Download Geochemistry 303

THERMOCALC Course 2006
Chemical systems, phase diagrams,
tips & tricks
Richard White
School of Earth Sciences
University of Melbourne
[email protected]
Outline
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What chemical system to use
differences between systems
Choosing a bulk rock composition
Getting started
The shape of lines & fields
Starting guesses
Problem solving
Using diagrams to interpret rocks
What diagrams to draw
What chemical system to use

Before you embark on calculating diagrams, you
need to work out what chemical system to use.
 It
must be able to allow you to achieve your aims
 Must be as close an approximation to nature as possible

Using a single system throughout a study provides a
level of consistency
 If
you are modelling both pelites and greywackes you could
use KFMASHTO for pelites and NCKFMASH for
greywackes BUT NCKFMASHTO for both is better
 With very different rock types (eg mafic & pelite) you may
have to use different systems
What chemical system to use

The system you choose also depends on what you
are trying to do
 Forward modelling theoretical scenarios and
processes in general

Simpler systems may be used to illustrate these more
clearly
 Inverse

modelling of rocks for P-T info
Larger systems should be used to get equilibria in the
right place
What chemical system to use

The rocks & minerals tell you what system you need to use





What elements are present in your minerals
Eg Grt in metapelite at Greenschist has Mn
 MnKFMASH better than KFMASH
Grt at high -P in metapelite may have significant Ca
 NCKFMASH better than KFMASH
Spinel bearing rocks-need to consider Ti & Fe3+
 KFMASHTO better than KFMASH
Getting this right at the beginning saves later problems


It may be tempting to try and use simple systems (less calculations)
If in doubt, the larger system is safer
What chemical system to use

When adding components, we need to consider
what minerals these components will go in
 THERMOCALC
has to be able to write reactions
between endmembers.
 Must have this component in more than 1
endmember and in reality as many as we can
 May involve us adding new phases to the modelling
that may or may not actually be in our rock. *
mineral stability is relative to other minerals.
 THERMOCALC is simply a tool. It can only give us
information within the parameters we decide.
What chemical system to use


An example
The effect of Fe3+ on spinel stability.
 Can
model spinel in KFMASH, but this doesn’t consider
Fe3+
 Could model in KFMASHO, but is this satisfactory?- NO
 Why? Must consider other minerals that take up Fe3+, eg
the oxides.
 When modelling the oxides, we should also consider Ti
(e.g. ilmenite, magnetite, haematite)
 So a better system is KFMASHTO
What chemical system to use

Why is the right system so important
 If
we are trying to model rocks, our model system must
approach that of the rock as closely as possible.
 Minor components can have a big influence on some
minerals & hence some equilibria.
 Minor minerals in a rock will change the reactions and
their positions on a petrogenetic grid
 Ignoring a component can artificially alter the bulk comp

Eg: a High-T granulite metapelite
 FMAS
will show relationships between many minerals but
they won’t be in the right P-T space or possibly the right
topology.
 The rock will not see any of the FMAS univariant equilibria
What chemical system to use

Eg: a High-T granulite metapelite cont.
 These
rocks will contain melt at peak, substantial K, some
Ca, Na, H2O (in melt & crd) and Ti & Fe3+ in biotite &
spinel if appropriate.
 KFMASH doesn’t do a bad job (backbone of the main
equilibria) but will make modeling melt & oxides
problematic and ignores plag.

So to do it properly we need to model our rocks in
NCKFMASHTO.
 Modeling
in these larger systems does have major
benefits for getting appropriate model bulk rock
compositions from real rocks

Thus size is important!
differences between systems

Will concentrate on going from smaller to larger systems.




