¹C.M.Corrigan,²K.Nagashima,^3,6C.Hilton,¹T.J.McCoy,³R.D.Ash,⁴H.A.Tornabene,³R.J.Walker,³W.F.McDonough,⁵D.Rumble
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2022.06.008]

¹Department of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, 10th Street and Constitution Ave. NW, Washington, DC 20560 USA
²Hawai‘i Institute of Geology and Planetology, University of Hawai’i at Manoa, 2020 Correa Rd, Honolulu, HI 96822 USA
³Department of Geology, University of Maryland , College Park, MD 20742 USA
⁴Department of Earth and Planetary Sciences, Rutgers University, Piscataway, NJ 08854 USA
⁵Carnegie Institution for Science, Earth and Planets Laboratory, 5241 Broad Branch Road NW, Washington DC 20015 USA
⁶Environmental Signatures Team, Pacific Northwest National Laboratory, Richland, WA 99354 USA
Copyright Elsevier

Ungrouped iron meteorites Tishomingo, Willow Grove, and Chinga, and group IVB iron meteorites, are Ni-rich. Similarities include enrichments of 10-100 × CI for some refractory siderophile elements, and equivalent depletions in more volatile siderophile elements. Superimposed on the overall enrichment/depletion trend, certain siderophile elements (P, W, Fe, Mo) are depleted relative to elements of similar volatility. All three ungrouped irons derive from parent bodies formed in the early Solar System. Willow Grove and Chinga are characterized by cosmic ray exposure corrected ¹⁸²W/¹⁸⁴W consistent with metal-silicate segregation on their parent bodies within 1-3 Myr of Solar System formation, within the age range determined for segregation of magmatic iron meteorite parent bodies, including group IVB irons. Tishomingo is characterized by a younger model age 4-5 Myr subsequent to Solar System formation, reflecting either late stage melting resulting from ²⁶Al decay, or an impact resetting. The discovery of stishovite in Tishomingo, indicating exposure to a minimum shock pressure of 8-9 GPa, is consistent with the latter.

Stishovite in Tishomingo and chromite included in troilite-daubréelite in IVB irons allows oxygen isotopic composition comparison between these meteorites. Different mass independent oxygen isotopic compositions of IVB irons and Tishomingo indicate genetically distinct parent bodies. By contrast, mass independent Mo isotopic compositions overlap within analytical uncertainties, indicating a similar, carbonaceous chondrite (CC) type genetic heritage. Molybdenum and ¹⁸³W isotopic data for Chinga and Willow Grove indicate derivation from CC type parent bodies. Willow Grove shares Mo and ¹⁸³W isotopic compositions with the Ni-rich South Byron Trio (SBT) grouplet and the Milton pallasite. These Ni-rich meteorites likely formed in the same general nebular environment as other CC planetesimals, likely the outer Solar System.

Highly siderophile element (HSE) abundances of Willow Grove and Tishomingo are similar to some IVB meteorites, consistent with formation by moderate degrees of fractional crystallization from initial metallic melts with low S and P, and modestly fractionated HSE. The comparable HSE abundances of Tishomingo and Willow Grove to some IVB irons, yet substantially higher Ni concentrations, indicate formation on parent bodies with lower bulk HSE abundances or HSE concentration in proportionally smaller volumes of metal. HSE abundances in Chinga are considerably lower than in IVB irons, highly fractionated, and processes responsible for these remain elusive.

For IVB irons and these ungrouped irons, high temperature condensation likely dominated the enrichment and depletion of the refractory and volatile siderophile elements, respectively. Parent body degassing may have also played a role. Relative depletion of volatile siderophile elements is not, however, a universal feature of high-Ni meteorites. The SBT and Milton pallasite are Ni-rich, but less depleted in the more volatile siderophile elements. Nickel enrichment was likely driven by oxidation of Fe metal during parent body accretion or core segregation. Oxidation of the Tishomingo and Willow Grove parent bodies may have occurred at ∼IW+1, indicated by relative Mo and W depletions due to metal/water reaction during differentiation. Late-stage reduction, indicated by the presence of Cr-bearing sulfides in Tishomingo and IVB irons, may have resulted from exhaustion of the oxidant.

