This is a rare occurrence in nature since hydrologic systems are neither commonly completely open nor completely closed systems.įigure 1. Hence, any process that can be modeled as a Rayleigh fractionation will not exhibit fractionation between the product and source if the process proceeds to completion (with 100% yield). Mass-balance considerations require that the isotope content of the total accumulated vapor approaches the initial water δ 18O value as f → 0. The integrated curve, giving the isotopic composition of the accumulated vapor thus removed, is shown as solid line C. For higher temperatures, the α value would be smaller and the curves closer together. The δ 18O of the instantaneously formed vapor (solid line B) describes a curve parallel to that of the remaining water, but lower than it (for all values of f) by the precise amount dictated by the fractionation factor for ambient temperature, in this case by 10‰. During open-system evaporation where the vapor is continuously removed (i.e., isolated from the water), as evaporation progresses (i.e., f → 0), the δ 18O of the remaining water (solid line A) becomes higher. For example, Figure 1 shows the changes in the δ 18O of water and vapor during both open-system (solid lines) and closed-system (dashed lines) evaporation with a constant fractionation factor α l− v=1.010 (i.e., the newly formed vapor is always 10‰ lower than the residual water). The isotope enrichment achieved can be very different in closed versus open systems. The Rayleigh equation can be employed for the investigation of a plethora of geological processes, where fluid and mineral phases are separated by fractionation, crystallization, evaporation, or condensation. However, the term is commonly used also to describe finite, closed reservoirs and kinetic fractionations as well, because the situations may be computationally identical ( Kendall and Caldwell, 1998). In a strict sense, the term Rayleigh fractionation should refer only to open systems, where the removed isotopic species are in the thermodynamic and isotopic equilibrium with those remaining in the system, the reactant reservoir is infinite and well mixed, and it did not re-react with the extract.
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