¹Claudia A. Trepmann,¹Fabian Dellefant,²Melanie Kaliwoda,¹Kai‐Uwe Hess,^1,2Wolfgang W. Schmahl,³Stefan Hölzl
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13590]
¹Department of Earth and Environmental Sciences, Ludwig‐Maximilians‐University, 80333 Munich, Germany
²Mineralogische Staatssammlung, Staatliche naturwissenschaftliche Sammlungen Bayerns, 80333 Munich, Germany
³RiesKraterMuseum, Staatliche naturwissenschaftliche Sammlungen Bayerns, 86720 Nördlingen, Germany
Published by arrangement with John Wiley & Sons

Quartz and cristobalite ballen aggregates surrounded by dendritic cristobalite in gneiss clasts of impact melt rocks from the Ries impact structure are analyzed by Raman spectroscopy, microscopy, and electron backscattered diffraction to elucidate the development of the characteristic polycrystalline ballen that are defined by curved interfaces between each other. We suggest that the investigated ballen aggregates represent former fluid inclusion‐rich quartz grains from the granitic gneiss protolith. Upon shock loading, they transformed into an amorphous phase that partly retained information on the precursor structure. Volatiles from inclusions dissolved into the amorphous phase. During decompression and cooling, dehydration takes place and causes fracturing of the amorphous phase and disintegration into small globular ballen, with the fluid being expelled along the fractures. A similar formation of small globules due to dehydration of silica‐rich glass is known for perlitic structures of volcanic rocks. Remnants of the precursor structure are present in the amorphous phase and enabled topotactic crystallization of quartz, leading to a crystallographic preferred orientation. Crystallization of more distorted parts of the amorphous phase led to random orientations of the quartz crystals. Ballen comprised of cristobalite formed from a dehydrated amorphous phase with no structural memory of the precursor. Dendritic cristobalite exclusively occurring at the rim of quartz ballen aggregate is interpreted to have crystallized directly from a melt enriched in fluids that were expelled during dehydration of the amorphous phase.

^1,2Bastien Soens,^3,4Martin D. Suttle,¹Ryoga Maeda,⁵Frank Vanhaecke,⁶Akira Yamaguchi,⁷Matthias Van Ginneken,²Vinciane Debaille,¹Philippe Claeys,¹Steven Goderis
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13599]
¹Analytical‐, Environmental‐, and Geo‐Chemistry, Vrije Universiteit Brussel, Pleinlaan 2, Brussels, 1050 Belgium
²Laboratoire G‐Time, Université Libre de Bruxelles 50, Av. F.D. Roosevelt CP 160/02, Brussels, 1050 Belgium
³Dipartimento di Scienze della Terra, Università di Pisa, Via S. Maria 53, Pisa, 56126 Italy
⁴Planetary Materials Group, Department of Earth Sciences, Natural History Museum, Cromwell Road, London, SW7 5BD UK
⁵Atomic & Mass Spectrometry – A&MS Research Group, Department of Chemistry, Ghent University, Krijgslaan 218 – S12, Ghent, 9000 Belgium
⁶National Institute of Polar Research, 10‐3 Midori‐cho, Tachikawa‐shi, Tokyo, 190‐8518 Japan
⁷Centre for Astrophysics and Planetary Science, University of Kent, Canterbury, Kent, CT2 7NZ UK
Published by arrangement with John Wiley & Sons

