Metamorphic Petrology GLY 712 Geothermo-barometry

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2 Metamorphic Petrology GLY 712 Geothermo-barometry

3 What is thermobarometry? Thermobarometry is concerned with estimating or inferring the temperatures and pressures at which a rock formed and/or subsequently evolved.

4 Why is this important? From an academic point of view thermobarometry can be used to make inferences about the evolution of metamorphic rocks, which in turn, yields information about tectonic processes From a practical point of view thermodynamics can be useful from an applied mineralogical perspective.

5 THERMOBAROMETRY The thermodynamic data (i.e. H, S etc) for a given reaction/equilibrium are extracted from directly calibrated experimental datasets and inserted into the master equation along with the K value that was determined from the a-x data. Geobarometers are relatively insensitive to temperature, therefore the master equation should be re-arranged to make P the subject then solve for a given temperature. Conversely, geothermometers are relatively insensitive to pressure changes, therefore the master equation should be re-arranged to make T the subject then solve for a given pressure.

6 For a balance reaction at equilibrium 0 = ΔG + ( P 1)( ΔV + Δ( αv )( T 298) Δ( βv ) P / 2) + ( r)1, T s nrt ln f + RT ln K where ΔG ( r)1, T = ΔH ( r)1,298 + M T ( ΔS ( r)1,298 + N)

7 THERMOBAROMETRY In order to obtain a reliable intersection of two equilibria, one must be sensitive to T i.e. a geothermometer (e.g. Garnet-biotite Fe-Mg exchange thermometer, which has large H, S and small V) and the other must be sensitive to pressure A geobarometer (e.g. GASP), has a large V, small H and S. Applying both GASP and Grt-Bt Fe-Mg exchange thermometry to a rock with a suitable mineral assemblage will yield a unique P and T.

8 THERMOBAROMETRY

9 Types of reaction There are two types of reaction, commonly used to estimate T and/or P conditions during metamorphism: Net-transfer reactions (e.g. the GASP geobarometer) Exchange reactions (e.g. the garnet-biotite Fe-Mg exchange thermometer)

10 Types of reaction 1) Net-transfer reactions (univariant, discontinuous reaction, F = 1 in a given system) result in a transfer of material between two or more minerals, i.e., the creation of new minerals (product) and the consumption of old minerals (reactant). e.g. The solid-solid reaction that contain garnet, plagioclase, aluminosilicate (Ky, Sill, And), and quartz (GASP geobarometer) 3CaAl 2 Si 2 O 8 = Ca 3 Al 2 Si 3 O Al 2 SiO 5 + SiO 2 Anorthite (Pl) = Grossular (Grt) + Ky/Sill/And + Quartz. This reaction is commonly used to estimate the P of metamorphism in pelitic rocks. The advantage of this type of reactions is that the T and P at which they occur are independent of fluid composition (solid-solid reaction)

11 Types of reaction 2) Exchange reactions are those that do not result in the formation of any new minerals, they just redistribute cations/elements between already pre-existing minerals (continuous reactions, F = 2, in a given system) e.g. the exchange of Fe and Mg between biotite and garnet. This can be represented by the reaction: KFe 3 AlSi 3 O 10 (OH) 2 + Mg 3 Al 2 Si 3 O 12 = KMg 3 AlSi 3 O 10 (OH) 2 + Fe 3 Al 2 Si 3 O 12 Annite + Pyrope = Phlogopite + Almandine As this reaction proceeds, the compositions of biotite and garnet change, but for every mole of annite consumed a mole of phlogopite is produced and for every mole of pyrope consumed a mole of almandine is produced. Essentially just Fe-Mg exchange

12 Errors and uncertainties There are two main sources of error in using geothermobarometry (1) uncertainties on the thermodynamic data (ΔS, ΔH etc) extracted from the experiments and (2) Analytical error on the compositions which are used to derive activities and K. Reported errors from the experiments and analytical data should be propagated to derive margins of error on P and/or T. There are other errors in the experimental calibration that are often ignored. For example the experiments are often conducted at certain P- T-X conditions. The thermodynamic data is then extrapolated to other P-T-X conditions a procedure which may not be appropriate. Furthermore, the experiments are often conducted with pure endmember phases. This approach ignores the possibility that minor chemical components, present in the naturally occurring phase, may drastically alter the P-T conditions under which a particular reaction will occur e.g. the presence of Fe 3+ in Fe-Mg exchange thermometers

