154-4 Incomplete Tetrataenite Disorder in Moderately Shocked Ordinary Chondrites: Insights from Rock Magnetism and Scanning Transmission Electron Microscopy

Session: Asteroid Observations, Return Missions, and Meteoritics: Interweaving Perspectives and Data (Posters)

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Abstract:

Tetrataenite (γ̋-ordered face-centered cubic, equiatomic FeNi) forms in equilibrated (type 4–6) ordinary chondrites as a result of thermal metamorphism on their parent bodies at temperatures of 700–950 °C, followed by slow cooling [1]. Due to its exceptional magnetic hardness, tetrataenite is readily detectable using magnetic methods. However, while non-shocked or weakly shocked (S1–S3) ordinary chondrites typically contain abundant tetrataenite, conventional magnetometry approaches often indicate an absence of the mineral in higher shock grade (S4–S6) meteorites (e.g., [2]). It is generally assumed that strong impact-related pressure and temperature surges lead to irreversible disordering of tetrataenite into magnetically soft taenite.

A recent study of the Daule chondrite (L5, S4, W0) [3] suggested differential post-shock survival of tetrataenite at the millimeter scale, indicating heterogeneity in impact-related thermal and pressure conditions. The study also proposed that remanent magnetization-based methods are more sensitive to detecting minor amounts of tetrataenite than induced magnetization approaches. These findings support the hypothesis that S4-grade metamorphism may not uniformly exceed the threshold for complete tetrataenite disordering [4].

To further test this hypothesis, we investigated twelve ordinary chondrites (shock grades S3–S6) using a comprehensive suite of rock magnetic techniques. These included thermomagnetic analysis at both high and cryogenic temperatures, magnetic hysteresis and backfield demagnetization, first-order reversal curve (FORC) measurements, and thermal and alternating field demagnetization of natural remanent magnetization. Samples exhibiting high-coercivity signals were further analyzed for tetrataenite using scanning transmission electron microscopy (STEM). For these analyses, the samples were crushed, magnetically separated from the matrix, and dispersed in fluid. Using high-angle annular dark-field (HAADF) imaging, we identified and indexed individual grains and imaged clusters of magnetic alloys.

[1] Yang et al. (1997), Geochim. Cosmochim. Acta 61, 2943–2956.

[2] Gattacceca et al. (2014), Meteorit. Planet. Sci. 49, 652–676.

[3] Bristol et al. (2023), Icarus, 404, 115684.

[4] Scorzelli et al. (1987), J. Phys. F: Met. Phys. 17, 1993–1997.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-10622

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