¹Romy D. Hanna, ¹Richard A. Ketcham
Earth and Planetary Science Letters 481, 201-211 Link to Article [https://doi.org/10.1016/j.epsl.2017.10.029]
¹Jackson School of Geosciences, University of Texas, Austin, TX 78712, USA
Copyright Elsevier

We use X-ray computed tomography (XCT) to examine the 3D morphology and spatial relationship of fine-grained rims (FGRs) of Type I chondrules in the CM carbonaceous chondrite Murchison to investigate the formation setting (nebular vs. parent body) of the FGRs. We quantify the sizes, shapes, and orientations of the chondrules and FGRs and develop a new algorithm to examine the 3D variation of FGR thickness around each chondrule. We find that the average proportion of chondrule volume contained in the rim for Murchison chondrules is 35.9%. The FGR volume in relation to the interior chondrule radius is well described by a power law function as proposed for accretion of FGRs in a weakly turbulent nebula by Cuzzi (2004). The power law exponent indicates that the rimmed chondrules behaved as Stokes number Stη>1 nebular particles in Kolmogorov η scale turbulence. FGR composition as inferred from XCT number appears essentially uniform across interior chondrule types and compositions, making formation by chondrule alteration unlikely. We determine that the FGRs were compressed by the impact event(s) that deformed Murchison ( Hanna et al., 2015), resulting in rims that are thicker in the plane of foliation but that still preserve their nebular morphological signature. Finally, we propose that the irregular shape of some chondrules in Murchison is a primary feature resulting from chondrule formation and that chondrules with a high degree of surface roughness accreted a relatively larger amount of nebular dust compared to smoother chondrules.

¹J. Labidi, ¹S. König, ¹T. Kurzawa, ¹A. Yierpan, ¹R. Schoenberg
Earth and Planetary Science Letters 481, 212-222 Link to Article [https://doi.org/10.1016/j.epsl.2017.10.032]
¹Isotope Geochemistry, Department of Geosciences, Eberhard Karls Universität Tübingen, Wilhelmstraße 56, 72074 Tübingen, Germany
Copyright Elsevier

Element transfer from the solar nebular gas to solids occurred either through direct condensation or via heterogeneous reactions between gaseous molecules and previously condensed solid matter. The precursors of altered sulfides observed in chondrites are for example attributed to reactions between gaseous hydrogen sulfide and metallic iron grains. The transfer of selenium to solids likely occurred through a similar pathway, allowing the formation of iron selenides concomitantly with sulfides. The formation rate of sulfide however remains difficult to assess. Here we investigate whether the Se isotopic composition of meteorites contributes to constrain sulfide formation during condensation stages of our solar system. We present high precision Se concentration and δ82/78Se data for 23 chondrites as well as the first δ74/78Se, δ76/78Se and δ77/78Se data for a sub-set of seven chondrites. We combine our dataset with previously published sulfur isotopic data and discuss aspects of sulfide formation for various types of chondrites.

Our Se concentration data are within uncertainty to literature values and are consistent with sulfides being the dominant selenium host in chondrites. Our overall average δ82/78Se value for chondrites is −0.21±0.43‰ (n=23, 2 s.d.), or −0.14±0.21‰ after exclusion of three weathered chondrites (n=20, 2 s.d.). These average values are within uncertainty indistinguishable from a previously published estimate. For the first time however, we resolve distinct δ82/78Se between ordinary (−0.14±0.07‰, n=9, 2 s.d.), enstatite (−0.27±0.05‰, n=3, 2 s.d.) and CI carbonaceous chondrites (−0.01±0.06‰, n=2, 2 s.d.). We also resolve a Se isotopic variability among CM carbonaceous chondrites. In addition, we report on δ74/78Se, δ76/78Se and δ77/78Se values determined for 7 chondrites. Our data allow evaluating the mass dependency of the δ82/78Se variations. Mass-independent deficits ro excesses of 74Se, 76Se and 77Se are calculated relative to the observed 82Se/78Se ratios, and were observed negligible. This rules out poor mixing of nucleosynthetic components to account for the δ82/78Se variability and implies that the mass dependent Se isotopic variations were produced in a once-homogeneous disk.