¹Alexander N.Krot,¹Kazuhide Nagashima,²Glenn J.MacPherson,³Alexander A.Ulyanov
Geochimica et Cosmochimica Act a(in Press) Link to Article [https://doi.org/10.1016/j.gca.2022.06.013]
¹Hawai‘i Institute of Geophysics and Planetology, University of Hawai‘i at Mānoa, Honolulu, Hawai‘i 96822, USA
²Department of Mineral Sciences, Museum of Natural History, Smithsonian Institution, Washington, D.C. 20560, USA
³Department of Geology, Moscow State University, Moscow, 119992, Russia
Copyright Elsevier

Coarse-grained igneous Ca,Al-rich inclusions (CAIs) in CV (Vigarano group) carbonaceous chondrites have typically heterogeneous O-isotope compositions with melilite, anorthite, and high-Ti (>10 wt% TiO2) fassaite being 16O-depleted (Δ17O up to ∼ −3±2‰) compared to hibonite, spinel, low-Ti (<10 wt% TiO2) fassaite, Al-diopside, and forsterite, all having close-to-solar Δ17O ∼ −24±2‰. To test a hypothesis that this heterogeneity was established, at least partly, during aqueous fluid-rock interaction, we studied the mineralogy, petrology, and O-isotope compositions of igneous CAIs CG-11 (Type B), TS-2F-1, TS-68, and 818-G (Compact Type A), and 818-G-UR (davisite-rich) from Allende (CV>3.6), and E38 (Type B) from Efremovka (CV3.1−3.4). Some of these CAIs contain (i) eutectic mineral assemblages of melilite, Al,Ti-diopside, and ±spinel which co-crystallized and therefore must have recorded O-isotope composition of the eutectic melt; (ii) isolated inclusions of Ti-rich fassaite inside spinel grains which could have preserved their initial O-isotope compositions, and/or (iii) pyroxenes of variable chemical compositions which could have recorded gas-melt O-isotope exchange during melt crystallization and/or postcrystallization exchange controlled by O-isotope diffusivity. If these CAIs experienced isotopic exchange with an aqueous fluid, O-isotope compositions of some of their primary minerals are expected to approach that of the fluid.

We find that in the eutectic melt regions composed of highly-åkermanitic melilite (Åk65−71), anorthite, low-Ti fassaite, and spinel of E38, spinel, fassaite, and anorthite are similarly 16O-rich (Δ17O ∼ −24‰), whereas melilite is 16O-poor (Δ17O ∼ −1‰). In the eutectic melt regions of CG-11, spinel and low-Ti fassaite are 16O-rich (Δ17O ∼ −24‰), whereas melilite and anorthite are 16O-poor (Δ17O ∼ −3‰). In TS-2F-1, TS-68, and 818-G, melilite and high-Ti fassaite grains outside spinel have 16O-poor compositions (Δ17O range from −12 to −3‰); spinel is 16O-rich (Δ17O ∼ −24‰); perovskite grains show large variations in Δ17O, from −24 to −1‰. Some coarse perovskites are isotopically zoned with a 16O-rich core and a 16O-poor edge. Isolated high-Ti fassaite inclusions inside spinel grains are 16O-rich (Δ17O ∼ −24‰), whereas high-Ti fassaite inclusions inside fractured spinel grains are 16O-depleted: Δ17O range from −12 to −3‰. In 818-G-UR, davisite is 16O-poor (Δ17O ∼ −2‰), whereas Al-diopside of the Wark-Lovering rim is 16O-enriched (Δ17O < −16‰). On a three-isotope oxygen diagram, the 16O-poor melilite, anorthite, high-Ti fassaite, and davisite in the Allende CAIs studied plot close to O-isotope composition of an aqueous fluid (Δ17O ∼ −3±2‰) inferred from O-isotope compositions of secondary minerals resulted from metasomatic alteration of the Allende CAIs. We conclude that CV igneous CAIs experienced post-crystallization O-isotope exchange that most likely resulted from an aqueous fluid-rock interaction on the CV asteroid. It affected melilite, anorthite, high-Ti fassaite, perovskite, and davisite, whereas hibonite, spinel, low-Ti fassaite, Al-diopside, and forsterite retained their original O-isotope compositions established during igneous crystallization of CV CAIs. However, we cannot exclude some gas-melt O-isotope exchange occurred in the solar nebula. This apparently “mineralogically-controlled” exchange process was possibly controlled by variations in oxygen self-diffusivity of CAI minerals. Experimentally measured oxygen self-diffusion coefficients in CAI-like minerals are required to constrain relative roles of O-isotope exchange during aqueous fluid-solid and nebular gas-melt interaction.