Module # 8 – Stable Isotope Geochemistry
1. Introduction
2. Definition of the δ (delta) notation
3. Isotope Fractionation and Exchange Reactions
4. Stable isotopic compositions of rocks and fluids
5. Application to climate change discussion
READINGS: pp 253 - 265 in “Introduction to
Geochemistry Principles and Applications” by Kula
Misra (2012).
Natural abundances (%) of commonly used stable
isotopes in Earth sciences; the reported isotopic ratio of
interest; and the % mass difference of the isotopes.
Hydrogen
Carbon
Nitrogen
Oxygen
Sulphur
1
H
2
H (D)
12
C
13
C
14
N
15
N
16
O
18
O
32
S
34
S
99.985 % D / H
0.015 %
98.90 % 13C / 12C
1.10 %
15
14
99.63 %
N/ N
0.37 %
99.762 % 18O / 16O
0.200 %
95.02 % 34S / 32S
4.21 %
100 %
8.4 %
7.1 %
12.5 %
6.2 %
Also: geochemists are using stable isotopes of B, Li, Cu
and Fe (among others) to study geochemical processes
Stable Isotope Fractionation
Chemical fractionation wherein elements partition themselves between
solids and melts.
Stable isotope fractionation
In the case of stable isotopes, we are concerned with the fractionation
(or partitioning) of different isotopes of the same element between coexisting minerals, co-existing minerals and fluids (or melts), and coexisting fluids and gases.
For example, the hydrogen and oxygen isotopic composition of surface
H2O varies significantly because H and O isotopes fractionate as a
function of temperature and therefore depend, in part, on the local
geography of where you live.
The partitioning of stable isotopes to form different "types" of water (or
any other substance) with characteristic isotopic signatures is an
example of isotopic fractionation (which is distinct from chemical
fractionation).
Example: Isotope Exchange Reaction between water and gas.
Isotope exchange reactions are a “special” type of chemical
equilibrium. For example,
H2O + CO2 --- H2O + CO2
Can be re-written in terms of the oxygen isotopes involved:
H218O + C16O16O --- H216O + C18O16O
K = (C18O16O) (H216O) = (18O/16O)CO2 =
(C16O16O) (H218O)
αCO2-H2O = 1.0412
(18O/16O)H2O
The equilibrium constant for an isotope exchange reaction is referred
to as a fractionation factor denoted by the symbol “α” (alfa). It
describes the partitioning (or fractionation) of isotopes between any
two species.
Example: Isotope Exchange Reaction between mineral and water.
For silicates and water (e.g., quartz – water):
H218O + Si16O16O --- H216O + Si18O16O
αSiO2-H2O = (18O/16O)SiO2 = 1.035 (at 25oC)
(18O/16O)H2O
Fractionation factor for quartz - water states that the oxygen in quartz
is enriched in 18O by 3.5 % compared to oxygen in co-existing
water.
Example: Isotope Exchange Reaction between two minerals.
For K-feldspar – quartz (i.e. no water):
KAlSi3O718O+ Si16O16O  KAlSi3O716O + Si18O16O
αSiO2-Kfeld = (18O/16O)SiO2 = 1.012 (at 25 C)
(18O/16O)Kfeld
Fractionation factor for quartz - feldspar states that oxygen in quartz is
enriched 1.2 % in 18O compared to oxygen in feldspar.
Module # 8 – Stable Isotope Geochemistry
1. Introduction
2. Definition of the δ (delta) notation
3. Isotope Fractionation and Exchange Reactions
4. Stable isotopic compositions of rocks and fluids
5. Application to climate change discussion
Standard notation (for oxygen) for reporting stable isotope compositions
and some useful equations:
δ18O = { (18O/16Osample - 18O/16Osmow)
} * 1000
(18O/16Osmow)
Isotopic measurements are reported in units of per mil (‰)
relative to an international reference material. For oxygen and
hydrogen, the reference is SMOW (Standard Mean Ocean Water):
δ18O of SMOW = 0 per mil (‰)
Note the difference b/w: per cent (%) versus per mil (‰)
Note the simplicity of δ versus 18O/16Osmow = 0.0020052
The relationship of α to δ :
α X-Y
= (18O/16O)X = [1000 + δ18OX]
(18O/16O)Y
[1000 + δ18OY]
The notation for reporting isotopic compositions for other stable
isotopes is similar; each is reported relative to a standard. For example:
δ34S = { (34S/32Ssample - 34S/32SCDT)
} * 1000
(34S/32SCDT)
CDT=Canyon Diablo Troilite
δ13C = { (13C/12Csample - 13C/12CPDB)
(13C/12CPDB)
} * 1000
PDB is a fossil from PD formation
δ15N = { (15N/14Nsample - 15N/14Natm)
} * 1000
(15N/14Natm)
δD = { (D/Hsample - D/HSMOW)
(D/HSMOW)
atm = atmosphere
} * 1000
SMOW= Standard Mean Ocean Water
The reference for sulfur isotopes is S in a meteorite CDT; carbon is C in
a fossil PDB; nitrogen is N in the atmosphere.