The mass-dependent isotopic difference between enstatite and ordinary chondrites may reflect the contribution of a kinetic sulfidation process at anomalously high H2S–H2Se contents in the region of enstatite chondrite formation. Experimental studies showed that high H2S contents favor the formation of compact sulfide layers around metallic grains. This decreases the reactive surface, which tends to inhibit the continuation of the sulfidation reaction. Under these conditions sulfide growth likely occurs under isotopic disequilibrium and favors the trapping of light S and Se isotopes in solids; This hypothesis provides an explanation for our Se isotope as well as for previously published S isotope data. On the other hand, high δ82/78Se values in carbonaceous chondrites may result from sample heterogeneities generated by parent body aqueous alteration, or could reflect the contribution of ices carrying photo-processed Se from the outer solar system.

¹Wataru Fujiya
Earth and Planetary Science Letters 481, 264-272 Link to Article [https://doi.org/10.1016/j.epsl.2017.10.046]
¹Faculty of Science, Ibaraki University, 2-1-1 Bunkyo, Mito, Ibaraki 310-8512, Japan
Copyright Elsevier

In this work, I estimate the δ18O and δ17O values of primordial water in CM chondrites to be 55 ± 13 and 35 ± 9‰, respectively, based on whole-rock O and H data. Also, I found that the O and/or H data of Antarctic meteorites are biased, which is attributed to terrestrial weathering. This characteristic O isotopic ratio of water together with corresponding water abundances in CM chondrites are consistent with the origin of water as ice processed by photochemical reactions at the outer regions of the solar nebula, where mass-independent O isotopic fractionation and water destruction may have occurred. Another possible mechanism to produce the inferred O isotopic ratio of water would be O isotopic fractionation between water vapor and ice, which likely occurred near the condensation front of H2O (snow line) in the solar nebula. The inferred O isotopic ratio of water suggests that carbonate in CM chondrites formed at low temperatures of <150 °C. The O isotopic ratios of primordial water in chondrites other than CM chondrites are not well constrained.

¹Yves Marrocchi, ¹David V. Bekaert, ¹Laurette Piani
Earth and Planetary Science Letetrs 482, 23-32 Link to Article [https://doi.org/10.1016/j.epsl.2017.10.060]
¹CRPG, CNRS, Université de Lorraine, UMR 7358, Vandoeuvre-les-Nancy, F-54501, France
Copyright Elsevier

The origin and abundance of water accreted by carbonaceous asteroids remains underconstrained, but would provide important information on the dynamic of the protoplanetary disk. Here we report the in situ oxygen isotopic compositions of aqueously formed fayalite grains in the Kaba and Mokoia CV chondrites. CV chondrite bulk, matrix and fayalite O-isotopic compositions define the mass-independent continuous trend (δ17O = 0.84 ± 0.03 × δ18O − 4.25 ± 0.1), which shows that the main process controlling the O-isotopic composition of the CV chondrite parent body is related to isotopic exchange between 16O-rich anhydrous silicates and 17O- and 18O-rich fluid. Similar isotopic behaviors observed in CM, CR and CO chondrites demonstrate the ubiquitous nature of O-isotopic exchange as the main physical process in establishing the O-isotopic features of carbonaceous chondrites, regardless of their alteration degree. Based on these results, we developed a new approach to estimate the abundance of water accreted by carbonaceous chondrites (quantified by the water/rock ratio) with CM (0.3–0.4) ≥ CR (0.1–0.4) ≥ CV (0.1–0.2) > CO (0.01–0.10). The low water/rock ratios and the O-isotopic characteristics of secondary minerals in carbonaceous chondrites indicate they (i) formed in the main asteroid belt and (ii) accreted a locally derived (inner Solar System) water formed near the snowline by condensation from the gas phase. Such results imply low influx of D- and 17O- and 18O-rich water ice grains from the outer part of the Solar System. The latter is likely due to the presence of a Jupiter-induced gap in the protoplanetary disk that limited the inward drift of outer Solar System material at the exception of particles with size lower than 150 μm such as presolar grains. Among carbonaceous chondrites, CV chondrites show O-isotopic features suggesting potential contribution of 17–18O-rich water that may be related to their older accretion relative to other hydrated carbonaceous chondrites.