Date of Award
Master of Science
Mantle xenoliths have been used to interpret the magnetic characteristics of the lithospheric mantle because their rapid rate of ascent to the surface does not allow enough time for alteration. Studies of mantle xenoliths have suggested that magnetite in the mantle contributes to long wave length magnetic anomalies over areas of suppressed geothermal gradients (cratons, forearcs). Magnetite will only form in oxidizing conditions. This leads to a debate if mantle xenoliths become altered through oxidation to form magnetite in the mantle before ascent, during ascent (decompression), or after ascent at the surface. Natural and experimental occurrences of magnetite forming as inclusions or dislocations in fosterite from Fe-impurities suggest that oxidation of olivine is a mechanism for magnetite formation in the mantle. To test this, high olivine samples were placed into a furnace at 600C and 900C in free air at 1bar of pressure, removing samples at different time increments ranging from 0.2 to 625 hours. These temperatures were used because they represent mantle like temperatures in oxidizing conditions. After oxidation, samples from the 900C experiment show evidence for magnetite/hematite formation after only 0.2 hours and the amount of Fe-oxide formation increases with time. Samples from the 600C experiment show significant alteration and Fe-oxide formation after 125 hours. These results suggest that it is possible for magnetite to form from the Fe in the fayalite component of fosterite in the mantle from oxidizing event such as metasomatism.
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