Fractionation factor between any two phases X and Y commonly
vary as a function of temperature:
103 ln (α X – Y) = (A x 106) + B
T2
(A, B are constants)
This expression can also be expressed in the form:
1000 ln (α X - Y) ~ ∆ x - y = δ18Ox - δ18Oy
E.g. Oxygen isotope exchange for calcite – water:
103 ln α Cc-H2O = (2.78 x 106) - 2.89
T2
(looks like y=mx + b)
δ18Occ - δ18OH2O ~ (2.78 x 106) - 2.89
T2
~ ∆18Omin-H20 = δ18Omin – δ18OH2O
Fractionation factors between Quartz (qtz) and Calcite (cc) as a
function of temperature: δ18Omin-δ18OH2O = [(A×106)/T2] + B
∆
qtz-H2O
= 20.5 (at 100oC)
∆
∆
cc-H2O
qtz - cc
= 4 (at 100oC)
= 16.5 (at 100oC)
Module # 8 – Stable Isotope Geochemistry
1. Introduction
2. Definition of the δ (delta) notation
3. Isotope Fractionation and Exchange Reactions
4. Stable isotopic compositions of rocks and fluids
5. Application to climate change discussion
Example 1: For the precipitation of quartz from water at 100 oC in a
vein, the oxygen isotope exchange reaction at equilibrium can be
written as:
H218O + Si16O16O ---
H216O + Si18O16O
The1000 ln (α QTZ - H2O) ~ 21.5 ‰ at 100 oC (see previous slide).
Therefore there is a difference of 21.5 ‰ between the δ18O value of
quartz and water from which the quartz precipitated
(i.e. δ18OQTZ - δ18OH2O = 21.5) at 100 oC.
If calcite were to precipitate in the same quartz vein at the same time
and from the same fluid, the diagram above indicates that, at 100oC,
1000 ln(αCC - H2O) ~ 16.5 ‰.
Therefore there is a difference in the δ18O value of calcite and water of
16.5 ‰ at 100 oC or,
Another way of looking at the diagram, is to say that there is a
difference in the δ18O value of quartz and calcite of 4 ‰ at 100 oC.
Question 1a: Use the above diagram to deduce
the temperature of vein formation if the quartz had
a δ18O value of 16 per mil and co-existing vein
calcite of 14 per mil.
Question 1b: How would you calculate (i.e. give
equation and steps) the temperature and the δ18O
value of the H2O that precipitated in a vein if it
were made of quartz and chlorite?
{hint: you would need to know the A and B constants for
quartz and water and chlorite and water – see Table 17.3
on next slide}.
Other fractionation factors between phases X and Y as a
function of temperature:
ISO.2 Temperature Dependence
of the O-isotope fractionation
3
6
10 ln α X - Y = (A x 10 ) + B
factor between minerals and
2
T
water in the form
103lnα=A*106/T2+Ba
ISO.1 Isotope fractionation
Mineral
A
B
factors for H and O between
Kaolinite
2.5
-2.87
clay minerials and water at
Illite
2.43
-4.82
Earth-surface temperatures
Smectite
2.67
-4.82
Mineral
H
O
Chlorite
1.56
-4.7
Montmorillonite 0.94 1.027
Quartz
3.38
-3.4
Kaolinite
0.97 1.027
Calcite
2.78
-2.89
Galucaonite
0.93 1.026
Illite
- 1.0236
Dolomite
3.14
-2
Gibbsite
0.984 1.018
Anhydrite
3.21
-4.72
Module # 8 – Stable Isotope Geochemistry
1. Introduction
2. Definition of the δ (delta) notation
3. Isotope Fractionation and Exchange Reactions
4. Stable isotopic compositions of rocks and fluids
5. Application to climate change discussion
Typical carbon isotope
compositions:
(characteristic
low “organic”
carbon values)
(oceanic or
seawater carbon
near 0 per mil)
Mantle value constant
near -6 +/- 2 per mil
Typical oxygen isotope compositions:
(Some granites and
metamorphic rocks
interact with meteoric
water)
“Normal” granites and
metamorphic rocks
“Normal” sedimentary
rocks
(igneous fractionation
and low T processes
trend to higher values)
Diagnostic low values
of meteoric water
Mantle value constant
near 5.7 per mil
“Types” of Water
defined by their
hydrogen and oxygen
isotope compositions.
