Chondrites metallic iron

Calculations predict that metallic iron should react with sulfur in nebular gas when temperatures drop below 650 to produce iron sulfide FeS (troilite). Indeed, brassy troilite grains are commonly observed in association with metal in chondrites. However, sulfur is so readily mobilized during later heating so that it is doubtful that troilite grains formed by nebular reactions have been preserved in their original form. 

Irons are non-chondritic meteorites that are predominantly metal. Iron meteorites formed by melting of, most likely, chondritic material and segregation of metal melt from silicate. Many apparently represent asteroidal cores, although some may have formed as dispersed metal pockets in the parent asteroids. 

The aubrites are the most reduced achondrites (Keil et al., 1989). Their silicates are essentially free of iron, and they contain minor metallic iron. A variety of unusual sulfides of calcium, chromium, manganese, titanium, and sodium - all usually lithophile elements -occur in aubrites. These unusual sulfides also characterize the highly reduced enstatite chondrites, which may have been precursors for these rocks. 

Concentrations of metallic iron and oxidized iron, normalized to silicon, indicate that both oxidation/reduction and loss of metal are required to explain the compositions of various chondrite classes. 

Another important chemical difference between chondrite groups is in their oxidation states. In Chapter 7, we learned that differences in chondrite compositions cannot be simply explained by fractionation of metallic iron, but also must involve oxidation or reduction of iron (see Fig. 7.14). 

Jarosewich, E. (1990) Chemical analyses of meteorites A compilation of stony and iron meteorite analyses. Meteoritics, 25, 323-337. A compilation of many years of painstaking wet-chemical analyses of representative chondrite powders. These analyses are especially useful because they distinguish the amounts of metallic iron and Fe2+. 

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. 

The stony-iron meteorites are intermediate between chondrites and irons. These very rare meteorites are equal mixtures of iron/nickel alloys and silicate minerals. Pallasites are striking examples of this type of meteorites, consisting of green olivine crystals in a matrix of metallic iron. Another type of stony-iron meteorite, called mesosiderites, contain pyroxene and plagioclase feldspars, minerals that are common on Earth. 

Fig. 13. MnO/FeO correlation first noted by Laul et al. 10°) for lunar samples. A similar correlation seems to hold for terrestrial and meteoritic samples, too. By oxidation of metallic iron the data point of a material with the composition an ordinary H-group chondrite (marked H) will move along the horizontal dashed line. The location of the point of maximum oxidation (Femetai = 0) is also indicated...

Enstatite chondrites comprise two groups with different contents of metallic iron EH and EL. Additionally, there is an ungrouped E chondrite, LEW87223 (Grossman et al., 1993). 

Figure 7 Urey-Craig diagram showing relative iron contents and oxidation states of the chondrite groups. Iron present in metal and sulfide phases is plotted versus iron present in silicate and oxide phases, for bulk chondrite compositions (after Brearley and Jones, 1998) (reproduced by permission of the Mineralogical Society of America from Reviews in Mineralogy 1998, 36, 1-398).

Chondritic meteorites, characterized by their relatively unfractionated chemical compositions, and usually consisting of chondrules and some matrix, can be subdivided into classes, as follows carbonaceous chondrites—Vigarano-type (CV), Ornans-type (CO), Mighei-type (CM), Renazzo-type (CR), Karoonda-type (CK), Bencubbin-type (CB), and ALH 85085-type (CH) (see Chapter 1.05 for details) ordinary chondrites—high-iron (H), low-iron (L), and low-iron, low-metal (FF) and enstatite chondrites—high-iron (EH), low-iron (EL) R-chondrites, characterized by olivine with very high ferrous iron content. 

Thus core-mantle equilibration can be excluded as the source of the HSEs in the Earth s mantle. It is more likely that a late accretionary component has delivered the HSEs to the Earth s mantle, either as single Moon-sized body which impacted the Earth after the end of core formation or several late arriving planetesimals. The impac-tors must have been free of metallic iron, or the metallic iron of the projectiles must have been oxidized after the collision(s) to prevent the formation of liquid metal or sulfide that would extract HSEs into the core of the Earth. The relative abundances of the HSEs in the Earth s mantle are thus the same as in the accretionary component, but may be different from those in the bulk Earth. The late addition of PGE with chondritic matter is often designated as the late veneer hypothesis (Kimura et al., 1974 Chou, 1978 Jagoutz et al., 1979 Morgan et al., 1981 O Neill, 1991). This model requires that the mantle was free of PGE before the late bombardment established the present level of HSEs in the Earth s mantle. 

The rock reservoirs on the modem Earth show a very narrow range of Mn/Fe ratios, ranging only from 0.016 to 0.019, which demonstrates how similar the two elements are in the normal terrestrial rock cycle. Carbonaceous chondrites, which are meteorites that presumably represent the primordial composition of the Earth as a whole, are enriched in iron relative to manganese compared to the Earth s cmst and mantle. This difference reflects the concentration of metallic iron, but not of manganese, in the Earth s core. Another variation in composition from the normal cmstal value of 0.017 is seen in Archean cmst, which averages 0.023, a value that is higher than any common igneous rocks. 

Types of meteorite Meteorites may be subdivided into two main categories - unmelted meteorites, that is those which come from a parent body which has not been fractionated since its aggregation early in the history of the solar system, and melted, or differentiated meteorites (Fig. 2.8). Unmelted meteorites are stony meteorites, the chondrites, and are made up of the same silicate minerals that are found on Earth. Melted meteorites are of three types. They include some stony meteorites (the achondrites), the iron meteorites, whose composition is dominated by a metallic iron-nickel alloy, and stony-iron meteorites, meteorites which are made up of approximately equal proportions of... 

Chondrites are ultramafic in composition and contain the minerals olivine, pyroxene, and metallic iron. They are composed of three main components (see Fig. 2.9), each of which represents a different component of primitive solar nebula material ... 

Geochemical constraints The siderophile elements provide an important control on the processes of core formation. Siderophile elements are those elements which have a chemical affinity for metallic iron (Section 2.3.2) and so might be expected to concentrate in the core. This is in fact what is observed, and siderophile element concentrations in the mantle are depleted relative to concentrations in chondritic meteorites, with... 

We can estimate how much water was dissociated by considering the oxidation of the mantle. If the Earth started out with the oxidation state of chondrites (although not necessarily of total chondritic composition), it would take the oxygen from about one present-day ocean volume to reach the oxidation state of the mantle inferred from mantle-derived rocks. Alternatively, if we start with a more reduced ensemble, suggested by condensation calculations from a solar nebula, we will have primarily enstatite (MgSiOs) and metallic iron as the original source to be oxidized. The oxidation reaction would then be as shown in eqn [1]. 

The chondrites include the ordinary and the carbonaceous varieties of stony meteorites each of which is further split into different groups. Most chondrites contain chondrules which are spherical particles about 1 mm or less in diameter composed of silicate minerals such as olivine, pyroxene, plagioclase, as well as troil-ite, spinel, and metallic iron (Hewins 1997). The chondmles formed in the solar nebula by accretion of dust particles which were subsequently melted and... 

Chlorine-36 is favored by investigators because the grains of metallic iron are easily concentrated from crushed samples of stony meteorites and because the concentration of 1 atoms per gram of metal can be determined routinely by accelerator mass-spectrometry. The corresponding rate of decay of Cl, also called the activity, is calculated from the measured concentration by means of the law of radioactivity. The calculation of the terrestrial 1 age of the LL6 chondrite ALH 78153 is demonstrated in Appendix 18.12.2. 

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).

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