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| Titel |
Single Crystal Paleointensity Analyses of Olivine-Diogenites: Implications for a Past Vestan Magnetic Field |
| VerfasserIn |
J. A. Tarduno, R. D. Cottrell |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250069331
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| Zusammenfassung |
| Pioneering studies of meteorites and recent investigations have presented paleomagnetic data
suggesting some parent bodies had dynamos. With this background, meteorites
of the Howardite-Eucrite-Diogenite (HED) group of anchondrites, linked to the
differentiated asteroid 4 Vesta, represent promising targets for magnetic investigation.
Prior studies of HED meteorites have yielded contrasting results. Cisowski [1]
reported low paleofields (1-5 μT) from two unbrecciated eucrites, whereas Morden [2]
reported paleointensities of up to 37 μT from Thellier analyses of the brecciated
Millbillillie eucrite; the latter were interpreted as indicative of a past dynamo. The
age of the Millbillillie magnetization might be approximately 3.55 Ga [3] when
the meteorite was heated by impact. Fu and Weiss [4-5] have recently reported a
study of fusion crust of the Millibillie eucrite, supporting the conclusion that the
meteorite preserves an ancient magnetization, but with very low (2-3 μT) paleointensity
values.
Here we discuss results from Northwest Africa (NWA) 5480, which is a olivine-diogenite
(or harzburgite), Â 57 vol% olivine and 42 vol% orthopyroxene. Olivine is found in bands that
have been interpreted as magmatic flow within the Vestan mantle [6-7]. We use single crystal
paleointensity analyses [8]. Olivine grains 1-2 mm in size were separated for analyses. We
specifically exclude grains with large visible inclusions as these may be multidomain
magnetic minerals which relax on relatively short timescales. Magnetic hysteresis
measurements suggest that olivine hosts single to pseudo-single domain magnetic inclusions
suitable for paleointensity analyses. Thermal demagnetization reveals removal of several
scattered magnetizations at low unblocking temperatures, followed by stable decay at higher
temperatures. Thellier-Coe paleointensity data suggest a field of approximately 36 μT.
These preliminary data, if confirmed, imply a Vestan dynamo because alternative
primary magnetic sources would have imparted much weaker fields. Consistency tests
are needed to further address potential magnetic contamination during meteorite
collection. In addition, definition of the nature and composition of the magnetic
inclusion carriers is needed. These studies are in progress, as are single crystal
paleointensity analyses of other HED meteorites. References [1] Cisowski, S.M.
(1991) Earth Planet Sci. Lett., 107, 173-181. [2] Morden, S.J. (1992) Meteoritics 27,
560-567. [3] Yamaguchi, A. et al. (1994) Meteoritics 29, 237-245. [4] Fu, R. and
Weiss B.P. (2011) EPSC, Abstract 1646. [5] Fu, R. and Weiss B.P. (2011) AGU
abstract P21E-08. [6] Irving, A.J. et al. (2009), LPSC Abstract 2466. [7] Tkalcec, B.J.
(2010), LPSC Abstract 5191. [8] Tarduno J. A. et al. (2006) Rev. Geophys., 44,
RG1002. |
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