New phases to add
New endmembers to existing phases
Start with petrogenetic grids & in particular invariant
points.
Need to consider the phase rule

Relationships are different for adding different numbers of phases
components
V = C - P +2
V, Variance; C, Number of components; P, Number of
phases
 And Schreinermakers rules
Some examples: I
Some examples: II
Building up to bigger systems: I




Building up from KFMASH for example to
KFMASHTO, NCKFMASH or NCKFMASHTO
requires several intermediate steps.
The grid can only be built up one component at a
time
Each of the new sub-system topologies has to be
determined
To go from KFMASH to KFMASHTO we have to
make the datafiles and calculate the grids for the
sub-systems KFMASHO & KFMASHT before we
make the KFMASHTO datafiles and grid.
Building up to bigger system: II
Building up to bigger systems: III
Building up to bigger systems: IV
Building up to bigger systems: V
Building up to bigger systems: VI
Building up to bigger systems: VII

On P-T grids we can get either more or less
invariants.
 KFMASH
to KFMASHTO = More
 KFMASH to NCKFMASH = Less


Overall more possibilities for more fields in
pseudosections
The controlling subsystem reactions are still
present but:
 may
involve additional phases, or
 be present as higher variance relations
 Will shift in P-T space
Bulk compositions


Pseudosections require that a bulk rock
composition in the model system is chosen.
For diagrams that are directly related to specific
rocks this bulk rock info should be derived from
the rocks themselves
 But
must reduce the measured bulk to the model
system-must be done with care

Thus, choosing a bulk rock composition will
depend on your interpretation of a ‘volume of
equilibrium’
 May
be different for different rocks
 May vary over the metamorphic history
Bulk compositions

Ways of estimating bulk rock composition
 XRF-
good if you have large volumes of equilibration.
 Quantitative X-Ray maps-good for analysing smaller
compositional domains. Clarke et al., 2001, JMG, 19, 635-644
 Modes and compositions-Less reliable,but can work on
simple rocks.

Wt% bulks have to be converted to mole % to use in
THERMOCALC
 Mol%


= wt% / mw
The amount of H2O has to generally be guessed if not
in excess.
Fe3+ may also require guess work, or measured
another way
Bulk compositions III

The bulk rocks we use in THERMOCALC are
approximations of the real composition as many
minor elements are ignored
 The
further our model system is from our real system
the harder it is to accurately reproduce the mineral
development of rocks.
 Eg. Using KFMASH to model a specific metapelite
raises problems with ignoring Na, Ca, Ti, Fe3+.
 Location and variance of equilibria, modifying our bulk
rock so it is in KFMASH.
Bulk compositions IV


Scales of equilibration we are trying to model.
Commonly we interpret the scale of equilibration
to be smaller than a typical XRF sample size
 Our
prograde and peak scale of equilibration may have
been large but if we are trying to model retrograde
processes this scale may be small
 Our rocks may contain distinct compositional domains,
driven by a slow diffuser eg. Al
 High-Mn garnet cores may be chemically isolated from
the rest of the rock

We need to adjust our bulk composition to
accommodate these features
Bulk compositions V


How do we adjust our bulk
Use a smaller scale method for estimating bulk
such as X-ray maps
 Useful
only on quite small scales
 Can directly relate measured compositions to textures
and hence effective bulk compositions

Modify the bulk composition using the modes &
compositions given by THERMOCALC
 Can
model progressive partitioning by doing this in
steps
 Cheap & simple, but still need to do the petrography &
mineral analyses to establish the nature of the element
distribution
Bulk compositions VI


Two examples involving removing the cores of
garnets from our bulk rock
E.g, 1. Using X-ray maps to remove garnet cores
in prograde-zoned garnets.
 Based
on a paper by Marmo et al 2002, JMG
 In this paper different amounts of core garnet are
removed to model the prograde mineral assemblage
development in the matrix.

