amari, S., Nittler, L.R., Zinner, E., Lodders, K., Lewis, R.S., 2001a. Presolar SiC grains of type A and B: their isotopic compositions and stellar origins. Apa. J. 559, 463–483. Amari, S., Nittler, L.R., Zinner, E., Gallino, R., Lugaro, M., Lewis, R.S., 2001b. SiC grains of type Y: origin from low-metallicity Giant Branch Stars. Apa. J. 546, 248–266. Andreasen, R., Sharma, M., 2006. Solar nebula heterogeneity in p-process samarium and neodymium isotopes. Science 314, 806–809. Andreasen, R., Sharma, M., 2007. Mixing and homogenization in the early solar system: clues from Sr, Ba, Nd, and Sm isotopes in Meteorites. Apa. J. 665, 874–883. Becker, H., Walker, R.J., 2003a. Efficient mixing of the solar nebula from uniform Mo isotopic composition of meteorites. Nature 425, 152–155. Becker, H., Walker, R.J., 2003b. In search of extant Tc in the early solar system: 98Ru and 99Ru abundances in iron meteorites and chondrites. Chem.Geol. 193, 43–56. Brandon, A.D., Humayun, M., Puchtel, I.S., Leya, I., Zolensky, M., 2005. Osmium isotope evidence for an s-process carrier in the primitive chondrites. Science 309, 1233–1236.

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