Iron meteorites, parent bodies

Using this eguation, the calculated radii of most iron meteorite parent bodies are found to have been -10-100 km. 

An interesting consequence of this model is that bodies from the terrestrial-planet region may be scattered outwards while the planets are forming and implanted in the Asteroid Belt. With dynamical and collisional modeling of this process, Bottke et al. (2006) find that this may be the origin of most iron meteorites. This would explain the diversity of iron-meteorite types, why there is little observational or meteoritical evidence of mantle material from differentiated bodies in the Asteroid Belt, and the fact that most iron-meteorite parent bodies appear to have formed >1 Myr before the parent bodies of the chondritic meteorites (Kleine et al. 2005). 

Snyder G. A., LeeD.-C., Ruzicka A. M., Taylor L. A., HaUiday A. N., and Prinz M. (1998) Evidence of late impact fractionation and mixing of silicates on iron meteorite parent bodies Hf—W, Sm—Nd, and Rb—Sr isotopic studies of sificate inclusions in HE irons. In Lunar Planet. Sci. XXIX, 1142. The Lunar and Planetary Institute, Houston (CD-ROM). 

Iron meteorite parent bodies experienced heating and melting at temperatures in excess of 1,500 °C (Taylor, 1992). What was the heat... 

After the cores of the differentiated asteroids had crystallized, a slow cooling period commenced. During this period the most prominent feature of iron meteorites evolved—the Widmanstatten pattern (Figure 8). Several characteristics of the Widmanstatten pattern may be used to constrain the thermal evolution and the sizes of the iron meteorite parent bodies. 

Qin, L., Dauphas, N., Wadhwa, M., Masarik, J., and Janney, P.E. (2008) Rapid accretion and differentiation of iron meteorite parent bodies inferred from Hf- W chronometry and thermal modeling. Earth Planet. Sci. Lett., 273, 94-104. 

Fig. 2. The plot of total reduced iron, Fe, and oxidized iron, Fe, normalized to Si abundance shows how the chondrite classes fall into groups distinguished by oxidation state and total Fe Si ratio. The soHd diagonal lines delineate compositions having constant total Fe Si ratios of 0.6 and 0.8. The fractionation of total Fe Si is likely the result of the relative efficiencies of accumulation of metal and siUcate materials into the meteorite parent bodies. The variation in oxidation state is the result of conditions in the solar nebula when the soHds last reacted with gas. Terms are defined in Table 1 (3).

As already noted, spectral similarities between the various asteroid classes and specific types of meteorites provide a way to identify possible meteorite parent bodies. The Tholen and Barucci (1989) asteroid taxonomy has been interpreted as representing the types of meteorites shown in Table 11.1. Using the Bus et al. (2002) taxonomy, the C-complex asteroids are probably hydrated carbonaceous chondrites (e.g. Cl or CM). These carbonaceous chondrite asteroids probably accreted with ices and will be considered in Chapter 12. Some S-complex asteroids are ordinary chondrite parent bodies, but this superclass is very diverse and includes many other meteorite types as well. The X-complex includes objects with spectra that resemble enstatite chondrites and aubrites, and some irons and stony irons, although other X-complex asteroids are unlike known meteorite types. A few asteroid spectra are unique and provide more definitive connections, such as between 4 Vesta and... 

Fig. 18.3 Iron meteorites have a chtiracteristic texture which appears on polished and etched surfaces. This texture, called the Widmanstatten pattern, formed during slow cooling and crystallization of liquid iron-nickel in the core of a meteorite parent body. The tear-shaped inclusion is composed of iron sulfide which is immiscible in iron-nickel liquid. The meteorite in this image is Carbo, a IID iron meteorite, which was not collected in Antarctica (Reproduced by permission of H. Haack from Htiack and McCoy (2005, Fig. 8, p. 337) and Elsevier, Inc. through the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01932)...

Meteorites can be classified into stony, stony iron and iron. The most common meteorites are chondrites which are stony. Radiometric dating indicate an age of about 4.5x10 years. Achondrites are also stony but they are considered differentiated or reprocessed matter. They are formed by melting and recrystallization on or within meteorite parent bodies. Pallasites are stony iron meteorites composed of olivine enclosed in metal. 

Stony Irons. The stony iron meteorites are composed of substantial iron and siUcate components. The paHasites contain cm-sized ohvine crystals embedded ia a soHd FeNi metal matrix and have properties consistent with formation at the core mantle boundary of differentiated asteroids. The mesosiderites are composed of metal and siUcates that were fractured and remixed, presumably ia the near-surface regions of their parent bodies. 

The rates at which parent bodies cooled also provide constraints on thermal models. A method for determining the cooling rates for iron meteorites is described in Box 11.2. A similar method for chondrite cooling rates is also based on the compositions of metal grains. Cooling rates can also be estimated from knowing the blocking temperatures of various radioisotope systems. 

Kleine, T., Mezger, K., Palme, H., Scherer, E. and Munker, C. (2005) Early core formation in asteroids and late accretion of chondrite parent bodies Evidence from Hf- W in CAIs, metal-rich chondrites and iron meteorites. Geochimica et Cosmochimica Acta, 69, 5805-5818. 

Irons. Approximately Tr of meieorile falls are irons, Because they are distinctive rocks and weather relatively slowly, most meleoriies that were not seen lo fall, bur were found accidentally, are irons. Iron meteorites are composed of metallic iron and sideraphile elements Ihut fractionated from molten parent bodies. They may have been cores of asteroids nr they may have only been localised inelal accumulations. 

The W isotopic compositions of various terrestrial samples, chondrites, iron meteorites, basaltic achondrites, lunar samples, and Martian meteorites are expressed as deviations in parts per 104 from the value for the silicate earth (such as the W in a drill bit or chisel), which are the same as those of average solar system materials, represented by carbonaceous chondrites. These values are summarized in Fig. 8.9, from which it can be seen that early segregated metals such as the iron meteorites and metals from ordinary chondrites have only unradiogenic W because they formed early with low Hf/W. The time differences between metal objects segregated from parents with chondritic Hf/W are revealed by the differences in W isotopic compositions between each of the metal objects and chondrites. The Hf-W model ages of all these metals indicate that all of their parent bodies formed within a few million years, implying rapid accretion in the early history of the solar system. 

Therefore these data for the short-lived chronometer Hf-W provide a consistent picture of rapid accretion, equilibration, and planetesimal differentiation in the early solar system with only small (106-year) time differences resolvable between some events for the parent bodies of chondrites, basaltic achondrites, and iron meteorites. 

Cosmogenic radioactivity 53Mn is created continuously as anuclear collision fragment in meteorites when cosmic rays strike the meteoritic iron nuclei during the meteorite s journey to the Earth. This radioactivity is counted in the lab, alive today in the meteorite, and gives information on the transit time of the meteorite from parent body to Earth. 

The terrestrial planets and the Moon are differentiated, with dense iron-rich cores and rocky mantles. The uncompressed densities of Earth and Venus are similar. Mercury has a high density which suggests it has relatively large core. Conversely, the Moon has a low density, indicating a very small core. There is little observational evidence that asteroids are differentiated except for Vesta and Ceres (Thomas et al. 2005). However, iron meteorites from the cores of differentiated asteroids are quite common, and the irons found to date come from several dozen different parent bodies (Meibom Clark 1999). Most meteorites come from asteroids that never differentiated. These chondritic meteorites consist of intimate mixtures of heterogeneous material millimeter-sized rounded particles that were once molten, called chondrules, similarly sized calcium-aluminum-rich inclusions (CAIs), and micrometer-sized matrix grains. 

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