What happens to our protolith when acted on by agents of change
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What happens to our PROTOLITH when acted on by AGENTS OF CHANGE??. Agents of Change  T, P, fluids, stress, strain Metamorphic Reactions!!!! Solid-solid phase transformation Solid-solid net-transfer Dehydration Hydration Decarbonation Carbonation. Solid-solid phase transformation.

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What happens to our protolith when acted on by agents of change
What happens to our PROTOLITH when acted on by AGENTS OF CHANGE??

  • Agents of Change  T, P, fluids, stress, strain

  • Metamorphic Reactions!!!!

    • Solid-solid phase transformation

    • Solid-solid net-transfer

    • Dehydration

    • Hydration

    • Decarbonation

    • Carbonation


Solid solid phase transformation
Solid-solid phase transformation CHANGE??

  • Polymorphic reaction  a mineral reacts to form a polymorph of that mineral

  • No transfer of matter, only a rearrangment of the mineral structure

  • Example:

    • Andalusite  Sillimanite

Al2SiO5

Al2SiO5


Solid solid net transfer
Solid-solid net-transfer CHANGE??

  • Involve solids only

  • Differ from polymorphic transformations: involve solids of differing composition, and thus material must diffuse from one site to another for the reaction to proceed

  • Examples:

  • NaAlSi2O6 + SiO2 = NaAlSi3O8

    Jd Qtz Ab

  • MgSiO3 + CaAl2Si2O8 = CaMgSi2O6 + Al2SiO5

    En An Di And


Solid solid net transfer ii

Solid-Solid Net-Transfer II CHANGE??

If minerals contain volatiles, the volatiles must be conserved in the reaction so that no fluid phase is generated or consumed

For example, the reaction:

Mg3Si4O10(OH)2 + 4 MgSiO3 = Mg7Si8O22(OH)2

Talc Enstatite Anthophyllite

involves hydrous phases, but conserves H2O

It may therefore be treated as a solid-solid net-transfer reaction


Hydration dehydration reactions
Hydration/ Dehydration Reactions CHANGE??

  • Metamorphic reactions involving the expulsion or incorporation of water (H2O)

  • Example:

    • Al2Si4O10(OH)2 <=> Al2SiO5 + 3SiO2 + H2O

      Pyrophyllite And/Ky Quartz water


Carbonation decarbonation reactions
Carbonation / Decarbonation Reactions CHANGE??

  • Reactions that involve the evolution or consumption of CO2

  • CaCO3 + SiO2 = CaSiO3 + CO2

    calcite quartz wollastonite

    Reactions involving gas phases are also known as volatilization or devoltilization reactions

    These reactions can also occur with other gases such as CH4 (methane), H2, H2S, O2, NH4+ (ammonia) – but they are not as common


Systems
Systems CHANGE??

  • Rock made of different minerals

  • Metamorphic agents of change beat on it  metamorphic reactions occur

  • A closed system does not gain or lose material of any kind

  • An open system can lose stuff – liquids, gases especially

Outside

world

Hunk o’ rock


Thermodynamics primer
Thermodynamics Primer CHANGE??

  • Thermodynamics describes IF a reaction CAN occur at some condition (T, P, composition typically)

  • Second Law of thermodynamics:

  • DG=DH – TDS

    • Where G, Gibb’s free energy determines IF the REACTION will go forward (-DG=spontaneous)

    • H is enthalpy – has to do with heat…

    • S is entropy – has to do with bonds and order…


Thermodynamics vs kinetics
Thermodynamics vs. Kinetics CHANGE??

  • Thermodynamics – comparing the potential ENERGY of things  what is more stable? Will a reaction occur at some T,P, soln, melt composition go or Not?

  • Kinetics  IF thermodynamics says YES, the reaction should occur (always toward lower energy!) kinetics determines how fast

  • Minerals out of equilibrium pass the thermodynamic test but the kinetics of their reaction is very slow…


Phase diagrams
Phase diagrams CHANGE??

  • Tool for ‘seeing’ phase transitions

  • H2Oice H2Oliquid

  • Reaction (line) governed

    by DG=DH – TDS

  • Phase Rule:

    • P+F=C+2

    • Phases coexisting + degrees of freedom = number of components + 2

    • Degree of freedom  2= either axis can change and the phase stays the same  where??


Phase diagrams1
Phase diagrams CHANGE??

  • Let’s think about what happens to water as conditions change…

  • P+F=C+2

  • Point A?

  • Point B?

  • Point C?

