Composition of chondrule silicates in LL3-5 chondrites and implications for their nebular history and parent body metamorphism

Journal Article · Fri Feb 01 00:00:00 EST 1991 · Geochimica et Cosmochimica Acta; (United States)

DOI:https://doi.org/10.1016/0016-7037(91)90015-W· OSTI ID:5504915

McCoy, T J ^[1]; Scott, E R.D.; Keil, K; Taylor, G J ^[2]; Jones, R H ^[3]

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1. Univ. of Hawaii, Honolulu (USA) Univ. of New Mexico, Albuquerque (USA)
2. Univ. of Hawaii, Honolulu (USA)
3. Univ. of New Mexico, Albuquerque (USA)

The authors petrologic studies of 75 type 1A and type 2 porphyritic olivine chondrules in nine selected LL group chondrites of type 3.3 to type 5 and comparisons with published studies of chondrules in Semarkona (LL3.0) show that compositions of silicates and bulk chondrules, but not overall chondrule textures, vary systematically with the petrologic type of the condrite. These compositional trends are due to diffusive exchange between chondrule silicates and other phases (e.g., matrix), such as those now preserved in Semarkona, during which olivines in both chondrule types gained Fe{sup 2+} and Mn{sup 2+} and lost Mg{sup 2+}, Cr{sup 3+}, and Ca{sup 2+}. In a given LL4-5 chondrite, the olivines from the two chondrule types are identical in composition. Enrichments of Fe{sup 2+} in olivine are particularly noticeable in type 1A chondrules from type 3.3-3.6 chondrites, especially near grain edges, chondrule rims, grain boundaries, and what appear to be annealed cracks. Compositional changes in low-Ca pyroxene lag behind those in coexisting olivine, consistent with its lower diffusion rates. With increasing petrologic type, low-Ca pyroxenes in type 1A chondrules become enriched in Fe{sup 2+} and Mn{sup 2+} and depleted in Mg{sup 2+}, Cr{sup 3+}, and Al{sub 3+}. These compositional changes are entirely consistent with mineral equilibration in chondritic material during metamorphism. From these compositional data alone they cannot exclude the possibility that chondritic material was metamorphosed to some degree in the nebula, but they see no evidence favoring nebula over asteroidal metamorphism, nor evidence that the chondrule reacted with nebular gases after crystallization.

• OSTI ID: 5504915
• Journal Information: Geochimica et Cosmochimica Acta; (United States), Vol. 55:2; ISSN 0016-7037
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
CHONDRITES
CHEMICAL COMPOSITION
GEOCHEMISTRY
METAMORPHISM
MINERALOGY
PETROLOGY
CRYSTALLIZATION
GEOLOGIC HISTORY
ION EXCHANGE
OLIVINE
PETROGENESIS
PYROXENES
ALKALINE EARTH METAL COMPOUNDS
CHEMISTRY
GEOLOGY
IRON COMPOUNDS
IRON SILICATES
MAGNESIUM COMPOUNDS
MAGNESIUM SILICATES
METEORITES
MINERALS
OXYGEN COMPOUNDS
PHASE TRANSFORMATIONS
SILICATE MINERALS
SILICATES
SILICON COMPOUNDS
STONE METEORITES
TRANSITION ELEMENT COMPOUNDS
580000* - Geosciences