E.g. 2. Using THERMOCALC to remove the cores
of large garnets so that the retrograde evolution
of a rock can be assessed.
 Will
show how this is done
Example 1
Example 1
Example 1
E.g. 2
E.g. 2
E.g. 2: removing the garnet cores

Calculate the ‘full bulk’ equilibria at the desired P
& T.
 There

is a new facility to change min props called rbi
We can use rbi to set our bulk comp via info on
the modes & compositions of minerals
 rbi
info can be output in the log file
Adjusting bulk from calculated modes

Bulks can be set/adjusted using the mineral modes(mole prop.) and the
mineral compositions

Uses the rbi code (rbi =read bulk info)
 You can make thermocalc output the rbi info into the log file using the
command “printbulkinfo yes”
Adjusting bulk from calculated modes

The bulk rock can be read from rbi code in the”tcd” file instead of the usual
mole oxide %’s
Bulk compositions

We can use the method shown in e.g. 2 for any
phase or groups of phases
 This
is how we make melt depleted compositions for
example.
 We can divide a bulk rock into model compositional
domains

Again, what we do here is determined by our
petrography & interpretation of what processes may
go on
Getting started

In most of the pracs you will be largely finishing
partly completed diagrams
 In

reality, you will need to start from scratch
Knowing where to start is not always straight-forward
 It
is easy to accidentally calculate a metastable higher
variance assemblage rather than the stable lower variance
one
 Some rocks are dominated by high variance assemblages
in big systems (eg greywackes, metabasics)
 If your system has lots of univariant lines you can look at
them
Getting started

In large systems, there are few if any univariant
reactions that will be seen
 Need

to look for higher variance equilibria
There are some smaller system equilibria that form
the backbone for larger systems
 The
classic KFMASH univariant equilibria occur as narrow
fields in bigger systems in pelites
 NCFMASH univariant equilibria in metabasics may still be
there in some form in bigger systems
Getting started

In most cases the broad topology of a pseudosection will
be well enough understood that you will know what some
of the equilibria will be.




Follow logic: most metapelites see the reaction bi + sill = g + cd in
some form
Look at diagrams in the same system and with similar bulks to
your samples
Sometimes you may be trying to calculate a diagram in an
unusual bulk or one that hasn’t be calculated by anyone
Diagrams that are dominated by high variance equilibria
may be hard to start.

What is the right equilibria to look for
Getting started

1.
There are two ways to approach this problem
Calculate part of a T-X or P-X diagram from a
known bulk to your unknown bulk

2.
Use the ‘dogmin’ code in THERMOCALC to try
and find the most stable assemblage at P-T



Work your way across the diagram, find an equilibria
that occurs in your new bulk and build up your P-T
pseudosection from there
This is a Gibbs energy minimisation method
May not be able to calculate the most stable
assemblage and your answer could be a red herring.
Method 1 is far more reliable, and if possible
should be used in preference to method 2
e.g.
Drawing up your diagram

It is always wise to sketch the diagram as you go
 No
need to make this sketch an in-proportion and precise
rendering of the phase diagram-that’s what drawpd is for

The sketch is there to help you draw the diagram
and for labelling
 Very
are
small fields have to be drawn bigger than they really
Shapes of fields & lines



Most assemblage field boundaries on a
pseudosection are close to linear
Strongly curved boundaries do occur and can be
difficult to calculate in one run
Very steep & very shallow boundaries & reactions
can also present problems
 For
shallow boundaries calculate P at a given T
calctatp ask
calctatp yes
calctatp no
You are prompted at
each calculation
You input P to get T
You input P to get T
Curved boundaries: I
Curved boundaries: II

In T-X & P-X sections, X is always a variable so
near vertical lines require very small X-steps to
find them.
 Curved
lines with two ‘X’ solutions have to be done
over small T or P ranges

Overall changing the P, T or X range will help as
will changing the variable being calculated
 Changing
from calc T at P to calc P at T.
Starting Guesses


THERMOCALC uses the starting guesses in the “tcd” file
as a point from which to begin the calculation.
These starting guesses have to:




Be reasonably close to the actual calculated results
Have common exchange variables in the right order for the
minerals eg. XFe g>bi>cd
This may mean having to change the starting guesses to
calculate different parts of the diagram
When changing starting guesses, it is best to create a new
“tcd” file and change the guesses in that so your original
file remains unchanged.

This way you will always have all the files needed to calculate the
whole diagram
Changing starting guesses

A good way to ensure starting
guesses are appropriate is to use
output comps as starting guesses.



