¹Kohei Fukuda,^1,2Travis J.Tenner,³Makoto Kimura,⁴Naotaka Tomioka,¹Guillaume Siron,⁴Takayuki Ushikubo,^1,5Noël Chaumard,^1,6Andreas T.Hertwig,¹Noriko T.Kita
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2021.12.027]
¹WiscSIMS, Department of Geoscience, University of Wisconsin-Madison, 1215 W. Dayton St., Madison, WI 53706, USA
²Chemistry Division, Nuclear and Radiochemistry, Los Alamos National Laboratory, MSJ514, Los Alamos, NM 87545, USA
³National Institute of Polar Research, Tokyo 190-8518, Japan
⁴Kochi Institute for Core Sample Research, JAMSTEC, Kochi 783-8502, Japan
⁵Fi Group, Direction scientifique, 14 terrasse Bellini, 92800, Puteaux, France
⁶Institute of Earth Sciences, Heidelberg University, Im Neuenheimer Feld 236, 69120 Heidelberg, Germany
Copyright Elsevier

Deciphering the spatial and temporal evolution of chondrules allows for a better understanding of how asteroidal seeds formed, migrated, and eventually accreted into parent asteroids. Here we report high precision Al-Mg ages and oxygen three-isotope ratios of fifteen FeO-poor chondrules from the least metamorphosed Mighei-like (CM) and Ornans-like (CO) carbonaceous chondrites, Asuka 12236 (CM2.9), Dominion Range 08006 (CO3.01), and Yamato-81020 (CO3.05). This is the first report of Al-Mg ages of chondrules from the CM chondrite group. All but one of the fifteen chondrules exhibit a restricted range of inferred initial ²⁶Al/²⁷Al ratios, and all ratios are ≤ 6.0 × 10⁻⁶, which is systematically lower than those of the majority of ordinary chondrite (OC) chondrules. These observations indicate that the majority of chondrules in the outer Solar System were produced ≥ 2.2 Ma after the formation of Ca-Al-rich inclusions (CAIs), which postdates OC chondrule formation in the inner Solar System (≤ 2.2 Ma after CAI formation). We propose that the discrete chondrule-forming events in different disk regions reflect a time difference in growth and orbital evolution of planetesimals within the first 4 Ma of the Solar System.

One chondrule from Asuka 12236 has an age of 1.9 Ma after CAI formation and is therefore significantly older than the other fourteen chondrules, meaning this chondrule formed contemporaneously with the majority of OC chondrules. This old chondrule also exhibits ¹⁶O-depleted oxygen isotope characteristics compared to the other chondrules, suggesting a distinct formation region, probably inside the disk region relative to where the majority of CM and CO chondrules formed. Our results indicate that this old chondrule has migrated from the inner to the outer part of the protoplanetary disk within ∼1 Ma and then accreted into the CM parent asteroid >3 Ma after CAI formation, although its formation exterior to the accretion region of the CM parent asteroid and subsequent inward migration cannot be ruled out completely.

^1,2Guy Libourel,²Kazuhide Nagashima,³Marc Portail,²Alexander N.Krot
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2021.12.026]
¹Université Côte d’Azur, OCA, CNRS, Laboratoire Lagrange, Boulevard de l’Observatoire, CS 34229, 06304 Nice Cedex 4, France
²Hawai‘i Institute of Geophysics and Planetology, School of Ocean, Earth Science and Technology, University of Hawai‘i at Mānoa, Honolulu, Hawai‘i 96821, USA
³CNRS-CRHEA (Centre de Recherches sur l’Hétéro-Epitaxie et ses Applications), Université Côte d’Azur, Sophia Antipolis, Rue Bernard Grégory, 06560 Valbonne, France
Copyright Elsevier

Using high-resolution cathodoluminescence (HR-CL) panchromatic imaging for the location of high-precision oxygen three-isotope analyses by secondary ion mass-spectrometry (SIMS), this study is aimed at characterizing the oxygen-isotope variations in Mg-rich olivines (≥ Fo99) of selected type I chondrules from the Yamato (Y) -81020 CO3.05 (Ornans-type) carbonaceous chondrite. Cathodoluminescence being extremely sensitive to faint changes in CL activator/quencher concentrations (Al, Cr, Mn, Fe) allows us to describe various overlooked cycles of growth and dissolution in Mg-rich olivines, which strongly suggest an intimate relationship with their gaseous environment during their formation. The present study confirms significant Δ17O variations of ten ‰ in Mg-rich olivines but does not support the relationship previously found between Mg# [MgO/(MgO+FeO)×100, mol%] and Δ17O among type I chondrules, nor the interpretation of redox changes that has been made of it. We instead show that Mg-rich olivines in Y-81020 chondrules exhibit a prominent 16O-enriched and 16O-depleted bimodal distribution, which is considered as the most primordial signature of type I chondrules from Y-81020 and very likely other carbonaceous chondrites. This signature is interpreted as a snapshot of the early stages of a mixing occurring between two clouds/environments in which chondrules formed and evolved by gas-melt interaction and mixed according to hydrodynamical instabilities imposed by the process responsible for the mixing. As far as this study allows, O-isotope variations of Mg-rich olivines seems to account for large scale dynamical instabilities while chemical variations highlighted by HR-CL (dissolution/growth) bear witness of smaller scale instabilities very likely occurring in the immediate vicinity of the chondrules. Without being able to decide on plausible astrophysical settings yet, we note however that processes like disruptive and vaporizing collisions between planetesimals offer a range of processes and physicochemical conditions, e.g., expansion, decompression, dynamical instabilities, that deserve to be explored in more detail, some of which resembling those highlighted in this study, e.g., gas-melt interaction, partial pressure fluctuations, heterogeneous materials, gas mixing.

