Physics
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufmgp21c0790k&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #GP21C-0790
Physics
1519 Magnetic Mineralogy And Petrology, 5420 Impact Phenomena, Cratering (6022, 8136)
Scientific paper
Shock-induced changes in magnetic properties of rocks, minerals and meteorites play an important role in modelling the magnetic anomalies of impact structures (e.g. Vredefort), in interpretation of the magnetic anomalies of planetary bodies (e.g. Mars) and in understanding the paleomagnetic data of meteorites. To shed further light on these problems we report results of experimentally shocked samples of synthetic fine grained magnetite. We used cylindrical surface-polished discs (d 10 mm, h 4 mm) of the well characterized magnetite with SD-PSD grain size range. The magnetite powder was mixed with Al2O3 and sintered into disktype pellets. A series of shock recovery experiments from 10 to 45 GPa (nominal pressure) using a conventional high-explosive set-up with a steel (ARMCO) sample container, surrounded by a momentum trap of the identical material. As the samples were shocked inside the highly magnetic containers, the prevailing magnetic field was roughly five times higher than the ambient field. After the shock, the containers cooled down slowly to ambient temperatures. The estimated post-shock temperatures of the samples range from nearly ambient temperature (10 GPa) up to about 1400 K (45 GPa). Evaluating the "real" pressures reached in these experiments requires a model to account for the high porosity of the pellets. The porosity also affects significantly the post-shock temperature. Independent of the fact that pressure, shock- and post- shock tmperatures are not sufficiently constrained yet, the experiments form a well-characterized series of shocks with systematically increasing pressure. Surprisingly enough, the sample discs were not friable and could be removed by retaining shape largely unchanged. The shock induced changes in sample properties show, with the exception of the 45 GPa sample, with increasing shock pressure: 1. Reduction of bulk density and significant increase in porosity 2. Minor increase in magnetic susceptibility (10 GPa, 15 GPa and 45 GPa) compared to pre-shock value and a decreasing trend in susceptibility as a function of shock (except 45 Gpa) 3. Decrease of the Median Destructive Field values of SIRM and ARM but simultaneously a progressive "hardening" of SIRM in shocked samples. 4. Decrease in SIRM but increase in ARM. 5. Progressive shock demagnetization of the pre-shock remanence (SIRM). 6. Decrease in the value of the coercive force (Hc) and a trend towards MD-field but no shock-dependent trend in the ratios of Mrs/Ms and Hcr/Hc (Day-plot parameters). While the first three results confirm our previous shock experiments on natural Laanila diabase (Pesonen et al., 1997), decrease of SIRM is opposite to what has been previously seen. We are now investigating what part of this decrease is due to shock. The results of the 45 GPa sample are not in line with those of the 10 GPa, 15 GPa, 25 GPa and 35 GPa samples. We are now investigating the reasons for these discrepancies. One possibility is that it is contaminated by the container material.
Deutsch Alex
Heikinheimo E.
Honnermann U.
Kohout Tomas
Pesonen Lauri
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