Magnesium chondrites

The largest class of meteorite finds is stony meteorites, made principally of stone. The general stony classification is divided into three subclasses called chondrites, carbonaceous chondrites and achondrites, and it is at this level of distinction at which we will stop. Before looking at their mineral and isotopic structure in more detail, it is useful to hold the composition of the Earth s crust in mind here for comparison. The Earth s crust is 49 per cent oxygen, 26 per cent silicon, 7.5 per cent aluminium, 4.7 per cent iron, 3.4 per cent calcium, 2.6 per cent sodium, 2.4 per cent potassium and 1.9 per cent magnesium, which must have formed from the common origin of the solar system. 

Hidaka H, Ohta Y, Yoneda S, DeLaeter JR (2001) Isotopic search for live Cs in the early solar system and possibility of Cs- Ba chronometer. Earth Planet Sci Lett 193 459-466 Hinton RW, Bischofif A (1984) Ion microprobe magnesium isotope analysis of plagioclase and hibonite from ordinary chondrites. Nature 308 169-172... 

Hoppe P, Amari S, Zinner E, Ireland T, Lewis RS (1994) Carbon, nitrogen, magnesium, silicon and titanium isotopic compositions of single interstellar silicon carbide grains from the Murchison carbonaceous chondrite. Astrophys J 430 870-890... 

Weber D, Zinner E, Bischoff A (1995) Trace element abundances and magnesium, calcium, and titanium isotopic compositions of grossite-containing inclusions from the carbonaceous chondrite Acfer 182. Geochim Cosmochim Acta 59 803-823... 

The only in situ chemical data for asteroids are from the NEAR Shoemaker spacecraft, which orbited 433 Eros in 2000-1, and from the Japanese Hayabusa spacecraft, which visited 25143 Itokawa in 2003. NEAR obtained numerous measurements of the surface composition using X-ray fluorescence and gamma-ray spectrometers, and Hayabusa carried an XRF. The magnesium/silicon and aluminum/siUcon ratios for both asteroids are consistent with the compositions of chondrites. However, sulfur is depleted in Eros relative to chondritic compositions, possibly due to devolatilization by impacts or small degrees of melting. 

Nagashima, K., Krot, A.N. and Chaussidon, M. (2007) Aluminum-magnesium isotope systematics of chondrules from CR chondrites (abstr.). Meteoritics and Planetary Science, 42, Supplement, All5. 

The most exotic materials in CP IDPs are particles of glass with embedded metal and sulfides, identified by the acronym GEMS (Fig. 12.8d). Although GEMS are commonly described as having chondritic bulk compositions, that description applies only for aggregates of these submicron particles. The nanometer-sized FeNi metal (kamacite) inclusions and pyrrhotite are clearly out of equilibrium with the magnesium-rich silicate... 

Silicates with olivine composition (MgxFe(i x))2Si04 are common in chondrites, comets, IDPs, and in protoplanetary disks. The Mg-rich end-member of the olivine family is forsterite, also often termed as Foioo the Fe-rich end-member is fayalite (Foo). The interstellar medium contains a similar concentration of the FeO- and MgO-rich silicates (see Chapter 2). Correspondingly, amorphous silicate grains frequently have similar magnesium and iron abundances in protoplanetary disks, in cometary dust, and in chondritic IDPs. In stark contrast, crystalline dust is almost always dominated by Mg-rich grains in protoplanetary disks (e.g. Malfait et al. 1998 Bouwman etal. 2008), comet tails (e.g. Crovisier el al. 1997 Wooden et al 2004 Harker et al. 2005 Lisse et al. 2006), in the most primitive and least processed chondritic matrices, and IDPs (for a review, see Wooden et al. 2007). 

New phosphoms and titanium XRF data for Cl chondrites were reported by Wolf and Palme (2001). The change in phosphorus is significant. The new Cl abundance is 926 ppm, which is much lower than the values in the older compilations, 1,105 ppm in Palme and Beer (1993) and 1,200 ppm in Anders and Grevesse (1989), respectively. The new phosphorus contents are considered to be more reliable. The changes in titanium are small. Wolf and Palme (2001) also reported major element concentrations of Cl meteorites and other carbonaceous chondrites. Their magnesium and silicon contents were almost identical to those given by Palme and Beer (1993) 10.69% versus 10.68% (for silicon) in Palme and Beer (1993) and 9.60% versus 9.61 % (for magnesium) in Palme and Beer (1993). The aluminum, calcium, and iron concentrations... 

Figure 2 Magnesium- and Cl-normalized bulk lithophile (a, c) and siderophile (b, d) element abundances of the carbonaceous (a, b) and noncarbonaceous (c, d) chondrite groups (sources Kallemeyn and Wasson, 1981,1982,1985 Kallemeyn et al., 1978, 1989, 1991, 1994, 1996 Kallemeyn, unpublished).

