Suitability of volcanic glasses with different compositions and evolution histories to gain paleointensity data
Final Report Abstract
The original concept of the proposal was to try to understand how different magma evolution histories (eruption type, devitirification, hydration etc.) influence the reliability of paleointensity records as determined from volcanic glass, to test the suitability of submarine basaltic glass for the reconstruction of the ancient geomagnetic field strength, and to produce high quality paleointensity records from different times and places. Upon transfer of the project, it was decided that instead of employing existing techniques to measure absolute paleointensity on specific materials, we initiated a new approach through the further development of a so called Triaxe, which can measure a sample’s magnetization up to the Curie temperature of magnetite or hematite (for example) in the presence of an applied field in any specified direction or in the presence of a null field. With this instrument, absolute paleointensity can be measured automatically on one sample in less than three hours. Here, we report our progress in developing the instrument and initial measurements from the Triaxe, which is now being used in the Munich paleomagnetic laboratory. The development was carried out in part by a PhD student, who in parallel performed relative paleointensity experiments using anisotropy of magnetic remanence (AMR), which holds promise to quantify relative paleointensity records from sedimentary rocks. A proof of concept was established in applied fields that exceeded the intensity of the geomagnetic field. At Earth-like fields, large uncertainties necessitate the development of a method to improve the estimation of the anisotropy tensor. Here, we designed a numeral method that demonstrates the applicability and resolution needed to optimize the experimental protocol. We then implemented a 30-position procedure for AMR measurements and compared the numerical and experimental data with a typically-used, 12-position procedure. Redeposition experiments with sediments rich in single domain magnetite were carried out in fields with intensities of 0, 10, 50 and 100 µT; 25 individual samples were redeposited and measured at each field condition. The 30-position protocol facilitates the isolation and resolution of the fieldaligned prolate fabric (1-2% of the total) from the oblate sedimentary fabric, and more so when applying tensor subtraction of the fabric obtained in a null field. Scatter of the anisotropy parameters is inversely proportional to field strength, which together with the high-resolution protocol and tensor subtraction method, should lead to robust relative paleointensity corrections and/or estimates.
Publications
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Error variance analysis of different measurement schemes to improve anisotropy measurements, Castle Meeting Abstracts (2022), p. 91-92, in: Franciskovic-Bilinski, S., Böhnel, H., Egli, R., Hirt, A. M., Petrovsky, E., Spassov, S., Werner, T. (Eds.), 17th Castle Meeting on Paleo, Rock and Environmental Magnetism Book of Abstracts (2022), Ruder Boskovic Institute, ISBN 978-953-7941-44-4
Ostermeier, F. K.; Gilder, S. A.; Wack, M. R.; Jezek, J. & Finn, D.
