Isotopic Evolution of the Inner Solar System Inferred from Molybdenum Isotopes in Meteorites

Several group IC, IIAB, IID, and IIIAB irons with variable CRE effects on Pt isotopes were selected for this study. Except for the IID irons, only NC iron meteorites were selected, because this study aims to assess potential isotopic changes in inner disk composition. The IID irons were incorporated because one of them (Carbo) is among the most strongly irradiated irons known (Kruijer et al. 2013; Qin et al. 2015). Combined, the investigated samples include strongly and weakly irradiated irons, which makes it possible to precisely quantify CRE effects on Mo isotopes.

Sample preparation and Mo and Pt isotope measurements followed previously established methods (Kruijer et al. 2013; Budde et al. 2019). Isotopic compositions were determined using a Thermo-Fisher Neptune Plus MC-ICP-MS at Münster and are reported in the ε-notation (parts per 10,000 deviations from terrestrial standard values) after mass bias correction to the terrestrial ⁹⁸Mo/⁹⁶Mo and ¹⁹⁸Pt/¹⁹⁵Pt, respectively.

Samples of a given iron group show variable εⁱMo values that correlate with ε¹⁹⁶Pt, indicating the presence of CRE effects (Figure 1). The εⁱMo–ε¹⁹⁶Pt correlations are best defined for the IIAB and IID irons, both of which include samples with large CRE effects. Nevertheless, the IC and IIIAB irons also display correlated εⁱMo–ε¹⁹⁶Pt variations, and the εⁱMo–ε¹⁹⁶Pt slopes are consistent for all groups. The pre-exposure εⁱMo (i.e., unaffected by CRE) for each iron group can either be obtained from the intercept value at ε¹⁹⁶Pt = 0, or by individually correcting each sample to ε¹⁹⁶Pt = 0 and using the mean εⁱMo–ε¹⁹⁶Pt slopes determined for the different iron groups. Both approaches yield identical results (Table 1) and provide precise pre-exposure εⁱMo values for the IC, IIAB, IID, and IIIAB irons. Pre-exposure εⁱMo values for the IIIE irons were calculated using previously published Mo and Pt isotopic data (Kruijer et al. 2017; Worsham et al. 2019; Table 1). The pre-exposure εⁱMo values of this study are a factor of ∼5 more precise than previous results (Bermingham et al. 2018), and only for IIAB irons have values with comparable precision been previously reported (Yokoyama et al. 2019; Table 1). Finally, pre-exposure εⁱMo values for IVA irons were calculated by averaging data for samples having no CRE effects (Poole et al. 2017) and CRE-corrected data (Bermingham et al. 2018).

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In a diagram of ε⁹⁵Mo versus ε⁹⁴Mo, bulk meteorites plot along two distinct and approximately parallel lines, which were termed the NC and CC lines (Budde et al. 2016). The Mo isotopic variations along the NC and CC lines are predominantly governed by s-process variations, whereas the offset between the two lines reflects the characteristic r-process excess of the CC over the NC reservoir. For distinguishing between these different Mo isotope variations, it is useful to define Δ⁹⁵Mo as the vertical deviation (in ppm) of a sample from an s-process mixing line passing through the origin (Budde et al. 2019):

$$\begin{eqnarray}&&{{\rm{\Delta }}}^{95}\mathrm{Mo}=({\varepsilon }^{95}\mathrm{Mo}-0.596\times {\varepsilon }^{94}\mathrm{Mo})\times 100.\end{eqnarray} \tag{ 1 }$$

The quantity 0.596 is the slope of s-process mixing lines defined by bulk samples and acid leachates from both NC and CC meteorites (Budde et al. 2019), which is indistinguishable from the slope obtained from mainstream presolar SiC grains (Stephan et al. 2019). Distinct Δ⁹⁵Mo values, therefore, indicate Mo isotope heterogeneities unrelated to pure s-process variations.

The precise pre-exposure ε⁹⁵Mo and ε⁹⁴Mo values from this study reveal that some NC irons plot below the NC line (Figure 2), and have slightly lower Δ⁹⁵Mo than the characteristic NC value (Δ⁹⁵Mo = −9 ± 2; Budde et al. 2019; Table 1). Linear regression of available ε⁹⁵Mo and ε⁹⁴Mo data for NC meteorites (Table A1 in the Appendix), including the precise data for NC irons from this study, yields a slope of 0.528 ± 0.045 (MSWD = 0.85), which is shallower than the slope of the CC line and the characteristic slope of a pure s-process mixing line (Figure 2). Including leachate data for NC chondrites (Budde et al. 2019) results in a steeper slope (m = 0.595 ± 0.011), which is consistent with that of the CC line and pure s-process variations. However, the higher MSWD of 1.6 for this regression is above the upper acceptable limit of 1.45 for N  = 41 (Wendt & Carl 1991), indicating additional scatter outside the analytical uncertainties. The ε⁹⁵Mo–ε⁹⁴Mo slope of bulk NC meteorites, therefore, is shallower than the predicted slope of a pure s-process mixing line. This results in a weak inverse correlation of Δ⁹⁵Mo with ε⁹⁴Mo (Figure 2) and indicates that the Mo isotope variations among NC meteorites do not solely reflect s-process but also additional r-process variations.

