Metamorphic Petrology GLY 262 P-T and T-X phase diagrams

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1 Metamorphic Petrology GLY 262 P-T and T-X phase diagrams

2 How do we estimate P-T conditions? Inverse modelling: (1) Look at our rock, identify the mineral assemblage and determine the compositions of the minerals (2) Use of conventional thermobarometry to determine P-T conditions e.g. On grt + bt Fe-Mg exchange thermometry to determine T PROBLEM(S): a) The thermodynamic data is taken from directly calibrated experiments which dealt with ideal or pure end-member minerals. Grt + Bt in real rocks are not pure end-members. Experiments have ignored minor components e.g. Fe 3+ in Grt or Ti in Bt. b) Thermodynamic data derived in experiments at a particular P-T are extrapolated to other P-T-x conditions.

3 How do we estimate P-T conditions? For a given model chemical system (e.g. KFMASH) grids will show ALL the possible phase relations / reactions / stabilities for a given set of minerals. There are over 150 possible reactions between just 8 KFMASH pure end-member minerals alone in P-T range (0.5-4 kbars o C). Spear, 1999

4 How do we estimate P-T conditions? Petrogenetic grids are not limited by compositional constraints such as only being able to represent a 4-component system HOWEVER Petrogenetic grids are constructed using the end-member minerals e.g. Fe-grt + Fe-chl + mu ----> Fe-staur + Fe-bt + qtz + H2O and Mg-grt + Mg-chl + mu ----> Mg-staur + Mg-bt + qtz + H2O What about intermediate compositions?

5 Consider the following mineral assemblage developed in a pelitic rock (MM184) from the Etive thermal aureole, Scotland: and + cd + ksp + bi. How can conventional thermobarometry determine the P-T conditions of equilibration for this assemblage?

6 There are no conventional thermometers or barometers e.g. Gt-Bt Fe-Mg exchange that we could use. What about petrogenetic grids?

MM184 At 1 kbar, under water-saturated conditions, temperature estimates range from 460-790 o C.

7 The use of petrogenetic grid or experimentally located equilibria will enable one to demonstrate that the sample equilibrated somewhere within the andalusite field and uptemperature of the muscovite + quartz breakdown reaction. MM184 At 1 kbar, under water-saturated conditions, temperature estimates range from o C. This approach yields little information regarding P-T conditions nor does it take into account the presence of cordierite and biotite in the assemblage.

8 MOST ROCKS CONTAIN HIGH VARIANCE ASSEMBLAGES LIKE THIS ONE AND PETROGENETIC GRIDS AND TRADITIONAL GEOTHERMOBAROMETRY CANNOT BE APPLIED So what now?

9 What is a pseudosection? Pseudosections a phase diagram that shows the stable mineral assemblage for a given bulk-rock composition. Bulkcompositions are generally determined using XRF analysis. Pseudosections can be constructed in P-T space or other variables can be used on the axes e.g. T-X, P-X diagrams where X represents a compositional variable such as XFe or XCa.

10 XFe (Chl) = 0.55 XFe (Mu) XFe (Chl) = % Chl

460-790 o C Pseudosection: At 1 kbar, under water-saturated conditions, the

11 Compare the results for sample MM184: cd + ksp + bi + and P-T grid: At 1 kbar, under water-saturated conditions, temperature estimates range from o C Pseudosection: At 1 kbar, under water-saturated conditions, the assemblage is stable from o C a drastic improvement on the conventional estimate!

12 What is THERMOCALC? THERMOCALC is a programme developed and maintained by Tim Holland (Cambridge) and Roger Powell (Melbourne) (Powell & Holland, 1988, Journal of Metamorphic Geology, 6, ). The word THERMOCALC is a contraction of thermodynamic calculator. The programme consists of two parts: (1) the application itself and (2) the thermodynamic dataset.

13 How does it work? THERMOCALC utilises the available thermodynamic data (i.e. ΔH, ΔS, f etc) for reactions (mineral equilibria) which are commonly taken from experimental studies. THERMOCALC uses this data to calculate the P-T conditions under which a mineral assemblage formed and subsequently evolved. It assumes measured values (from experimental work) of S, V, f and other thermodynamic variables such as thermal expansion and compression terms for minerals are essentially correct. It optimises the P-T sensitive parameter of enthalpy. The enthalpies of formation (which are extracted from experimental work) for all minerals were juggled simultaneously using a supercomputer to get a best fit between calculated reaction/equilibrium curves and the experimental brackets (or margins of error) for those curves. This resulted in a self-consistent data-set i.e. the numbers are consistent with themselves and each other.

14 (a) Show P-T stability fields for a given mineral assemblage. Identify the correct equilibrium assemblage and you have a P-T fix!

15 (b) Can contour fields for mol proportion of each phase present (i.e. how much) which in turn can be used to deduce reactions: e.g. modes of qtz, plag, bt and sill decrease up-temperature as melt + g increase consistent with qtz + sillimanite + plagioclase + biotite = melt + garnet

16 (c) Can contour fields for any compositional parameter for any phase Compare measured values of mineral composition to modelled isopleths

17 (d) Can compare contoured compositional isopleths to measured zoning profiles

18 (e) Can calculate P-X or T-X diagrams

19 The assemblage actually contains q + ilm + pl + sill + g + bi + mu + ksp This does not appear on the diagram. Why?

20 Petrography is still crucial Some minerals are metastable, prograde relicts and some are retrograde. Thus thin-section work is still crucial in identifying the equilibrium assemblage at various times during the metamorphic history.

21 How can we do this?

22 Thin-section petrography determines that the mineral assemblage includes bi, g, sill, pl, q Where does it plot?

23 However on closer inspection you find garnet and K-feldspar is partially replaced by muscovite This texture implies garnet is not stable! But muscovite is!

24 Where does it plot?

25 Leucosome are crystallized melt. When melt crystallizes it expels free H 2 O leading to retrogression of adjacent minerals. In this case K-feldspar to muscovite

26 Where does it plot?

27 Further petrographic analysis reveals inclusions of kyanite within garnet INCLUSIONS WITH OTHER MINERALS (USUALLY INSIDE GARNET) PROVIDE INFORMATION ON THE PROGRADE P-T EVOLUTION

28 Where does it plot?

29 Even without using mineral chemistry modelled isopleths we have defined a rough P-T path!

30 IF THE PETOGRAPHY IS WRONG THE INTERPRETATION OF THE THERMODYNAMIC MODELLING WILL ALSO BE WRONG

Metamorphic Petrology GLY 712 Geothermo-barometry

Metamorphic Petrology GLY 712 Geothermo-barometry What is thermobarometry? Thermobarometry is concerned with estimating or inferring the temperatures and pressures at which a rock formed and/or subsequently