Can use oxygen isotopes
to calculate the
temperature of formation
of minerals, but also often
want to calculate the
isotopic composition of
the water that formed the
minerals. The "type" of
water can be linked to
geologic processes.
GMWL δD = 8 δ18O + 10
Why does the oxygen isotope composition of meteoric water differ from
that of seawater?
Evaporation preferentially removes water molecules containing lighter
16O isotopes, leaving heavier 18O isotopes behind so that the ocean
becomes enriched in 18O relative to 16O. The same is true for
deuterium and hydrogen (i.e. lighter H fractionates into atmosphere)
Progressive changes in oxygen isotopic composition of meteoric water
as water vapour passes from the ocean over land. What controls the
changes in isotopic composition?
H216O (vapour)  H218O (liquid)
αliq-vap = 1.0092
δ18O(liquid) - δ18O(vapour)~ 9.2 per mil
-18 ‰
-12 ‰
-3 ‰
-9 ‰
0‰
Ocean
(Small kinetic effect
during initial evap.)
Continent
-29 ‰
-20 ‰
Interior or
far north of
continent
Geography plays a role in
defining the δD value of
precipitation
(e.g., proximity to oceans and
mountains effects isotope
compositions)
This reflects progressive
depletion in the heavier
isotope (deuterium) of cloud
vapour during rainout as the
cloud migrates across the
continent
Module # 8 – Stable Isotope Geochemistry
1. Introduction
2. Definition of the δ (delta) notation
3. Isotope Fractionation and Exchange Reactions
4. Stable isotopic compositions of rocks and fluids
5. Application to climate change discussion
Oxygen Isotopes of Calcium Carbonate as
Paleo-Thermometers
24
Fractionation factor between calcite and water at low temperature:
How to use
isotopes to
calculate changes
in temperature of
seawater – global
cooling or
warming in the
past.
∆
The δ18O of
calcite yields
temperature if
∆
cc-H2O
cc-H2O
= 31 (at 10oC)
= 28.2 (at 25oC)
δ18O of the
ocean is fixed at
near zero per mil.
Recall:
∆
cc-H2O
= δ18Occ – δ18OH2O
Can use corals and forams (CaCO3) whose 18O / 16O ratio in carbonate reflects the
temperature of the seawater in which they grew.
Forams
Coral
Foraminifera (microfossils) are found in deep-sea
sediments. Lower ratios of 18O / 16O imply
warmer temperatures of formation. Sediment
cores from the sea bottom or fossils preserved in
limestone in the rock record preserve a 18O / 16O
temperature record of climate change (oceanic)
over the past 500 million years.
Like trees, corals add seasonal layers
which appear as bands in their skeletons. Each of the
light-dark bands in this X-ray image (left) formed
during one year of growth. This produces a continuous
oxygen isotope record of temperature.
A 20-year oxygen isotope record for a coral from the SW Pacific (Guam).
Note that the data is reported in per mil relative to SMOW.
Corresponds to what temperature? (see plot of fractionation factor)
δ18Occ
27.80
27.90
28.00
28.10
28.20
28.30
28.40
28.50
1980
1985
1990
YEAR
Corresponds to what temperature?
1995
2000
Guam coral data - SW Pacific ocean
The δ18O of
calcite yields
temperature if
δ18O of the
ocean is fixed at
zero per mil.
Recall:
∆
cc-H2O
= δ18Occ – δ18OH2O
Oxygen
Isotopes of H2O
as PaleoThermometers
Dansgaard (1964). Tellus.
16(4): 377-381
29
• Ice located high in
mountains and in polar
ice caps has
accumulated from
snowfall occurring over
long periods of time.
Want to collect an Ice Core
• Being made of H2O,
these ice cores also
contain a useful record
of oxygen isotope
variation over time.
Edge of Greenland ice sheet
• By drilling into ice caps it is possible to collect ice cores that
preserve proxy climate information going back hundreds of
thousands of years.
• Ice cores also contain trapped air bubbles, which directly
preserve samples of the atmosphere including its CO2 content.
Drilling camp on the ice sheet
of central Greenland
Ice core
Question
• If you had a water sample from an ice core
dated 50,000 years ago with a δ18O
isotope value of -25 ‰, what was the
annual atmospheric temperature 50,000
years ago (or at the time the ice was
formed?
32
Vostok ice core
*
*
*
*
*
Determined
using δ18O of
water
Temp
**
**
**
**
Determined
in gas
bubbles
CO2
Dust
* Interglacial
** Glacial
33