Meteorites, sulfur isotopes

What are the relative contributions of these two sources Two approaches have been taken. One is to establish the geology and hydrology of a basin in great detail. This has been carried out for the Amazon (Stallard and Edmond, 1981) with the result that evaporites contribute about twice as much sulfate as sulfide oxidation. The other approach is to apply sulfur isotope geochemistry. As mentioned earlier, there are two relatively abundant stable isotopes of S, and The mean 34/32 ratio is 0.0442. However, different source rocks have different ratios, which arise from slight differences in the reactivities of the isotopes. These deviations are expressed as a difference from a standard, in the case of sulfur the standard being a meteorite found at Canyon Diablo, Arizona. 

Beaudoin G, Taylor BE, Rumble III D, Thiemens M (1994) Variations in the sulfur isotope composition of troilite from the Canon Diablo iron meteorite. Geochim Cosmochim Acta 58 4253-4255 Birck JL, Allegre CJ (1988) Manganese-chromium isotope systematics and the development of the early solar system. Nature 331(6157) 579-584... 

Jensen ML, Nakai N (1962) Sulfur isotope meteorite standards, results and recommendations. In Jensen ML (ed) Biogeochemistry of sulfur isotopes, NSE Symp Vol, p 31 Jia Y (2006) Nitrogen isotope fractionations during progressive metamorphism a case study from the Paleozoic Cooma metasedimentary complex, southeastern Australia, Geochim Cosmochim Acta 70 5201-5214... 

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... 

Sulfur isotopes also show mass-independent effects that are probably produced by the same photochemical mechanism as oxygen effects in the Earth s upper atmosphere. Mass independent variations in sulfur from Martian meteorites have been interpreted to result from volcanic injections of SO2 and H2S into the Martian atmosphere followed by photolysis, which fractionates the sulfur isotopes. There is also evidence from ancient terrestrial sediments that the same photo lytic process was operating on sulfur in the Earth s atmosphere prior to 2.4 Ga, before oxygen began to accumulate in the atmosphere (see review by Thiemens, 2006). 

Figure 7.4 shows the reduction in sulfates and the corresponding growth of both the parent carbonates and the offspring methane with subbottom depth. The methane production is parallel but lower in isotope production than the carbonates. In Figure 7.4 the sulfur isotope (< 34S) content is defined in an identical manner to Equation 7.2 with the replacement of the fraction 13C/12C by 34S/32S in both the numerator and the denominator, using Canon Diablo meteoritic troilite as a standard. The < 34S value increases from 20-60%c before substantial biogenic methane is produced. 

Review of Isotope Concepts. The average relative concentration of the two most abundant stable isotopes of sulfur, 32S and S, are 95.0% and 4.2%, respectively. Ratios of these two isotopes are measured with specialized isotope ratio mass spectrometers, normalized to standard Canyon Diablo meteoritic sulfur, and expressed as a delta value (634S) according to the relation ... 

Sulfur Isotope Composition and the Sources of Sulfur in Coal. Sulfur has four stable isotopes of atomic masses 32, 33, 34, and 36. The sulfur isotopic composition of a sample is generally characterized by its 34S/32S ratio, expressed in terms of 834S. It is the permil (%o) deviation in the 34S/32S ratio of a sample from a standard (troilite in the Canon Diablo meteorite) (102). 

MIF in oxygen isotopes (O-MIF) is well known in primitive meteorites [1] and in atmospheric O3 [2,3], and MIF in sulfur isotopes (S-MIF) has been discovered in Archean rocks [4] and modern ice core sulfates [5]. In terrestrial environments... 

There are several review papers on mass-independent chemical processes and their applications. Thiemens and Weston reviewed the progress in understanding the physical chemistry of gas-phase mass-independent processes and their observation on Earth and meteorites. Thiemens et al. (2001) reviewed the observations of mass-independent isotopic composition in various solid reservoirs of Earth and Mars, including both oxygen and sulfur isotopes. A more recent review (Thiemens, 2002) has summarized the theoretical and laboratory studies of the physical chemistry of mass-independent isotope effects and their observation on Earth and Mars, subsequent to the review of Thiemens et al. (2001). 

