L7 - Introduction to Metamorphism

Introduction to Metamorphic Processes

What is Metamorphism?

Metamorphism is the transformation of rocks due to changes in:
- Chemical environment
- Temperature
- Pressure
The mineral assemblage changes as phases become unstable.
Texture changes via recrystallization.
- The size, shape, and orientation of minerals provide clues to formation conditions.
Example:
- Chloritization of biotite in a granite.
- Muscovite crenulation cleavage in a schist.
Different minerals form under different pressure-temperature (P-T) conditions.
The phase with the lowest Gibbs free energy will be stable.
Polymorphs are minerals with the same chemical composition that crystallize in different forms depending on P and T.
- Kyanite, Sillimanite, and Andalusite are polymorphs of $Al_2SiO_5$ .
  - Kyanite: density $(\rho) = 3.61$ (Triclinic)
  - Sillimanite: density $(\rho) = 3.24$ (Orthorhombic)
  - Andalusite: density $(\rho) = 3.15$ (Orthorhombic)

Why do we see metamorphic rocks at all?

Prograde metamorphism: P and T increase.
Retrograde metamorphism: P and T decrease.
Reasons for seeing high P-T assemblages at the surface:
1. Chemical reactions happen faster at higher T.
2. Fluids lost during prograde metamorphism via devolatilization, hindering the formation of hydrous minerals and carbonates.
3. Fluids catalyze reactions; their absence makes retrograde metamorphism slower.

Types of Metamorphism

Thermal: Temperature changes in a near-static stress field.
Dynamothermal: Both pressure and temperature change.
Dynamic: Deviatoric (shear) stress is dominant.
Chemical: Fluid-driven recrystallization.

Metamorphic Environments

Adjacent to igneous intrusions: contact metamorphism.
In response to collisional tectonics: regional metamorphism.
Along fault zones: dynamic metamorphism.
In hydrothermal systems: chemical metamorphism.
Associated with impact events: shock metamorphism.

Contact Metamorphism

Requires a high geothermal gradient produced by hot, intruding magma.
Usually occurs at shallow depths where there is a large temperature contrast between the hot magma and the cold crust.
Heat flow is described by the equation:
- $Q = -k \cdot \frac{dT}{dz}$
  - $Q$ is heat flow ( $\text{W m}^{-2}$ ).
  - $k$ is the thermal conductivity ( $\text{W m}^{-1} \text{K}^{-1}$ ).
  - $\frac{dT}{dz}$ is the geothermal gradient ( $\text{K m}^{-1}$ ).
The transport of heat from an igneous intrusion into the surrounding country rock can be described by Fourier’s law of heat conduction:
- $\frac{dT}{dt} = \kappa \cdot \frac{\delta^2T}{\delta x^2}$
  - $\kappa$ is thermal diffusivity.
  - $\kappa = \frac{k}{\rho C_p}$
    - $k$ is thermal conductivity.
    - $\rho$ is density.
    - $C_p$ is specific heat capacity.
The characteristic time it takes for an intrusion to cool is governed by:
- $t \sim \frac{l^2}{\pi \kappa}$
  - $t$ is the characteristic time ( $\text{s}$ ).
  - $l$ is the length scale of the intrusion ( $\text{m}$ ).
  - $\kappa$ is the thermal diffusivity (≈ $10^{-6} \text{ m}^2 \text{ s}^{-1}$ ).
The size of the contact aureole depends on:
- The size of the intrusion.
- The temperature of the intrusion.
- Whether the country rock is wet or dry.
  - If the country rock is wet, heat is lost by convection.
    - Shallower temperature gradient.
    - Heat removed from the intrusion more effectively.
    - Narrower contact aureole.
  - If the country rock is dry, heat is lost by conduction.
    - Steeper temperature gradient.
    - Heat removed from the intrusion less effectively.
    - Wider contact aureole.

