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.

Representing Mineral Reactions albite  jadeite + quartz

From Hacker, B.R.,

Let’s put it all together…

What if we had staurolite and andalusite together What if we had staurolite and andalusite together? What conditions would that indicate?

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 (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 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 T O - T O - T O Serpentine: Mg3 [Si2O5] (OH)4 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

Chrysotile does not do this and tends to roll into tubes Serpentine Octahedra are a bit larger than tetrahedral match, so they cause bending of the T-O layers (after Klein and Hurlbut, 1999). Antigorite maintains a sheet-like form by alternating segments of opposite curvature Chrysotile does not do this and tends to roll into tubes

Serpentine 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 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) Clinochlore, chamosite, pennantite, nimmite – end members Chloritoid - Similar in appearance to chlorite, but different 2V and relief

Prehnite-Pumpellyite Low-grade metamorphic minerals Minerals related to chlorite, form at slightly lower P-T conditions Prehnite is also green, pumpellyite green too, varies based on Fe content Prehnite + chlorite  pumpellyite + quartz

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

Metamorphic chain silicates Actinolite and tremolite are chain silicates derived from dolomite and quartz and common in low-mid grade metamorphic rocks Riebeckite and Glaucophane are also chain silicates – higher grade minerals, often a blue color These minerals usually lower P, higher T conditions

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? The boundaries between metamorphic facies represent T-P conditions in which key minerals in mafic rocks are either added or removed, thus changing the mineral assemblages observed They are thus separated by mineral reaction isograds The limits are approximate and gradational, because the reactions vary with rock composition and the nature and composition of the fluid phase The 30oC/km geothermal gradient is an example of an elevated orogenic geothermal gradient.

Metamorphic Facies Table 25-1. The definitive mineral assemblages that characterize each facies (for mafic rocks). The definitive mineral assemblages that characterize each facies (for mafic rocks) are listed in Table 25-1 (details and variations later)

Let’s put it all together…

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) The contrast between Miyashiro’s higher-T-lower-P Ryoke-Abukuma belt (Section 21.6.3) and the classical Barrovian sequence also led him to suggest that more than one type of facies series is probable Metamorphic field gradients are highly variable (even in the same orogenic belt) and transitional series abound Miyashiro later chose to limit the number instead to the three broad major types of facies series (also called “baric types”) illustrated in Fig. 25-3 Although the boundaries and gaps between them are somewhat artificial, the three series represent common types of tectonic environments

Fig. 25-3. 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. The high P/T series, for example, typically occurs in subduction zones where “normal” isotherms are depressed by the subduction of cool lithosphere faster than it can equilibrate thermally The facies sequence here is (zeolite facies) - (prehnite- pumpellyite facies) - blueschist facies - eclogite facies. The medium P/T series is characteristic of common orogenic belts (Barrovian type) The sequence is (zeolite facies) - (prehnite-pumpellyite facies) - greenschist facies -amphibolite facies - (granulite facies) Crustal melting under water-saturated conditions occurs in the upper amphibolite facies (the solidus is indicated in Fig. 25-2) The granulite facies, therefore, occurs only in water-deficient rocks, either dehydrated lower crust, or areas with high XCO2 in the fluid The low P/T series is characteristic of high-heat-flow orogenic belts (Buchan or Ryoke-Abukuma type), rift areas, or contact metamorphism The sequence of facies may be a low-pressure version of the medium P/T series described above (but with cordierite and/or andalusite), or the sequence (zeolite facies) - albite-epidote hornfels facies - hornblende hornfels facies - pyroxene hornfels facies Sanidinite facies rocks are rare, requiring the transport of great heat to shallow levels

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 ‘staurolite-out 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?