¹Josefine A.M.Nanne,²Francis Nimmo,³Jeffrey N.Cuzzi,¹Thorsten Kleine
Earth and Planetary Science Letters 511, 44-54 Link to Article [https://doi.org/10.1016/j.epsl.2019.01.027]
¹Institut für Planetologie, University of Münster, Wilhelm-Klemm-Straße 10, 48149 Münster, Germany
²Department of Earth and Planetary Sciences, University of California Santa Cruz, Santa Cruz, CA 95064, USA
³Space Science Division, Ames Research Center, Moffett Field, CA 94035, USA
Copyright Elsevier

The isotopic composition of meteorites reveals a fundamental dichotomy between non-carbonaceous (NC) and carbonaceous (CC) meteorites. However, the origin of this dichotomy—whether it results from processes within the solar protoplanetary disk or is an inherited heterogeneity from the solar system’s parental molecular cloud—is not known. To evaluate the origin of the NC–CC dichotomy, we report Ni isotopic data for a comprehensive set of iron meteorites, with a special focus on groups that have not been analyzed before and belong to the CC group. The new Ni isotopic data demonstrate that the NC–CC dichotomy extends to Ni isotopes, and that CC meteorites are characterized by a ubiquitous ⁵⁸Ni excess over NC meteorites. These data combined with prior observations reveal that, in general, the CC reservoir is characterized by an excess in nuclides produced in neutron-rich stellar environments, such as ⁵⁰Ti, ⁵⁴Cr, ⁵⁸Ni, and r-process Mo isotopes. Because the NC–CC dichotomy exists for refractory (Ti, Mo) and non-refractory (Ni, Cr) elements, and is only evident for nuclides produced in specific, neutron-rich stellar environments, it neither reflects thermal processing of presolar carriers in the disk, nor the heterogeneous distribution of isotopically anomalous Ca–Al-rich inclusions (CAI). Instead, the NC–CC dichotomy reflects the distinct isotopic composition of later infalling material from the solar system’s parental molecular cloud, which affected the inner and outer regions of the disk differently. Simple models of the infall process by themselves can support either infall of increasingly NC-like material onto an initially CC-like disk, or infall of increasingly CC-like material in the absence of disk evolution by spreading. However, provided that CAIs formed close to the Sun, followed by rapid outward transport, their isotopic composition likely reflects that of the earliest infalling material, implying that the composition of the inner disk (i.e., the NC reservoir) is dominated by later infalling material, whereas the outer disk (i.e., the CC reservoir) preserved a compositional signature of the earliest disk. The isotopic difference between the inner and outer disk was likely maintained through the rapid formation of Jupiter, which prevented complete homogenization between material from inside (NC reservoir) and outside (CC reservoir) its orbit.

^1,5Noriyuki Kawasaki,²Changkun Park,³Naoya Sakamoto,²Sun Young Park,⁴Hyun Na Kim,⁵Minami Kuroda,^5,3,1Hisayoshi Yurimoto
Earth and Planetary Science Letters 511, 25-35 Link to Article [https://doi.org/10.1016/j.epsl.2019.01.026]
¹Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Sagamihara 252-5210, Japan
²Division of Earth-System Sciences, Korea Polar Research Institute, Incheon 21990, Republic of Korea
³Isotope Imaging Laboratory, Creative Research Institution, Hokkaido University, Sapporo 001-0021, Japan
⁴Department of Geoenvironmental Sciences, Kongju National University, Chungcheongnam-do 32588, Republic of Korea
⁵Natural History Sciences, Hokkaido University, Sapporo 060-0810, Japan
Copyright Elsevier

Al–Mg mineral isochrons of three Ca–Al-rich inclusions (CAIs) that formed primarily by condensation, one fine-grained, spinel-rich inclusion and two fluffy Type A CAIs, from the reduced CV chondrites Efremovka and Vigarano were obtained by in situ Al–Mg isotope measurements using secondary ion mass spectrometry. The slope of the isochron obtained for the fine-grained, spinel-rich inclusion gives an initial ²⁶Al/²⁷Al value, (²⁶Al/²⁷Al)₀, of (5.19 ± 0.17) × 10⁻⁵. This is essentially identical to the Solar System initial ²⁶Al/²⁷Al determined by whole-rock Al–Mg isochron studies for CAIs in CV chondrites. In contrast, the isochron slopes for the two fluffy Type A CAIs from their Al–Mg mineral isochrons, (4.703 ± 0.082) × 10⁻⁵ and (4.393 ± 0.084) × 10⁻⁵, are significantly lower than the Solar System initial value. The range of (²⁶Al/²⁷Al)₀ values of the three CAIs, from (5.19 ± 0.17) to (4.393 ± 0.084) × 10⁻⁵, corresponds to a formation age spread of 0.17 ± 0.04 Myr. This formation age spread is similar to that of igneous CAIs from CV chondrites. The data suggest that condensation and melting of minerals occurred in the hot nebular gas contemporaneously for ∼0.2 Myr at the very beginning of our Solar System, if ²⁶Al was distributed homogeneously in the CAI forming region. Alternatively, the observed variations in (²⁶Al/²⁷Al)₀ among fluffy Type A CAIs would also raise a possibility of heterogeneous distributions of ²⁶Al in the forming region.