1. 第五節 氧同位素
氧同位素的基本特征
测量方法
国际标准
分馏系数
常见应用

2. 氧同位素特征
16O, %
17O, %
18O, %
18O/16O, (Garlick, 1969)
Species: H2O, H3O+, OH-, CO2, HCO3-, CO3-, SiO42-
Dm/m = for 18O/16O, O is the most abundant element in the crust

3. 氧同位素范围

4. 氧同位素测量方法
Equilibration of ml-sized samples with CO2. Traditional method.
Water is equilibrated with CO2, then on-line to IRMS.
Water is converted into pure O2 then CO2 via combustion, reduction, and pyrolysis reactions in the presence of catalysts. On-line

5. Glass line, equilibrium method, CO2 to IRMS (ml-size), ±0.15-0.2‰
EA-HT-IRMS, pyrolysis reaction (>1450oC), He carrier, ml, ±0.15‰
GB-IRMS, equilibrium method, He carrier, ml, ±0.15‰
TC/EA-IRMS, combustion, pyrolysis (>1450oC), He carrier, 10ml, ±0.15‰
CO2+ (mass 44), 13CO2+ (mass 45) and C18O2+ are measured by IRMS.

6. Standards V-SMOW R(18O/16O) = 2005.20±0.4310-7 0‰ GISP -24.75‰ (SMOW)
V-SLAP ‰ (SMOW)
NBS-19 Marble ‰ (PDB)
NBS-18 Carbonatite ‰ (PDB)
NBS-28 Quartz ‰ (SMOW)
NBS-30 Biotite ‰ (SMOW)
Working standards
PDB Belemnitella americana from the Cretaceous Peedee formation, South Carolina
SMOW
PDB

7. 氧同位素分馏
O isotope variations are due to: 1. phase transitions of water between vapor, liquid, and ice through evaporation/precipitation and/or boiling/condensation in the atmosphere, at the earth’s surface, and in the upper part of the crust. 2. Isotopic fractionation between CaCO3 and CO32-during carbonate precipitation. 3. Oxygen isotopic exchange during rock-water interaction. 4. Oxygen isotopic exchange between minerals.

9. Equilibrium fractionation between water and calcite
T（oC） = 17.0 – 4.52(d18Oc – d18Ow) (d18Oc – d18Ow)2
where d18Oc is the O-isotope composition of CO2 derived from carbonate and d18Ow is the O-isotope composition of CO2 in equilibrium with water at 25oC ‰/ oC
dcalcite – dwater = – 0.232T（oC）
daragonite – dwater = – 0.221T（oC）

10. Equilibrium Isotopic Fractionation
Raoult’s Law :the partial pressure of a species
above a solution is equal to its molar concentration in the solution times the partial pressure exerted
by the pure solution.
The fractionation factor for oxygen between vapor and liquid is just the ratio of the standard state partial pressures
Over a sufficiently small range of temperature, we can assume that ΔH is independent of temperature.
Over a larger range of temperature, ΔH is not constant.
The log of the fractionation factor in that case depends on the inverse square of temperature,

11. aeq = exp (1137/Tk2 – /Tk – )
Majoube (1977)

12. Rayleigh fractionation
动力分馏
平衡分馏

13. Example D = d water – d vapor
Calculate the d18O of raindrops forming in a cloud after 80% of the original vapor has already condensed assuming (1.) the water initial evaporated from the ocean with d18O = 0, (2.) the liquid-vapor fractionation factor, a =
D = d water – d vapor

14. Influence of geographic and meteorological factors (Rainout, temperature, humidity)
纬度效应、高程效应、温度效应、雨量效应

16. But the slope and intercept vary depending on the conditions of evaporation, vapor transport and precipitation. Global Meteoric Water Line

17. Ice core (Temperature record)
南极-East Antarctic
北极-Greenland

18. Isotope Geothermometry

20. Isotopic mass balance
f1, d1
f2, d2 d
d = f1d1 + f2d2

21. Examples
Glaciers presently constitute about 2.1% of the water at the surface of the Earth and have a d18OSMOW of  –30‰. The oceans contain essentially all remaining water. If the mass of glaciers were to increase by 50%, how would the isotopic composition of the ocean change (assuming the isotopic composition of ice remains constant)?
A lake that has no outlet receives 95% of its water from river inflow and the remaining 5% from rainfall. The river water has d18O of –10‰ and the rain has d18O of -5‰. What is the initial d18O of the lake (assuming no evaporation)?

22. Kinetic effect
O isotope fractionation occurs during photosynthesis such that 18O is depleted in the organically-bound oxygen. The complexity of the various reactions during cellular metabolism involved in this fractionation makes the quantitative modeling on-going. (tree-ring)
In salt solutions, isotopic fractionations can occur between water in the “hydration sphere” and free water, affected by charge and radius of salt ions.

23. 盐度效应

24. 含盐水阳离子及其浓度的影响

25. Evaporation effect
In an evaporative environment, enrichment of d18O is relatively larger than that of dD. This is because molecular diffusion adds a non-equilibrium fractionation term and the limited isotopic enrichment occurs as a consequence of molecular exchange with atmospheric vapor.

26. 水-岩反应的氧同位素不平衡分馏

27. 方解石与其它含氧矿物间的分馏

28. Formation water
Formation waters are salines with salt contents ranging from ocean water to very dense Ca-Na-Cl brines.
O and H isotopes are a powerful tool in the origin of subsurface waters. Although formation waters show a wide range in isotopic composition, waters within a sedimentary basin are usually isotopically distinct.
Fluids most distant from the MWL tend to be the most saline. d value decrease with altitude.
Water-rock interaction leads to d value shift.

30. Upper Mantle/Magmatic system

31. Ore deposits and hydrothermal system