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| Titel |
Chondritic models of 4 Vesta: Comparison of data from the Dawn mission with predicted internal structure and surface composition/mineralogy. |
| VerfasserIn |
M. J. Toplis, H. Mizzon, O. Forni, M. Monnereau, J. A. Barrat, T. H. Prettyman, H. Y. McSween, T. J. McCoy, D. W. Mittlefehldt, M. C. De Sanctis, C. A. Raymond, C. T. Russell |
| 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 |
250070722
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| Zusammenfassung |
| While the HEDs provide an extremely useful basis for interpreting data from the Dawn
mission, there is no guarantee that they provide a complete vision of all possible crustal (and
possibly mantle) lithologies that are exposed at the surface of Vesta. With this in
mind, an alternative approach is to identify plausible bulk compositions and use
mass-balance and geochemical modelling to predict possible internal structures and
crust/mantle compositions and mineralogies. While such models must be consistent
with known HED samples, this approach has the potential to extend predictions to
thermodynamically plausible rock types that are not necessarily present in the HED
collection.
Nine chondritic bulk compositions are considered (CI, CV, CO, CM, H, L, LL, EH, EL).
For each, relative proportions and densities of the core, mantle, and crust are quantified. This
calculation is complicated by the fact that iron may occur in metallic form (in the core) and/or
in oxidized form (in the mantle and crust). However, considering that the basaltic crust has
the composition of Juvinas and assuming that this crust is in thermodynamic equilibrium with
the residual mantle, it is possible to calculate a single solution to this problem for a given bulk
composition.
Of the nine bulk compositions tested, solutions corresponding to CI and LL groups
predicted a negative metal fraction and were not considered further. Solutions for enstatite
chondrites imply significant oxidation relative to the starting materials and these
solutions too are considered unlikely. For the remaining bulk compositions, the relative
proportion of crust to bulk silicate is typically in the range 15 to 20% corresponding to
crustal thicknesses of 15 to 20 km for a porosity-free Vesta-sized body. The mantle is
predicted to be largely dominated by olivine (>85%) for carbonaceous chondrites,
but to be a roughly equal mixture of olivine and pyroxene for ordinary chondrite
precursors. All bulk compositions have a significant core, but the relative proportions
of metal and sulphide can be widely different. Using these data, total core size
(metal+ sulphide) and average core densities can be calculated, providing a useful
reference frame within which to consider geophysical/gravity data of the Dawn
mission.
Further to these mass-balance calculations, the MELTS thermodynamic calculator
has been used to assess to what extent chondritic bulk compositions can produce
Juvinas-like liquids at relevant degrees of partial melting/crystallization. This work will
refine acceptable bulk compositions and predict the mineralogy and composition of
the associated solid and liquid products over wide ranges of partial melting and
crystallization, providing a useful and self-consistent reference frame for interpretation
of the data from the VIR and GRaND instruments onboard the Dawn spacecraft. |
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