These can be written to the log file in the
form shown on the left
To do this the following script
“printguessform yes”
goes into the tcd file
There are a few tricks to remember
when doing this, especially with
phases with the same coding
separated by a solvus

Have to ensure the starting guess is on
the right side of the solvus
Common problems with starting
guesses


THERMOCALC won’t calculate all or part of a given
equilibria
THERMOCALC gives the same composition for two
‘similar’ minerals that should be separated by a solvus



Eg. Ilm-hem, mt-sp, pl-ksp
THERMOCALC sometimes gives a different answer to
one calculated earlier with different starting guesses or
even with the same starting guesses
THERMOCALC gives a bomb message regarding chl
starting guesses.
THERMOCALC won’t calculate all or
part of a given equilibria

Four problems can cause this:
1.
2.
3.
4.


Your line is outside your specified P-T range
Your P-T range is too broad
Your line is very steep/flat or is curved
Your starting guesses are too far from a solution
The solution to problem 4 is to use the compositions
from the log file on the part of the equilibria you can
calculate or from a nearby equilibria you can calculate.
If it’s the first line on a diagram, have a guess from
another “tcd” file in the same system or use your rock
info


You can also calc part of a T/P-x section from a known bulk that
works with your starting guesses
Adjust you starting guesses as you work across the diagram
liq 8
q(L)
fsp(L)
na(L)
an(L)
ol(L)
x(L)
h2o(L)
0.1825
0.2236
0.5086
0.003065
0.001511
0.9256
0.6519
-------------------------------------------------------------------P(kbar) T(°C) q(L) fsp(L) na(L) an(L)
ol(L)
x(L) h2o(L)
6.82 820.0 0.1837 0.3422 0.3649 0.01560 0.004747 0.6510 0.4315
mode
liq
ksp
pl
0.2253 0.1498 0.08311
cd
g
ilm
sill
q
0 0.1392 0.01302 0.05505 0.3345
THERMOCALC gives the same composition for two
‘similar’ minerals that should be separated by a
solvus




Restricted to minerals that have identical coding
but rely on distinct starting guesses to get each of
the 2 solutions.
Particularly problematic close to the solvus top
Caused by the starting guesses generally being
too similar or both too close to only one of the
solutions
Solution: Change starting guesses so they are
less similar and on opposite sides of the solvus
A univariant example in KFMASHTO
P(kbar) T(°C) x(he) y(he) z(he)
x(mt) y(mt) z(mt)
2.60
877.9 0.9464 0.8203 0.06514 0.9750 0.1730 0.4069
209sp + 167opx + 29liq + 10ilm + 220q = 56mt + 35cd + 129g + 11ksp
2.70
544.0 0.9913 0.03152 0.1763 0.9913 0.03152 0.1763
mt = sp
2.80
548.1 0.9908 0.03197 0.1751 0.9908 0.03197 0.1751
mt = sp
In the last two results both spinel and magnetite
have a magnetite composition
In pseudosections this feature can cause the calculation to fail or
may give perfectly sensible looking P-T conditions for an equilibria
if it is near the solvus top, but with the wrong composition
THERMOCALC sometimes gives a different answer
to one calculated earlier with different starting
guesses or even with the same starting guesses

Different starting guesses may give different P-T answers




Especially when you have some very complex phases where the
G-x surface is ‘bumpy’ (gets stuck in a hole)
Also a problem when you have a mineral that may have a solvus
(composition flicks from one side of the solvus to the other)
Solution: Go back to well behaved equilibria that lead to
your trouble area. Follow the compositions carefully (tco)
the change in P-T should be accompanied with a sudden
change in some of the mineral compositions. Change
starting guesses to close to the right answers, with
allowances for solvii.
If problem persists email roger with the tcd and log files
THERMOCALC gives a bomb message
regarding chl starting guesses.

