Fayalite-rich rims, veins, and halos around and in forsteritic olivines in CAIs and chondrules in carbonaceous chondrites: Types, compositional profiles and constraints of their formation

Conference · Wed Jun 01 04:00:00 EDT 1988 · Geochimica et Cosmochimica Acta; (USA)

OSTI ID:5596335

Hua, X; Adam, J; Palme, H; Goresy, A E ^[1]

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1. Max-Planck-Institut fuer Kernphysik, Heidelberg (Germany, F.R.)

Fayalite-rich rims, veins, and halos around and in forsteritic olivines are a wide-spread phenomenon in chondrules, Ca, Al-rich inclusions (CAIs), and single grains in carbonaceous chondrites. The presence of fayalite rod-like crystals and laths in rims, veins, in wall of pores, and as fluffy network bridging neighboring olivines, pyroxenes, feldspars, etc. is strongly suggestive that the fayalitic olivine was formed by condensation presumably from the solar nebula gas. The formation of the fayalitic olivine was probably caused by an increase in the H{sub 2}O/H{sub 2} ratio (to a ratio between 0.1-1) subsequent to condensation of forsterite. At that stage, FeNi inclusions in olivine were also oxidized and fayalitic halos around the metal were then formed Fe diffusion along with addition of SiO{sub 2} from the solar gas or loss of M{sub g}O to the solar gas. The Fa-rich olivine rims and veins display a narrow compositional variation between Fa{sup 34} and Fa{sup 46}. Subsequent to condensation of Fa-rich olivine and oxidation of FeNi metal, Fe diffused in forsterite. This diffusion was probable enhanced due to the presence of point defects in olivine or the formation of a nonstoichiometric phase analogous to laihunite enriched in Al{sub 2}O{sub 3} and Cr{sub 2}O{sub 3}. However, the presence of Al{sub 2}O{sub 3{minus}} and Cr{sub 2}O{sub 3{minus}} rich discrete domains cannot by excluded. Cooling rates calculated by modeling of the diffusion profiles are indicative of rapid cooling subsequent to the condensation of fayalitic olivines. The authors obtain cooling rates ranging from 2000{degree}/day and 10{degree}C/day at an initial temperature of 1200C{degree} and 900C{degree}, respectively.

• OSTI ID: 5596335
• Report Number(s): CONF-8703325--
• Conference Information: Journal Name: Geochimica et Cosmochimica Acta; (USA) Journal Volume: 52:6
• Country of Publication: United States
• Language: English

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ALKALINE EARTH METAL COMPOUNDS
CARBONACEOUS MATERIALS
CHALCOGENIDES
CHEMICAL COMPOSITION
CHEMISTRY
CHONDRITES
COOLING
GEOCHEMISTRY
GEOLOGIC HISTORY
INCLUSIONS
IRON COMPOUNDS
IRON SILICATES
MAGNESIUM COMPOUNDS
MAGNESIUM OXIDES
MAGNESIUM SILICATES
MATERIALS
METEORITES
MINERALIZATION
MINERALOGY
MINERALS
NEBULAE
OLIVINE
OXIDES
OXYGEN COMPOUNDS
PETROGENESIS
SILICATE MINERALS
SILICATES
SILICON COMPOUNDS
SOLAR NEBULA
SOLAR SYSTEM EVOLUTION
STONE METEORITES
TRANSITION ELEMENT COMPOUNDS