Mineralogical records of early planetary processes on the HED parent body with reference to Vesta

Details Mineralogical records of early planetary processes on the HED parent body with reference to Vesta Mineralogical records of early planetary processes on the HED parent body with reference to Vesta

Nov 1997

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997m%26ps...32..841t&link_type=abstract

Meteoritics & Planetary Science, vol. 32, no. 6, pages 841-853

Mathematics

Logic

36

Scientific paper

Mineralogical information recovered from the HED meteorites was employed to reconstruct its history of their parent body and relate it to 4 Vesta. These interpreted crustal evolution processes were then compared to the expected geological features on the surface of a likely protoplanet, 4 Vesta. The original crustal materials of the HED parent body were preserved as mineral grains and lithic clasts, but in many eucrites, Fe/Mg ratios in pyroxenes were homogenized by diffusion after crystallization. The crystallization trend of the protocrust has been deciphered by (1) examining monomict and crystalline samples and using their mineralogical and chemical information to formulate a sequence of crystallization and cooling trends; and by (2) reconstructing the original crust prior to cratering events from lithic clasts and mineral fragments in polymict breccias such as howardites and polymict eucrites. Mineral components are identical, both in the individual HED and in polymict breccias, and no remnants of primitive materials were preserved in the polymict breccias. A layered crust model reconstructed from such breccias consists of an upper crust with extrusive lava-like eucrites that have been brecciated and metamorphosed, diogenite mantle, and cumulate eucrites of varying thickness between them. This model can be used to explain the surface geological features of Vesta observed from the Hubble Space Telescope. A large crater with diogenitic orthopyroxene at the crater floor are consistent with the deepest diogenitic layer of the layered crust model; and an underlying olivine layer is expected from early crystallized olivine in the crystal fractionation model. The old terrain of eucritic surface materials of Vesta can be howardites, polymict eucrites, or regolith-like eucrites produced from eucrites extruded and impacted on the surface. Partial melting models of eucrites seems to be favored by the REE chemistry and experimental studies. Unfortunately, partial melting models have not been demonstrated to explain how the HED parent body is converted to a layered crust without producing any metamorphosed primitive material in the layered crust. The origin of cumulate eucrites with systematic variation of textures and chemistries of pyroxene can be explained by the layered crust model with excavation and mixing of trapped liquid. Discovery of basaltic materials with Na-rich plagioclase and augite in iron meteorites, which are the products of partial melting, suggests that eucrites may be unique to a body that underwent large scale differentiation and metamorphism.

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