Tomographic study of shapes and metal abundances of Renazzo chondrules

Conference · Mon May 19 04:00:00 EDT 2003

OSTI ID:1008889

Hertz, J; Ebel, Denton; Weisberg, W K ^[1]

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1. Columbia Preparatory School

Analysis of 3-dimensional tomographic data for 3 Renazzo chondrules shows that 2-D thin section methods are inadequate to quantify 'convolution index', grain sizes and distributions, or modal metal abundance, but 3-D methods are more promising. The origin of metal in the metalrich, highly primitive, CR2 chondrites is vigorously debated. In some Renazzo chondrules, metal has an approximately solar Ni:Co ratio which led to suggest that it is a product of solar nebula condensation. Additionally, in many chondrules, metal occurs in two locations: as one or two large metal grains in the chondrule interior and as numerous smaller metal grains along the chondrule rim. In other chondrules, metal is more evenly dispersed in smaller grains. Interior metal generally has higher concentrations of the more refractory siderophile elements than metal in the rim, which tends to be enriched in volatile metals. This difference may be due to (1) partial evaporation and rapid recondensation of metal; (2) condensation of core metal at higher temperatures, suggesting accretionary growth of the chondrules as temperature decreased; or (3) late Fe addition to the metal on the chondrule rims due to FeO reduction from the adjoining silicates. [4] analyzed PGE distribution in CR chondrite metal and argued that rim metal may have formed by a reaction with the surrounding silicates at the time of chondrule formation. [5] showed that Ni and Co concentrations in the metal grains of the least circular, finest-grained chondrules do not follow a condensation trend. This implies that the relative amounts of Ni and Co in the interior grains were gradually established during chondrule melting due to Fe oxidation or reduction. More recently, observed that chondrules that appear more circular in thin section outline also have coarser metal and silicate grains and a more clearly defined compositional differentiation between rim and core metal grains. These textures were interpreted as reflecting a higher degree of partial melting and 'maturation' of the chondrules, as measured by lower fayalite content in olivine and higher P and Ni concentrations of interior metal. To quantify the degree of melting of chondrules, measured a convolution index (CVI) for each chondrule. The CVI, ranging from 1 to 2, is defined as the ratio of the measured perimeter of the chondrule to the perimeter of a circle with the same area as that measured for the chondrule. Correlation of the CVI with chemical data led to conclude that Renazzo chondrules formed 'by aggregation of numerous droplets in a dust-rich environment'.

• Research Organization: Advanced Photon Source (APS), Argonne National Laboratory (ANL), Argonne, IL (US)
• Sponsoring Organization: USDOE
• OSTI ID: 1008889
• Country of Publication: United States
• Language: ENGLISH

Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
ABUNDANCE
CHONDRITES
DISTRIBUTION
EVAPORATION
GRAIN SIZE
MELTING
OLIVINE
ORIGIN
OXIDATION
SILICATES
SOLAR NEBULA