CHROMIUM ISOTOPE SYSTEMATICS OF ACHONDRITES: CHRONOLOGY AND ISOTOPIC HETEROGENEITY OF THE INNER SOLAR SYSTEM BODIES

All absolute ages reported here are calculated relative to the angritic achondrite D'Orbigny, which has a Pb–Pb age of 4564.42 ± 0.12 Ma (Amelin 2008), and an initial ⁵³Mn/⁵⁵Mn ratio of (3.24 ± 0.04) × 10⁻⁶ (Glavin et al. 2004). If we calculate the absolute age using the LEW 86010 angrite instead of D'Orbigny, the ages will be ∼0.8 Ma younger (Table 2). All data from NWA 766, excluding the first leachate (LE 1), define a linear correlation on an isochron plot (Figure 1(a)). However, since the materials in the reduction rim and vein are likely a product of a later process that occurred in the ureilite parent body (UPB; Walker & Grove 1993), the calculation for the age was made by eliminating the data points from LE 1 to LE 3. While the NWA 766 silicates and whole rock contain materials from the reduction rim and vein, their influence on the Cr isotope is small, as estimated from mass balance, and thus they were included in the calculation of the isochron (See Table 3). The slope of the obtained isochron translates to an absolute age of 4564.60 ± 0.67 Ma (Figure 1(b)), which we consider the best estimate for the differentiation age of NWA 766. The data for the whole rock samples are also scattered along the isochron (Figure 1(c)), suggesting that ureilites are derived from an isotopically uniform reservoir. The NWA 1241, on the other hand, exhibits a uniform ε⁵³Cr value of ∼+0.12. This implies that both the differentiation and the reduction of NWA 1241 took place close to or after the extinction of ⁵³Mn. The regression through all the data points shows a nearly flat slope with a corresponding age of <4561 Ma (Figure 1(b)). These age constraints indicate that the UPB forming process started by 4564.60 ± 0.67 Ma and continued for at least 3.6 Ma. It is important to note that the age of NWA 766 is similar to the age of oldest eucrite and whole rock angrites using the same age anchor (4565.57 ± 0.39 Ma for eucrite Asuka 881394 (Wadhwa et al. 2005) and 4564.68 ± 0.26 Ma for whole rock angrites (Shukolyukov & Lugmair 2007); Table 2). This indicates that the differentiation of achondrite asteroids of several hundred kilometers in size began at about the same time of ∼4565 Ma in the inner solar system.

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The additional advantage of investigating the Cr isotopic signature of meteorites is the presence of distinct ⁵⁴Cr isotopic signature in various meteorite classes. Since each meteoritic group possesses its own ε⁵⁴Cr value (Shukolyukov & Lugmair 2006a; Trinquier et al. 2007; Qin et al. 2010), our data may provide an insight into a genetic relationship between ureilites and other meteoritic groups. The obtained ε⁵⁴Cr values from the whole rock samples and the leachates and residues of NWA 766 and NWA 1241 all possess identical ε⁵⁴Cr with a mean value of −0.92 ± 0.02 (Table 1), the lowest deficit found for meteoritic samples (cf. Shukolyukov & Lugmair 2006b; Ueda et al. 2006). Previous studies have proposed a genetic link between ureilites and carbonaceous chondrites based on petrography, oxygen isotopes, planetary-type noble gases within carbon and metal with high abundances of siderophile trace elements (e.g., Goodrich 1992; Mittlefehldt et al. 1998). It has been suggested that ureilites may be a cumulate crystallized from a melt derived from the partial melting of a chondritic source, or a residue after such a partial melting event(s). However, our ε⁵⁴Cr isotopic data are distinct from those of bulk carbonaceous chondrites (Shukolyukov & Lugmair 2006a; Trinquier et al. 2007; Yin et al. 2009). Although the bulk carbonaceous chondrites are characterized by positive ε⁵⁴Cr values, they are mixtures of phases with both positive and negative ε⁵⁴Cr values when investigated in mineralogical scale (Rotaru et al. 1992; Trinquier et al. 2007, 2008). Therefore, it may be possible to produce melt with ε⁵⁴Cr of −0.9 by preferentially melting phases with negative ε⁵⁴Cr (low temperature phases such as carbonates, sulfides, sulfates, and metals, plus some silicates), leaving high ε⁵⁴Cr phases (silicates and refractory phases such as spinel) as a residue. Using the data of Rotaru et al. (1992) and Trinquier et al. (2008), simple mass balance calculation will imply an equilibration of Cr, which resides in all the low temperature phases, plus 45%–100% of silicate (and in some cases, additional contribution from the equilibration of refractory phases is also required) to produce melt with ε⁵⁴Cr of −0.9 if the ureilite precursor material was a CI-CM-CV chondrite-like material. However, such a high degree of processing will likely eliminate the oxygen isotope heterogeneity of the precursor material but this is not the case for ureilites (Clayton & Mayeda 1988). The residue after such a melting event, on the other hand, will be characterized by ε⁵⁴Cr value greater than that of the initial carbonaceous chondrite. This is at odds with the Cr isotopic signature of ureilites. Therefore, our data suggest that a genetic link between the carbonaceous chondrite and ureilite is unlikely.