13 Pseudosection Mineral stability diagrams calculated for a particular bulk rock composition. Provide means for predicting stable mineral assemblages in P-T space They provide a more complete chemistry of metamorphism than traditional geothermobarometry s Can also be used to obtained estimate of composition and relative abundance of all minerals in each field. Measured X can be compared to predicted X to obtained P-T estimates under which the rock formed

14 Pseudosection are built on assumptions Minerals within the same sample must grow in equilibrium with each other, from the same bulk X However, 1. Metamorphic cycles occur over wide P-T range and mineral assemblages will reflect a part of the range 2. Minerals with solid solution between end members are rarely compositionally homogenous 3. Components making up the chemical system all have different diffusivities and will equilibrate over varying length scales in the rock

15 The fundamental assumption seem shaky at best. However, careful application of the pseudosection calculations to specific equlibria within each sample can provide useful thermobarometric information that does not contravene the fundamental assumption

16 Garnet composition isopleth thermobarometry Garnet composition is frequently used in these calculations because the mineral commonly preserved it original composition over a wide P- T range. It the only common mineral in metapelites that can have more that 2 independent compositional variables, That can be measured by EPMA analysis making it ideal for isopleth thermobarometry It preserve it growth due to its very low diffusivity below C

17 Low intra-granular diffusivity of garnet causes the mineral to be effectively removed from the bulk rock Fe, Mg, Ca, Mn, Al and Si that are present in garnet only and not available for growing other minerals Therefore, the bulk rock X will not be an accurate chemical system operating after garnet growth Only the earliest growth garnet can be used for isopleth thermobarometry Unless calculation's are made to estimate the effect of crystal fractionation

18 Element distribution maps through a garnet porphyroblast

19 Compositional variation across a garnet porphyroblast

20 Compositional variation across a garnet porphyroblast

21 THERMOCALC and Perple_X THERMOCALC is a programme developed and maintained by Tim Holland (Cambridge) and Roger Powell (Melbourne) (Powell & Holland, 1988, Journal of Metamorphic Geology, 6, ). Perple_X was developed by Connolly(1990) and also used the same Thermodynamic database of Powell & Holland, 1998 The word THERMOCALC is a contraction of thermodynamic calculator. The programme consists of two parts: (1) the application itself and (2) the thermodynamic dataset.

22 How does it work? They both utilises the available thermodynamic data (i.e. ΔH, ΔS, f etc) for reactions (mineral equilibria) which are commonly taken from experimental studies. Using 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.

23 What can it do? There are several modes to both these programs, principally: (1) End-member reaction mode (2) Average P-T mode and (3) Phase diagram mode - ergh! (1) End-member reaction mode. You input the end-member names and the activities of those end-members, which you calculated from the probe analyses using a-x models. The output you get is the P-T co-ordinates of that reaction. For example you could calculate the positions of all the Al 2 SiO 5 polymorph reactions: And = Sill, Sill = Ky and And = Ky

24 What can it do? You can also control parameters such as ah 2 O (activity of water) and calculate how changes in such a parameter would change the position of the reaction in P-T space. For example the following equilibrium: Chl + Ky + Qtz = Crd + H 2 O at various ah 2 O activities. At a given temperature of 600 o C and under water-saturated conditions (i.e. ah 2 O = 1) this reaction will proceed at pressures of ~ 4 kbar. If our rock was essentially dry (ah 2 O = <<1) reaction 2 would only proceed under increased pressure conditions at the same temperature i.e. 600 o C ah 2 O = 1 Pressure 5 4 Chl + Ky + Qtz Crd + H 2 O Temperature

25 What can it do? (2) Average P-T mode or werami You need to input the end-member names of the phases in your rock, plus their activities and the uncertainties associated with the activities (i.e. standard deviations etc).

26 (3) Phase Diagram Mode 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

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