Isotopic evolution of the inner Solar System inferred from molybdenum isotopes in meteorites

The fundamentally different isotopic compositions of non-carbonaceous (NC) and carbonaceous (CC) meteorites reveal the presence of two distinct reservoirs in the solar protoplanetary disk that were likely separated by Jupiter. However, the extent of material exchange between these reservoirs, and how this affected the composition of the inner disk are not known. Here we show that NC meteorites display broadly correlated isotopic variations for Mo, Ti, Cr, and Ni, indicating the addition of isotopically distinct material to the inner disk. The added material resembles bulk CC meteorites and Ca-Al-rich inclusions in terms of its enrichment in neutron-rich isotopes, but unlike the latter materials is also enriched in s-process nuclides. The comparison of the isotopic composition of NC meteorites with the accretion ages of their parent bodies reveals that the isotopic variations within the inner disk do not reflect a continuous compositional change through the addition of CC dust, indicating an efficient separation of the NC and CC reservoirs and limited exchange of material between the inner and outer disk. Instead, the isotopic variations among NC meteorites more likely record a rapidly changing composition of the disk during infall from the Sun's parental molecular cloud, where each planetesimal locks the instant composition of the disk when it forms. A corollary of this model is that late-formed planetesimals in the inner disk predominantly accreted from secondary dust that was produced by collisions among pre-existing NC planetesimals.


INTRODUCTION
Nucleosynthetic isotope anomalies reveal a fundamental dichotomy between non-carbonaceous (NC) and carbonaceous (CC) meteorites (Budde et al. 2016;Warren 2011), which sample two spatially distinct reservoirs that coexisted in the early Solar System for several million years (Ma) (Kruijer et al. 2017). The prolonged spatial separation of the NC and CC reservoirs most likely reflects the formation of Jupiter, which acted as an efficient barrier against material exchange either by its growth itself (Kruijer et al. 2017;Morbidelli et al. 2016) or through a pressure maximum in the disk near the location where Jupiter later formed (Brasser & Mojzsis 2020).
Although there is little doubt that the NC and CC reservoirs were spatially separated, the extent of material exchange between them remains poorly constrained. For example, the Jupiter barrier may have resulted in a filtering effect by which the inward drift of large grains was efficiently blocked, while smaller dust grains may have passed the barrier as part of the gas flow (Haugbølle et al. 2019;Weber et al. 2018). On this basis, it has been argued that the inner disk's isotopic composition was modified through the addition of inward drifting CC dust (Schiller et al. 2018. This interpretation, however, depends on the assumed starting composition of the inner disk, and on the unknown efficiency of the Jupiter barrier over time. Thus, understanding and quantifying any compositional evolution of the NC reservoir is of considerable interest, as it would allow reconstructing the structure and temporal evolution of the solar accretion disk, and the importance of Jupiter for separating the NC and CC reservoirs. The NC-CC dichotomy has been identified for several elements and so far holds for all analyzed meteorites (Kleine et al. 2020;Kruijer et al. 2020). The dichotomy is particularly exploitable for Mo, which can distinguish between isotope variations arising from the heterogeneous distribution of matter produced by the p-, s-, and r-processes of stellar nucleosynthesis (Burkhardt et al. 2011). While there are large s-process Mo isotope variations among meteorites within both the NC and CC groups, all CC meteorites are characterized by an approximately constant r-process excess over NC meteorites (Budde et al. 2016;Kruijer et al. 2017;Poole et al. 2017;Worsham et al. 2017). This difference makes Mo isotopes ideally suited to identify any compositional change of the NC reservoir, because the continuous addition of CC dust to the NC reservoir would result in a characteristic isotopic shift of the NC composition towards an enrichment in r-process Mo isotopes over time. For identifying such a potential isotopic shift in the NC reservoir, iron meteorites are particularly important, because they derive from some of the Isotopic evolution of the inner Solar System 4 earliest planetesimals formed within the NC reservoir ) and, therefore, may have a distinctly different Mo isotopic composition compared to later-formed NC planetesimals.
Until now, no systematic Mo isotopic difference between early-and late-formed NC bodies has been identified . This might in part be due to the overall small Mo isotopic offset between the NC and CC reservoirs, but for iron meteorites may also reflect the modification of their Mo isotopic compositions by neutron capture reactions induced during cosmic ray exposure (CRE) (e.g., Worsham et al. 2017). Here, we employ Pt isotopes to quantify CREeffects (Kruijer et al. 2013;Wittig et al. 2013) on Mo isotopes with unprecedented precision and use these data, combined with published data for other meteorite groups, to assess any compositional heterogeneity within the inner disk that may have arisen through material exchange between the NC and CC reservoirs.

Correction of Cosmic Ray Effects
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). Isotopic compositions were determined using a Thermo-Fisher Neptune Plus MC-ICP-MS at Münster and are reported in the ε-notation (parts-

Mo Isotope Variations among NC Meteorites
In a diagram of ε 95 Mo versus ε 94 Mo, bulk meteorites plot along two distinct and approximately parallel lines, which were termed the NC (Non-Carbonaceous) and CC (Carbonaceous (  (Table S1), 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 (Fig. 2). Including leachate data for NC chondrites ) 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 ε 95 Mo-ε 94 Mo 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 Δ 95 Mo with ε 94 Mo (Fig. 2) and indicates that the Mo isotope variations among NC meteorites do not solely reflect s-process but also additional r-process variations.

