A ciência meteorítica e a sua aplicação em dois condritos brasileiros (Campos Sales/CE e Santa Vitória do Palmar/RS)

Abstract

The science that studies the meteorites is called Meteoritics, which are properly one of the branches of Planetary Geology. This science deals with the composition, formation and geologic, geochemical and isotopic evolution of the meteorites, which constitutes samples of the remnant materiais of the planetary formation. Meteorites are, in fact, geologic samples of materiais that represents asteroids and planetary bodies that had been participated on the Solar System formation and consequently on the Earth formation. The most primitive meteorites of the Solar System are those that retain the proportions of the chemical elements, with the exception of the extremely volatiles, of the materials that early formed the planets and the Sun, these are called chondrites. These meteorites are samples from the asteroids originated at the Main Belt and those materiais possesses a chondritic texture in which consists of spherules of ferromagnesian minerais in an igneous crystallized texture supported by a fine grained matrix of silicate composition and amorphous nature, containing certain proportions of iron-nickel metallic minerais, sulfides like troilite and oxides like chromite and spinel. The meteorites that derive from partial melting, planetary and asteroidal differentiation and shock metamorphism occurred on the original chondritic materiais are called the nonchondritic. Those of silicate composition are called achondrites, in which most of them consists of igneous rocks, the rocky-iron meteorites are called siderolites and the metallic meteorites are called siderites, in which the latter one represent fragments of ancient cores of differentiated asteroids and protoplanets. All of these types of meteorites are genetically related on the context of the accreted planetesimal materiais in the early Solar System in the so called protoplanetary disk and they are truly fossil vestiges of the planetary formation. The classification of these geologic materiais is important to understand their relation with the chondrites. Most of the chondrites are classified as ordinary, because they represent at least 80% of ali collected meteorites, representing a high frequency asteroid population in our Solar System. Most of the chondrites were modified by thermal metamorphism in their parent asteroids. The two ordinary chondrites studied are the Campos Sales and Santa Vitória do Palmar, in which the first one is a witnessed fali in 1991 at the Campos Sales city, Ceará and the latter one is a probable witnessed fali in 1997 that was found in 2003 near the city of Santa Vitória do Palmar, Rio Grande do Sul. These two chondrites are different, principally with respect to thermal metamorphism and shock stage. The Campos Sales chondrite is highly metamorphosed but yet preserves its relic chondritic texture with a matrix, originally amorphous, totally recrystallized and with a low shock stage. The Santa Vitória do Palmar chondrite represents a well preserved chondritic material containing the chondrules supported by an opaque and amorphous matrix called mesostasis. Furthermore, this chondrite presents a high shock stage presenting planar fractures and undulatory extinction on its olivine crystals. The goal here is to demonstrate the chondrite classification method of these two ordinary chondrites with the use of the petrographic analysis and SEM/EDS as a complementary analysis of mineral chemistry. 9 The comparative study of these meteorites, between an ordinary chondrite of preserved texture which was submitted to an incipient thermal metamorphism and an ordinary chondrite with highly modified texture by thermal metamorphism, presenting a totally recrystallized matrix and containing relict chondrules, permits to observe the geologic evolution of the asteroidal bodies in our Solar System.