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The Δ⁹⁵Mo and ε⁹⁴Mo values of NC meteorites are not only correlated with another, but also with ε⁵⁰Ti, ε⁵⁴Cr, and ε⁶²Ni (Figure 3). These correlations involve lithophile (Ti, Cr) and siderophile (Ni, Mo) as well as refractory (Ti, Mo) and non-refractory (Cr, Ni) elements, indicating that the isotopic variations do not reflect the heterogeneous distribution of individual presolar carriers (e.g., SiC) or chemically fractionated components (e.g., refractory inclusions, silicates, metal). Instead, they are indicative of mixing between two isotopically distinct components with similar bulk chemical compositions. One of the mixing endmembers has the characteristic isotopic composition of the NC reservoir (low Δ⁹⁵Mo, ε⁵⁰Ti, ε⁵⁴Cr, and ε⁶²Ni), while the other has high Δ⁹⁵Mo, ε⁵⁰Ti, ε⁵⁴Cr, and ε⁶²Ni (Figures 3(a)–(c)), which are the isotopic characteristics of bulk CC meteorites and Ca–Al-rich inclusions (CAIs).

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However, unlike for Δ⁹⁵Mo (Figures 3(a)–(c)), NC meteorites, CC meteorites, and CAIs do not define a single mixing line in ε⁹⁴Mo versus ε⁵⁰Ti–ε⁵⁴Cr–ε⁶²Ni diagrams (Figures 3(d)–(f)). Instead, NC meteorites plot along a trend toward more positive ε⁵⁰Ti, ε⁵⁴Cr, and ε⁶²Ni, but negative ε⁹⁴Mo (Figures 3(d)–(f)). By contrast, bulk CC meteorites and typical CAIs are characterized by positive ε⁹⁴Mo and, therefore, plot off this trend (Figures 3(d)–(f)). This also includes CI chondrites, which have been suggested to represent the material that was added to the inner disk and continuously changed its composition (Schiller et al. 2020). Thus, although one of the endmembers defining the NC mixing trend has some isotopic characteristics of CC meteorites and CAIs, compared to these samples this material is characterized by negative ε⁹⁴Mo, which is indicative of an excess in s-process Mo. The only known meteoritic materials with such a composition are the matrix of the CV3 chondrite Allende (Budde et al. 2016) and some fine-grained CAIs (Brennecka et al. 2017). We emphasize that this does not imply that these materials physically represent one of the endmembers defining the NC mixing trend, but it merely reveals that material with appropriate isotopic compositions existed in the disk at various times.

Like the NC mixing trend, the NC–CC dichotomy probably also results from mixing between two reservoirs with overall chondritic chemical but distinct isotopic compositions (Burkhardt et al. 2019; Nanne et al. 2019). In this model, the earliest disk, which formed by viscous spreading of early infalling material (Yang & Ciesla 2012; Jacquet et al. 2019), was characterized by a CAI-like isotopic composition (inclusion-like chondritic reservoir (IC); Burkhardt et al. 2019), while the later infall had a NC-like isotopic composition and provided most of the material in the inner disk. Mixing within the disk then gave rise to the CC reservoir, whose isotopic composition is intermediate between those of the IC and NC reservoirs (Burkhardt et al. 2019; Nanne et al. 2019). Thus, similar mixing processes that produced the NC–CC dichotomy also seem to be responsible for the isotopic variations within the NC reservoir, with the important difference that the material that produced the NC mixing trend is enriched in s-process Mo compared to the material that produced the NC–CC dichotomy (Figure 3). Consequently, to account for both the isotopic variations in the NC reservoir and the NC–CC dichotomy requires at least three components: (1) the characteristic starting composition of the NC reservoir (e.g., as given by magmatic irons); (2) s-process-depleted IC material (as observed for most CAIs); and (3) s-process-enriched IC or CC material. Mixing between the first two of these components (i.e., between NC and IC) resulted in the characteristic composition of the CC reservoir, whereas mixing between the first and the third component (i.e., between NC and s-enriched IC or CC) produced the isotopic variations within the NC reservoir.

