Oxygen isotopes mass-independent fractionation

Thiemens MH, Heidenreich JE (1983) The mass independent fractionation of oxygen a novel isotope effect and its possible cosmochemical implications. Science 219 1073-1075 Thiemens MH, Jackson TL, Brenninkmeijer CAM (1995) Observation of a mass-independent oxygen isotopic composition in terrestrial stratospheric COj, the link to ozone chemisdy, and the possible occurrence in the Martian atmosphere. Geophys Res Lett 22 255-257 Timmes FX, Woosley SE, Weaver TA(1995) Galactic chemical evolution hydrogen through zinc. Astrophys J Suppl 98 617-658... 

Thiemens, M. H. and Heidenreich, J. E. 1 (1983) The mass-independent fractionation of oxygen a novel isotope effect and its possible cosmochemical implications. Science, 219, 1073-1075. 

A chemically based, mass-independent fractionation process was first observed during ozone formation through the gas-phase recombination reaction (Thiemens Heidenreich 1983) O + O2 + M - O3 + M. The product ozone possesses equally enriched heavy-oxygen isotopes I7 IS0. by approximately lOO /oo with respect to the initial oxygen, with a slope value of unity in a three-isotope oxygen plot. This discovery led to the conclusion that a symmetry-dependent reaction can produce meteoritic isotopic anomalies (Thiemens 1999, 2006). Recently, theoretical calculations of Gao Marcus (2001) established the major role of symmetry in isotopolog-specific stabilization of vibrationally excited ozone molecules that give rise to the mass-independent compositions. 

FIGURE 2.3 Heavy oxygen isotope enriched mass independent fractionation, experiment [59] and theory [13]. 

The CO + OH CO2 + H reaction has been reported as showing mass independent fractionation [11]. This term, in the strict sense of "mass-independence" used earlier means that the slope of a O/ O fractionation plotted versus the corresponding 0/ 0 fractionation would be about unity, rather than having the conventional mass-dependent value of 0.52. However, what is frequently meant by MIF of oxygen instead is that the quantity A O = S O — 0.525 0 is different from zero. The number of experiments needed to determine is only one, instead of the number needed to determine the slope of a 3-isotope and is commonly used in the literature. Indeed, sometimes the data are such that a three-isotope plot cannot be obtained from the available data. To avoid possible confusion one might term reactions that have a non zero value for A O but for which the vs. slope has not been measured, as being mass-anomalous fractionated, MAF. However, the term MIF is in widespread usage, and includes both the strictly MIF and any mass anomalous fractionation (MAF) reactions. 

The origin of mass-dependent fractionation (MDF) in isotope systems lies in the mass dependence of the molecular properties (e.g., zero-point energy) and physical processes (e.g., evaporation) affecting the compound. If a compound comprised of atoms with 3 or more stable isotopes, such as oxygen or sulfur, deviates from a mass-dependent relationship, the compound is said to exhibit mass-independent fractionation (MIF). MIF signatures are not affected by mass-dependent processes, and so are excellent tracers of the small number of mass-independent processes that exist in nature. 

Kasting (2001) argues in support of the view of Farquhar et al (2000) (but see also Ohmoto et al, 2001) that sulfur isotope fractionation changed around 2.3 Ga. This opinion is based on the claim, from comparison of sulfur isotopes, that so-called mass independent fractionation occurred as a result of gas-phase photochemical reactions, particularly photolysis of SO2. Such fractionation would be much more likely to occur in a I0W-O2 atmosphere in which sulfur was present in a variety of oxidation states. Thus, the claim that fractionation changed around 2.3 Ga ago can be seen as supporting the notion that there was a substantial rise in O2 around this time. This, however, raises the question if cyanobacterial oxygen production had been sufficient to create the mbisco fingerprint in carbonates as early as 2.7-3.0Ga ago, why did the rise of free O2 only occur 400-700 Myr later ... 

Farquhar et al. (2000) investigated mass-independent fractionation in sulphur isotopes from across the period when oxygen levels are thought to have risen. They found a major change somewhere between 2090 and 2450 Ma ago. In rocks older than this, gas-phase atmospheric reactions may have influenced the sulphur cycle, playing a role in determining the oxidation state of sulphur. This would imply that atmospheric oxygen had low partial pressures, and microbial oxidation and reduction of sulphur were... 

Oxygen consists of three stable isotopes 0, " O and " O. For most applications only the ratio between the more abundant Lso-topes, 0 and O, is measured. If all fractionation processes in the environmental system were purely mass-dependent, measurements of 0/ 0, would be redundant, as they could be predicted from the 0/ 0 ratio. However, it has recently been ob.served that photochemical exchange between O2, Oi, and CO, in the stratosphere involves mass-independent fractionation among the oxygen isotopes (1990 Thiemens et al., 1993a, b Thiemens, 1999). Thereby O2 becomes anomalously depleted, while CO2 becomes anomalously enriched. Because of this, measurements of 0/ 0 in atmospheric O2 and/or CO2 provide an independent piece of information. Conveniently, one may define an O anomaly tracer (A 0) (Thiemens et n/., 1995b)... 

