THERMOCALC Course 2009: Day 3. Chemical systems, phase diagrams, tips & tricks. Richard White. Institute for Geosciences University of Mainz

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1 THERMOCALC Course 2009: Day 3 Chemical systems, phase diagrams, tips & tricks Richard White Institute for Geosciences University of Mainz

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 & Fe 3+ 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 An example What chemical system to use The effect of Fe 3+ on spinel stability. Can model spinel in KFMASH, but this doesn t consider Fe 3+ Could model in KFMASHO, but is this satisfactory?- NO Why? Must consider other minerals that take up Fe 3+, 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, H 2 O (in melt & crd) and Ti & Fe 3+ in biotite & spinel if appropriate. KFMASH doesn t do a bad job (backbone of the main equilibria) but will make modelling melt & oxides problematic and ignores plag. So to do it properly we need to model our rocks in NCKFMASHTO. Modelling 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 Schreinemakers 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 Building up to bigger systems: VII In pseudosections the issues regarding different systems are twofold The different thermo changing the overall stability of different assemblages How to deal with the bulk rock composition These two effects are intimately interconnected Phases are not just different in the extra components Ie if a particular equilibria shifts in P-T then each phase changes its overall composition (eg Xfe changes)

21 The changes between systems Considerations The expansion of one field is often matched by the constriction of another Ignoring components can have many unexpected results

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23 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

24 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, 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 H 2 O has to generally be guessed if not in excess. Fe 3+ may also require guess work, or measured another way

25 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, Fe 3+. Location and variance of equilibria, modifying our bulk rock so it is in KFMASH.

26 Bulk compositions IV Reducing compositional space: options Simply ignore given component Eg P 2 O 5, which may be mostly in apatite. But this leaves an excess of Ca which must go somewhere (affects the stability of other Ca-bearing phases) Effect may be small (eg in a metabasic rock) or surprisingly large (in an otherwise Ca-poor metapelite). Ignore Ti will change the bulk XFe if ilmenite present Fe 2+ versus Fe 3+ Difficult to sensibly adjust a bulk composition this way

27 Reducing compositional space: options Remove a phase component Bulk compositions IV Eg for apatite could remove both P 2 O 5 & CaO in the proportion they occur in apatite This is sensible as most of the P 2 O 5 is in apatite Ti is more complex especially in rocks with more than one Ti-oxide Ti also in biotite Can over or under adjust a bulk composition (eg by removing an ilm component) But there are more complex thermodynamic effects. Ti in bi changes its stability (reactions move up T) Thus the composition (eg. XFe) of the other phases also changes. By ignoring one component thermodynamically we can create problems The model system issue has both a bulk rock and thermodynamic component

28 Bulk rock example Each of these diagrams represents NCKFMASHTO bulk rock or a reduction from that system via simply ignoring components The differences are startling

29 Bulk compositions V 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

30 Bulk compositions VI 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

31 Bulk compositions VII 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

32 Example 1

33 Example 1

34 Example 1

35 E.g. 2

36 E.g. 2

37 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

38 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

39 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

40 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

41 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

42 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

43 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

44 There are two ways to approach this problem Getting started 1. 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 more reliable, and if possible should be used in preference to method 2 However method 2 is getting better

45 e.g.

46 Getting started One approach is to use a mix of methods 1 & 2 Whilst you construct your T-X section you can do some dogmin A reasonable double check that you have not missed the appearance of a phase The T-X diagram gives you some reasonable starting guesses Remember dogmin can miss the most stable eqm if the starting guesses are not good enough Starting with the wrong equilibria and then calculating a whole lot of metastable equilibria is really annoying If this happens go and have a drink and calm down If you do dogmin at a single P-T-x then thermocalc produces a list of assemblages from lowest G

47 Dogmin output <==================================================> phases : liq, bi, cd, g, ksp, pl, mt, q, H2O, (ilm) or [and] <== nothing found ==> <==================================================> phases : liq, bi, cd, g, ksp, pl, and, q, H2O, (ilm) or [mt] <== nothing found ==> <==================================================> phases : liq, bi, cd, g, ksp, mt, and, q, H2O, (ilm) or [pl] <== nothing found ==> <==================================================> phases : liq, bi, cd, g, pl, mt, and, q, H2O, (ilm) or [ksp] P(kbar) T( C) q(l) fsp(l) na(l) an(l) ol(l) x(l) h2o(l) x(bi) y(bi) f(bi) t(bi) Q(bi) x(cd) h(cd) x(g) z(g) f(g) ca(pl) k(pl) x(mt) y(mt) z(mt) x(ilm) Q(ilm) <==================================================> phases : liq, bi, cd, ksp, pl, mt, and, q, H2O, (ilm) or [g] P(kbar) T( C) q(l) fsp(l) na(l) an(l) ol(l) x(l) h2o(l) x(bi) y(bi) f(bi) t(bi) Q(bi) x(cd) h(cd) na(ksp) ca(ksp) ca(pl) k(pl) x(mt) y(mt) z(mt) x(ilm) Q(ilm) <==================================================>. Always check all of the output carefully especially the compositions of phases (look at starting guesses as well)-also use you intuition, does the result look sensible?. Gibbs energy minimisation info P(kbar) T( C) liq bi cd g ksp pl mt ilm and q H2O n G del (Use these results at your own risk!) liq bi cd g ksp pl mt ilm and q H2O

