¹Dafilgo Fernandes,^1,2N. G. Rudraswami,^1,2V. P. Singh
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.70206]
¹National Institute of Oceanography (Council of Scientific and Industrial Research), Dona Paula, India
²Academy of Scientific and Innovative Research (AcSIR), Ghaziabad, India
Published by arrangement with John Wiley & Sons

We report 14 Antarctic and 15 deep-sea partially melted micrometeorites containing ~71 to 458 μm rounded, porous, and intact igneous objects. These objects likely represent non-porphyritic igneous chondrules. Analyzing these objects allows us to better relate them to the constituents of their parent bodies, thereby improving our understanding of the chondrule properties inherent to micrometeorite precursors. Seven identified Antarctic spherules and all deep-sea spherules primarily exhibit radial pyroxene (Rp) textures; one Antarctic composite spherule contains an Rp object embedded within an olivine matrix. Additionally, four Antarctic spherules show barred olivine (Bo) textures, two of which are surrounded by igneous rims, while two other Antarctic spherules show cryptocrystalline (Cc) textures. The bulk major and minor element oxides for the Rp objects vary significantly: MgO ~25.4 to 39.0 wt%, Al₂O₃ ~ 0.03 to 3.23 wt%, SiO₂ ~ 44.7 to 55.1 wt%, CaO ~0.02 to 2.72 wt%, Cr₂O₃ ~ 0.18 to 1.44 wt%, MnO ~0.18 to 1.51 wt%, and FeO ~11.4 to 22.1 wt%. The chemical compositions of the pyroxene within the Rp spherules suggest they originate primarily from unequilibrated–equilibrated ordinary chondrites (UOC–EOC) rather than carbonaceous chondrites. Conversely, the glass chemical compositions of the Cc spherules (MgO ~32.9 to 42.4 wt% and FeO ~0.73 to 10.5 wt%; En98-83) largely support an origin from chondritic carbonaceous materials. Atmospheric entry heating has progressively altered the chemical composition of the Bo spherules beyond recognition from their original chondrule states. Ultimately, their collective chemical compositions suggest that these spherules may consist of chondrules similar to non-porphyritic chondrules in carbonaceous and ordinary chondrites. Based on their textures and mineralogy, these spherules indicate that the parent sources of these micrometeorites are chondrule-bearing asteroid bodies.

^1,2Elizabeth J. Catlos,¹Andrew F. Parisi,¹Michael E. Brookfield,³Timmons Erickson,^1,2Sean P.S. Gulick,^4,5Axel K. Schmitt,^1,2Daniel F. Stockli,⁶Daniel P. Miggins,⁷Ben Ruchte,¹Mark Cloos
Meteoritics & Planetary Science (in Press) Open Access Link to Article [https://doi.org/10.1111/maps.70191]
¹Department of Earth & Planetary Sciences, Jackson School of Geosciences, The University of Texas at Austin, Austin, Texas,USA
²Center for Planetary Systems Habitability, The University of Texas at Austin, Austin, Texas, USA
³Amentum – JETS II, Astromaterials Research and Exploration Science Division, NASA Johnson Space Center, Houston,Texas, USA
⁴Institute of Earth Sciences, Heidelberg University, Heidelberg, Germany
⁵John de Laeter Centre, Curtin University, Bentley, Western Australia, Australia
⁶College of Earth, Ocean, and Atmospheric Sciences, Oregon State University, Corvallis, Oregon, USA
⁷IXRF, Inc., Austin, Texas, USA
Published by arrangement with John Wiley & Sons

The Ames impact structure (Oklahoma) is thought to have formed during the Ordovician Meteor Event, based on conodont biostratigraphy of its crater fill. Here, U–Pb zircon dates from its impact-melt portion, conducted using secondary ion mass spectrometry and laser ablation–inductively coupled plasma–mass spectrometry (n = 37 spots), yield a Mesoproterozoic emplacement age for the impacted granodiorite (1401.2 ± 8.1 Ma, ±2σ, upper Concordia intercept). However, the youngest zircon dates define a weighted mean age of 369.7 ± 5.9 Ma (n = 10/11), with MSWD = 0.71 and p(χ²) = 0.7. Cathodoluminescence and electron backscattered diffraction images reveal that most zircons, including the youngest Devonian-age grains, show primary oscillatory zoning and lack deformation. However, two have impact-related textures, including regions of low-angle grain boundaries within microcracks and discrete arrays of granular zircon crosscutting oscillatory growth zoning. Plagioclase (n = 6 samples, ⁴⁰Ar/³⁹Ar) yields Late Carboniferous (~310.5 Ma) and Permian (~250.5 Ma) approximate total fusion dates that overlap the timing of heating and hydrocarbon maturation in the crater, suggesting the argon system records postimpact thermal overprinting. Based on the youngest zircon dates, the Ames impact structure may record activity near the Frasnian–Famennian boundary, contemporaneous with other North American impacts.