Download Lecture 7: Chemography II A system AO2-BO

Lecture 7: Chemography II
Chemography: Graphical representation of mineral and bulk compositions to aid in
determining parageneses, mineral reactions and the impacts of protolith composition
C2O
min-3
P = 2 kbar
T = 400oC
We have 7 possible
minerals; 4 minerals of
fixed composition and
3 solid solutions
+ H2O
B A
min-7
min-6
min-4
C
min-5
D
AO2
min-1
BO
min-2
At this P + T, min-4 is
not stable and the solid
solutions have only a
limited compositional
range
So what are the mineral
parageneses for bulk
compositions A → D ?
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A system AO2-BO-C2O + H2O at 500oC
At 500oC, min-4 has become stable and the compositional ranges have changed:
C2O
min-3
P = 2 kbar
T = 400oC
+ H2O
P = 2 kbar
T = 500oC
B A
+ H2O
min-7
min-6
B A
min-4
C
min-7
min-5
min-6
D
AO2
min-1
C2O
min-3
min-4
C
BO
min-2
min-5
D
AO2
min-1
BO
min-2
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A system AO2-BO-C2O + H2O at 500oC
The appearance of min-4 means that there is no longer a unique solution to the tielines between the minerals:
P = 2 kbar
T = 500oC
Such an intersecting tieline points to a possible
reaction in the rock;
C2O
min-3
+ H2O
min-5 + min-7 → B A
min-4 + min-6
min-7
min-6
min-4
C
This is called a crossing
tie-line reaction
min-5
D
AO2
min-1
BO
min-2
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A system AO2-BO-C2O + H2O at 525oC
The appearance of min-4 means that there is no longer a unique solution to the tielines between the minerals:
P = 2 kbar
T = 525oC
Such an intersecting tieline points to a possible
reaction in the rock;
C2O
min-3
+ H2O
min-5 + min-7 → B A
min-4 + min-6
min-7
min-6
min-4
C
This is called a crossing
tie-line reaction
min-5
D
AO2
min-1
BO
min-2
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A system AO2-BO-C2O + H2O at 550oC
At even higher temperature, min-6 is no longer stable and breaks down
P = 2 kbar
T = 550oC
This type of reaction is
called a terminal stability
reaction;
C2O
min-3
+ H2O
min-6 → B A
min-3 + min-1 + min-5
min-7
min-6
min-4
C
min-5
D
AO2
min-1
BO
min-2
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Terminal and crossing tie-lines: reaction-isograds
Reactions in compositional space result in changes in mineralogy and these can be
recognized in the field. This allows these reactions to be mapped out and these
reaction lines in the field are called reaction-isograds
Because reactions are mostly related to a change in grade, this allows you to map out
metamorphic grade in the field (assuming that the bulk composition remains constant)
A+B=C+D
below the isograd
at the isograd
above the isograd
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Crossing tie-line reaction in AKF
Metamorphic rocks around Oslo contain Crd + Kfs, but at Orijärvi Ms + Bt + Qtz
2 Bt + 6 Ms + 15 Qtz = 3 Crd + 8 Kfs + 8 H2O
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Rules for choosing the projection variables
Metamorphic rocks commonly contain 9 major components and many more minor
components, so how do you choose what to project from ?
Philpotts specified 5 rules that you can use as a guide:
1. Components that occur as pure phases can be ignored (= projected from)
2. Components whose chemical potential is externally controlled can be ignored
(for example H2O and CO2)
3. Components occurring in just one phase can be ignored if we also ignore that
phase (for example ZrO2 if it is only present in zircon)
4. Components that are not sufficiently abundant to stabilize their own phase can
be ignored. This gets rid of most of the minor and trace elements
5. If a component occurs in two or more phases it can’t be ignored. However, if
one of these phases is always present; use it to project from
Let’s apply this to the pelites
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Meta-pelitic rocks: the KFMASH system
Meta-pelitic rocks are predominantly composed of: SiO2 - Al2O3 - MgO - FeO - K2O
and H2O. We call this the KFMASH compositional system.
To work with this system, need to reduce the number of components from 6 to 3
1. Project from H2O, because there’s always a fluid present
2. Project from SiO2, because will always have quartz which
buffers the activity of SiO2
3. Meta-pelitic rocks contain either muscovite or K-feldspar,
depending on grade: qtz + ms = Kfs + Al-sil + H2O → can
project from muscovite at low grade and from K-feldspar at
high grade
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KFMASH projections onto the AFM plane
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The AFM diagram changes with grade
As metamorphic grade changes
the mineralogy changes, as well
as the composition of solid
solutions:
minerals will disappear and new
minerals appear, and the field for
solid solutions will change shape
and size
y
At this grade, bulk composition
z
x:
sill + bt + st
y:
st + gt + bt
z:
bt only
different bulk compo = different
minerals at the same grade!
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Continuous reactions
Systematic variations in mineral composition as grade changes because of exchange
of Fe and Mg among the phases:
low grade
gt
A
A
medium grade
chl
gt
F
M
bt
chl
F
M
bt
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