There now exist numerous observations of mass-independent isotopic compositions in nature. Most of these have recently been reviewed and will not be repeated here. When the first laboratory measurements of the mass-independent isotope effect were reported by Thiemens and Heidenreich (1983), their occurrence in nature was not expected, except possibly for the early solar system to produce the observed meteoritic CAI data. It is significant to note that, at present, all oxygen-bearing molecules in the atmosphere (except water) possess mass-independent isotopic compositions. These molecules include O2, O3, CO2, CO, N2O, H2O2, and aerosol nitrate and sulfate. Mass-independent sulfur isotopic compositions are also observed in aerosol (solid) sulfates and nitrates and sulfide and sulfate minerals from the Precambrian, Miocene volcanic sulfates, Antarctica dry valley sulfates, Namibian Gypretes, and Chilean nitrates. In addition, martian (SNC meteorites) carbonates and sulfates possess both mass-independent sulfur and oxygen isotopic compositions. These studies have been reviewed recently (Thiemens et al., 2001 Thiemens, 1999). 

MacNamara J. and Thode H. G. (1950) Comparison of the isotopic composition of terrestrial and meteoritic sulfur. Phys. Rev. 78, 307-308. 

There are four stable isotopes of sulfur as listed in Table 1. The isotopic abundances vary slightly and this is frequently used to distinguish the source of the element. Because measurement of absolute isotope abundance is difficult, relative isotopic ratios are measured by comparison with the abundance of the natural isotopes in a standard sample. The Canyon Diablo meteorite has been used as a standard for sulfur isotopes. 

M.L. Jensen, N. Nakai. (1962) Sulfur isotope meteorite standards Results and recommendations. In M.L. Jensen (ed.) Biogeochemistry of Sulfur Isotopes. NSF Symposium, New Haven, Ct, pp. 30-35... 

McSween HY, Riciputi LR (1997) Fractionated sulfur isotopes in sulfides of the Kaidun meteorite. Meteoritics Planetary Sci 32 51-54... 

Fig. 4. The quantity of meteorite-derived sulfur, Sm, in the samples illustrated in Fig. 3, as a function of nitrogen content. Values of Sm were calculated from observed sulfur contents using a model based on mixing of indigenous and meteoritic sulfur followed by isotopically competitive loss of sulfur from the lunar surface, see text. Nitrogen content is employed here as a surrogate for soil maturity, see for example, Kerridge (1993). The clear correlation of Sm with maturity constitutes evidence for addition of meteoritic sulfur to the lunar regolith. From Kerridge et al (1975).

Nothing published since 1975 calls into question our model invoking influx of meteoritic sulfur and isotopically competitive loss of part of the resulting sulfur population. Curiously, a paper appeared shortly afterwards purporting to show, without benefit of isotopic data, that the lunar regolith contained meteoritic sulfur, but without the isotopic evidence for the substantial amount of missing sulfur, it represented an exercise in futility. Our model remains the best available description of the behavior of sulfur on the lunar surface, with the caveat that it is still... 

Shearer CK, Lane GD, Papike JJ, Spilde MN (1996) Sulfur isotope systematics in alteration assemblages in martian meteorite ALH 84001. Geochim Cosmochim Acta 60 2921-2926... 

Standards The international sulfur isotope scale is based on a troilite sample from the Canyon Diablo meteorite, termed CDT. The International Atomic Energy Agency (IAEA) in Vienna has promoted the use of a more accurate sulfur isotope scale, V-CDT, to replace CDT because of isotopic inhomogenities of +0.4(i in the original sample. A new Ag2S standard, IAEA S-1, has a defined value of -0.301 V-CDT. 

Clayton DD (1999) Radiogenic iron. Meteor Planet Sci 34 A145-A160 Clayton RN (1993) Oxygen isotopes in meteorites. Ann Rev Earth Planet Sci 21 115-149 Clayton RN, Mayeda TK, Hurd JM (1974) Loss of oxygen, silicon, sulfur, and potassiimi from flie limar regolith. Proc Lunar Sci Conf 5 1801-1809... 

There are many sulfur components in meteorites which may occur in all possible valence states (—2 to +6). TroUite is the most abundant sulfur compound of iron meteorites and has a relatively constant S-isotope composition (recall... 

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