Origin of zoned fine-grained inclusions in the Allende meteorite

Journal Article · Mon May 01 04:00:00 EDT 1989 · Geochimica et Cosmochimica Acta; (USA)

DOI:https://doi.org/10.1016/0016-7037(89)90218-4· OSTI ID:5127236

McGuire, A V; Hashimoto, Akihiko ^[1]

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1. Harvard-Smithsonian Center for Astrophysics, Cambridge, MA (USA)

Many fine-grained Ca-Al-rich inclusions in the Allende meteorite show a consistent pattern of mineralogic and textural zonation. The center, zone A, consists mostly of spinel, nepheline, Al-diopside and salite, with minor amounts of olivine, sodalite, grossular, anorthite, perovskite, and ilmenite. In most inclusions, these minerals occur as loosely packed, discrete anhedral to subhedral grains with only minor intergrowth between phases. The textural and mineralogical similarity of zone A, except for the rimmed spinel objects, to altered areas of coarse-grained melilite-rich CaIs suggests that a similar open system alteration affected the fine-grained inclusions and that zone A's contained melilite before being completely altered to anorthite, grossular, nepheline and sodalite. Under conditions of low temperatures (< 1,000 K) and/or at H/O ratios lower than the cosmic abundance, Ca is more volatile than Si due to the formation of gaseous Ca-hydroxides. The thermodynamic stability of minerals in zones A and B also indicates alteration at temperatures less than 1,000 K. The mode of occurrence of hedenbergite and andradite in zone B implies that they are condensates deposited on the edge of zone A. It is likely that the Ca migrating out of the center of the inclusions and the Si and Fe migrating inward from the surrounding nebular gas coprecipitated according to the thermodynamic stability of these minerals. Zone C, on the other hand most likely existed even before alteration, surrounding what is now zone A. However, the mineralogy of zone C appears to have changed by alteration: the zone C consisting of olivine, nepheline and ilmenite was probably a spinel-rich belt mantling the illusion.

• OSTI ID: 5127236
• Journal Information: Geochimica et Cosmochimica Acta; (USA), Journal Name: Geochimica et Cosmochimica Acta; (USA) Vol. 53:5; ISSN GCACA; ISSN 0016-7037
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
580000* -- Geosciences
ALKALINE EARTH METAL COMPOUNDS
ALUMINIUM COMPOUNDS
ALUMINIUM OXIDES
ANORTHITE
CHALCOGENIDES
CHEMICAL COMPOSITION
CHEMISTRY
DIOPSIDE
FELDSPARS
GEOCHEMISTRY
ILMENITE
INCLUSIONS
IRON COMPOUNDS
IRON OXIDES
IRON SILICATES
MAGNESIUM COMPOUNDS
MAGNESIUM OXIDES
MAGNESIUM SILICATES
METEORITES
MINERALOGY
MINERALS
OLIVINE
ORIGIN
OXIDE MINERALS
OXIDES
OXYGEN COMPOUNDS
PETROGENESIS
PETROGRAPHY
PHYSICAL PROPERTIES
PYROXENES
SILICATE MINERALS
SILICATES
SILICON COMPOUNDS
SPINELS
THERMODYNAMIC PROPERTIES
TITANIUM COMPOUNDS
TITANIUM OXIDES
TRANSITION ELEMENT COMPOUNDS