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| Titel |
Mineralogical and Geochemical Analysis of Howardite DaG 779: understanding geological evolution of asteroid (4) Vesta |
| VerfasserIn |
Christian Marcel Müller, Kurt Mengel, Guneshwar Singh Thangjam, Gerd Weckwerth |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250133159
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| Publikation (Nr.) |
 EGU/EGU2016-13737.pdf |
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| Zusammenfassung |
| The HED meteorites, a clan of stony achondrites, are believed to originate from asteroid (4)
Vesta (e.g. Mittlefehldt et al. (2015)). Recent evolution models (e.g. Toplis et al. (2013)) and
observations from Dawn spacecraft data (e.g., Prettyman et al. (2013)) indicate that
diogenites form the lower crust and uppermost mantle of (4) Vesta. Deep seated material
excavated by large impacts such as the Rheasilvia- and Veneneiaforming event should be
present in howardites.
We analysed a slice of howardite DaG 779 which had been recovered from the Libyan
Desert in 1999 and was briefly described by Grossmann (2000). The data presented here
include electron microprobe, bulk-rock XRD and XRF as well as trace element analysis by
ICP-MS and INA. The petrographic results confirm earlier observations that DaG 779 is
polymict and mainly contains diogenite and eucrite clasts. Mass balance calculations using
bulk-rock and microprobe major element data reveal a modal mineralogy of 77%
orthopyroxene, 8% plagioclase, 7% clinopyroxene and 2% spinels, the rest being
olivine, SiO2-phases, sulphides, and native Fe(Ni). When compared with the element
compilation recently reported by Mittlefehldt (2015) the 39 trace element analysed
here (including REE and PGE) confirm that this howardite is clearly dominated by
diogenite.
Beside the modal petrographic information, a number of more detailed observations
obtained from microprobe investigations reveal fresh and recrystallized glasses,
troilite-orthopyroxene symplectites from a mixed silicate-sulphide melt giving rise to graphic
intergrowths as well as vermicular and reticular FeS in highly disrupted clasts. While the
origin of the FeS in these clasts is not clear yet, its particular shape and distribution indicates
that this mineral has been (partially) molten and recrystallized from a sulphide melt. The
silicate minerals around these FeS occurrences are recrystallized but there is no indication for
a partial silicate melt.
Further metasomatic reactions were observed between clinopyroxene (pigeonite) and a
sulphide-bearing agent, according to the principal reaction Pigeonite (Fe-rich) + S2 ↔ FeS +
Augit (Mg-rich) + SiO2. This type of metasomatism (Zhang et al. (2013)) is not well
understood yet.
References:
Grossman, J. N. (2000): The Meteoritical Bulletin, No. 84, 2000 August. Meteoritics &
Planetary Science, 35: A119–A225. doi: 10.1111/j.1945-5100.2000.tb01797.x.
Toplis, M.J. et al. (2013): Chondritic models of 4 Vesta: Implications for geochemical
and geophysical properties. Meteoritics & Planetary Science, 48: 2300–2315. doi:
10.1111/maps.12195.
Zhang, A. et al. (2013): Record of S-rich vapors on asteroid 4 Vesta: Sulfurization in the
Northwest Africa 2339 eucrite. Geochim. Cosmochim. Acta 109, 1–13.
Mittlefehldt, D.W., (2015): Asteroid (4) Vesta: I. The howardite-eucrite-diogenite (HED)
clan of meteorites. Chemie Erde-Geochem. 75, 2, 155–183.
Prettyman, T.H. et al. (2013): Neutron absorption constraints on the composition of 4
Vesta. Meteoritics & Planetary Science 48:2211-2236. |
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