The Limits of Metamorphism Low-temperature limit grades into diagenesis –The boundary is somewhat arbitrary Diagenetic/weathering processes are indistinguishable from metamorphic Metamorphism begins in the range of o C for the more unstable types of protolith Some zeolites are considered diagenetic and others metamorphic – pretty arbitrary

The Limits of Metamorphism High-temperature limit grades into melting Over the melting range solids and liquids coexist If we heat a metamorphic rock until it melts, at what point in the melting process does it become “igneous”? Xenoliths, restites, and other enclaves are considered part of the igneous realm because melt is dominant, but the distinction is certainly vague and disputable Migmatites (“mixed rocks”) are gradational

Metamorphic Agents and Changes Temperature: typically the most important factor in metamorphism Figure 1-9. Estimated ranges of oceanic and continental steady-state geotherms to a depth of 100 km using upper and lower limits based on heat flows measured near the surface. After Sclater et al. (1980), Earth. Rev. Geophys. Space Sci., 18,

Metamorphic Agents and Changes Increasing temperature has several effects 1) Promotes recrystallization  increased grain size Larger surface/volume ratio of a mineral  lower stability Increasing temperature eventually overcomes kinetic barriers to recrystallization, and fine aggregates coalesce to larger grains

Metamorphic Agents and Changes Increasing temperature has several effects 2) Drive reactions that consume unstable mineral(s) and produces new minerals that are stable under the new conditions 3) Overcomes kinetic barriers that might otherwise preclude the attainment of equilibrium

Metamorphic Agents and Changes Pressure –“Normal” gradients may be perturbed in several ways, typically: High T/P geotherms in areas of plutonic activity or rifting Low T/P geotherms in subduction zones

Metamorphic Agents and Changes Stress is an applied force acting on a rock (over a particular cross-sectional area) Strain is the response of the rock to an applied stress (= yielding or deformation) Deviatoric stress affects the textures and structures, but not the equilibrium mineral assemblage Strain energy may overcome kinetic barriers to reactions

Metamorphic Agents and Changes Fluids Evidence for the existence of a metamorphic fluid: –Fluid inclusions –Fluids are required for hydrous or carbonate phases –Volatile-involving reactions occur at temperatures and pressures that require finite fluid pressures

The Types of Metamorphism Different approaches to classification 2. Based on setting –Contact Metamorphism Pyrometamorphism –Regional Metamorphism Orogenic Metamorphism Burial Metamorphism Ocean Floor Metamorphism –Hydrothermal Metamorphism –Fault-Zone Metamorphism –Impact or Shock Metamorphism

The Progressive Nature of Metamorphism Prograde: increase in metamorphic grade with time as a rock is subjected to gradually more severe conditions –Prograde metamorphism: changes in a rock that accompany increasing metamorphic grade Retrograde: decreasing grade as rock cools and recovers from a metamorphic or igneous event –Retrograde metamorphism: any accompanying changes

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 Al 2 SiO 5

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: NaAlSi 2 O 6 + SiO 2 = NaAlSi 3 O 8 Jd QtzAb MgSiO 3 + CaAl 2 Si 2 O 8 = CaMgSi 2 O 6 + Al 2 SiO 5 EnAn 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: Mg 3 Si 4 O 10 (OH) MgSiO 3 = Mg 7 Si 8 O 22 (OH) 2 TalcEnstatite Anthophyllite involves hydrous phases, but conserves H 2 O It may therefore be treated as a solid-solid net-transfer reaction

Hydration/ Dehydration Reactions Metamorphic reactions involving the expulsion or incorporation of water (H 2 O) Example: –Al 2 Si 4 O 10 (OH) 2 Al 2 SiO 5 + 3SiO 2 + H 2 O Pyrophyllite And/Ky Quartz water

Carbonation / Decarbonation Reactions Reactions that involve the evolution or consumption of CO 2 CaCO 3 + SiO 2 = CaSiO 3 + CO 2 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 CH 4 (methane), H 2, H 2 S, O 2, NH 4 + (ammonia) – but they are not as common

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)

The Phase Rule in Metamorphic Systems Consider the following three scenarios: C = 1 (Al 2 SiO 5 ) s F = 1 common s F = 2 rare s F = 3 only at the specific P-T conditions of the invariant point (~ 0.37 GPa and 500 o C) Figure The P-T phase diagram for the system Al 2 SiO 5 calculated using the program TWQ (Berman, 1988, 1990, 1991). Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Let’s put it all together…

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

From Hacker, B.R.,