Скачать или смотреть MI2021 - 16 Ana Pagu: The hunt for tetrataenite

The hunt for tetrataenite: microscopy vs. magnetism

Ana Pagu, University of Oxford, UK

Extensive studies of the magnetic properties of meteorites have been performed in recent years to explore early Solar System dynamos, providing a wealth of information on the formation, thermal evolution, differentiation, and internal dynamics of planetesimals. However, interpretation of paleomagnetic data from meteorites is challenging owing to their complex histories, which often involve aqueous alteration, impacts, and metamorphism. Robust magnetic recorders are therefore required to ensure that primary magnetic signals can be recovered.
Tetrataenite (ordered FeNi with 50-55% Ni) has been identified as a stable magnetic carrier in numerous meteorite groups, and it has been shown to be abundant in mesosiderites. We applied and compared a range of experimental techniques to identify tetrataenite in the Estherville mesosiderite: microscopy techniques (scanning electron microscopy, electron backscatter diffraction, and electron microprobe) and palaeomagnetic techniques (alternating field demagnetisation and IRM acquisition experiments).
Owing to the compositional overlap between tetrataenite and taenite (its disordered, lower-coercivity form), identification of tetrataenite was only possible through structural analysis using EBSD, and only where tetrataenite was present as micron-sized, multidomain grains within the silicate matrix. We identified nanometric high-Ni inclusions in pyroxene as candidates for reliable magnetic carriers because of their small sizes (within single-domain or pseudo-single-domain range), but we could not distinguish between taenite and tetrataenite using EBSD due to resolution challenges.
AF demagnetisation revealed a low-coercivity component that demagnetises by 12mT, and no stable high-coercivity signal. IRM acquisition experiments show that the sample saturates at 600mT, with IRM acquisition peaks suggesting that the signal is dominated by multidomain FeNi, with limited single-domain or pseudo-single-domain tetrataenite present. This shows that tetrataenite is more readily identifiable using paleomagnetic techniques than using microscopy techniques, since very high resolution structural data would be required to identify tetrataenite within the size range required for palaeomagnetism.
The absence of a high-coercivity signal in Estherville can be interpreted in several ways. Firstly, it is possible that the mesosiderite parent body never produced a magnetic field (e.g., if it experienced incomplete differentiation), or that its field was too short-lived (i.e., it shut off before the mesosiderites reached the tetrataenite ordering temperature). It is also possible that the field intensity was too weak to be recorded by these meteorites. Future work will include ARM fidelity tests to determine the minimum field intensity that is expected to have imparted a remanence to Estherville, and using this to explore the evolution of the mesosiderite parent body.