This is a minor, specific problem that commonly pops
up with highly ordered phases chl and some of the
carbonates

THERMOCALC can’t handle exact solutions (ie.
output results from a log file) as starting guesses in
chlorite.
Simply nudge the numbers slightly and it should
work

Other common problems

There are a range of things that can go wrong with
calculating mineral equilibria and drawing phase
diagrams
 These
have an equally broad range of sources ranging
from user errors to bugs in the code

Remember there are uncertainties in every
calculation
 The
standard deviation on each calculation can be provide
by thermocalc using “calcsdnle yes” in the tcd file
 These are 1 errors given so they should be doubled to
give 2 uncertainties- based on uncertainty of enthalpy
only
Other problems

Here I calculated T at P so we only have an uncertainty on T


2 uncertainty is ± 18°
Notice we also have uncertainties on mineral composition and mineral
modes

Can be considered when contouring diagrams
Other problems

Thermocalc does not reproduce my assemblages
 How
different are they (one phase extra or missing)
 Is it a minor or major phase (look at the rocks)
 Eg in modelling some metagranites I found that thermocalc
calculated a small amount of sillimanite (0.2-0.6%) that
wasn’t in the rock, same problem with plag in some pelites
 This is not the end of the world but the diagram looks a
bit wrong
 Look at the uncertainties on the modes, are they bigger
than the mode itself
Other problems


In this metapelite, the presence or absence of minor plag is
not constrained
Similar problems can occur with any mineral
Other problems

What causes a discrepancy between observed and
modelled assemblages
 The
modelling is not in the right system
 There is a component and phase we can’t model that is in
the rock
 Our method for estimating bulk has problems (look at
analytical uncertainties)
 The thermo and or a-x relationships are incorrect
 The eqm assemblage in the rocks has been misidentified

Always go back and look at the rocks again, have a
good look for that mineral, there may only be a few
grains of it
Other problems

In the case of minor sill in a metagranite, I found that
the measured biotite was a little more aluminous
than the calculated biotite
 A rock
made up of bi-pl-ksp-q-ilm plotted in the bi-pl-kspilm-sill field

A very minor adjustment to the bulk rock composition
gets rid of sill
 Remember
bulks
there are analytical uncertainties in measuring
Other problems

Crashes!!!
 These
still occasionally occur
 Look at the error output, is the cause obvious from this and
can you fix it
 If not, contact Roger, with an explanation of what
happened, your tcd file, the log file, and information of what
version of thermocalc you were using and on what platform

Thermocalc can’t find a solution
 Just
returns a series of numbers
 Commonly this is a starting guess issue, or choice of PT window
Other problems

I get a solution but it is in the wrong P-T area
 Generally
this reflects 2 solutions, one is metastable
 Common on curved equilibira
 Can generally be avoided by either changing the P-T
window or by changing from calc T at P to calc P at T or
vice versa

Can also occur if you have accidentally changed
some of the a-x relations
 Always
keep spare original tcd files
Other problems

You just can’t calculate the equilibria you know is
there, or can’t calculate all of it
 Barring
starting guess or slope of line problems, sometime
thermocalc just may struggle with a particular calc

Look at the part you can calculate
 There
is info in the output that can help
 Try changing the P-T window and P/T increments

Can sometimes set a mode or composition
parameter
Other problems
What diagrams to draw: I


It is not always obvious what diagrams to draw to
show a particular feature of our rocks or to
highlight a given process
Our basic pseudosections are:
 P-T
pseudosections
 T-X/P-X pseudosections
 Compatibility diagrams

More complex diagrams include
 X-X
pseudosections (constructed by hand)
 M-X pseudosections (constructed by hand)
 T-V pseudosections
 T-a & P-a pseudosections
What diagrams to draw: II

P-T pseudosections
 A series
of these diagrams can show the textural
development in different rocks/domains
 The compositions of the different bulks can be shown
on a compatibility diagram e.g. AFM
 Open system processes and mineral fractionation can
be shown on a series of P-T pseudosections

P-T pseudosections are the mainstay diagram for
analysing rocks
 But
some other diagrams can show much
What diagrams to draw: III