48 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

49 Drawing up your diagram A common issue for many is trying to guess what phases may appear THERMOCALC is a program for petrologists Ie you are expected to have a reasonable handle on what minerals commonly coexist in rocks What fundamental changes occur in rocks as P, T and X change What basic equilibria are stable The advantage of drawing a T-X and or P-X diagram to start is that some of these changes will be sorted out then Though it is possible to miss a new phase. This is quite problematic in mafic rocks where lots of higher variance fields can exist. Sometimes you just have to use trial & error seeing which of several possible minerals appears first Again, starting guesses are important

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51 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

52 Curved boundaries: I

53 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.

54 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. X Fe 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 save the old guesses in the storage area. This way you will always have all the starting guesses needed to calculate the whole diagram

55 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 script 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

56 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.

57 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 range 2. Your P-T range is too broad 3. Your line is very steep/flat or is curved 4. 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

58 liq 8 q(l) fsp(l) na(l) an(l) ol(l) x(l) h2o(l) P(kbar) T( C) q(l) fsp(l) na(l) an(l) ol(l) x(l) h2o(l) mode liq ksp pl cd g ilm sill q

59 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

60 A univariant example in KFMASHTO P(kbar) T( C) x(he) y(he) z(he) x(mt) y(mt) z(mt) sp + 167opx + 29liq + 10ilm + 220q = 56mt + 35cd + 129g + 11ksp mt = sp 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

61 solvi P(kbar) T( C) x(act) y(act) z(act) a(act) c(act) f(act) Q1(act) Q2(act) # x(hb) y(hb) z(hb) a(hb) c(hb) f(hb) Q1(hb) Q2(hb) x(di) j(di) f(di) Q(di) Qaf(di) Qfm(di) x(chl) y(chl) Q(chl) x(g) z(g) f(g) mode act hb di chl g ru Need to look at the output if there is the possibility of solvi Often get significant changes in the modes in going from good results to bad ones Be careful that you don t use dodgy results as starting guesses

62 Apparent solvi P(kbar) T( C) x(act) y(act) z(act) a(act) c(act) f(act) Q1(act) Q2(act) # x(hb) y(hb) z(hb) a(hb) c(hb) f(hb) Q1(hb) Q2(hb) x(di) j(di) f(di) Q(di) Qaf(di) Qfm(di) x(chl) y(chl) Q(chl) x(g) z(g) f(g) mode act hb di chl g ru Hard to tell if this is a real solvus or just some small bump on the G-x surface

63 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 roger with the tcd and log files

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65 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

66 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

67 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

68 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 ( %) 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

69 Other problems In this metapelite, the presence or absence of minor plag is not constrained Similar problems can occur with any mineral

70 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

71 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 there are analytical uncertainties in measuring bulks

72 Crashes!!! These still occasionally occur Other problems 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 input files, 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

73 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 a-x files

74 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

75 Other problems

76 If your calculation fails I recommend the following steps 1. Look at the output. The numbers given may lead you to a solution. 2. Look at the assemblage, zero mode phase(s), P-T window and P/T increments you entered. Do they look right? 3. If they look ok run the equilibria again to see if it will work the second time 4. Change your P-T window and/or P/T increments (especially if you think the line is short) and try again. 5. If you are calculating a line and it could be short, try calculating the points that may terminatre it. 6. If you think the reaction could be very flat in P-T try calculating P at T rather than T at P 7. Change starting guesses and try again 8. If still not working try steps 1-5 again with the new starting guesses 9. If that still is not working, try calculating the equilibria with no modes set to zero to see if you can find the equilibria at all (this is particularly useful if you have got the zero mode phase wrong). Try calculating different possible boundaries (ie different zero-mode phases) for this field. 10. A final trick is to set a composition isopleth or a series of compositions as well as a zero mode for a phase and calculate a series of points. 11. If all else fails, leave that section of the diagram and calculate the equilibria all around it, this may give you an idea of the size & shape of the field you are having trouble with as well as supplying different possible starting guesses. Other times an equilibrium may be hard to calculate includes where a number of phases have near zero modes.

77 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

78 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

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80 T-X & P-X pseudosections What diagrams to draw: III 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. X Fe or complex e.g. X melt-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

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82 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

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85 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

86 AFM + qtz +ksp +liq sp sill cd g F opx bi M

87 More complex diagrams What diagrams to draw: V 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

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92 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 %

93 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

94 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

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96 E. g. 1

97 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!

98 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

99 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

100 E. g. 1

101 E. g. 1

102 E. g. 1

103 E. g. 1

104 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

105 E.g. 2