CV (Vigarano-like) and ungrouped CV-like chondrites. The CV chondrites (Figure 12) are characterized by (i) millimeter-sized chondrules with mostly porhyritic textures, most of which are magnesium rich and —50% of which are surrounded by coarse-grained igneous rims ... 

H, L, and LL chondrites. Ordinary chondrites (Figure 13(b)) are characterized by (i) magnesium-normalized refractory lithophile abundances —0.85 X Cl chondrites (Figure 2) ... 

Based on the bulk chemistry, IDPs are divided into two groups (i) micrometer-sized chondritic particles and (ii) micrometer-sized nonchondritic particles. A particle is defined as chondritic when magnesium, aluminum, silicon, sulfur, calcium, titanium, chromium, manganese, iron, and nickel occur in relative proportions similar (within a factor of 2) to their solar element abundances, as represented by the Cl carbonaceous chondrite composition (Brownlee et al., 1976). Chondritic IDPs differ significantly in form and texture from the components of known carbonaceous chondrite groups and are highly enriched in carbon relative to the most carbon-rich Cl carbonaceous chondrites (Rietmeijer, 1992 Thomas et al., 1996 Rietmeijer, 1998, 2002). 

The nonchondritic IDPs consist mostly of magnesium-rich olivines and pyroxenes, and FeNi-sulfides (Rietmeijer, 1998). Chondritic materials are often attached to the surface of nonchondritic IDPs, which suggests that the groups have a common origin. For more on IDPs, see Chapter 1.26. 

Figure 1 Maps showing magnesium concentrations in two chondrites (a) PCA91082, a CR2 carbonaceous chondrite, and (b) Tieschitz, an H/L3.6 ordinary chondrite. In CR chondrites, as in most carbonaceous chondrites, nearly all chondrules have porphyritic textures and are composed largely of forsterite (white grains), enstatite (gray), and metallic Fe,Ni (black). The subscripts show type I chondrules, which are common, and type II, which are FeO-rich and rare in this chondrite. Tieschitz, like other ordinary chondrites, is composed of all kinds of chondrules with diverse FeO concentrations. Key to chondrule t3fpes BO, barred olivine C, cryptocrystalline, PO, porphyritic olivine POP, porphyritic olivine-pyroxene PP, porphyritic pyroxene RP, radial pyroxene. These maps were made with an electron microprobe from Mg Ka X-rays.

Mean ratio of refractory lithophiles relative to magnesium, normalized to Cl chondrites. Includes chondrule fragments and silicates inferred to be fragments of chondrites, for all matrix in CH and CBb chondrites (Greshake et at, 2002). Fall frequencies based on 918 falls of differentiated meteorites and classified chondrites (Grady, 2000). 

Figure 3 Mean abundances of lithophile elements normalized to Cl chondrites and silicon arranged in order of increasing volatility in seven chondrite groups (Wasson and Kallemeyn, 1988). Refractories (elements condensing above V) are uniformly enriched in CO, CM, and CV chondrites and depleted in H, L,and EH chondrites. Moderately volatile elements, which condense below magnesium and silicon, are all depleted relative to Cl chondrites. These fractionations are related in poorly understood ways to the formation of CAIs and chondrules (reproduced by permission of The Royal Society from Phil. Trans. Roy. Soc. London, 1988, A325, p. 539).

Figure 11 Compositions of olivine in AO As from the reduced CV chondrites Vigarano (a), Leoville (b), Efremovka (c), oxidized CV chondrite Allende (d), CR chondrites (e), CH and CB chondrites (f), unique carbonaceous chondrites Adelaide (g), and Acfer 094 (h). Olivines in CRs, Adelaide, and Acfer 094 are magnesium-rich compared to olivines in CV AO As. Fayalite contents in olivines from CV AO As increase in the order Leoville and Vigarano, Efremovka, Allende this is correlated with the degree of secondary alteration and thermal metamorphism experienced by CV chondrites. Data for Allende AO As are from Hashimoto and Grossman (1987) and for Efremovka, Leoville, and Vigarano AOAs from Komatsu et al (2001).

Aluminum-magnesium isotope measurements of aluminum-rich chondrules in ordinary and carbonaceous chondrites were summarized by Huss et al. (2001). About 10-15% of all aluminum-rich chondrules smdied show Mg. The inferred ( Al/ Al)o ratio of these chondrules is (0.5-2) X 10 , which is consistent with the values found in ferromagnesian chondrules from type 3.0-3.1 unequilibrated ordinary chondrites (Kita et al, 2000 McKeegan et al, 2000b Mostefaoui et al, 2000 2002). 

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