THERMOCALC Course 2006 Chemical systems, phase diagrams, tips & tricks Richard White School of Earth...

THERMOCALC Course 2006

Chemical systems, phase diagrams, tips & tricks

Richard White

School of Earth Sciences

University of Melbourne

[email protected]

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.


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


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


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

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

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

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