Understanding the enhanced magnetization intensities of post-20 Ma oceanic basalts through magneto-mineratogical experiments
Zusammenfassung der Projektergebnisse
The magnetization intensities of oceanic basalts vary through time. Going from the present to the past, intensities decrease until around 20 Ma. For older ages the tendency is less clear, but on average there seems to be an increase of magnetization again. The initial decrease in intensity can be explained by low temperature oxidation of titanomagnetite to titanomaghemite; however, the mechanisms causing the subsequent changes in magnetization for oceanic basalts older than 20 Ma are still under debate. Thus it is essential to characterize these basalts with respect to their mineralogy and their magnetic properties. Titanomagnetites with various composition were examined with the spatially very highly resolving methods of Magnetic Force Microscopy (MFM) and Atomic Force Microscopy (AFM). Particular emphasis was on the compositional characterization by Raman spectroscopy. Raman spectra of titanomagnetites vary in peak position, intensity and width a function of titanium concentration and degree of oxidation. Hence, Raman spectroscopy can identify the composition and oxidation state of minerals of the titanomagnetite-ülvospinel solid solution series. We find that depending on the titanium content the mineral starts to oxidize beyond a certain laser power threshold. Adjusting the laser power just below the threshold for immediate oxidation is not sufficient because still a time-dependent oxidation processes can occur. Oxidation could even continue for weeks after laser application. These results also imply that the tendency towards oxidation in the deep sea depends on the specific composition. Increasing Titanium content may be correlated with a lower oxidation threshold. In deep sea basalts we found mineral phases that can be attributed to a titanomagnetite with a titanium content of about 20 % - 40 %. In summary our results paved the way to a combined chemometric and magnetic characterization of titanomagnetites in deep sea basalts. With Raman spectroscopy most phases can be identified, we also defined experimental conditions which allow for a well defined spectroscopic characterization. With an integrated Raman – Magnetic Force Microscope the identical specimen can be investigated correlating such chemical composition with magnetic properties.
Projektbezogene Publikationen (Auswahl)
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Application of Raman Spectroscopy to Identification of Titanomagnetites, American Geophysical Union (AGU), Fall Meeting Abstracts, 2008, A791+
Lisa Tatsumi-Petrochilos, Stuart A. Gilder, Pavel Zinin, Julia E. Hammer, and Michael D. Fuller
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(2010): Raman spectroscopy of laser induced material alterations. Dissertation, LMU München
Bauer, Michael
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Identification of iron oxide phases in thin films grown on Al2O3(0 0 0 1) by Raman spectroscopy and X-ray diffraction. Surf. Sci. 604 (7-8), 679-685 (2010)
M. Lübbe, A. M. Gigler, R. W. Stark, and W. Moritz
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Raman spectroscopy of titanomagnetites: Calibration of the intensity of Raman peaks as a sensitive indicator for their Ti content. Am. Mineralogist, 96, 1537–1546, (2011)
Pavel Zinin, Lisa Tatsumi-Petrochilos, Lydie Bonal, Tayro Acosa, Julia Hammer, Stuart Gilder, and Mike Fuller
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Raman spectroscopy of laser-induced oxidation of titanomagnetites. J. Raman Spectrosc. 42 (6), 1413-1418 (2012)
M. Bauer, P. Davydovskaya, M. Janko, M. Kaliwoda, N. Petersen, S. Gilder, and R. W. Stark
