Sulfur chondrites

Kerridge JF (1983) Isotopic composition of carbonaceous-chondrite kerogen evidence for an interstellar origin of organic matter in meteorites. Earth Planet Sci Lett 64 186-200 Kerridge JF, Haymon RM, Kastner M (1983) Sulfur isotope systematics at the 21°N site. East Pacific Rise. Earth Planet Sci Lett 66 91-100... 

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. 

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. 

Weisberg et al. 2006 Chapter 1). The key point is that subdivision of chondrites is based on differences in the basic chemical, mineralogical, and textural properties that are unique to each class. Chondrites also record difference in redox conditions before and during their accretion providing important clues to disk environments and properties such as oxygen, carbon, and sulfur abundances in these regions. 

Troilite (FeS) is also a common mineral in chondrites. Chondrules sometimes have rims of troilite, which may be recondensates of evaporated sulfur from chondrule precursors. The recondensation behavior of sulfur onto chondrules would be different from that of moderately volatile alkali elements described above. During cooling, sulfur would not re-enter the melt because the chondrule melt would have solidified by the time sulfur began to recondense. Instead, sulfur would recondense as sulfide veneers around chondrules (Zanda et al. 1995) or as opaque assemblages... 

Moreover, phyllosilicate is immune to sulfur poisoning. Indeed, Studier et al. (1968) showed that the Cold Bokkeveld carbonaceous chondrite, despite its sulfur content of 3 %, was able to catalyze an FTT reaction. 

In Figure 3, sodium, zinc, and sulfur are representative of the abundances of moderately volatile elements (Figure 2 and Table 2). Abundance variations reach a factor of 5 for sulfur and 10 for zinc. All three elements show excellent agreement of solar with Cl abundances, in contrast to other groups of chondritic meteorites, except for the enstatite chondrites, which reach the level of Cl abundances. However, enstatite chondrites... 

The abundances of 39 nongaseous elements in the Sun have assigned errors below 30%. Only the four elements sulfur, manganese, scandium, and strontium differ by more than 20% from Cl abundances. The difference is below 10% for 27 of these elements. The agreement between meteoritic and solar abundances must therefore be considered excellent and there is not much room left for further improvements. Obvious candidates for redetermination of solar abundances are manganese and sulfur. The hmiting factor in the accuracy of meteorite abundances is the inherent variability of Cl chondrites, primarily the Orgueil meteorite. 

Dreibus G., Palme H., Spettel B., Zipfel J., and Wanke H. (1995) Sulfur and selenium in chondritic meteorites. 

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). 

Silicates provide further evidence for the unusual origin of lAB-IIICD. While differentiated silicates might be expected in association with iron meteorites, silicates in lAB-IIICD irons are broadly chondritic (Mittlefehldt et al., 1998 Benedix et al., 2000 see Chapter 1.11). Models for the origins of lAB-IIICD iron meteorites include crystallization of a sulfur- and carbon-rich core in a partially differentiated object (Kracher, 1985 McCoy et al., 1993), breakup and reassembly of a partially differentiated object at its peak temperature (Benedix et al., 2000), or crystal segregation in isolated impact melt pools on the surface of a porous chondritic body (Wasson and Kallemeyn, 2002). A recent compilation of the chemical compositions of lAB and IIICD iron meteorites may be found in Wasson and KaUemeyn (2002). 

Figure 6 Volatile/refractory element ratio-ratio plots for chondrites and the silicate Earth. The correlations for carbonaceous chondrites can be used to define the composition of the Earth, the Rb/Sr ratio of which is well known, because the strontium isotopic composition of the BSE represents the time-integrated Rb/Sr. The BSE inventories of volatile siderophile elements carbon, sulfur, and lead are depleted by more than one order of magnitude because of core formation. The values for Theia are time-integrated compositions, assuming time-integrated Rb/Sr deduced from the strontium isotopic composition of the Moon (Figure 8) can be used to calculate other chemical compositions from the correlations in carbonaceous chondrites (Halliday and Porcelli, 2001). Other data are from Newsom (1995).

From the budgets of potassium, silicon, carbon, and sulfur extrapolated from carbonaceous chondrite compositions, one can evaluate the amounts of various light elements possibly incorporated in... 

The major-element compositions of 200 chondritic IDPs were measured by EDS (Table 1 and Figure 12). All of the particles were identified as extraterrestrial because they have approximately chondritic compositions or consist predominantly of a single mineral grain like forsterite or pyrrhotite (commonly found within chondritic IDPs) 37% of the particles are CSIDPs, 45% are CP IDPs, and 18% IDPs composed predominantly of a single mineral. Table 1 summarizes the compositions of the IDPs. Within a factor of 2 the abundances of oxygen, magnesium, aluminum, sulfur, calcium, chromium, manganese, iron, and nickel are approximately chondritic. CP IDPs are a closer match to Cl carbonaceous chondrites than CS IDPs, and they are closer to Cl bulk than to Cl... 

Figure 12 Cl chondrite-normalized element to silicon ratios for CS and CP IDPs. The solid line represents frequency of CS IDPs and the dotted line frequency of CP IDPs. Numbers in upper right of each histogram are the number of CS and CP IDPs, respectively, with element to silicon ratios >3 CL CS IDPs are systematically depleted in calcium and magnesium while CP IDPs are only slightly depleted in calcium, aluminum, sulfur, and iron relative to Cl (vertical dotted line) (source Schramm et al., 1989).

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