Geothermometry

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Geothermometry paleoclimate proxies 10/22/07


The JOIDES Resolution drillship
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Temperature-dependent fractionation - recap
Equilibrium fractionation is temperature-dependent
, always. - weve discussed the liquid-vapor
fractionation for precipitation - today
carbonate-liquid fractionation
any solid phase
redwarm bluecold
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Carbonate d18O introduction
Minerals (e.g. carbonate, quartz, barite, etc)
form from super-saturated solution. d18O of
these minerals is a fxn of d18O of solution and
temperature of solution Remember the d18O of a
solid phase is usually reported in PDB (heavy
standard) while d18O of liquid phase is usually
reported in SMOW (light standard) interconversion
equation Important You need to know the
d18O of the solution to derive temperature from
d18Osolid The ocean d18O is defined as 0
(Standard Mean Ocean Water), and its a big
volume, so how do you change d18O of seawater?
Friedman and ONeil (1977)
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Carbonate d18O temperature relationships
NOTE dc must be wrt PDB, dw must be wrt
SMOW good for low T, paleoceanography
The relationship between water-d18O, temperature,
and the equilibrium d18O of calcite was
determined empirically by Sam Epstein et al.,
(1953) and later modified by Craig
(1965) ONeil et al. (1969) determined an
experimental relationship for the
temperature-dependence of a for the calcite-water
system
T in Kelvin good for high T
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Aragonite d18O temperature relationships
Why is the d18Oarag-water a different than the
d18Ocal-water a?
Is the a larger for aragonite or calcite?
T-dependence of a (T in Kelvin)
Zhou Zheng, GCA, 2003
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Glacial-Interglacial foraminifera d18O, revisited
LGM
Data from deep-sea (benthic) foraminifera show
1.5 d18O shift during LGM
The million-dollar question in paleoceanography H
ow much of this shift was due to ice volume (sea
level change) and how much was due to temperature
change? Schrag modelled the glacial-interglacial
shift in porewater d18O (1.0), so we have 0.5
left over for temperature change. How much did
bottom water temperatures change during the
LGM? Or you could measure temperature (trace
metal concentrations in carbonates), and obtain a
residual d18O that gives you the d18OSW change.
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Complications Kinetic effects, vital effects
and carbonate d18O
Fact very few organisms precipitate carbonate
in isotopic equilibrium with the surrounding
water (the vital effect) One problem
skeletons are precipitated in super-saturated
micro-environments, with sources from
surrounding water metabolic products Another
problem isotopic exchange may be rate-limited
in biological reactions
Kinetic isotope effects underlie vital effect
Can track kinetic effects with isotope-isotope plo
t (d13C vs. d18O), check for slope 2
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Carbonate ion effect on foram d18O
Not a very big effect, but casts further doubt
on inferring G-I T changes from forams
Results from culturing living forams increase
CO32-, d18O foram decreases
Spero et al., 1997
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Glacial-Interglacial climate reconstruction

• In order to reconstruct surface temperatures from
  carbonate d18O formed
• during the LGM, you need to
• remove the ice-volume effect
• constrain the d18O of your water mass
• apply the paleo-temperature equation

• However, people can use other proxies to get at
  temperature
• foraminifera assemblage data (CLIMAP)
• tree lines and snow lines will be lower during
  cold times
• trace metals in carbonates (Mg/Ca and Sr/Ca)
• alkenones (saturation index of long-chained
  alkanes in coccolithophores)

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Geothermometry Part II

• Lecture outline
• oxygen isotopes in the lithosphere
• high-temperature fractionation
• mineral-mineral fractionation
• water-rock alteration

spectrometer light intake
A black smoker from the East Pacific Rise
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Oxygen Isotopic compositions of geological
materials
Why does eclogite have a heavier d18O than MORB?
Lunar rocks MORB basic lavas mantle
nodules eclogites andesites ophiolites rhyolites
tuffs granitic rocks altered igneous
rocks metamorphic rocks clastic sediments marine
limestones
Why do metamorphic rocks exhibit such a range of
d18O?
If carbonates precipitate from a light
ocean, why are they so heavy?
composition of lunar rocks, carbonaceous chondrite
s, and MORB
What scale is this?
Why is the ocean so light compared to MORB?
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Principle of Geothermometry
The fractionation of oxygen or hydrogen in
different minerals of a rock can be used as a
geothermometer, provided that 1. minerals
deposited at same time, at equilibrium 2. no
subsequent alteration 3. fractionation factors
and T-dependence known experimentally NOTE
Using multiple mineral pairs will increase
confidence in the calculated temperature, if the
mineral pair temperatures agree i.e. they are
concordant.
For phases m and n
T in Kelvin
General form of geothermometry fractionation
equations.
Remember from last lecture we talked about the
high-T water-calcite equation?
Handy conversions
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T-dependent fractionation in various mineral pairs
Where must these lines converge?
NOTE These slopes are different so all you
need to determine T is Dm-n
The highest fractionation is between quartz and
magnetite. In general, 18O is increasingly
favored in higher-quartz minerals, and
less favored in hydrous minerals
(magnetite, amphibole, chlorite).
How could we determine the slope of the
Quartz-Muscovite fractionation?
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Tables of T-dependent Fractionation
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Example
Quartz, calcite, and chlorite were all
precipitated in a hydrothermal vent setting.
Measured d18Os Quartz 5.1 SMOW Calcite
3.8 SMOW Chlorite -1.5 SMOW
Why does quartz have the heaviest d18O, and
chlorite the lightest? And why is the quartz only
5.1 heavier than SMOW?
Did these minerals precipitate at the same
temperature? How would you begin to solve this
problem?
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Metamorphism Water-Rock interactions
Fact In several places it is possible to
measure igneous rocks with d18O values of
-5! These are places were fluid has interacted
with the rock (usually at high T) to change the
isotopic composition of the rock. We can use a
mass balance approach to calculate the amount of
water that has reacted with a host rock (or
water/rock ratio) over time (assuming
equilibrium)
is the equilibrium values for water and
mineral, (need to know temperature independently)
for a closed system
What does a closed system mean?
cw conc. of O in water cr conc. of O in
rock W mass water R mass rock superscript I
initial superscript f final
and mass balance equation
and combining these
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Water-Rock interactions II
for an open system
Now we only have a small parcel of water (dW)
interacting at any given time, but new water
parcels are injected continuously in time,
causing ddr in this scenario we need to
integrate to calculate W/R ratios.
Probably much more realistic, because water flows
through the rock.

• In order to solve for W/R interactions, you need
  to know
• the temperature of the interaction (hopefully you
  can get that by a mineral-mineral pair)
• the mineral phases that experienced fluid
  alteration
• the isotopic composition of the water before it
  interacted with the rock (dD of rock why?)
• the isotopic composition of the rock before it
  interacted with the water (unaltered samples)

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A real-world example
A characteristic signature of hydrothermal activit
y is a bulls-eye pattern of d18O values, with
low values in the middle. Alteration occurs
along an established conduit of weakened
structures. Most gems are the product of low-T,
high-fluid metamorphism 1M worth of gold
mined in the Bohemia complex between 1870 and
1940 - happy hunting!
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A cool early Earth?
Idea measure U-Pb dates and d18O of old
zircons - if you find low d18O relative to
todays primitive mantle, then that implies
interaction with meteoric waters at low
temperatures
This work is done using a laser flourination
line plumbed to a dual inlet mass spec
A Cool Early Earth, (2002) Geology. 30 351-354.
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So whats causing the relatively high zircon d18O
values at 4.2Ga?