Regional Metamorphism

Occurs across large areas of the crust and is typically the result of orogenic events.
Rocks are metamorphosed within the cores of mountain belts and subsequently exposed by erosion.
Pressure and temperature increase simultaneously due to burial and radioactive decay.
The pressure a rock experiences is a function of the overlying rock column.
- The lithostatic pressure is given by:
  - $P = \rho g h$
    - $P$ is the lithostatic pressure ( $\text{Pa}$ ).
    - $\rho$ is the density of the overlying rock ( $\text{kg m}^{-3}$ ).
    - $g$ is gravity ( $\text{m s}^{-2}$ ).
    - $h$ is the height of the overlying rock column ( $\text{m}$ ).
Crustal densities range from $2700 \text{ kg m}^{-3}$ (felsic) to $3300 \text{ kg m}^{-3}$ (ultramafic).
Crustal thickness varies from ~ 7 – 35 km under stable conditions.
Therefore, the base of the crust experiences pressures of ~0.2 – 1 GPa ≡ 2 – 10 kbar.
Rule of thumb: 10 km of crust is ~ 0.3 GPa of pressure.
In subduction zones, rocks can reach P > 3 GPa.
At the base of mountains, rocks can reach P ~ 2 GPa.

Regional Metamorphism and Uplift

In ‘normal’ crust, the isotherms increase in temperature with depth.
During compression, the isotherms are buried - P increases as the crust thickens downwards.
T increases faster than P as the burial rate decreases, allowing thermal equilibration. Heat production from Th, K, and U.
Erosion is faster than thermal relaxation: T continues to increase as the rock is exhumed (P is lowered).
The system reaches equilibrium, i.e., thermal steady state. Different to original P and T because the crust is thicker, and there is redistribution of heat-producing elements.
Maximum P and T conditions don’t have to occur at the same time; it Depends on the rate of erosion vs. thermal equilibration.
Isograds are the boundaries between regions of different metamorphic grade, which are identified by the appearance of a new metamorphic phase.

In the 1890s, Barrow mapped the isograds over Scotland, which are now known as Barrow zones.

P-T-t paths

Clockwise P-T paths:
- Rocks exhumed during tectonic and metamorphic event.
- Erosion plays a key role.
Anti-clockwise P-T paths:
- Rocks exhumed after tectonic and metamorphic event.
- Usually exhumed by a later tectonic event.
P-T-t paths can also tell us whether similar/different processes buried and exhumed the rocks
If the maximum P and T occur at different times it implies burial/exhumation was slower than thermal relaxation.
If maximum P and T occur at the same time it implies burial/exhumation was faster than thermal relaxation.
Mineral inclusions can be used to partly constrain the direction of P-T paths.
- In a rock which has experienced decreasing P and/or increasing T, for example – the high P, low T phase (kyanite) is preserved in inclusions, but the rest has transformed to the high T phase (sillimanite).
Reaction textures as evidence for incomplete reactions, or the presence of pseudomorphs.
- Pseudomorphs are elements which take the shape of another by filling in the hole left due to incorrect conditions. (Sillimanite taking up the shape that a andalusite crystal left behind despite not making that shape naturally).
Element zoning can also partly constrain P-T paths
- Garnet with Mn-rich cores and Mn-poor rims indicate an increase in T.
- Mn is preferentially partitioned into the early-growing garnet and is therefore depleted in the rim as garnet growth continues.
- Zoning indicates low T conditions, or a short duration at high T – otherwise, zoning would become homogenized.

Chemical Metamorphism

Oceanic crust undergoes extensive chemical exchange with seawater – the resulting chemical and mineralogical transformations are known as metasomatism.
The extent of metasomatism is a function of fluid flux, temperature, and pressure.
Alteration is often incomplete.
Example: partially serpentinized harzburgite from the Oman ophiolite
- olivine and/or pyroxene + water -> serpentine + hydrogen + methane + heat (+ magnetite)
Produce greenhouse gases – helped keep Mars warm and wet?
At slow spreading ridges, extension is accommodated by faulting. These faults enhance hydration of the oceanic crust, resulting in a higher degree of metasomatism.
This alteration makes the lithosphere mechanically weak and influences tectonic behavior.

Shock Metamorphism

Shatter cones form when shock waves pass through the rock, generating pressures between 2 – 30 GPa.
Pseudotachylites are generated by fast frictional sliding and are associated with impact structures and fault zones.