Mixing Trends in the NC Reservoir
The Δ 95 Mo and ε 94 Mo values of NC meteorites are not only correlated with another, but also with ε 50 Ti, ε 54 Cr, and ε 62 Ni (Fig. 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 Δ 95 Mo, ε 50 Ti, ε 54 Cr, and ε 62 Ni), while the other has high Δ 95 Mo, ε 50 Ti, ε 54 Cr, and ε 62 Ni ( Fig. 3a-c), which are the isotopic characteristics of bulk CC meteorites and Ca-Al-rich inclusions (CAIs).
However, unlike for Δ 95 Mo ( Fig. 3a-c), NC meteorites, CC meteorites, and CAIs do not define a single mixing line in ε 94 Mo versus ε 50 Ti-ε 54 Cr-ε 62 Ni diagrams ( Fig. 3d-f). Instead, NC meteorites plot along a trend towards more positive ε 50 Ti, ε 54 Cr, and ε 62 Ni, but negative ε 94 Mo ( Fig. 3d-f). By contrast, bulk CC meteorites and typical CAIs are characterized by positive ε 94 Mo and, therefore, plot off this trend ( Fig. 3d-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 . 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 ε 94 Mo, 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 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 (Fig. 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.

Spatial and Temporal Variations in 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 and Rumuruti chondrites) appear to cover a similar range of isotopic anomalies (Fig. 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 this would, as noted above, 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.

IMPLICATIONS FOR PLANETESIMAL FORMATION IN THE INNER DISK
As noted in prior studies, the clear compositional gap between the NC and CC reservoirs in multi-element isotope space (Fig. 3)  topic mixing trend reflects spatial heterogeneities within the inner disk, then these isotopic variations must also be preserved for the ~2 Ma period of NC planetesimal formation. Together, these observations imply either that dust in the inner disk was somehow stored for at least ~2 Ma, or that later-formed NC planetesimals predominantly accreted from secondary dust produced by collisions among pre-existing planetesimals.
Pressure maxima in the inner disk are a potential way for storage of dust and would have also prevented mixing of dust across the resulting gap. However, a pressure bump would have also resulted in dust pile-up and, ultimately, its rapid accretion into planetesimals (e.g., Morbidelli et al. 2020). As such, it is unclear why some of these putative pressure maxima in the inner disk would have converted dust into planetesimals very rapidly (e.g., NC iron parent bodies), while others preserved dust for ~2 Ma until accretion into planetesimals (e.g., NC chondrite parent bodies). This would require different efficiencies with which pressure maxima resulted in the concentration of dust, but whether this is feasible is unknown. Thus, although we cannot exclude that pressure maxima in the inner disk resulted in a prolonged preservation of dust, the distinct accretion times of NC meteorite parent bodies make this scenario less likely.
By contrast, secondary dust would be produced naturally in the inner disk during the later stages of its evolution, when the damping effect of gas on the planetesimals' velocity dispersion becomes weaker and protoplanets become more massive so that they can scatter planetesimals more efficiently (Gerbig et al. 2019). Moreover, the lower amount of gas remaining at later stages favors planetesimal formation by the streaming instability, because the dust-to-gas ratio is high even for low amounts of dust (Carrera et al. 2017). Thus, from a dynamical standpoint the formation of planetesimals from secondary dust is expected, and so we consider it the more likely mechanism to account for the prolonged interval of planetesimal formation in the inner disk. . We note, however, that the formation of NC chondrites from collisionally-produced dust does not necessarily imply that the chondrules themselves formed as a result of these collisions. It is also possible that the chondrule-melting events occurred later by another process, and were unrelated to the collisions that produced their precursor dust. Distinguishing between these different models is not possible using the data of this study, but will require a better understanding of the underlying mechanisms that produced chondrules and whether or not this process was different in the inner and outer Solar System.
Finally, the formation of NC chondrites from secondary dust implies that their isotopic composition does not provide a snapshot of inner disk composition at the time of parent body accretion at ~2 Ma, but instead reflects those of pre-existing planetesimals and, therefore, records an earlier time of disk evolution. As such, there is no need to preserve spatial isotopic variations within the NC reservoir for a period of ~2 Ma. Instead, the isotopic variations among NC meteorites were likely generated over a much shorter time interval and, as such, may record a rapidly changing composition of the disk, where each planetesimal locks the instant composition of the disk when it forms.  Notes. The Mo and Pt isotope ratios were normalized to 98 Mo/ 96 Mo = 1.453173 and 198 Pt/ 195 Pt = 0.2145 using the exponential law, respectively. The ε-notation is the parts per 10 4 deviation relative to the terrestrial bracketing Alfa Aesar solution standard. The uncertainties for N ≤ 3 represent the 2 standard deviations (2 s.d.) of repeated analyses of the NIST 129c metal standard or the internal precision (2 standard errors [2 s.e.]), whichever is larger. The uncertainties for N ≥ 4 represent the Student-t 95% confidence intervals, i.e.,   Summary of Mo, Ti, Cr, Ni, and accretion age literature data for selected meteorites Notes. The ε-notation is the parts per 10 4 deviation relative to the terrestrial bracketing solution standard. The uncertainties represent the 2 standard deviations (2 s.d.) for samples with N ≤ 3 and Student-t 95% confidence intervals, i.e., (t 0.95,N-1 × s.d.)/√N, for N ≥ 4. a after the formation of Ca-Al-rich inclusions (CAI)