Stable isotope analysis of Earth, Moon, and meteorite samples provides important information concerning the origin of the solar system. 8lsO values of terrestrial and lunar materials support the old idea that earth and moon are closely related. On the other hand three isotope plots for oxygen fractionation in certain meteoric inclusions are anomalous. They show unexpected isotope fractionations which are approximately mass independent. This observation, difficult to understand and initially thought to have important cosmological implications, has been resolved in a series of careful experimental and theoretical studies of isotope fractionation in unimolecular kinetic processes. This important geochemical problem is treated in some detail in Chapter 14. 

Abstract The theoretical framework needed for interpretation of kinetic isotope effects on unimolecular reactions is reviewed. Application to the satisfactory rationalization of the theoretically puzzling mass independent isotope effect observed for oxygen isotope fractionation in extraterrestrial samples is described. 

Fig. 14.4 Oxygen isotopic composition of atmospheric species measured to date (After Thiemens, M., Ann. Rev. Earth Planet. Sci. 34, 217 (2006)). For these data, m, Equation 14.31 is approximately (0.7 < m < 0.9) indicating for these materials the fractionation is better described as anomalous rather than mass independent ...

The work of Thiemens, Mauersberger, and their collaborators has now definitively established a pervasive mass independent isotope fractionation for oxygen containing molecules found in the atmosphere, upper atmosphere, and in space. These results, initially surprising because they are inconsistent with ordinary equilibrium... 

Mass-independent isotopic fractionations are widespread in the earth s atmosphere and have been observed in O3, CO2, N2O, and CO, which are all linked to reactions involving stratospheric ozone (Thiemens 1999). For oxygen, this is a characteristic marker in the atmosphere (see Sect. 3.9). These processes probably also play a role in the atmosphere of Mars and in the pre-solar nebula (Thiemens 1999). Oxygen isotope measurements in meteorites demonstrate that the effect is of significant importance in the formation of the solar system (Clayton et al. 1973a) (Sect. 3.1). 

Another oxygen isotope fractionation effect is documented in CO2 samples collected between 26 and 35 km altitude, which show a mass - independent enrichment in both 0 and 0 of up to about 15%c above tropospheric values (Thiemens et al. 1995). The enrichment of stratospheric CO2 relative to tropospheric CO2 should make it possible to study mixing processes across the tropopause. 

Clayton RN, Goldsmith JR, Karel KJ, Mayeda TK, Newton RP (1975) Limits on the effect of pressure in isotopic fractionation. Geochim Cosmochim Acta 39 1197-1201 Clayton RN, Onuma N, Grossman C, Mayeda TK (1977) Distribution of the presolar component in Allende and other carbonaceous chondrites. Earth Planet Sd Lett 34 209-224 Clayton RN, Goldsmith JR, Mayeda TK (1989) Oxygen isotope fractionation in quartz, albite, anorthite and caldte. Geochim Cosmochim Acta 53 725-733 Cliff SS, Thiemens MH (1997) The 0/ 0 and 0/ 0 ratios in atmospheric nitrous oxide a mass independent anomaly. Science 278 1774-1776 Cliff SS, Brenninkmeijer CAM, Thiemens MH (1999) First measurement of the 0/ 0 and ratios in stratospheric nitrous oxide a mass-independent anomaly. J Geophys Res 104 16171-16175... 

In cosmochemistry, we use stable-isotope fractionations to study evaporation and condensation in the solar nebula, aqueous processes on asteroids, and even ion-molecule reactions to form organic molecules in interstellar clouds. The oxygen isotopes also show large mass-independent shifts that may be related either to chemical or physical processes or to incomplete mixing of the products of nucleosynthesis. These topics will be covered in detail in later chapters. 

When differences in the oxygen isotopic composition of CAIs were first measured in 1973, Robert Clayton and his coworkers attributed these mass-independent variations to mixing of normal solar system gas (plotting on or above the terrestrial mass-fractionation... 

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). 

Explain how mass-dependent and mass-independent isotopic fractionation of oxygen isotopes interact to produce the range of compositions that we observe in cosmochemical materials. 

However, the mechanism through which this anomaly is sequestered inside the solids is not clear. It is postulated that, as the nebular disk evolved, this isotopically enriched ice migrates inward (and mixes with the isotopically normal water) and volatilizes in the inner nebula, driving gas-phase oxidation processes to form solids, during which the isotopic anomaly is passed to condensates (Yurimoto Kuramoto 2004). A major issue is that the silicate materials do not show the required level of heavy-oxygen fractionation, so another mass-independent mechanism is required. 

Mass-Independent Oxygen Isotope Fractionation in Selected Systems. Mechanistic Considerations... 

Abstract Studies of the mass-independent effect on oxygen isotope fractionation... 

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