¹Ryan C. Ogliore,²Donald E. Brownlee,³Kazuhide Nagashima,²David J. Joswiak,¹Josiah B. Lewis,³Alexander N. Krot,¹Kainen L. Utt,³Gary R. Huss
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13364]
¹Department of Physics, Washington University in St. Louis, St. Louis, Missouri, 63130 USA
²Department of Astronomy, University of Washington, Seattle, Washington, 98195 USA
³Hawai’i Institute of Geophysics and Planetology, University of Hawai’i at Mānoa, Honolulu, Hawaii, 96822 USA
Published by arrangement with John Wiley & Sons

Filamentary enstatite crystals are found in interplanetary dust particles (IDPs) of likely cometary origin but are very rare or absent in meteorites. Crystallographic characteristics of filamentary enstatites indicate that they condensed directly from vapor. We measured the O isotopic composition of an enstatite ribbon from a giant cluster IDP to be δ¹⁸O = 25 ± 55, δ¹⁷O = −19 ± 129, ∆¹⁷O = −32 ± 134 (2σ errors), which is inconsistent at the 2σ level with the composition of the Sun inferred from the Genesis solar wind measurements. The particle’s O isotopic composition, consistent with the terrestrial composition, implies that it condensed from a gas of nonsolar O isotopic composition, possibly as a result of vaporization of disk region enriched in ¹⁶O‐depleted solids. The relative scarcity of filamentary enstatite in asteroids compared to comets implies either that this crystal condensed from dust vaporized in situ in the outer solar system where comets formed or it condensed in the inner solar system and was subsequently transported outward to the comet‐forming region.

¹Cosette M. Gilmour,¹Christopher D. K. Herd,²Pierre Beck
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13362]
¹Department of Earth and Atmospheric Sciences, University of Alberta, 1‐26 Earth Sciences Building, Edmonton, Alberta, T6G 2E3 Canada
²UJF‐Grenoble 1, CNRS‐INSU, Institut de Planétologie et d’Astrophysique de Grenoble (IPAG), UMR 5274, Grenoble, F‐38041 France
Published by arrangement with John Wiley & Sons

Here, we evaluate the extent of aqueous alteration among five pristine specimens of the ungrouped Tagish Lake carbonaceous chondrite (TL5b, TL11h, TL11i, TL4, and TL10a) using thermogravimetric analysis (TGA) and infrared (IR) transmission spectroscopy. Both TGA and IR spectroscopy have proven to be reliable methods for determining the extent of aqueous alteration among different carbonaceous chondrites, in particular the CM chondrites (e.g., Garenne et al. 2014), with which Tagish Lake shares some affinities. Using these two methods, our goal is to incorporate TL4 and TL10a into the known alteration sequence of TL5b < TL11h < TL11i (Herd et al. 2011; Blinova et al. 2014a). This study highlights the compositional variability of the Tagish Lake specimens, which we ascribe to its brecciated nature. Our TGA and IR spectroscopy results are congruent with the reported alteration sequence, allowing us to introduce the TL4 and TL10a specimens in the following order: TL4 < TL5b ≤ TL10a < TL 11h < TL11i. Notably, these two specimens appear to be similar to the least altered lithologies previously reported, and the alteration of Tagish Lake is similar to that experienced by lesser altered members of the CM chondrites (>CM1.6). Based on these findings, Tagish Lake could be considered a 1.6–2.0 ungrouped carbonaceous chondrite. Visible and near‐IR reflectance measurements of Tagish Lake were also acquired in this study to revisit the Tagish Lake parent body connection. While other studies have paired Tagish Lake with D‐ and T‐type asteroid parent bodies, the reflectance spectra acquired in this study are variable among the different Tagish Lake specimens in relation to their alteration sequences; results match with spectra characteristic of C‐, X‐, Xc‐, and D‐type asteroids. The heterogeneity of Tagish Lake coupled with its low albedo makes the parent body connection a challenge.

^1,2Chen Zhao,¹Katharina Lodders,¹Hannah Bloom,¹Heng Chen,¹Zhen Tian,¹Piers Koefoed,³Mária K. Pető,¹Kun Wang (王昆)
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13358]
¹Department of Earth and Planetary Sciences and McDonnell Center for the Space Sciences, Washington University in St. Louis, Campus Box 1169, One Brookings Drive, St. Louis, Missouri, 63130 USA
²Faculty of Earth Sciences, China University of Geosciences, Wuhan, Hubei, 430074 China
³Konkoly Observatory, Research Center for Astronomy and Earth Sciences, Hungarian Academy of Sciences, H‐1121 Budapest, Hungary
Published by arrangement with John Wiley & Sons

Enstatite chondrites and aubrites are meteorites that show the closest similarities to the Earth in many isotope systems that undergo mass‐independent and mass‐dependent isotopic fractionations. Due to the analytical challenges to obtain high‐precision K isotopic compositions in the past, potential differences in K isotopic compositions between enstatite meteorites and the Earth remained uncertain. We report the first high‐precision K isotopic compositions of eight enstatite chondrites and four aubrites and find that there is a significant variation of K isotopic compositions among enstatite meteorites (from −2.34‰ to −0.18‰). However, K isotopic compositions of nearly all enstatite meteorites scatter around the bulk silicate earth (BSE) value. The average K isotopic composition of the eight enstatite chondrites (−0.47 ± 0.57‰) is indistinguishable from the BSE value (−0.48 ± 0.03‰), thus further corroborating the isotopic similarity between Earth’s building blocks and enstatite meteorite precursors. We found no correlation of K isotopic compositions with the chemical groups, petrological types, shock degrees, and terrestrial weathering conditions; however, the variation of K isotopes among enstatite meteorite can be attributed to the parent‐body processing. Our sample of the main‐group aubrite MIL 13004 is exceptional and has an extremely light K isotopic composition (δ41K = −2.34 ± 0.12‰). We attribute this unique K isotopic feature to the presence of abundant djerfisherite inclusions in our sample because this K‐bearing sulfide mineral is predicted to be enriched in 39K during equilibrium exchange with silicates.