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UVM GEOL 135 - Lecture 27- Isotope geochemistry I

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Isotope GeochemistryMeasuring IsotopesFractionationTemperature effects on fractionationEquilibrium vs. Kinetic fractionationFRACTIONATION DURING PHYSICAL PROCESSESSlide 7Equilibrium FractionationWHY IS K DIFFERENT FROM 1.0?Slide 10Equilibrium Fractionation IIEmpirical Relationship between Temp. & Oxygen Isotope Ratios in CarbonatesDistillationRAYLEIGH DISTILLATIONSlide 15Using isotopes to get information on physical and chemical processesSlide 17Slide 18VolatilizationSlide 20Slide 21Slide 22Slide 23Slide 24Slide 25Slide 26Slide 27ISOTOPE FRACTIONATION IN THE HYDROSPHERESlide 29Slide 30Slide 31Deviation from MWLKinetic FractionationKinetic fractionation I – SO42- reductionS isotopes and microbesIron IsotopesMass-independent fractionationS-isotopic evidence of Archaen atmosphereIsotope GeochemistryMeasuring Isotopes•While different, isotopes of the same element exist in certain fractions corresponding to their natural abundance (adjusted by fractionation)•We measure isotopes as a ratio of the isotope vs. a standard material (per mille ‰)31810standardstandardsampleRRRObaabRRWhere Ra is the ratio of heavy/light isotope and  is the fractionation factor‰abbaabln103Fractionation•A reaction or process which selects for one of the stable isotopes of a particular element•If the process selects for the heavier isotope, the reaction product is ‘heavy’, the reactant remaining is ‘light’•Isotope fractionation occurs for isotopic exchange reactions and mass-dependent differences in the rates of chemical reactions and physical processesabbaabln103Temperature effects on fractionation•The fractionation factors, , are affected by T (recall that this affects EA) and defined empirically:•Then,•As T increases,  decreases – at high T  goes to zeroBTAab26310ln10Where A and B are constants determined for particular reactions and T is temp. in Kelvinsabbaabln103Equilibrium vs. Kinetic fractionation•Fractionation is a reaction, but one in which the free energy differences are on the order of 1000x smaller than other types of chemical reactions•Just like other chemical reactions, we can describe the proportion of reactants and products as an equilibrium or as a kinetic functionFRACTIONATION DURING PHYSICAL PROCESSES•Mass differences also give rise to fractionation during physical processes (diffusion, evaporation, freezing, etc.).•Fractionation during physical process is a result of differences in the velocities of isotopic molecules of the same compound.•Consider molecules in a gas. All molecules have the same average kinetic energy, which is a function of temperature. 221mvEkineticBecause the kinetic energy for heavy and light isotopes is the same, we can write:In the case of 12C16O and 13C16O we have:Regardless of the temperature, the velocity of 12C16O is 1.0177 times that of 13C16O, so the lighter molecule will diffuse faster and evaporate faster.LHHLmmvv0177.1994915.2799827.28HLvvEquilibrium Fractionation•For an exchange reaction:½ C16O2 + H218O ↔ ½ C18O2 + H216O•Write the equilibrium:• Where activity coefficients effectively cancel out•For isotope reactions, K is always small, usually 1.0xx (this K is 1.047 for example))()()()(2182121621621218OHOCOHOCK WHY IS K DIFFERENT FROM 1.0?Because 18O forms a stronger covalent bond with C than does 16O.The vibrational energy of a molecule is given by the equations:HOHhElvibrationa21mk21kxF Thus, the frequency of vibration depends on the mass of the atoms, so the energy of a molecule depends on its mass.•The heavy isotope forms a lower energy bond; it does not vibrate as violently. Therefore, it forms a stronger bond in the compound.•The Rule of Bigeleisen (1965) - The heavy isotope goes preferentially into the compound with the strongest bonds.Equilibrium Fractionation II•For a mass-dependent reaction:•Ca2+ + C18O32-  CaC18O3•Ca2+ + C16O32-  CaC16O3•Measure 18O in calcite (18Occ) and water (18Osw)•Assumes 18O/16O between H2O and CO32- at some equilibriumT ºC = 16.998 - 4.52 (18Occ - 18Osw) + 0.028 (18Occ-18Osw)2Empirical Relationship between Temp. &Oxygen Isotope Ratios in CarbonatesAt lower temperatures, calcitecrystallization tends to incorporate arelatively larger proportion of 18Obecause the energy level (vibration)of ions containing this heavier isotopedecreases by a greater amount than ionscontaining 16O.As temperatures drop, the energy levelof 18O declines progressively by anamount that this disproportionatelygreater than that of the lighter 16O.Distillation•2 varieties, Batch and Rayleigh distillation dependent on if the products stay in contact and re-equilibrate with the reactants•Batch Distillation: f = i – (1 – F) 103lnCO2-Rock where the isotope of the rock (i) depends on it’s initial value (f) and the fractionation factor•Rayleigh Distillationf - i =103(F( – 1) – 1)RAYLEIGH DISTILLATIONIsotopic fractionation that occurs during condensation in a moist air mass can be described by Rayleigh Distillation. The equation governing this process is:where Rv = isotope ratio of remaining vapor, Rv° = isotope ratio in initial vapor, ƒ = the fraction of vapor remaining and a = the isotopic fractionation factor1fRRovvEffect of Rayleigh distillation on the 18O value of water vapor remaining in the air mass and of meteoric precipitation falling from it at a constant temperature of 25°C. Complications:1) Re-evaporation2) Temperature dependency of Using isotopes to get information on physical and chemical processes•Fractionation is due to some reaction, different isotopes can have different fractionation for the same reaction, and different reactions have different fractionations, as well as being different at different temperatures and pressures•Use this to understand physical-chemical processes, mass transfer, temperature changes, and other things…Volatilization•calcite + quartz = wollastonite + carbon dioxideCaCO3 + SiO2 = CaSiO3 + CO2 •As the CO2 is produced, it is likely to be expelled•Other volatilization reaction examples…ISOTOPE FRACTIONATION IN THE HYDROSPHEREEvaporation of surface water in equatorial regions causes formation of air masses with H2O vapor depleted in 18O and D compared to


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UVM GEOL 135 - Lecture 27- Isotope geochemistry I

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