1340h
AN: GP53A-0956
TI: Magnetic Record of Ordinary Chondrite
Chondrules
AU: * Kohout, T
EM: tomas.kohout@helsinki.fi
AF: Division of Geophysics, University of
Helsinki, Helsinki, 00014, Finland
AU: * Kohout, T
EM: tomas.kohout@helsinki.fi
AF: Department of Applied Geophysics, Charles
University in Prague, Prague, 12843, Czech
Republic
AU: * Kohout, T
EM: tomas.kohout@helsinki.fi
AF: Institute of Geology, Academy of Sciences
of the Czech Republic, Prague, 16500, Czech
Republic
AU: Kletetschka, G
EM: gunther.kletetschka@gsfc.nasa.gov
AF: Institute of Geology, Academy of Sciences
of the Czech Republic, Prague, 16500, Czech
Republic
AU: Kletetschka, G
EM: gunther.kletetschka@gsfc.nasa.gov
AF: Department of Physics, Catholic
University of America, Washington DC, DC 20064, United
States
AU: Kletetschka, G
EM: gunther.kletetschka@gsfc.nasa.gov
AF: NASA/GSFC, Code 691, Greenbelt, 20771,
United States
AB:
The magnetic record of individual chondrules has been examined on two
low-metamorphic grade ordinary
chondrites – Bjurböle (L4) and Avanhandava (H4). The chondrule
magnetic conglomerate study was done by
removing oriented chondrules from the meteorite. The magnetic
directions of the chondrules in both meteorites
seems to be randomly oriented within both meteorites.
The paleofield method based on the REM ratio (NRM/SIRM; Natural
Remanent Magnetization / Saturation
Isothermal Remanent Magnetization) calibrated for the TRM (Thermal
Remanent Magnetization; Kletetschka et al., 2004) magnetization process
reveals approximate paleofields between 5 μT and 20 μT (REM ~
0.002) for Avanhandava chondrules and between 12 μT and 45 μT (REM ∼
0.0015–0.0048) for
Bjurböle chondrules.
However, the timing of the magnetizing process remains unclear. The
random pattern of the individual chondrule
magnetic directions suggest that the chondrules were not remagnetized
by recent terrestrial weathering nor by
the exposure to the artificial magnetic fields. However the effect of
post aggregation metamorphism (reaching 400 – 600°C) and impact
related shock demagnetization on the parent body may be responsible for
partial or
complete loss of the magnetic information carried by the lower
coercivity grains.
In the case of Avanhandava meteorite this can be observed in the REM
ratio. The values of REM ratio calculated in low AF (alternating Field)
demagnetizing fields are progressively lower than in the higher AF
fields.
The Bjurböle meteorite contains significant fraction of the
tetrataenite mineral. The origin of tetrataenite is related
to the slow ordering of the FeNi lattice at temperatures below ~
350°C). Thus the origin of tetrataenite must be
post-metamorphic ruling out the high temperature TRM scenario. The
origin of the magnetization in the Bjurböle
chondrules are most likely associated to the post-metamorphic low
temperature processes (i.e. electric
discharges or impact related shock remanent magnetization on the parent
body). The random magnetic
directions in this meteorite can be partly controlled by the strong
magnetic anisotropy of tetrataenite. Moreover, the paleofield method
based on REM ratio (Kletetschka et al., 2004) is calibrated for the TRM
magnetizing scenario
and not for low temperature acquisition processes and thus of limited
use in the Bjurböle case.
The results of the paleofield studies of the Avanhandava meteorite
shows that the paleofield carried out by its
chondrules can be significantly underestimated due to post-formation
metamorphic and impact events. Those
processes can in the same way influence the paleointensity results
previously published for other chondritic
meteorites. The interpretation of Bjurböle data may be even more
difficult due to the presence of tetrataenite.
Acknowledgements: Authors would like to thank to Peter J. Wasilewski
from NASA/GSFC for constructive
consultations and access to the laboratory facilities.
References: Kletetschka, G., Acuna, M. H., Kohout, T, Wasilewski, P.
J., and Connerney, J. E. P. (2004), An
empirical scaling law for acquisition of thermoremanent magnetization,
Earth Planet. Sci. Lett., 226, 521– 528.
UR: http://www.volny.cz/tomkohout/meteo/
DE: 1519 Magnetic mineralogy and petrology
DE: 1521 Paleointensity
DE: 1540 Rock and mineral magnetism
DE: 6030 Magnetic fields and magnetism
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2007 Fall Meeting