Suitability of chondrules for studying the magnetic field of the early solar system: an examination of synthetically produced dusty olivine

• Main Author: Hu, Yifan
• Format: Thesis
• Language: English
• Published: 2009
• Subjects: Chondritic meteorites; Paleointensity; High temperature; Olivine; Geophysics; Kya; Iceland
• Online Access: http://purl.umn.edu/59879

University of Minnesota M.S. thesis. December 2009. Major: Geophysics. Advisors: Bruce M. Moskowitz, Joshua M. Feinberg. 1 computer file (PDF); viii, 46 pages. Chondritic meteorites are rare, yet incredibly valuable windows into the geophysical and geochemical environment of the early solar system. Dusty olivine grains containing exsolved nanometer scale iron nickel alloy inclusions are present in many chondritic meteorites and their remanent magnetization may give insight into the strength of the solar dynamo at the time of chondrule formation. Laboratory methods for determining the paleointensity of these rare materials must be optimized prior to conducting experiments on actual meteorite samples. To this end, we have used high temperature recrystallization techniques to produce synthetic dusty olivine samples with textures remarkably similar to those observed in chondritic meteorites. The olivine grains used in these annealing experiments are from the 13 kya Haleyjabunga picritic basalt flow in Iceland and have compositions of Fo90, which closely resembles the olivine composition observed in chondritic meteorites. Samples were annealed at 1350 ˚C, 1315 ˚C and 1425 ˚C either under vacuum in the presence of graphite or under controlled oxygen fugacity using pure CO gas. The laboratory produced magnetic mineral assemblages in 4 different types of samples as well as the starting material have been characterized using low and high-temperature magnetic measurements, hysteresis loops, FORC diagrams, and scanning electron microscopy. The room temperature remanence properties of these materials have been explored using stepwise IRM and ARM acquisition and alternating field demagnetization. These synthesis techniques allow us to produce a wide range of iron nickel grain sizes with correspondingly large variations in coercivity (between 0 and 500 mT). High-temperature measurements of saturation magnetization show that all the samples reach their Curie temperatures at ~760 ˚C, consistent with kamacite, a low Ni high Fe metal alloy. Multiple experiments have shown that care must be taken to rigorously control the atmosphere in which the samples are heated and cooled in order to avoid forming trace amounts of magnetite on the surface of the samples. Future research will explore the feasibility of using modified Thellier protocols or the Shaw method to determine the paleointensity of laboratory induced thermoremanent magnetizations.