Metamorphic Petrology GLY 712Geothermo-barometry

What is thermobarometry?

•

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

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.

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.

For a balance reaction at equilibrium

)(where

lnln)2/)()298)(()(1(0

298,1)(298,1)(,1)(

,1)(

NSTMHG

KRTfnRTPVTVVPG

rrTr

sTr

+Δ−+Δ=Δ

++Δ−−Δ+Δ−+Δ= βα

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.

THERMOBAROMETRY

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)

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)

3CaAl2

Si2

O8

= Ca3

Al2

Si3

O12

+ 2Al2

SiO5

+ SiO2

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)

Types of reaction2) 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:

KFe3

AlSi3

O10

(OH)2

+ Mg3

Al2

Si3

O12

= KMg3

AlSi3

O10

(OH)2

+ Fe3

Al2

Si3

O12

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

Errors and uncertaintiesThere 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 end- member 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 Fe3+

in Fe-Mg exchange thermometers

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

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

•

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

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 7000C

•

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

Element distribution maps through a garnet porphyroblast

Compositional variation across a garnet porphyroblast

Compositional variation across a garnet porphyroblast

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, 173-204).

Perple_X

was developed by Connolly(1990) and also used the same Thermodynamic database of Powell & Holland, 1998The word THERMOCALC is a contraction of thermodynamic calculator. The programme consists of two parts: (1) the application itself and (2) the thermodynamic dataset.

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.

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 Al2

SiO5

polymorph reactions: And = Sill, Sill = Ky

and And

= Ky

What can it do?•

You can also control parameters such as aH2

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

+ H2

O at various aH2

O activities. At a given temperature of 600oC and under water-saturated conditions (i.e. aH2

O = 1) this reaction will proceed at pressures of ~ 4 kbar. If our rock was essentially dry

(aH2

O = <<1) reaction 2 would only proceed under increased pressure conditions at the same temperature i.e. 600oC.

0.2 0.3

aH2O = 1

0.4

Crd + H 2O

Chl + Ky + Qtz

2

3

4

5

6

7

8

500 550 600 650 700Temperature

Pres

sure

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 theiractivities and the uncertainties associated with the activities (i.e. standard deviations etc).

(3) Phase Diagram Mode

For a given model chemicalsystem (e.g. KFMASH) gridswill show ALL the possible phase relations / reactions /stabilities for a given set of minerals.

There are over 150 possiblereactions between just 8KFMASH pure end-member minerals alone in P-T range(0.5 -

4 kbars

400-1000oC).

Spear, 1999

XFe (Chl) = 0.45

XFe (Chl) = 0.55

XFe

(Mu)

10 % Chl