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The Vicencia meteorite fall: A new unshocked (S1) weakly metamorphosed (3.2) LL chondrite

Type of publication Peer-reviewed
Publikationsform Original article (peer-reviewed)
Publication date 2015
Author Keil Klaus, Riebe My, Wieler Rainer,
Project High energy particle irradiation in the early solar system and stellar nucleosynthesis studied with noble gases in primitive meteorites
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Original article (peer-reviewed)

Journal Meteoritics and Planetary Science
Volume (Issue) 50
Page(s) 1089 - 1111
Title of proceedings Meteoritics and Planetary Science
DOI 10.1111/maps.12456

Open Access

URL http://onlinelibrary.wiley.com/doi/10.1111/maps.12456/full
Type of Open Access Publisher (Gold Open Access)

Abstract

The Vicencia meteorite, a stone of 1.547kg, fell on September 21, 2013, at the village Borracha, near the city of Vicencia, Pernambuco, Brazil. It was recovered immediately after the fall, and our consortium study showed it to be an unshocked (S1) LL3.2 ordinary chondrite. The LL group classification is based on the bulk density (3.13gcm(-3)); the chondrule mean apparent diameter (0.9mm); the bulk oxygen isotopic composition (O-17=3.768 +/- 0.042 parts per thousand, O-18=5.359 +/- 0.042 parts per thousand, O-17=0.981 +/- 0.020 parts per thousand); the content of metallic Fe,Ni (1.8 vol%); the Co content of kamacite (1.73 wt%); the bulk contents of the siderophile elements Ir and Co versus Au; and the ratios of metallic Fe-0/total iron (0.105) versus total Fe/Mg (1.164), and of Ni/Mg (0.057) versus total Fe/Mg. The petrologic type 3.2 classification is indicated by the beautifully developed chondritic texture, the standard deviation (similar to 0.09) versus mean Cr2O3 content (similar to 0.14 wt%) of ferroan olivine, the TL sensitivity and the peak temperature and peak width at half maximum, the cathodoluminescence properties of chondrules, the content of trapped Xe-132(tr) (0.317x10(-8)cm(3)STPg(-1)), and the Raman spectra for organic material in the matrix. The cosmic ray exposure age is similar to 72Ma, which is at the upper end of the age distribution of LL group chondrites. The meteorite is unusual in that it contains relatively large, up to nearly 100m in size, secondary fayalite grains, defined as olivine with Fa(>75), large enough to allow insitu measurement of oxygen and Mn-Cr isotope systematics with SIMS. Its oxygen isotopes plot along a mass-dependent fractionation line with a slope of similar to 0.5 and O-17 of 4.0 +/- 0.3 parts per thousand, and are similar to those of secondary fayalite and magnetite in the unequilibrated chondrites EET 90161, MET 96503, and Ngawi. These data suggest that secondary fayalite in Vicencia was in equilibrium with a fluid with a O-17 of similar to 4 parts per thousand, consistent with the composition of the fluid in equilibrium with secondary magnetite and fayalite in other unequilibrated ordinary chondrites. Secondary fayalite and the chondrule olivine phenocrysts in Vicencia are not in isotopic equilibrium, consistent with low-temperature formation of fayalite during aqueous alteration on the LL parent body. That alteration, as dated by the Mn-53-Cr-53 chronology age of secondary fayalite, took place 4.0-1.1+1.4 Ma after formation of CV CAIs when anchored to the quenched angrite D'Orbigny.
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