The Vicência meteorite fall: A new unshocked (S1) weakly metamorphosed (3.2)LLchondrite

Abstract

The Vicência meteorite, a stone of 1.547 kg, fell on September 21, 2013, at the village Borracha, near the city of Vicência, 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 (3.13 g cm À3 ); the chondrule mean apparent diameter (0.9 mm); the bulk oxygen isotopic composition (d 17 O = 3.768 AE 0.042&, d 18 O = 5.359 AE 0.042&, D 17 O = 0.981 AE 0.020&); 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 (~0.09) versus mean Cr 2 O 3 content (~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 132 Xe tr (0.317 9 10 À8 cm 3 STP g À1 ), and the Raman spectra for organic material in the matrix. The cosmic ray exposure age is~72 Ma, 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 100 lm in size, secondary fayalite grains, defined as olivine with Fa >75 , large enough to allow in situ measurement of oxygen and Mn-Cr isotope systematics with SIMS. Its oxygen isotopes plot along a mass-dependent fractionation line with a slope of~0.5 and D 17 O of 4.0 AE 0.3&, 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 Vicência was in equilibrium with a fluid with a D 17 O of~4&, 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 Vicência 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 53 Mn-53 Cr chronology age of secondary fayalite, took place 4:0 þ1:4 À1:1 Ma after formation of CV CAIs when anchored to the quenched angrite D'Orbigny.

Origin of mass‐independent oxygen isotope variation among ureilites: Clues from chondrites and primitive achondrites

“…Evidence for aqueous alteration by isotopically heavy H 2 O has been reported in several other studies of LL3 chondrites (Krot et al 1997;Choi et al 1998;Bridges et al 1999;Sears et al 2009;Doyle et al 2014;Keil et al 2015). The alteration evidently took place at low temperatures; in one case, a temperature of around 100°C has been inferred for fayalite formation (Zolotov et al 2006).…”

“…; Keil et al. ). The alteration evidently took place at low temperatures; in one case, a temperature of around 100 °C has been inferred for fayalite formation (Zolotov et al.…”

“…(), Keil et al. () and references therein. Also shown is the field for bulk R chondrites (pale gray) whose original chondrules occupy the same field as the LL 3 chondrules (Kita et al.…”

“…), and Vicência (Keil et al. ). In all cases, secondary fayalite has compositions that cluster close to a slope 1 line that passes through the initial chondrule silicates (Fig.…”

“…Solid black circle is the mean bulk composition of LL chondrites, which is "heavier" than the average of the original chondrules, probably because H 2 O from ice, with very high Δ 17 O located at some place up the pale gray slope 1 trajectory shown, has altered the chondrules and oxidized metal to make the magnetite and fayalite. Fields are from Kita et al (2010), Keil et al (2015) and references therein. Also shown is the field for bulk R chondrites (pale gray) whose original chondrules occupy the same field as the LL3 chondrules .…”