¹Craig R. Walton,²Ioannis Baziotis,³Ana Černok,⁴Ludovic Ferrière,⁵Paul D. Asimow,^1,6Oliver Shorttle,^3,7Mahesh Anand
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13648]
¹Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ UK
²Department of Natural Resources Management and Agricultural Engineering, Agricultural University of Athens, IeraOdos 75, 11855 Athens, Greece
³Department of Physical Sciences, Open University, Walton Hall, Milton Keynes, MK7 6AA UK
⁴Natural History Museum, Burgring 7, A‐1010 Vienna, Austria
⁵Division of Geological and Planetary Sciences, California Institute of Technology, 1200 E California Blvd, Pasadena, California, 91125 USA
⁶Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge, CB3 OHA UK
⁷Department of Earth Sciences, The Natural History Museum, London, SW7 5BD UK
Published by arrangement with John Wiley & Sons

The geochemistry and textures of phosphate minerals can provide insights into the geological histories of parental asteroids, but the processes governing their formation and deformation remain poorly constrained. We assessed phosphorus‐bearing minerals in the three lithologies (light, dark, and melt) of the Chelyabinsk (LL5) ordinary chondrite using scanning electron microscope, electron microprobe, cathodoluminescence, and electron backscatter diffraction techniques. The majority of studied phosphate grains appear intergrown with olivine. However, microtextures of phosphates (apatite [Ca5(PO4)3(OH,Cl,F)] and merrillite [Ca9NaMg(PO4)7]) are extremely variable within and between the differently shocked lithologies investigated. We observe continuously strained as well as recrystallized strain‐free merrillite populations. Grains with strain‐free subdomains are present only in the more intensely shocked dark lithology, indicating that phosphate growth predates the development of primary shock‐metamorphic features. Complete melting of portions of the meteorite is recorded by the shock‐melt lithology, which contains a population of phosphorus‐rich olivine grains. The response of phosphorus‐bearing minerals to shock is therefore hugely variable throughout this monomict impact breccia. We propose a paragenetic history for P‐bearing phases in Chelyabinsk involving initial phosphate growth via P‐rich olivine replacement, followed by phosphate deformation during an early impact event. This event was also responsible for the local development of shock melt that lacks phosphate grains and instead contains P‐enriched olivine. We generalize our findings to propose a new classification scheme for Phosphorus‐Olivine‐Assemblages (Type I–III POAs). We highlight how POAs can be used to trace radiogenic metamorphism and shock metamorphic events that together span the entire geological history of chondritic asteroids.

^1,2Caroline E. Caplan,²Gary R. Huss,²Hope A. Ishii,²John P. Bradley,^2,3Birger Schmitz,¹Kazuhide Nagashima
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13649]
¹Department of Earth Sciences, University of Hawai‘i at Mānoa, 1680 East‐West Road, Honolulu, Hawai‘i, 96822 USA
²Hawai‘i Institute of Geophysics and Planetology, University of Hawai‘i at Mānoa, 1680 East‐West Road, Honolulu, Hawai‘i, 96822 USA
³Astrogeobiology Laboratory, Department of Physics, Lund University, Lund, Sweden
Published by arrangement with John Wiley & Sons

Remnant extraterrestrial chrome spinels from terrestrial sediments provide information on how the mixture of meteoritic materials falling to Earth has changed over Earth’s history. The parent meteorite type of each grain can be identified by characteristic elemental and oxygen‐isotope abundances. Some meteorite types can be difficult to classify because their chrome‐spinel compositional ranges overlap. Silicate inclusions within chrome spinels of modern ordinary chondrites have been shown to have discriminating power among meteorite subclasses. We employed energy‐dispersive X‐ray spectroscopy in a scanning electron microscope (SEM) and in a (scanning) transmission electron microscope (S/TEM) to investigate inclusions in chrome‐spinel grains from Ordovician and Jurassic sediments. Unaltered Ordovician inclusions allowed us to establish the size limits for reliable SEM analysis of inclusions. The Jurassic grains were more altered, but the use of STEM techniques on small inclusions (<3 μm diameter at their polished surfaces) allowed us to determine chemical compositions and mineral structures of inclusions in three chrome spinels. The parent meteorite type was determined for one Jurassic grain based on its inclusion compositions. Our study confirms that silicate inclusions can be used to classify parent meteorite types of chrome‐spinel grains, but the size of the inclusions and the complex effects of terrestrial alteration must be taken into account. During our study, we also found some interesting exsolution phenomena in the host chrome‐spinel grains.

¹Akimasa Suzumura,²Noriyuki Kawasaki,³Yusuke Seto,²Hisayoshi Yurimoto,¹Shoichi Itoh
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2021.03.030]
¹Department of Earth and Planetary Sciences, Kyoto University, Kyoto 606-8502, Japan
²Department of Natural History Sciences, Hokkaido University, Sapporo 060-0810, Japan
³Department of Planetology, Kobe University, Kobe 657-8501, Japan
Copyright Elsevier

We report the in-situ oxygen isotopic distributions corresponding to the petrographic-mineralogical observation on a compact Type A (CTA) Ca-Al-rich inclusion (CAI), KU-N-02, from a reduced CV3 chondrite, Northwest Africa 7865. The CTA has an igneous texture and mainly consists of spinel, melilite, and Al-Ti-rich clinopyroxene (fassaite). Oxygen isotopic compositions of the constituent minerals plot along the carbonaceous chondrite anhydrous mineral line. The spinel grains are poikilitically enclosed in the melilite and fassaite and are uniformly 16O-rich (Δ17O = approximately −23‰). The fassaite is texturally classified into two types: blocky fassaite and intergranular fassaite. The blocky fassaite crystals exhibit growth zoning as they change from Ti-rich to Ti-poor along the inferred directions of crystal growth from core to rim, while the oxygen isotopic compositions change from 16O-poor (Δ17O = approximately −6‰) to 16O-rich (Δ17O = approximately −23‰) with crystal growth. The intergranular fassaite crystals exist between the melilite crystals and exhibit variable Ti abundance and oxygen isotopic compositions. Additionally, their relationships between Ti contents and oxygen isotopic composition are similar to those of the blocky fassaite. The melilite grains are homogeneously 16O-poor (Δ17O = approximately −2‰), irrespective of their åkermanite (Åk) content. Each melilite grain generally exhibits growth zoning with increasing Åk contents from core to rim, although the melilite contains Åk-rich patches within single crystal. Åk-rich patches often include two types of fassaite: small blebby crystals attached to spinel crystals and round crystals. The oxygen isotopic compositions of the Åk-rich patch and blebby fassaite are 16O-poor (Δ17O = approximately −2‰), similar to that of the host melilite. On the other hand, the round fassaite exhibits significant variation in oxygen isotopic compositions ranging from Δ17O = −23‰ to −4‰, which are different from those of the host melilite. These petrographic textures and oxygen isotopic variations indicate the presence of a solid precursor with variable oxygen isotopic compositions for the CTA. The spinel and round fassaite grains are relicts of the precursor that melted in the 16O-poor nebular gas, resulting in the crystallization of the host melilite from the 16O-poor melt. The Åk-rich patches and blebby fassaite crystallized from melts trapped by the growing host melilite crystals. The blocky and intergranular fassaite crystallized after the melilite did, and the oxygen isotopic composition of the melt changed to 16O-rich during the crystallization process, suggesting that the oxygen isotopic composition of the surrounding nebular gas could be varied. The inferred oxygen isotopic evolution for CTA is consistent with those inferred for Type B CAIs, suggesting that coarse-grained igneous CAIs formed in a similar nebular environment regardless of precursor chemistry.