We report the discovery of a unique, refractory phase‐bearing micrometeorite (WF1202A‐001) from the Sør Rondane Mountains, East Antarctica. A silicate‐rich cosmic spherule (~400 µm) displays a microporphyritic texture containing Ca‐Al‐rich inclusion (CAI)‐derived material (~5–10 area%), including high‐Mg forsterite (Fo_98‐99) and enstatite (En_98‐99, Wo_0‐1). The micrometeorite also hosts a spherical inclusion (~209 µm), reminiscent of chondrules, displaying a barred olivine texture. Oxygen isotopic compositions of the micrometeorite groundmass (δ¹⁷O = –3.46‰, δ¹⁸O = 10.43‰, ∆¹⁷O = –1.96‰) are consistent with a carbonaceous chondrite precursor body. Yet, a relict forsterite grain is characterized by δ¹⁷O = –45.8‰, δ¹⁸O = –43.7‰, ∆¹⁷O = –23.1‰, compatible with CAIs. In contrast, a relict low‐Ca pyroxene grain (δ¹⁷O = –4.96‰, δ¹⁸O = –4.32‰, ∆¹⁷O = –2.71‰) presumably represents a first‐generation silicate grain that accreted ¹⁸O‐rich gas or dust in a transient melting scenario. The spherical inclusion displays anomalous oxygen isotope ratios (δ¹⁷O = –0.98‰, δ¹⁸O = –2.16‰, ∆¹⁷O = 0.15‰), comparable to anhydrous interplanetary dust particles (IDPs) and fragments from Comet 81P/Wild2. Based on its major element geochemistry, the chondrule size, and oxygen isotope systematics, micrometeorite WF1202A‐001 likely sampled a carbonaceous chondrite parent body similar to, but distinct from CM, CO, or CV chondrites. This observation may suggest that some carbonaceous chondrite bodies can be linked to comets. The reconstructed atmospheric entry parameters of micrometeorite WF1202A‐001 suggest that the precursor particle originated from a low‐inclination, low‐eccentricity source region, most likely either the main belt asteroids or Jupiter family comets (JFCs).

¹P. M. Thesniya,¹V. J. Rajesh,²J. Flahaut
Meteoritics & Planetary Science (in Press) Link to Artuicle [https://doi.org/10.1111/maps.13579]
¹Department of Earth and Space Sciences, Indian Institute of Space Science and Technology, Valiamala (P. O.), Thiruvananthapuram, 695547 India
²Centre de Recherches Pétrographiques et Géochimiques (CRPG)—CNRS/Université de Lorraine, 15 rue Notre Dame des Pauvres, 54500 Vandoeuvre les Nancy, France
Published by arrangement with John Wiley & Sons

Lunar mare basalts represent flood volcanism between ~4.0 and 1.2 Ga, therefore, providing insights into the thermal and volcanic history of the Moon. The present study investigates the spectral and chemical characteristics as well as ages of the nearside mare basaltic units from the Grimaldi basin, namely Mare Grimaldi and Mare Riccioli, using a wealth of orbital remote sensing data. This study delineated distinct basaltic units of varying albedo, mineralogy, and titanium contents in both Mare Grimaldi and Mare Riccioli. The crater size–frequency distribution technique revealed that at least two phases of basaltic magmatism spanning ~3.5 to 1.5 Ga (Late Imbrian‐Eratosthenian) have occurred in the Grimaldi basin. High‐Ti olivine basalts dated at 2.05 Ga are found to be surrounded by the Late Imbrian (~3.47 Ga) low‐ to intermediate‐Ti basalts in Mare Grimaldi. Low‐ to intermediate‐Ti basalts observed in Mare Riccioli date back to two different volcanic events at ~3.5 Ga and ~3.2 billion years, while patches of basalts having remarkably higher titanium content within the Mare Riccioli record the youngest age of ~1.5 Ga. The chemical trend of the pyroxenes from distinct basaltic units also revealed that multiple events of volcanism have occurred in the Grimaldi basin. The high‐Ti basalts in the Mare Grimaldi crystallized from an Fe‐enriched late‐stage magma while the low‐Ti basalts crystallized from an Mg‐ and Ca‐rich initial magma that experienced an ultra‐late stage quenching. The low‐ to intermediate‐Ti basaltic magma erupted in both the units was derived by partial melting of early cumulate materials from the hybrid source region in the post‐overturn upper mantle and made its way to the surface through dikes that propagated by excess pressures accumulated in the diapirs stalled at the base of the crust due to buoyancy trap. The high‐Ti magma erupted in the Mare Grimaldi was generated by a hot plume ascended from deeper clinopyroxene–ilmenite‐rich cumulate layer near the core–mantle boundary. However, the Eratosthenian (~1.5 Ga) intermediate‐Ti volcanic activity in the Mare Riccioli rather sourced from the ilmenite–clinopyroxene cumulate materials thet remained in the upper mantle after mantle overturn. The new results suggest that volcanism had not ceased in the Grimaldi basin at 3.27 Ga, rather it was active and fed by different mantle sources until 1.5 Ga for a period spanning ~2 billion years.