¹Francis M.McCubbin,¹Jonathan A.Lewis,²Jessica J.Barnes,³Stephen M.Elardo,¹Jeremy W.Boyce
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2021.08.021]
¹NASA Johnson Space Center, Mailcode XI2, 2101 NASA Parkway, Houston, Texas 77058, USA
²Lunar and Planetary Laboratory, University of Arizona, 1629 E University Blvd, Tucson, AZ 85721, USA
³Department of Geological Sciences, University of Florida, Gainesville, FL 32611, USA
Copyright Elsevier

We conducted a petrologic study of apatite within eight unbrecciated, non-cumulate eucrites and two monomict, non-cumulate eucrites. These data were combined with previously published data to quantify the abundances of F, Cl, and H2O in the bulk silicate portion of asteroid 4 Vesta (BSV). Using a combination of apatite-based melt hygrometry/chlorometry and appropriately paired volatile/refractory element ratios, we determined that BSV has 3.0–7.2 ppm F, 0.39–1.8 ppm Cl, and 3.6–22 ppm H2O. The abundances of F and H2O are depleted in BSV relative to CI chondrites to a similar degree as F and H2O in the bulk silicate portion of the Moon. This degree of volatile depletion in BSV is similar to what has been determined previously for many moderately volatile elements in 4 Vesta (e.g., Na, K, Zn, Rb, Cs, and Pb). In contrast, Cl is depleted in 4 Vesta by a greater degree than what is recorded in samples from Earth or the Moon. Based on the Cl-isotopic compositions of eucrites and the bulk rock Cl/F ratios determined in this study, the eucrites likely formed through serial magmatism of a mantle with heterogeneous δ37Cl and Cl/F, not as extracts from a partially crystallized global magma ocean. Furthermore, the volatile depletion and Cl-isotopic heterogeneity recorded in eucrites is likely inherited, at least in part, from the precursor materials that accreted to form 4 Vesta and is unlikely to have resulted solely from degassing of a global magma ocean, magmatic degassing of eucrite melts, and/or volatile loss during thermal metamorphism. Although our results can be reconciled with the past presence of wide-scale melting on 4 Vesta (i.e., a partial magma ocean), any future models for eucrite petrogenesis involving a global magma ocean would need to account for the preservation of a heterogeneous eucrite source with respect to Cl/F ratios and Cl isotopes.

¹Maxime Piralla,¹Johan Villeneuve,²Valentina Batanova,³Emmanuel Jacquet,¹Yves Marrocchi
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2021.08.007]
¹Université de Lorraine, CNRS, CRPG, UMR 7358, Vandœuvre-lès-Nancy 54500, France
²Université Grenoble Alpes, ISTerre, CNRS, UMR 5275, Grenoble 38000, France
³Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie (IMPMC), Muséum national d’Histoire naturelle, Sorbonne Université, CNRS, UMR 7590, CP52, 57 rue Cuvier, Paris 75005, France
Copyright Elsevier

Chondrules are sub-millimetric spheroids that are ubiquitous in chondrites and whose formation mechanism remains elusive. Textural and oxygen isotopic characteristics of chondrules in carbonaceous chondrites (CCs) suggest that they result from the recycling of isotopically heterogeneous early-condensed precursors via gas-melt interactions. Here, we report high-resolution X-ray elemental maps and in situ O isotopic analyses of FeO-poor, olivine-rich chondrules from ordinary chondrites (OCs) to compare the conditions of chondrule formation in these two main classes of chondrites. OC chondrules show minor element (e.g., Ti, Al) zonings at both the chondrule and individual olivine grain scales. Considering the entire isotopic data set, our data define a mass-independent correlation, with olivine grains showing O isotopic variations spanning more than 40 ‰. Though 16O-rich relict olivine grains were identified in OC chondrules, they are much less abundant than in CC chondrules. They appear as two types: (i) those with low minor element abundances and Δ17O < −15 ‰ and (ii) those with varying minor element abundances and less negative Δ17O values averaging −5.5 ‰. The host olivine grains exhibit mass-dependent O isotopic variations within individual chondrules. Our results reveal that similar processes (precursor recycling and interactions between chondrule melts and a SiO- and Mg-rich gas) established the observed features of OC and CC chondrules. The mass-dependent isotopic variations recorded by host olivine grains result from kinetic effects induced by complex evaporation/recondensation processes during the gas-melt interactions. This suggests that OC chondrules formed through enhanced recycling processes, in good agreement with the lower abundances of relict olivine grains in OC chondrules compared to CC chondrules. We use the Δ18O = δ18O − δ17O parameter to demonstrate that there is no genetic relationship between CC and OC chondrules, suggesting limited radial transport in the protoplanetary disk. Finally, to the first order, the Δ18O−Δ17O diagram may allow the non-carbonaceous vs. carbonaceous origin of a given chondrule to be deciphered.

¹C.Deligny,¹E.Füri,¹E.Deloule
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2021.07.038]
¹Université de Lorraine, CNRS, CRPG, F-54000 Nancy, France
Copyright Elsevier

Angrites are derived from the earliest generation of differentiated planetesimals that accreted sunward of Jupiter’s orbit, and are, thus, key to constraining the timing and source(s) of volatile delivery to planetary bodies in the inner solar system. Here we investigate the nitrogen and hydrogen isotopic signatures of angrite melts by in situ secondary ion mass spectrometry (SIMS) analyses of mineral-hosted melt inclusions and interstitial glass in two of the oldest volcanic angrites: D’Orbigny and Sahara 99555. The most primitive melt trapped in Mg-rich olivines in D’Orbigny is characterized by δ15N values ranging from 0 ± 25 to +56 ± 29‰ and δD values between −348 ± 53 and −118 ± 31‰. This shows that the angrite mantle source sampled by D’Orbigny has a N-H isotopic composition that is similar to that of CM carbonaceous chondrites, whose parent bodies are thought to have accreted in the outer solar system. The low nitrogen and water contents measured in Sahara 99555 possibly indicate that its parental melt underwent a higher degree of degassing compared to D’Orbigny or, alternatively, that the two angrites do not sample the same volatile reservoir within the angrite parent body. Given the very old crystallisation age of D’Orbigny, our findings imply that nitrogen- and water-rich objects, presumably formed beyond the orbit of Jupiter, must have been present in the terrestrial planet-forming region within the first ~4 Ma after the formation of Ca-Al-rich inclusions (CAIs, the oldest materials in the solar system).