Magmatic iron meteorites

Chemical evidence indicates that magmatic iron meteorite groups formed by fractional crystallization, most likely in the cores of differentiated asteroids. 

Chabot N. L. and Drake M. J. (1999) CrystaUization of magmatic iron meteorites the role of mixing in the molten core. Meteorit. Planet. Sci. 34, 235—246. 

Malvin D. J. (1988) Assimilation-fractional crystalhzation of magmatic iron meteorites. Lunar Planet. Sci. XIX. Lunar and Planetary Instimte, Houston, pp. 720-721. 

Magmatic iron meteorites are thought to be samples from the metal cores of differentiated asteroids (18) and as such are ideally suited for application of Hf-W chronometry. Iron meteorites contain virtually no Hf (i.e., HfrW 0), such that the timing of core formation in their parent bodies can be calculated from their alone. Tungsten isotope data are now available for a vast number... 

The Hf-W systematics of the metal cores (represented by magmatic iron meteorites, see above) and silicate mantles (represented by eucrites) of differentiated asteroids as well as of the Martian, lunar and terrestrial mantles are summarized in Figure 4. The model ages for core formation calculated from these parameters and equation 7 are shown in Figure 5. 

Iron meteorites consist mainly of Fe-Ni metal with some troilite (FeS) inclusions. There are more than 10 different groups of irons, for example, lAB, IIAB, and IIIAB (Table 10.1), that are defined based on chemical differences, and each may be derived from a distinct parent body [22, 23]. The magmatic iron meteorites (Table 10.1), which encompass most groups of irons, are thought to represent the cores of differentiated asteroids. In contrast, the non-magmatic irons (lAB, IIICD) are probably derived from Fe-rich melts that formed on a chondritic parent body. 

The LAB-IIICD irons are non-magmatic irons that often contain silicate inclusions. Tungsten is significantly more radiogenic in these meteorites than in the IIAB irons because... 

Wasson, J.T., Choi, B.-G., Jerde, E.A. and Ulff-M0ller, F. (1998) Chemical classification of iron meteorites XII. New members of the magmatic groups. Geochimica et Cosmochimica Acta, 62(4), 715-24. 

Ulff-M0ller F., Rasnussen K. L., Prinz M., Palme H., Spettel B., and Kallemeyn G. W. (1995) Magmatic activity on the IVA parent body evidence from silicate-bearing iron meteorites. Geochim. Cosmochim. Acta 59, 4713-4728. 

Differentiated achondrites represent the products of classical igneous processes acting on the silicate-oxide system of asteroidal bodies— partial to complete melting, and magmatic crystallization. Iron meteorites represent the complementary metal-sulfide system products of this process. 

He) were able to show that some iron meteorites and lunar rocks also have cosmogenic W isotope anomalies that are superimposed on the radiogenic isotope effects. For the lunar rocks, these anomalies were shown to be due to the Ta(n,Y) a reaction, induced by cosmic radiation, whereby the short-lived isotope a (T = 114 days) subsequently decays to [141]. For the iron meteorites, cosmic ray-induced reactions lead to both the production and consumption of various W isotopes. As a result of these reactions, many samples of both magmatic and non-magmatic irons appear to have ratios that are... 

The differentiated nonchondrites in Table 18.1 consist primarily of achondrites which are stony meteorites that lack chondrules and contain evidence of magmatic differentiation, including the segregation of liquid iron and nickel which are insoluble in silicate... 

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