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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 • 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 • 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 • 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 • 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 • 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 • 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 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 – 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 • 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 diagrams • Let’s think about what happens to water as conditions change… • P+F=C+2 A C • Point A? • Point B? • Point C? B Mineral Assemblages in Metamorphic Rocks • 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 • 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 Systems Consider the following three scenarios: 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) 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 • 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 • SILLIMANITE: Orthorhombic: Octahedral Al chains (6-fold) are crosslinked by both Si and Al tetrahedra (4-fold). • ANDALUSITE: Orthorhombic: 5-coordinated Al; Same octahedral (6fold) chains. • KYANITE: Triclinic: All the Al is octahedrally coordinated (6- and 6fold). 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 Minerals • 3 polymorphs of Al2SiO5 are important metamorphic minerals Andalusite Kyanite Sillimanite Topaz • Aluminosilicate mineral as well, one oxygen substituted with OH, F • Al2SiO4(F,OH)2 • Where do you think Topaz forms?? Serpentine Minerals • 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 Yellow = (OH) Serpentine: Mg3 [Si2O5] (OH)4 T-layers and triocathedral (Mg2+) layers (OH) at center of T-rings and fill base of VI layer weak van der Waals bonds between T-O groups T O T O T O vdw vdw Serpentine Antigorite maintains a sheet-like form by alternating segments of opposite curvature Chrysotile does not do this and tends to roll into tubes Octahedra are a bit larger than tetrahedral match, so they cause bending of the T-O layers (after Klein and Hurlbut, 1999). Serpentine Nagby and Faust (1956) Am. Mineralogist 41, 817-836. Veblen and Busek, 1979, Science 206, 1398-1400. S = serpentine T = talc The rolled tubes in chrysotile resolves the apparent paradox of asbestosform sheet silicates Chlorite • 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 • Minerals related to chlorite, form at slightly lower P-T conditions • Prehnite is also green, pumpellyite Micas • 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 • 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 • Sorosilicates (paired silicate tetrahedra) • Include the mineral Epidote Ca2FeAl2Si3O12(OH), Zoisite (Ca2Al3Si3O12(OH) and clinozoisite (polymorph) Garnets Garnet: A2+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 Garnet (001) view blue = Si purple = A turquoise = B Staurolite • 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 Metamorphic Facies • 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 Facies • Table 25-1. The definitive mineral assemblages thatTable characterize eachAssemblages facies (for mafic Facies rocks). 25-1. Definitive Mineral of Metamorphic Facies Zeolite Definitive Mineral Assemblage in Mafic Rocks zeolites: especially laumontite, wairakite, analcime Prehnite-Pumpellyite prehnite + pumpellyite (+ chlorite + albite) Greenschist chlorite + albite + epidote (or zoisite) + quartz ± actinolite Amphibolite hornblende + plagioclase (oligoclase-andesine) ± garnet Granulite orthopyroxene (+ clinopyrixene + plagioclase ± garnet ± hornblende) Blueschist glaucophane + lawsonite or epidote (+albite ± chlorite) Eclogite pyrope garnet + omphacitic pyroxene (± kyanite) Contact Facies After Spear (1993) Mineral assemblages in mafic rocks of the facies of contact metamorphism do not differ substantially from that of the corresponding regional facies at higher pressure. Facies Series • 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. Temperaturepressure 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 • Lines (on a map) or Surfaces (in the 3D world) marking the appearance or disappearance of the Index minerals in rocks of appropriate composition e.g. the ‘garnet-in isograd’; the ‘stauroliteout isograd’ Complicated by the fact that most of these minerals are solid solutions • Isograds for a single shale unit in southern Vermont • Which side reflects a higher grade, or higher P/T environment?