¹Scott A.Whattam,^2,3Roger H.Hewins,⁴Jieun Seo,⁵Bertrand Devouard
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2021.12.022]
¹Department of Geosciences, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia
²IMPMC, Sorbonne Univ., MNHN, UPMC Paris 06, UMR CNRS 7590, 75005 Paris, France
³Department of Earth and Planetary Sciences, Rutgers University, Piscataway, NJ 08854, United States
⁴Department of Earth and Environmental Sciences, Korea University, Seoul 02841, Korea
⁵Aix-Marseille Université, CNRS, IRD, CEREGE UM34, BP 80 Aix en Provence, 13545 France
Copyright Elsevier

The formation of chondrules involved major processes in the protoplanetary disk and therefore needs to be understood. Identifying possible precursors and the conditions of their transformation into chondrules is an essential step. Here we investigate whether refractory inclusions (RI) can be converted into Type IA chondrule analogs by isothermal heating and dynamic crystallization experiments, and report a new constraint on chondrule peak temperatures. We prepared synthetic calcium-aluminum-rich inclusions (CAI) by sintering <20 µm An + Di + Sp powder at 1200 °C and synthetic AOA analogs from crushed <5 µm Fo gel or San Carlos olivine mixed with nuggets of synthetic CAI. We used the AOA analogs as starting materials in experiments and were able to reproduce the textures and mesostasis compositions of Type IA chondrules. However, in the charges, the olivine lacks asymmetric zonation and our mesostasis compositions show olivine fractionation trends, two differences from Type I chondrules indicating the requirement of condensation of Mg and SiO in the latter. Relict spinel is present in isothermal runs up to 1550 °C, but is totally resorbed by 1600 °C. We conclude that CAI and AOA were sintered essentially at their condensation temperatures and are appropriate precursors for chondrules. Chondrules with relict spinel must have formed at <1600 °C, much lower than their liquidus temperatures (∼1750 °C). Such peak temperatures are consistent with models of condensation during chondrule formation. In typical chondrules with no inclusions of AOA or CAI, spinel is an indicator of their near complete assimilation. Grains of spinel (sensu stricto) in chondrules are relicts of RI and constitute a largely untapped cosmochemical resource for the investigation of chondrule provenance.

^1,2Andrea Patzer,¹Emma S.Bullock,¹ConelM. O’D. Alexander
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2021.12.021]
¹Earth and Planets Laboratory, Carnegie Institution for Science, 5241 Broad Branch Rd. NW, Washington D.C. 20015, USA
²Geosciences Center, University of Goettingen, Goldschmidtstr. 1, 37077 Goettingen, Germany
Copyright Elsevier

Knowing how the major chondritic components evolved and what their initial compositions were is pivotal for our understanding of the processes that shaped the early Solar System. Here, we have extended to the CR chondrites our testing of chondrule-matrix complementarity and the four-component model, i.e., two very different explanations for the bulk compositions of the carbonaceous chondrites and their components. Combining point-counting with electron microprobe analyses, we have analyzed four relatively primitive Antarctic CRs and the fall Renazzo. Our results for the abundances of chondrules and matrix are in good agreement with literature data, and confirm that these abundances vary considerably amongst the CRs (80.4 ±2.3 wt.% and 18.5 ±2.8 wt.%, respectively, in the four Antarctic CRs vs. 62.3 ±3.4 wt.% and 33.2 ±2.2 wt.% in Renazzo). The significant differences make the determination of the average properties and bulk compositions of the CRs problematic. This is particularly true for the volatile elements that were predominantly accreted in matrix. Nevertheless, all major and many minor element concentrations reported in the literature for average bulk CRs are reproduced here to better than 10 %. By comparing our results to conventionally determined bulk compositions, we were able to verify the accuracy of our approach and identify elements likely affected by alteration or analytical artifacts (e.g., Ti, K, Co). Two particular compositional details of the CR chondrites investigated are (a) the relatively high contents of Mn in the chondrules compared to CO chondrules, and (b) the depletion of S in the matrix, relative to CI. In terms of the major elements Mg, Al, Si and Ca, our data suggest that unaltered chondrules and matrix exhibited CI-like relative abundances, supporting previous conclusions for the CO chondrites. Where observed, deviations of element abundances in the matrix from CI (Na, Mg, S, Ca, Fe, Ni) can be explained in terms of alteration (parent body and terrestrial) and pre-accretionary loss of forsterite and, possibly, sulfides. Overall, our results are more consistent with the predictions of the four-component model than they are with chondrule-matrix complementarity.