When the ε⁵⁴Cr isotope data from the Earth, Mars, and Vesta (HED parent body) are used as anchors, we noted that the ε⁵⁴Cr value of the inner solar system bodies decreases linearly with heliocentric distance in the terrestrial planet region (Figure 2(a)). If this linear trend extends outward into the exterior of the asteroid belt, the ε⁵⁴Cr isotopic signature of ureilites will place the location of UPB at around 2.8 AU. This is consistent with the findings of Jenniskens et al. (2009), who suggested that the F-type asteroids, located in the region between 2.5 and 3.5 AU (Gradie & Tedesco 1982), are possible candidates of the UPB. If we accept the location of the UPB to be around 2.8 AU, a very similar linear trend is observed for ⁵⁰Ti (Figure 2(b)), an isotope that is produced in Type 1a supernovae along with ⁵⁴Cr and ⁴⁸Ca (Clayton 2003; Hartman et al. 1985). Because Cr and Ti with negative ε⁵⁴Cr and ε⁵⁰Ti values do not always reside in the same mineral phase within the primitive meteorites (Trinquier et al. 2008, 2009), the observed ⁵⁴Cr and ⁵⁰Ti correlation with heliocentric distance is unlikely to be explained by preferential addition or removal of selected mineral phases into or from the chondritic precursor prior to the accumulation of these parent bodies. Rather, it is more likely to represent the heterogeneous distribution of these isotopes in the region of the inner solar system where these parent bodies evolved. Since the mixing timescale of injected nuclides is believed to be very short, ranging from 1000 years up to 1 million years (Ouellette et al. 2009), the presence of ⁵⁴Cr–⁵⁰Ti heterogeneity in the inner solar system hints toward the possible injection of isotopes from Type 1a supernovae shortly before the accumulation of inner solar system materials (i.e., 10³–10⁶ years before the formation of the inner solar system bodies).

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The Cr isotopic signature of carbonaceous chondrites that have reflective spectra similar to C-type asteroids located in the outer region of the asteroid belt beyond ∼2.7 AU (Pang et al. 1978), on the other hand, clearly deviates from this linear trend. The reason for this is not clear at this stage, and may imply that the linear decrease in the ε⁵⁴Cr (as well as ε⁵⁰Ti) value is valid only for differentiated bodies. However, recent numerical calculation has demonstrated that a migration of primordial trans-Neptunian objects into the outer asteroid belt was plausible as a result of dynamical evolution of the giant-planet orbit (Levison et al. 2009). If this is the case, the current position of the carbonaceous chondrite parent body (C-type and possibly some D/P-type asteroids) may not represent the location of their original formation. This issue clearly deserves further investigation.