Tremolite (orange) is replacing diopside (bright, yellow), which is most commonly present in the cores of elongate grains. (c) Colorized X-ray image (Red=Si, Green=Ca, Blue=Al, Cyan=S) of a tremolite-bearing area in and around a small chondrule (ref. Arrows denote region where tremolite is replacing diopside, which largely only remains in the core of the grain. Serpentine (Srp) is dark grey, clinochlore (Clc) is medium grey, tremolite (Tr) is medium-light grey, diopside (Di) is lightest grey, and magnetite (Mag) is bright white, as are sulfides (not identified). (b) High-resolution BSE image showing textural details of area in Fig. de Maranho ) March, 1879, chondrite, weight 2.024 grams, spec. Tremolite is darker yellow-green, diopside is brighter yellow-green, serpentine is red, clinochlore is magenta, magnetite is black, and sulfide is cyan the white box delineates the area shown in (b). Seven Brazilian meteorites are presented in the collection of the National. (a) Colorized X-ray image (Red=Mg, Green=Ca, Blue=Al, Cyan=S) of a tremolite-bearing area adjacent to a large magnetite spherule (ref. X-ray elemental and backscattered electron (BSE) images of minerals and textures in AhS 202. They have formation ages dating to at least 4.56 billion years before the. Because Allende and AhS 202 represent different asteroidal parent bodies, intermediate conditions may have been more widespread in the early Solar System than recognized from known carbonaceous chondrite meteorites, which are likely a biased sampling. unequilibrated chondrite Chondrites are the most primitive group of meteorites. Formation of these minerals requires prolonged metamorphism in a large (~640-1800 km diameter), unknown asteroid. Amphiboles are rare in carbonaceous chondrites, having only been identified previously as a trace component in Allende (CV3 oxA) chondrules 2. Here we show that an ungrouped, aqueously altered carbonaceous chondrite fragment (numbered 202) from the Almahata Sitta (AhS) meteorite contains an assemblage of minerals, including amphibole, that reflect fluid-assisted metamorphism at intermediate temperatures and pressures on the parent asteroid. Evidence of hydrous metamorphism at intermediate pressures and temperatures in carbonaceous chondrite parent bodies has been virtually absent. Others are dominated by anhydrous minerals and textures that indicate high temperature metamorphism in the absence of aqueous fluids 1. Some carbonaceous chondrites are volumetrically dominated by hydrated minerals, providing evidence for low temperature and pressure aqueous alteration 1. Carbonaceous chondrite meteorites record the earliest stages of Solar System geo-logical activities and provide insight into their parent bodies' histories.
0 Comments
Leave a Reply. |