Isotope Fractionations during Evaporation

This model qualitatively explains the deviation of isotopic compositions away from the Meteoric Water Line because molecular diffusion adds a non-equilibrium fractionation term and the limited isotopic enrichment occurs as a consequence of molecular exchange with atmospheric vapor. It is mainly the humidity which controls the degree of isotope enrichment. Only under very arid conditions, and only in small water bodies, really large emichments in D and are observed. For example, Gonfiantini (1986) reported a 5 0-value of +31.3%c and a 8D-value of +129%c for a small, shallow lake in the western Sahara. 

The isotopic composition of ocean water has been discussed in detail by Craig and Gordon (1965), and Broecker (1974). It is governed by fractionation during evaporation and sea-ice formation and by the isotope content of precipitation and runoff entering the ocean. 

Surface W rs NASW North Atlantic SPSW South Pacific NPSW North Pacifiv iOSW indian Ocean AESW Atiantfc Equatorial RSSW Red Sea MW Mediterranean Sea 

Deep Water Masses AABW Arvtarctic Bottom Water PDW Pacific Deep Water lODW Indian Ocean Deep Water NADW North Atlantic Deep Water 

Ocean water with 3.5% salinity exhibits a very narrow range in isotopic composition. There is, however, a strong correlation with salinity because evaporation, which increases salinity, also concentrates and D. Low salinities, which are caused by freshwater and melt water dilution, correlate with low D and 0 concentrations. As a consequence modem ocean waters plot along two trends that meet at an inflection point where salinity is 3.55% and 5 0 is 0.5%c (Fig. 3.18). 

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Earlier in this chapter, we discussed isotopic fractionation during evaporation. Under appropriate conditions, where the condensed phase remains isotopically well mixed and the gas phase is removed from the system to prevent back reaction, Rayleigh distillation will occur (Box 7.2), resulting in a condensed phase that is isotopically heavy relative to the starting composition (Fig. 7.9). Isotopic fractionation can occur during both condensation and evaporation, as demonstrated by experiments (Richter el al., 2002). But it is not necessary that isotopes fractionate during evaporation or condensation. It depends on the details of the process. If evaporation occurs into a gas phase that is sufficiently dense, back reactions between gas and liquid can reduce the isotopic fractionation to near the equilibrium value, which is very small. For example, sulfur in chondrules does not show the isotopic fractionation (Tachibana and Huss, 2005) expected during evaporation from a liquid. Also, evaporation from a solid does not produce isotopic fractionation in the solid because diffusion is much too slow to equilibrate the few layers of surface atoms that are fractionated with the bulk of the material. 

Isotopic Fractionation During Evaporation and Some Hydrological Applications... 

Isotopic fractionation during evaporation causes fractionation during cloud formation the vapor in the clouds has a lighter isotopic composition than the ocean that supplied the water. Upon condensation from the cloud, during rainformation, the reverse is true the heavy water molecules condense more efficiently, leaving the cloud residual vapor depleted of deuterium and lsO. 

Hydrogen and oxygen isotopes fractionate during evaporation. This affects the isotopic... 

Figure 2. Cartoon of isotopic fractionation during evaporation and precipitation over ocean, and additional precipitation on land. The isotopic changes are illnstrative only, and depend on the temperature at the time of condensation, the amonnt of moisture removed, whether the eondensation occurs to water or ice, and other factors. The general trends are aeeurate.

Wombacher, F., Rehkamper, M., and Mezger, K. (2004) Determination of the mass-dependence of cadmium isotope fractionation during evaporation. Geochim. Cosmochim. Acta, 68, 2349-2357. 

Cygan RT, Wright K, Fisler DK, Gale JD, Slater B (2002) Atomistic models of carbonate minerals bulk and surface structures, defects, and diffusion. Mol Simul 28 475-495 Davis AM, Hashimoto A, Clayton RN, Mayeda TK (1990) Isotope mass fractionation during evaporation of Mg2Si04. Nature 347 655-658... 

Davis AM, Hashimoto A, Clayton RN, Mayeda TK (1990) Isotope mass fractionation during evaporation of Mg SiO. Nature 347 655-658... 

Table 4.2 gives current estimates for the relative abundances of the isotopes in the solar system. The isotopic compositions of most elements, especially those that exist as solids, come from measurements of terrestrial materials. Because the Earth has experienced extensive melting and differentiation, it can be considered a homogeneous isotopic reservoir. However, each of the elements can experience both equilibrium and kinetically based isotopic fractionations during igneous, evaporative, and aqueous processes. The range of compositions introduced by such processes is small for most elements and so does not obscure the overall picture. 

Isotope mass fractionation during evaporation of Mg2Si04. Nature 347, 655—658. 

Wang J., Davis A. M., Clayton R. N., Mayeda T. K., and Hashimoto A. (2001) Chemical and isotopic fractionation during the evaporation of the FeO-MgO-Si02-CaO-Al203-Ti02-REE melt system. Geochim. Cosmochim. Acta 65, 479-494. 

Elemental and isotopic fractionations by evaporation of silicate liquids, in particular limiting circumstances, can be simulated by equilibrium calculations, provided that an adequate thermodynamic model of the melt is available. In this approach, a particular starting temperature, pressure, and initial composition of condensed material are chosen and the gas in equilibrium with the melt is calculated from thermodynamic data. The gas is then removed from the system and equilibrium is recalculated. Repeated small steps of this sort can simulate the kinetic behavior during vacuum evaporation (i.e., the limit of fast removal of the gas relative to the rate it is generated by evaporation). This approach has been taken by Grossman et al. (2000, 2002) and Alexander (2001, 2002). 

Alexander CMOD, Grossman JN, Wang J, Zanda B, Bourot-Denise M, Hewins RH (2000) The lack of potassium-isotopic fractionation in Bishunpur chondrules. Meteor Planet Sci 35 859-868 Alexander CMOD, Wang J (2001) Iron isotopes in chondrules implications for the role of evaporation during chondrule formation. Meteor Planet Sci 36 419-428... 

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