Fe-Zn-S system and the sphalerite geobarometer

Geothermometry/Barometry
Exsolution, Equilibration and
Stable Isotopes
Stable Isotopes
� Same # protons, different # neutrons
� Different isotopes of the same element will
behave differently due to this mass
difference
� In Economic Geology we use these
differences for thermometry, tracking
extent of equilibrium, water/rock
interactions, sources of fluids and ore
components
Temperatures/Pressures
� Fluid Inclusions
� Stable isotopes
� Sulfide thermometry/barometry
� Inversions - Polymorphs
� Exsolution relationships
Important Systems
O 16 99.756 O 17 0.039 O 18 0.205
H 1 99.985 H 2 =D 0.015 H 3
12.26 yr
half life
C12 98.89 C13 1.11 C14 5700 yr
half life
S 32 95.02 S 33 0.75 S 34 4.21 S 36 0.02
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How do we get them out?
� Carbonates: phosphoric acid -> CO 2
� Silicates: reaction w/ BrF 5, F 2, ClF 3 -> O
� O + hot graphite -> CO 2
� Sulfides: heat w/ oxidant e.g. CuO
� S + O -> SO 2
� Or S + F source -> SF 6
� OH or H 2O: reaction w/ hot U or Zn -> H 2
� Now: Direct laser fluorination
Ratios: Del/Delta Terminology
� δO 18 , δD, δC 13 , δS 34
� δ = (R sample/R std -1) * 1000
� where
� R sample = (O 18 sample /O16 sample )
� Ex: a δ of +10 => sample 1% heavier than
the standard
Standards
� Very difficult to measure absolute
amounts so always use standards
allowing ratio measurements
� O & H: referred to SMOW
� Soft rockers may refer O to PDB
� C: belemnite from the Pee Dee Fm: PDB
� S: Canyon Diablo meteorite troilite: CDT
Fractionation Factors
� See reading for relationship of the delta
notation to more rigorous derivation of the
fractionation factor between 2 minerals or
phases yielding diagrams with axis of:
� 10 3 lnα A-B
� 10 3 lnα A-B ~ δ A - δ B = Δ AB
2

See Figures for:
� Equipment schematic
� Natural Ranges
� Causes?
� Gas/Liquid, Liquid/Solid, Solid/Solid
� Problems:
� Calibrations, Analytical uncertainty,
Retrogression, Non-contemporaneous phases,
Scale of equilibrium, possible effects of salinity,
pH, ƒ(CO 2 ), …
Traditional Extraction Line
Schematic Mass Spec. Natural Ranges on Earth
3

Fractionations Among Sulfides Sulfide Thermometry
Sulfide/Fluid Speciation
Fractionations
Seawater
Sulfate vs
Time
4

Sediment Hosted Orogenic Au
Progressive Oxygen Isotope
Fractionation
Progressive Oxygen & Hydrogen
Isotope Fractionation D/O 18 - Water Reservoirs
5

D/O 18 - Geothermal Fluids Oxygen Isotope Thermometry
Fe-Zn-S system and the
sphalerite geobarometer!
� Iron content of sphalerite depends on aS 2 and
pressure!
� Phase equilibria studies have revealed that the iron content of
sphalerite equilibrated with pyrite and pyrrhotite, although
temperature independent between about 300° and 550°C, is
pressure dependent. !
� This relationship has been defined (Scott and Barnes, 1971;
Scott 1973) and thus allows the sphalerite composition in this
assemblage to serve as a geobarometer. The equation below
relates iron content (as FeS) to the pressure of equilibration
(Hutchison and Scott 1980): !
� P bar =42.30-32.10 log mole% FeS. !
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Sphalerite
geobarometer
Plot of the FeS content
of sphalerite coexisting
with pyrite and
hexagonal pyrrhotite at
0, 2.5, 5, 7.5, and 10
kbars at temperatures
from 300° to 700°C.
Sphalerite with “chalcopyrite disease”:
Exsolution? Diffusion? Replacement?!
FeS content of sphalerite in equilibrium with
pyrite and pyrrhotite as a function of P at 300°C!
Geothermometers
� Geothermometers are of two types
� 1. Sliding scale
� 2. Fixed point
� Sliding scale geothermometers are based on the temperature
dependence of the composition of a mineral or pair of minerals
when they are part of a specified assemblage.
� Arsenopyrite geothermometer:
� when equilibrated with pyrite, pyrrhotite, loellingite as shown in
next figure
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Fe-As-S system and the arsenopyrite
geothermometer!
� Barton and Skinner (1979) and Vaughan
and Craig (1978) have prepared extensive
lists of reaction points that serve as
potentially useful fixed point
geothermometers. Fig. 4 shows the
invariant points of S-Sb, S-Se, and S-Sn
systems that are of possible interest to the
geothermometry of an ore deposit.
� Fixed point geothermometers are minerals or
mineral assemblages that undergo a reaction
(e.g. melting, inversion, reaction to form a
different assemblage) at a defined temperature.
For example, crystals of stibnite must have
formed below its melting point (556°C) and the
mineral pair pyrite+arsenopyrite must have
formed below 491°C. The fixed points thus do not
sharply define the temperature of equilibrium but
rather set upper and lower limits.
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