THERMOCALC Course 2006

1. THERMOCALC Course 2006 Chemical systems, phase diagrams, tips & tricks Richard White School of Earth Sciences University of Melbourne rwwhite@unimelb.edu.au

2. 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

3. 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

4. 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

5. 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

6. 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.

7. 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

8. 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

9. 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!

10. 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

11. Some examples: I

12. Some examples: II

13. 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.

14. Building up to bigger system: II

15. Building up to bigger systems: III

16. Building up to bigger systems: IV

17. Building up to bigger systems: V

18. Building up to bigger systems: VI

19. 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

20. 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

21. 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

22. 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.

23. 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

24. 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

25. 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

26. Example 1

27. Example 1

28. Example 1

29. E.g. 2

30. E.g. 2

31. 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

32. 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”

33. 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

34. 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

35. 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

36. 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

37. 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

38. Getting started • 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 • Work your way across the diagram, find an equilibria that occurs in your new bulk and build up your P-T pseudosection from there • 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

39. e.g.

40. 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

42. 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 at each calculation calctatp yes You input P to get T calctatp no You input P to get T

43. Curved boundaries: I

44. 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.

45. 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

46. 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

47. 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.

48. THERMOCALC won’t calculate all or part of a given equilibria • Four problems can cause this: • 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

49. 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

50. 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