Magnetite dissolution,diachronous greigite formation,and secondary magnetizations from pyrite oxidation: Unravelling complex magnetizations in Neogene marine sediments from New Zealand |
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| Institution: | 1. State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China;2. Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200, Australia;1. National Centre for Antarctic and Ocean Research, Earth System Sciences Organization (ESSO), Ministry of Earth Sciences, Government of India, Headland Sada, Vasco 403804, Goa, India;2. Department of Marine Geology, Mangalore University, Mangalagangotri 574199, Karnataka, India;3. Human Resource Development Division, CHQ 27, J.L.N. Road, Kolkata-700016, India;4. Earth System Sciences Organization (ESSO), Ministry of Earth Sciences, Government of India, Prithvi Bhawan, Lodhi Road, New Delhi 110003, India |
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| Abstract: | Detailed rock magnetic and electron microscope analyses indicate that the magnetic signature of Neogene marine sediments from the east coast of New Zealand is dominated by the authigenic iron sulphide greigite. The greigite is present as a mixed population of stable single domain and superparamagnetic grains, which is consistent with authigenic growth from solution. This growth can result from pyritization reactions soon after deposition, which also leads to dissolution of most detrital magnetite; however, where constrained by field tests, our data suggest that remanence acquisition can occur > 1 Myr after deposition, and can vary in timing at the outcrop scale. Strong viscous overprints result from oxidation of the iron sulphides, probably during percolation of oxic ground water. This process can sometimes destroy any ancient remanent magnetization. This complex magnetic behaviour, particularly the presence of late-forming magnetizations carried by greigite, means that the remanence in New Zealand Cenozoic sediments, and in similar sediments elsewhere, cannot be assumed to be primary without confirmation by field tests. The reversals test should be employed with caution in such sediments, as patchy remagnetizations can lead to false polarity stratigraphies. |
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