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

2. 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 oC for the more unstable types of protolith
Some zeolites are considered diagenetic and others metamorphic – pretty arbitrary
Metamorphism begins in the range of oC 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

3. 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

4. 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,

5. 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)

6. 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

7. 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”

8. 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 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.

9. 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

10. 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

11. 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

12. 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

13. 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

14. 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

15. 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

16. 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

17. 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

18. 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