The addition of s-process-enriched IC or CC material to the inner disk may have occurred by different processes and at different times. For instance, isotopic heterogeneities in the inner disk may be inherited from the molecular cloud and would then reflect the changing isotopic composition of infalling matter from IC to NC at a very early stage when the NC reservoir was still forming (Burkhardt et al. 2019; Jacquet et al. 2019; Nanne et al. 2019). Alternatively, the outward transport of isotopically anomalous refractory material (e.g., CAIs) through the inner disk may have led to isotopic heterogeneities, because the fraction of CAIs remaining in the inner disk is expected to be higher at early times (Desch et al. 2018). Finally, the isotopic composition of the inner disk may have changed over time through the addition of CC-like dust from the outer solar system after the NC–CC dichotomy had been established (Schiller et al. 2018). We note, however, that the NC mixing trend points toward s-enriched IC or CC material, rather than to the s-depleted IC or CC compositions as sampled by typical CAIs and bulk CC meteorites, respectively (Figure 3). Thus, the NC mixing trend cannot result from the addition of these latter materials, but it may still reflect the addition of s-enriched IC or CC material. These additions would have likely resulted in distinct isotopic compositions for early- and late-accreted NC planetesimals, and so assessing whether the isotopic variations among NC meteorites are correlated with the accretion ages of their parent bodies may help to identify the underlying mechanisms that produced the NC mixing trend.

The accretion ages of NC iron meteorite and chondrite parent bodies are reasonably well established. For instance, ¹⁸²Hf–¹⁸²W ages for most NC iron meteorites (i.e., group IC, IIAB, IIIAB, and IVA irons) indicate parent body accretion within <0.5 Ma after CAI formation (Kruijer et al. 2014, 2017). Only the IAB and IIE iron meteorite parent bodies may have accreted slightly later (Hunt et al. 2018; Kruijer & Kleine 2019), but their younger Hf–W ages may also reflect resetting during impact events, in which case the original accretion age is unknown (Kruijer & Kleine 2019). The parent bodies of NC chondrites accreted at ∼2 Ma after CAI formation (Blackburn et al. 2017; Hellmann et al. 2019; Pape et al. 2019) and, therefore, later than those of the irons. Although accretion ages are only available for ordinary chondrites, it is reasonable to assume that the enstatite and Rumuruti chondrite parent bodies accreted at about the same time, given that all these bodies remained unmelted and, therefore, accreted later than ∼1.5 Ma after CAI formation to avoid melting by ²⁶Al decay (Hevey & Sanders 2006). Thus, when only iron meteorites and chondrites are considered, a temporal trend appears to exist in the isotopic composition of NC meteorites from lower to higher values of Δ⁹⁵Mo, ε⁵⁰Ti, ε⁵⁴Cr, and ε⁶²Ni (Figure 4). This trend is opposite to the expected isotopic variations for the incorporation of different amounts of refractory inclusions (e.g., CAIs) in NC planetesimals, which predicts more elevated Δ⁹⁵Mo, ε⁵⁰Ti, ε⁵⁴Cr, and ε⁶²Ni in early-formed objects, reflecting the larger fraction of refractory inclusions in the inner disk at early times (Desch et al. 2018). Instead, the apparent temporal trend defined by iron meteorites and chondrites appears consistent with the expected effects of CC dust addition to the inner disk, which should have produced more CC-like isotopic compositions in later-accreted NC bodies.

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However, some other NC meteorites do not seem to fit the trend of isotope anomalies versus accretion ages very well. For instance, the ureilite parent body may have accreted as late as 1.5 Ma after CAIs (Budde et al. 2015), yet seems to have the lowest contribution of CC material among all NC meteorites. Moreover, acapulcoites-lodranites, whose parent body likely accreted at ∼1.5 Ma after CAIs (Touboul et al. 2009), are also characterized by lower Δ⁹⁵Mo, ε⁵⁰Ti, and ε⁵⁴Cr values than, for instance, the angrites, whose parent body likely accreted within the first ∼0.5–1 Ma of the solar system (Kleine et al. 2012; Hans et al. 2013). Finally, aubrites and enstatite chondrites have very similar isotopic compositions, but the aubrite parent body likely accreted earlier, well within ∼1.5 Ma after CAI formation (Sugiura & Fujiya 2014). Differentiated meteorites, therefore, appear to cover most of the isotopic range observed among NC meteorites, yet these meteorites probably derive from bodies that accreted rather early. Similarly, later-accreted NC planetesimals (e.g., parent bodies of acapulcoites-lodranites and enstatite, ordinary, and Rumuruti chondrites) appear to cover a similar range of isotopic anomalies (Figure 4). Together, these observations reveal that the NC isotopic mixing trend cannot solely reflect a temporal evolution of inner disk composition by addition of s-enriched CC dust from the outer solar system.

The lack of a clear temporal trend in the inner disk's isotopic composition suggests that the NC mixing trend at least partially reflects spatial variations. These are unlikely to result from mixing between NC and CC materials, because, as noted above, this would lead to a temporal trend in the isotope anomalies. Instead, spatial variations within the inner disk more likely result from mixing between s-enriched IC and NC material, which occurred during infall from the Sun's parental molecular cloud and the associated early stages of disk building. It has been shown theoretically that infall from an isotopically zoned molecular cloud may result not only in an isotopically distinct outer disk (i.e., the CC reservoir), but also in spatial isotopic heterogeneities within the inner disk (Jacquet et al. 2019). The NC mixing trend may, therefore, at least in part reflect mixing of s-enriched IC and NC materials during infall and the early stages of disk building.