A

B

C


Mineral assemblages in metamorphic rocks
Mineral Assemblages in Metamorphic Rocks CHANGE??

  • Equilibrium Mineral Assemblages

  • At equilibrium, the mineralogy (and the composition of each mineral) is determined by T, P, and X

  • Relict minerals or later alteration products are thereby excluded from consideration unless specifically stated


The phase rule in metamorphic systems
The Phase Rule in Metamorphic Systems CHANGE??

  • Phase rule, as applied to systems at equilibrium:

    F = C - P + 2 the phase rule

P is the number of phases in the system

C is the number of components: the minimum number of chemical constituents required to specify every phase in the system

F is the number of degrees of freedom: the number of independently variable intensive parameters of state (such as temperature, pressure, the composition of each phase, etc.)


The phase rule in metamorphic systems1
The Phase Rule in Metamorphic Systems CHANGE??

C = 1 (Al2SiO5)

  • F = 1 common

  • F = 2 rare

  • F = 3 only at the specific P-T conditions of the invariant point

    (~ 0.37 GPa and 500oC)

Consider the following three scenarios:

Figure 21-9. The P-T phase diagram for the system Al2SiO5 calculated using the program TWQ (Berman, 1988, 1990, 1991). Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.


Metamorphic facies
Metamorphic facies CHANGE??

  • P-T conditions, presence of fluids induces different metamorphic mineral assemblages (governed by thermodynamics/ kinetics)

  • These assemblages are lumped into metamorphic facies (or grades)


Aluminosilicate minerals
Aluminosilicate Minerals CHANGE??

  • SILLIMANITE: Orthorhombic: Octahedral Al chains (6-fold) are crosslinked by both Si and Al tetrahedra (4-fold).

  • ANDALUSITE: Orthorhombic: 5-coordinated Al; Same octahedral (6-fold) chains.

  • KYANITE: Triclinic: All the Al is octahedrally coordinated (6- and 6-fold).

Andalusite

Kyanite

Sillimanite

  • Clearly, changes in structure are in response to changing P and T. Result is changes in Al coordination.

  • Phase transformations require rebonding of Al. Reconstructive polymorphism requires more energy than do displacive transformations. Metastability of these 3 are therefore important (Kinetic factors limit equilibrium attainment).

  • All 3 are VERY important metamorphic index minerals.


Aluminosilicate minerals1
Aluminosilicate Minerals CHANGE??

  • 3 polymorphs of Al2SiO5 are important metamorphic minerals

Andalusite

Kyanite

Sillimanite


Topaz
Topaz CHANGE??

  • Aluminosilicate mineral as well, one oxygen substituted with OH, F

  • Al2SiO4(F,OH)2

  • Where do you think Topaz forms??


Serpentine minerals
Serpentine Minerals CHANGE??

  • Mg3Si2O5(OH)4 minerals (principally as antigorite, lizardite, chrysotile polymorphs)

  • Forms from hydration reaction of magnesium silicates

    • Mg2SiO4 + 3 H2O  Mg3Si2O5(OH)4+ Mg(OH)2

      forsterite serpentine brucite

  • Asbestosform variety is chrysotile (accounts for 95% of world’s asbestos production  MUCH LESS DANGEROUS than crocidolite)


Phyllosilicates CHANGE??

T O - T O - T O

Yellow = (OH)

vdw

Serpentine: Mg3 [Si2O5] (OH)4

T-layers and triocathedral (Mg2+) layers

(OH) at center of T-rings and fill base of VI layer 

vdw

weak van der Waals bonds between T-O groups


Octahedra are a bit larger than tetrahedral match, so they cause bending of the T-O layers (after Klein and Hurlbut, 1999).

Serpentine

Antigorite maintains a sheet-like form by alternating segments of opposite curvature

Chrysotile does not do this and tends to roll into tubes


Serpentine cause bending of the T-O layers (after Klein and Hurlbut, 1999).

Veblen and Busek, 1979, Science 206, 1398-1400.

S = serpentine T = talc

Nagby and Faust (1956) Am. Mineralogist 41, 817-836.

The rolled tubes in chrysotile resolves the apparent paradox of asbestosform sheet silicates


Chlorite
Chlorite cause bending of the T-O layers (after Klein and Hurlbut, 1999).

  • Another phyllosilicate, a group of difficult to distinguish minerals

  • Typically green, and the dominant and characteristic mineral of greenschist facies rocks

  • Forms from the alteration of Mg-Fe silicates (pyroxenes, amphiboles, biotite, garnets)


Prehnite pumpellyite
Prehnite-Pumpellyite cause bending of the T-O layers (after Klein and Hurlbut, 1999).

