Fusion crust, meteorites

Figure 6.10 Meteorite ALH84001 - 1.93 kg of the most studied rock of all time (a) as it was found with the outer edge showing a fusion crust (b) after the sample was cut for analysis of the interior. (Reproduced from photos by courtesy of NASA)...

Ablative deceleration leaves a smoothed dark brown to black fusion crust on most meteorites (Plate If) translucent, pale crusts may occur on Fe-poor stony meteorites which may be olive-green on Lunar ones. Very rarely, an appropriately-shaped meteoroid may assume a quasi-stable orientation during late atmospheric traversal. In such a case, material ablated from the front can redeposit as delicate droplets or streamlets on its sides and rear (Plate 2). The fusion crust is an unambiguous indicator that an object in question is a meteorite. Also, nearly all meteorites contain iron and will therefore attract a magnet however, some terrestrial materials will too. 

Plate 2. Fusion crust of Lafayette Martian meteorite. Lafayette exhibits very delicate, redeposited droplets on its sides, indicating an orientation with its front pointing Earthward late in its atmospheric traversal, (Reproduced with permission from reference 1. Copyright 2006 Elsevier.) (See page 12 of color inserts.)... 

The first Lunar meteorite, found in 1981 in Antarctica, was immediately identified as such by. its fusion crust and mineralogic and chemical resemblance to samples returned by the Apollo program. The properties of the 31 Lunar meteorites are more characteristic of the whole Moon, because they are random samples, than are the Apollo samples which derive from Lunar sites mainly selected for landing safety reasons 10). 

J) Noblesville H chondrite - note that the fusion crust is thin and smooth (as it is on most meteorites) and has an usual dark brown color on top which grades to black on the other exposed surfaces (contrast with the Lafayette fusion crust in Plate 2). The broken surface next to the 1-cm cube shows the interior of this gas-rich regolith breccia. (Reproduced with permission from reference 1. 

The presence of a fusion crust, the color and shape of the surface, the presence of metallic grains, and the unusual densities, in most cases, permit meteorite specimens to be distinguished from terrestrial boulders. These criteria apply to meteorite specimens that occur not only in the cold desert of Antarctica, but also to those that are collected in the hot deserts of the world, and in all other places where meteorites can be found. Even though tens of thousands of meteorite specimens have been collected on the East Antarctic ice sheet, Antarctica is not receiving a higher flux of meteorites compared to other areas of the Earth. The apparent abundance of meteorites is caused primarily by their better preservation in the ice and by the dynamics of the East Antarctic ice sheet. 

The first evidence for the conversion of silicate minerals of stony meteorites in Antarctica was reported by Gooding (1986a) who detected the presence of clay mineraloids, gypsum, K-Fe sulfates (jarosite ), and rust on the surfaces and in cracks of achondrites and chondrites from Elephant Moraine (EET) and the Allan Hills. These weathering products had formed primarily from glass and plagioclase in the fusion crust and in cracks in the interiors of the meteorite specimens. 

Fig. 18.22 This specimen of the Derrick Peak iron meteorite has a mass of 10 kg and is highly corroded as a result of chemical weathering after it fell about 1 million years ago. The upper surface is dark brown in color and is characterized by protruding crystals of schreibersite [(Fe.NiljP] which are more resistant to chemical weathering than kamacite (Fe-Ni alloy). The original fusion crust and heat-altered surface layer are no longer present. The Derrick Peak meteorite is a member of group IlAB (coarse octahedrite) (Courtesy of NASA, JSC, Houston, Texas)...

Strange meteorite. Thin, tan-green fusion crust, -50%, with possible ablation features. Interior is dark grey with numerous white to grey breccia ( ) fragments. Somewhat equidimensional at 3 cm. 

The lunar meteorites have aerodynamically rounded shapes and are covered by a fusion crust which they acquired while they passed through the atmosphere of the Earth. Lunar meteorites also contain cosmogenic radionuclides (e.g., °Be, Al, Cl, and Mn) because they were exposed to cosmic rays on the surface of the Moon and while they were in transit from the Moon to the Earth. Many lunar meteorites are breccias and contain evidence of shock metamorphism. Like the other nonchondrites in Table 18.1, the lunar meteorites lack chondrules and do not contain grains of metallic Fe-Ni alloys. Because of their similarities to achondrite meteorites and to pebbles or cobbles of terrestrial rocks, lunar meteorites are not easy to identify in the field and, in some cases, were only collected in response to the ANSMET imperative When in doubt, collect... . 

Differentiated meteorites they come from asteroids which have been through a fusion process which led to a more or less clear separation into nucleus, mantle and crust. 

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