¹Lionel G.Vacher,¹Ryan C.Ogliore,²Clive Jones,²Nan Liu,²David A.Fike
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2021.06.026]
¹Department of Physics, Washington University in St. Louis, St. Louis, MO, USA
²Department of Earth and Planetary Sciences, Washington University in St. Louis, St. Louis, MO, USA
Copyright Elsevier

The Sun’s astrophysical birth environment affected the formation and composition of the Solar System. Primitive meteorites display mass-independent oxygen isotope anomalies that were likely caused by ultraviolet (UV) photochemistry of CO gas-phase molecules, either (i) in the outer solar nebula by light from the young Sun or (ii) in the parent molecular cloud by light from nearby stars. However, measurements of oxygen isotopes alone cannot unambiguously constrain the UV spectrum of the source responsible for the photochemistry. Sulfur, with four stable isotopes, can be used as a more direct probe of the astrophysical environment of mass-independent photochemistry. Here, we report the in situ isotopic analysis of paired oxygen and sulfur isotope systematics in cosmic symplectite (COS), magnetite-pentlandite intergrowths, in the primitive ungrouped carbonaceous chondrite Acfer 094. We show that COS grains contain mass-independent sulfur isotope anomalies (weighted means of Δ33S = +3.84 ± 0.72‰ and Δ36S = −6.05 ± 2.25‰, 2SE) consistent with H2S photochemistry by UV from massive O and B stars close to the Solar System’s parent molecular cloud, and inconsistent with UV from the protosun. The presence of coupled mass-independent sulfur and oxygen (Δ17O = 86 ± 6‰, 2SE) isotope anomalies in COS imply that these anomalies originated in the same astrophysical environment. We propose that this environment is the photodissociation region (PDR) of the Solar System’s parent molecular cloud, where nearby massive stars irradiated the edge of the cloud. We conclude that the Sun’s stellar neighbors, likely O and B stars in a massive-star-forming region, affected the composition of the Solar System’s primordial building blocks.

¹Anna Barbaro,¹Maria Chiara Domeneghetti,^2,3Konstantin D.Litasov,⁴Ludovic Ferrière,⁴Lidia Pittarello,⁵Oliver Christ,⁵Sofia Lorenzon,¹Matteo Alvaro,^5,6Fabrizio Nestola
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2021.06.022]
¹Dipartimento di Scienze della Terra e dell’Ambiente, Università degli Studi di Pavia, Via Ferrata 1, I-27100, Pavia, Italy
²Vereshchagin Institute for High Pressure Physics RAS, Troitsk, Moscow, 108840, Russia
³Fersman Mineralogical Museum RAS, Moscow, 115162, Russia
⁴Natural History Museum, Department of Mineralogy and Petrography, Burgring 7, 1010 Vienna, Austria
⁵Dipartimento di Geoscienze, Università degli Studi di Padova, Via G. Gradenigo 6, I-35131 Padova, Italy
⁶Geoscience Institute, Goethe-University Frankfurt, Altenhöferalee 1, 60323 Frankfurt, Germany
Copyright Elsevier

The occurrence of shock-induced diamonds in ureilite meteorites is common and is used to constrain the history of the ureilite parent bodies. We have investigated a fragment of the Kenna ureilite by micro-X-ray diffraction, micro-Raman spectroscopy and scanning electron microscopy to characterize its carbon phases. In addition to olivine and pigeonite, within the carbon-bearing areas, we identified microdiamonds (up to about 10 μm in size), nanographite and magnetite. The shock features observed in the silicate minerals and the presence of microdiamonds and nanographite indicate that Kenna underwent a shock event with a peak pressure of at least 15 GPa. Temperatures estimated using a graphite geothermometer are close to 1180 °C. Thus, Kenna is a medium-shocked ureilite, yet it contains microdiamonds, which are typically found in highly shocked carbon-bearing meteorites, instead of the more common nanodiamonds. This can be explained by a relatively long shock event duration (in the order of 4-5 seconds) and/or by the catalytic effect of Fe-Ni alloys known to favour the crystallization of diamonds. For the first time in a ureilite, carletonmooreite with formula Ni3Si and grain size near 4-7 nm, was found. The presence of nanocrystalline carletonmooreite provides further evidence to support the hypothesis of the catalytic involvement of Fe-Ni bearing phases into the growth process of diamond from graphite during shock events in the ureilite parent body, enabling the formation of micrometer-sized diamond crystals.