  • Minerals related to chlorite, form at slightly lower P-T conditions

  • Prehnite is also green, pumpellyite


Micas
Micas cause bending of the T-O layers (after Klein and Hurlbut, 1999).

  • Biotite and Muscovite are also important metamorphic minerals (muscovite often the principle component of schists)

  • Phlogopite – similar to biotite, but has little iron, forms from Mg-rich carbonate deposits and a common mineral in kimberlites (diamond-bearing material)

  • Sericite – white mica (similar to muscovite) – common product of plagioclase feldspar alteration at low grades


Zeolites
Zeolites cause bending of the T-O layers (after Klein and Hurlbut, 1999).

  • Diverse group of minerals forming at lower metamorphic grades

  • Framework silicas, but characteristically containing large voids and highly variable amounts of H2O

    • Name is from the greek – meaning to boil stone as the water can de driven off with heat

    • Voids can acts as molecular sieves and traps for many molecules

    • Diversity of minerals in this group makes a for a wide variety of sieve and trapping properties selective for different molecules


Epidote group
Epidote Group cause bending of the T-O layers (after Klein and Hurlbut, 1999).

  • Sorosilicates (paired silicate tetrahedra)

  • Include the mineral Epidote Ca2FeAl2Si3O12(OH), Zoisite (Ca2Al3Si3O12(OH) and clinozoisite (polymorph)


Garnets

  • Garnet: A cause bending of the T-O layers (after Klein and Hurlbut, 1999).2+3 B3+2 [SiO4]3

  • “Pyralspites” - B = Al

    • Pyrope: Mg3 Al2 [SiO4]3

    • Almandine: Fe3 Al2 [SiO4]3

    • Spessartine: Mn3 Al2 [SiO4]3

  • “Ugrandites” - A = Ca

    • Uvarovite: Ca3 Cr2 [SiO4]3

    • Grossularite: Ca3 Al2 [SiO4]3

    • Andradite: Ca3 Fe2 [SiO4]3

  • Occurrence:

    • Mostly metamorphic

    • Some high-Al igneous

    • Also in some mantle peridotites

Garnets

Garnet (001) view blue = Si purple = A turquoise = B


Staurolite
Staurolite cause bending of the T-O layers (after Klein and Hurlbut, 1999).

  • Aluminosilicate - Fe2Al9Si4O22(OH)2

  • Similar structure to kyanite with tetrahedrally coordinated Fe2+ easily replaced by Zn2+ and Mg2+

  • Medium-grade metamorphic mineral, typically forms around 400-500 C

    • chloritoid + quartz = staurolite + garnet

    • chloritoid + chlorite + muscovite = staurolite + biotite + quartz + water

  • Degrades to almandine (garnet at higher T)

    • staurolite + muscovite + quartz = almandine + aluminosilicate + biotite + water


Actinolite
Actinolite cause bending of the T-O layers (after Klein and Hurlbut, 1999).


Metamorphic facies1
Metamorphic Facies cause bending of the T-O layers (after Klein and Hurlbut, 1999).

  • Where do we find these regimes of P-T ‘off’ of the typical continental isotherms??

  • How is the environment that forms a blueschist facies rock different from one forming a hornfels?


Metamorphic facies2

Metamorphic Facies cause bending of the T-O layers (after Klein and Hurlbut, 1999).

Table 25-1. The definitive mineral assemblages that characterize each facies (for mafic rocks).


Facies series

Facies Series cause bending of the T-O layers (after Klein and Hurlbut, 1999).

Miyashiro (1961) initially proposed five facies series, most of them named for a specific representative “type locality” The series were:

1. Contact Facies Series (very low-P)

2. Buchan or Abukuma Facies Series (low-P regional)

3. Barrovian Facies Series (medium-P regional)

4. Sanbagawa Facies Series (high-P, moderate-T)

5. Franciscan Facies Series (high-P, low T)


Fig. 25-3. cause bending of the T-O layers (after Klein and Hurlbut, 1999).Temperature-pressure diagram showing the three major types of metamorphic facies series proposed by Miyashiro (1973, 1994). Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.


Isograds
Isograds cause bending of the T-O layers (after Klein and Hurlbut, 1999).

  • Lines (on a map) or Surfaces (in the 3D world) marking the appearance or disappearance of the Index minerals in rocks of appropriate compositione.g. the ‘garnet-in isograd’; the ‘staurolite-out isograd’Complicated by the fact that most of these minerals are solid solutions



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