Isograds for a single shale unit in southern Vermont Which side reflects a higher grade, or higher P/T environment?

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 100-150oC for the more unstable types of protolith Some zeolites are considered diagenetic and others metamorphic – pretty arbitrary Metamorphism begins in the range of 100-150oC for the more unstable types of protolith Marked by the formation of minerals such as laumontite, analcime, heulandite, carpholite, paragonite, prehnite, pumpellyite, lawsonite, glaucophane or stilpnomelane

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 We may all recognize a melt, but we may not be so good at recognizing the solid products crystallized from one Small, elongate, fairly coarse-grained and cross-cutting segregations of granitoid material in gneisses: Thin dikes of melt or precipitates from fluids, or fluid-enhanced recrystallization along fluid-filled fractures? The distinction between a silicate-saturated aqueous fluid and a fluid-saturated silicate melt

Metamorphic Agents and Changes Temperature: typically the most important factor in metamorphism Continental geotherm is higher than oceanic due to concentration of radioactive (LIL) elements 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, 269-311.

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 Especially for fine-grained and unstable materials in a static environment (shear stresses often reduce grain size)

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 2) Heating to conditions outside the stability range of some mineral(s) may cause a reaction to take place that consumes the unstable mineral(s) and produces new minerals that are stable under the new conditions 3)… Disequilibrium is relatively common in sediments and diagenesis Mineral assemblages are usually simpler at higher grades and the phase rule is applicable

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 Temperature rarely increases without an accompanying increase in pressure (geothermal gradients) Most disturbances are transient and eventually return to “normal”

Fig. 21-1 = estimates of metamorphic temperature-pressure relationships from ancient orogenic belts Based on P-T estimates for rocks exposed at the surface in these areas along a traverse from lowest to highest metamorphic conditions: metamorphic field gradients – not same as geotherms Figure 21-1. Metamorphic field gradients (estimated P-T conditions along surface traverses directly up metamorphic grade) for several metamorphic areas. After Turner (1981). Metamorphic Petrology: Mineralogical, Field, and Tectonic Aspects. McGraw-Hill.

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 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