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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
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氧同位素特征
 16O,
99.763%
 17O, 0.0375%
 18O, 0.1995%
 18O/16O, 0.002 (Garlick, 1969)
 Species: H2O, H3O+, OH-, CO2, HCO3-,
CO3-, SiO42 Dm/m = 0.125 for 18O/16O, O is the most
abundant element in the crust
氧同位素范围
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
Glass line, equilibrium method, CO2 to
IRMS (ml-size), ±0.15-0.2‰
 EA-HT-IRMS, pyrolysis reaction (>1450oC),
He carrier, 100-200ml, ±0.15‰
 GB-IRMS, equilibrium method, He carrier,
100-200ml, ±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.
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Standards
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V-SMOW R(18O/16O) = 2005.20±0.4310-7
0‰
GISP
-24.75‰ (SMOW)
V-SLAP
-55.50‰ (SMOW)
NBS-19
Marble
-2.2‰ (PDB)
NBS-18
Carbonatite
-23.0‰ (PDB)
NBS-28
Quartz
9.60‰ (SMOW)
NBS-30
Biotite
5.10‰ (SMOW)
Working standards
PDB
Belemnitella americana from the Cretaceous Peedee
formation, South Carolina
SMOW
PDB
氧同位素分馏
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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 CO32during carbonate precipitation. 3. Oxygen
isotopic exchange during rock-water
interaction. 4. Oxygen isotopic exchange
between minerals.
Equilibrium fractionation between
water and calcite
T（oC） = 17.0 – 4.52(d18Oc – d18Ow) +
0.13 (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. -0.26‰/ oC
 dcalcite – dwater = 3.945 – 0.232T（oC）
 daragonite – dwater = 4.675 – 0.221T（oC）
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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,
aeq = exp
(1137/Tk2 –
0.4156/Tk –
0.00207)
Majoube
(1977)
Rayleigh fractionation
动力分馏
平衡分馏
Example
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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 = 1.0092.
D = d water – d vapor
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
But the slope and intercept vary depending on
the conditions of evaporation, vapor transport
and precipitation. Global Meteoric Water Line
Ice core (Temperature record)
南极-East Antarctic
北极-Greenland
Isotope Geothermometry
Isotopic mass balance
f1, d1
f2, d2
d
d = f1d1 + f2d2
Examples
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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)?
Kinetic effect
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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.
盐度效应
含盐水阳离子及其浓度的影响
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.
水-岩反应的氧同位素不平衡分馏
方解石与其它含氧矿物间的分馏
Formation water
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Formation waters are salines with salt contents
ranging from ocean water to very dense Ca-NaCl 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.
Upper Mantle/Magmatic system
D
Ore deposits and hydrothermal
system