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Volk, Michael (2016): Influence of pressure, temperature and composition on magnetic recording in meteorites. Dissertation, LMU München: Faculty of Geosciences
Volk_Michael .pdf



Meteorites potentially record the magnetic fields present during the nascent formation of the early solar system. However, the interpretation of the magnetic records contained within them is complicated by a myriad of variables including poorly understood magnetic minerals and events occurring after the lock-in of the original magnetization such as shock, decompression and low-temperature cycling from space to the Earth's surface. This dissertation attempts to place constraints on how some of these factors influence paleomagnetic recording in meteorites. Through the development of a pressure cell, we show that even low pressures (< 2 GPa) will lower the remanent magnetization intensity in rocks. The magnetic field strength determined from shocked or decompressed material can be appreciably underestimated and should only be taken as a lower limit. Monoclinic pyrrhotite is is commonly found in certain achondritic meteorites, especially those from Mars. Monoclinic pyrrhotite undergoes a magnetic phase transition at ca. 30 K, called the Besnus transition, whose mechanism is poorly understood and highly debated. We provide evidence that this magnetic phase transition is due to a crystallographic reorientation in symmetry. Furthermore, our experiments show that low temperature cycling from their thermal equilibrium temperature in space to ambient surface temperatures on Earth will not significantly alter the paleointensity value recorded in pyrrhotite-bearing meteorites. The most common magnetic mineral in meteorites are body centered cubic iron-nickel alloys; however, the magnetic properties of these alloys as a function of grain size is virtually unknown. As grain size bears drastically on the ability to carry and retain magnetic remanence, filling this gap in our knowledge is crucial to properly interpret the paleomagnetic recording of FeNi alloys that are so ubiquitous in nature. For this reason, we initiated a research program to synthesize FeNi alloys with well constrained compositions and grain size distributions through mechanical alloying. Mechanically alloyed powders show similar magnetic properties to published results on metal bearing meteorites, but the size threshold for stable single domain particles-- the domain state essential for paleomagnetism-- was never identified. Finally, the design, building and implementation of a unique paleomagnetic thermal demagnetizing oven is presented.