Metamorphic Petrology GLY 262 Metamorphic fluids

‘The metamorphic fluid is arguably the most geologically important phase’

Spear (1993)

• The great volumetric abundance of hydrate-rich and carbonate-rich minerals in the Earth’s continental crust implies that H2O and CO2 are among the most important volatile species in crustal rocks. An appreciation of the behaviour of these two components is thus crucial to geologists’ understanding of crustal processes.

The importance of fluids 1. Controls the P-T positions of reactions 2. Enhance kinetics and catalyzes reactions 3. Enhances deformation 4. Responsible for mass transfer in the crust,

some of which may have economic ramifications e.g. fluids produced via dehydration reactions during burial metamorphism in the Wits basin may have been responsible for the re-mobilization of the Au

• Metamorphism and anatexis are two processes strongly dependent on the roles of H2O and CO2 as many of the reactions involved either release volatiles (e.g. prograde devolatilisation reactions) or consume them (e.g. retrograde hydration reactions; prograde ‘wet’ melting reactions).

FLUID-PRESENCE

Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

• Reactions involving H2O are the most common metamorphic reactions. The hydrates contained within many rocks are successively removed by dehydration reactions, as heat is added to the rocks.

• This ensures that a free H2O fluid phase is always present during the progress of dehydration reactions.

• Similarly, where carbonate-rich rocks are being heated for the first time they will undergo extensive decarbonation, ensuring a free CO2 fluid is present in the rocks.

• However, the free fluid is only present if the rate of H2O and/or CO2 production exceeds the rate at which it can escape by diffusion along grain boundaries, or by mass transfer through fractures and fissures

• The presence of a distinct, physically separable, fluid phase accompanying metamorphism, particularly where hydrate- and carbonate-rich rocks are being heated for the first time, implies fluid-present or fluid-saturated conditions

• The rate of fluid production is controlled in part by the proportions and types of hydrate and carbonate minerals and in part by the rate and changes of pressure and temperature along a particular P-T path.

Internal versus external fluid buffering

Internal buffering: Fluid composition will remain constant until the reaction takes place. CO2 is produced therefore as T increases, XCO2 also increases. At point A either calcite or quartz may run out. Therefore the reaction stops, and as T increases further no more fluid is produced.

A

Internal versus external fluid buffering

External buffering: Fluid influx from adjacent dehydrating pelites or crystallizing magmatic bodies maybe able to flush away the fluid produced from the reaction

Fluid influx at a particular T may induce the reaction

FLUID INFILTRATION

T-XH2O diagram illustrating the shapes and relative locations of the reactions for the isograds mapped in the Whetstone Lake area. Reactions 1, 2, and 4 are dehydration reactions and reaction 3 is the Ky = Sil transition, all in metapelites. Reaction 5 is a dehydration-decarbonation in calcic rocks with a temperature maximum at XH2O = 0.25. b. Isograds mapped in the field. Note that isograd (5) crosses the others in a manner similar to that in part (a). This behavior is attributed to infiltration of H2O from the syn-metamorphic pluton in the area, creating a gradient in XH2O across the area at a high angle to the regional temperature gradient, equivalent to the T-X diagram. After Carmichael (1970) J. Petrol., 11, 147-181.

Fluid-absence • Thompson (1983) discussed the implications of fluid-absent metamorphism, pointing out that while extensive devolatilisation does unequivocally occur, at least in the first episode of prograde metamorphism, that there are some time periods or temperature intervals along a P-T-t path where no fluid is present in the rock. The absence of a fluid phase will have profound implications for the interpretations of mineral assemblages and metamorphic processes.

Fluid-absence • Thompson (1983) lists these consequences as follows:

• 1) Conventional graphical analysis of mineral parageneses is not possible, where normally the projection points include H2O.

• 2) Most laboratory data for mineral equilibrium, kinetic and deformation studies are determined under fluid-present conditions, which are not applicable to fluid-absent conditions.

• 3) Many crosscutting or irregular isograd patterns mapped on the basis of fluid-present reactions may attain different meaning under fluid-absent metamorphism.

Fluid-absence versus fluid-presence • In theory, both fluid-present and fluid-absent processes

could operate within the same metamorphic terrane. • Rocks heated to a modest T might undergo

devolatilisation and initial partial melting reactions in the presence of hydrous fluid.

• The consumption of H2O by these ‘wet’ melting reactions (e.g. ms + pl + kfs + qtz + H2O = melt) could potentially exhaust the local fluid supply, leading to melting at higher T by fluid-absent melting reactions.

Devolatilisation Initial wet melting Fluid absent melting+ =

Fluid consumed

TEMPERATURE

(1) qtz + kfs + H2O +/-pl +/- bi +/- Al-silicate = melt (fluid-present melting at ‘wet’ granite solidus) = <2% (2) ms + pl + qtz = kfs + Al-silicate + melt (fluid-absent melting). Generally ~5-10% (3) bi + Al-silicate + plagioclase + qtz = garnet + kfs + melt +/- crd (fluid-absent melting). Generally, 20-40%

Mel

t vol

%

(1)

(2)

(3)

White, R.W. and Powell, R., 2002. Melt loss and the preservation of granulite facies mineral assemblages. Journal of Metamorphic Geology, 20, 621-632

The release of H2O from crystallizing melts causes the retrogression of granulite-facies assemblages. Well preserved granulite facies assemblages implies that melt was extracted from the system and was therefore unable to re-hydrate the assemblages when it finally crystallizes