T-X & P-X pseudosections






A series of these diagrams can show the effects of a progressive
process e.g melt loss
The compositions of the different bulks can be shown on a
compatibility diagram e.g. AFM
The X-axis can be simple e.g. XFe or complex e.g. Xmelt-loss,
between two bulk rock compositions
Open system processes and mineralogical fractionation can be
shown on a T-X or P-X pseudosections
If the P-T path can be simplified to vertical and horizontal
segments then the P-T path can be shown for a range of rocks on
a single diagram
T-X & P-X pseudosections are a very flexible and
adaptable diagram
Lack of
retrogression



Lets look at how much
melt must be lost from
granulites to allow the
preservation of
dominantly anhydrous
assemblages
For most rocks >70%
of the melt produced
has to be lost
Look at simple 1melt
loss event scenario
What diagrams to draw: IV

Compatibility diagrams
 The
compositions of the different bulks can be shown
on a compatibility diagram e.g. AFM
 Use is limited by having enough phases to project from
 A series of diagrams can illustrate the assemblage
development on a wide range of rocks
 The diagrams can use complex axes

Good summary diagrams
AFM
+ qtz
+ksp
+liq
sill
cd
sp
g
bi
F
opx
M
What diagrams to draw: V



More complex diagrams
These diagrams are relatively uncommon and
many are constructed by hand using
THERMOCALC output
Some of these, e.g. X-X pseudosections, will
become more common when their construction is
automated in THERMOCALC
Contours

Phase diagrams can contoured for mineral modes
and mineral compositions
 These
are very useful for illustrating more information
about changes that occur in rocks
 Remember there are uncertainties on these calculations,
so avoid taking the numbers too literally
 Mode contours are mole% or mole proportion-Not
Volume%
 The mineral modes are calculated on a one oxide total
basis to normalise the effects of molecular oxide sums
 this normalisation serves to make them approximate to
volume %
Contours

Composition contours use the composition variables
in the a-x relationships
 To
compare with analysed minerals you may have to
rework your analysis into thermocalc style
 Some are proportions eg XFe (opx) some are site fractions
eg yAl (opx)

The “number of oxygens” in some endmembers may
differ from that commonly reported in analyses tables
 Eg
micas in thermocalc are calculated on 11ox, analyses
commonly given as 22ox- this affects mole fraction
numbers
Contours


Contouring can be enabled using the scripts
setiso yes or setiso x(bi), for composition
or
setmodeiso yes
zeromodeiso no
setmodeiso bi
zeromodeiso no
You will then be prompted for some values either as
a list of numbers or start end interval
E. g. 1
Using diagrams to interpret rocks: I

We can use phase diagrams to interpret rocks in
many ways
 Constraining
P-T conditions, P-T paths
 Interpreting reaction textures
 Modeling open & closed system processes
 Fluid/ melt generation

But just because you can explain your rocks
using a particular diagram doesn’t mean that
explanation is the right one.
 We
can explain many reaction textures in metapelites
using only a P-T grid, but this does not mean a rock
actually experienced any of the univariant equilibria!
Using diagrams to interpret rocks: II

The best way to avoid a specious interpretation of
your rocks is to use as much rock-based
information as possible
 Pseudosections
based on real compositions
 Contouring diagrams for modal proportions
 Using a realistic chemical system
 Detailed petrography

There are a number of useful ways to more
closely model rocks
Interpreting rocks: e.g. 1




Interpretation of some reaction textures in some
Fe-rich metapelites.
The rocks developed distinct compositional
domains
Each domain preserves a slightly different
metamorphic history
We can use the information from different
domains to better constrain our history
E. g. 1
E. g. 1
E. g. 1
E. g. 1
E.g 2

Take an
anticlockwise P-T
path



Convert to linear
segments
Can see effects on a
range of bulk rock
comps
Allows us to infer
more of the P-T path
and reconfirm a path
derived from one
bulk with evidence
from another
E.g. 2