thermocalc course 2006 chemical systems, phase diagrams, tips & tricks richard white school of...
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THERMOCALC Course 2006
Chemical systems, phase diagrams, tips & tricks
Richard White
School of Earth Sciences
University of Melbourne
rwwhite@unimelb.edu.au
Outline 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 doForward 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 +2V, 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 There are two ways to approach this problem1. Calculate part of a T-X or P-X diagram from a
known bulk to your unknown bulk Work your way across the diagram, find an equilibria
that occurs in your new bulk and build up your P-T pseudosection from there
2. Use the ‘dogmin’ code in THERMOCALC to try and find the most stable assemblage at P-T 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 small fields have to be drawn bigger than they really
are
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 You are prompted ateach calculation
calctatp yes You input P to get Tcalctatp no 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. Your line is outside your specified P-T range2. Your P-T range is too broad3. Your line is very steep/flat or is curved4. 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) 0.1825 fsp(L) 0.2236 na(L) 0.5086 an(L) 0.003065 ol(L) 0.001511 x(L) 0.9256 h2o(L) 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 cd g ilm sill q 0.2253 0.1498 0.08311 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
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
A univariant example in KFMASHTO
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 equilibriaif 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-ksp-
ilm-sill field
A very minor adjustment to the bulk rock composition gets rid of sill Remember there are analytical uncertainties in measuring
bulks
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 P-T 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
opx
g
cd
bi
sillAFM + qtz+ksp+liq
F M
sp
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 scriptssetiso yes or setiso x(bi), for composition
orsetmodeiso yes zeromodeiso nosetmodeiso 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
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