LAVRAS DO SUL: A NEW EQUILIBRATED ORDINARY CHONDRITE FROM RIO

ZUCOLOTTO M.E.; ANTONELLO, LOIVA; VARELA M.E.; SCORZELLI R; LUDKA, IZABEL; MUNAYCO P.; DOS SANTOS E.

EARTH, MOON, AND PLANETS

SPRINGER

Año: 2012 vol. 108 p. 139 - 150

0167-9295

Abstract The new Brazilian chondrite, Lavras do Sul, was found in 1985 at Lavras do Sul, Rio Grande do Sul State-Brazil (333004800S; 535406500W). It consists of a single mass weighing about 1 kg, covered by a black fusion crust with grayish interior. Four polished thin sections were prepared from a slice weighing 67 g on deposit at the Museu Nacional/ UFRJ. It consists mostly of chondrules and chondrule fragments dispersed in a recrystallized matrix. Most chondrules are poorly defined and range in size from 300 to 2,000 lm, although some of them show distinct outlines, particularly when viewed under cross-polarized transmitted and reflected light. The texture of chondrules varies from nonporphyritic (e.g., barred-olivine, radial-pyroxene) to porphyritic ones (e.g., granular olivine as well as olivine-pyroxene). The meteorite contains mainly olivine (Fa24.9), low-Ca pyroxene (Fs22.6) and metal phases, with minor amounts of plagioclase, chromite and magnetite. Mo¨ssbauer Spectroscopy studies indicate that the metal phase is kamacite, tetrataenite and antitaenite. Veins of secondary iddingsite crosscut the thin section and some ferromagnesian silicates. The chemical composition indicates that Lavras do Sul is a member of the low iron L chondrite group. The poorly delineated chondritic texture with few well-defined chondrules, the occurrence of rare clinopyroxene and plagioclase (and maskelynite) with apparent diameters ranging from 5 to 123 lm led us to classify Lavras do Sul as an equilibrated petrologic type 5.