Alteration of Rocks by Temperature and Pressure Lecture 7 Metamorphism: Alteration of Rocks by Temperature and Pressure

About Metamorphism Changes in heat, pressure, and the chemical environment of rocks can alter mineral compositions and crystalline textures, making them metamorphic. Metamorphic changes occur in the solid state, so there is no melting.

● internal heat of Earth ● internal pressure of Earth 1. Causes of Metamorphism ● internal heat of Earth ● internal pressure of Earth ● fluid composition inside Earth

● temperature increases with depth ● rate = 20º to 60ºC per km 1. Causes of Metamorphism ● temperature increases with depth ● rate = 20º to 60ºC per km ● at 15 km depth: 450ºC

Temperature increases with depth =Geothermal gradient

1. Causes of Metamorphism ● pressure and temperature increase with depth in all regions

● the role of temperature ● geothermal gradient ● shallow (20ºC / km) 1. Causes of Metamorphism ● the role of temperature ● geothermal gradient ● shallow (20ºC / km) ● steep (50ºC / km)

● the role of pressure (stress) ● confining pressure 1. Causes of Metamorphism ● the role of pressure (stress) ● confining pressure ● directed pressure

● the role of pressure (stress) ● rate of increase = 1. Causes of Metamorphism ● the role of pressure (stress) ● rate of increase = 0.3 to 0.4 kbar / km ● minerals are geobarometers

Pressure: Increases with depth Different types of pressure result in different rearrangement of minerals (compaction or elongation) Preferred mineral orientation Denser atomic structures

First type of metamorphism is called LOW GRADE metamorphism: Takes place around 150 degree C Which is typically at depths of 5km below the surface (and up to ~400C) At this temperature and pressure new minerals are formed  change from one mineral to another that is stable within the new range of pressure-temperature

Second type of metamorphism is called HIGH GRADE metamorphism: Takes place at temperature > 400C which is typically > 15 km below the surface At this temperature melting of mineral occurs

1. Causes of Metamorphism

● accelerated chemical reactions 2. Types of Metamorphism ● the role of fluids ● metasomatism ● accelerated chemical reactions

Hot water dissolves mineral and deposits minerals that it carries (metals) Metamorphism by hot fluids leads to concentration of precious metals (gold, silver, copper) ...

2. Types of Metamorphism Depth, km Oceanic crust 35 Continental crust 35 Continental crust Oceanic lithosphere 75 Continental mantle lithosphere Asthenosphere

Shock metamorphism Depth, km Oceanic crust 35 Continental crust Oceanic lithosphere 75 Continental mantle lithosphere Asthenosphere Heat and shock wave from impact transform rocks at impact site

Shock metamorphism Regional metamorphism Depth, km Oceanic crust 35 Continental crust Oceanic lithosphere 75 Continental mantle lithosphere Asthenosphere At convergent plate boundaries, occurs du to pressure and temperature increase

Regional Metamorphism: The pre-existing rock is subject to intense stresses and strains (deformation) usually from forces of mountain building Involves: High heat/ high pressure Wide spread geographically Slow process

Shock metamorphism Regional metamorphism High-pressure metamorphism Depth, km Oceanic crust 35 Continental crust Oceanic lithosphere 75 Continental mantle lithosphere Asthenosphere At convergent plate boundaries, occurs due to very high pressure at depth

Shock metamorphism Regional metamorphism High-pressure metamorphism Contact metamorphism Depth, km Oceanic crust 35 Continental crust Oceanic lithosphere 75 Continental mantle lithosphere Asthenosphere At any area in the vicinity of magma: Affects a thin zone of country rock next to an intrusion due to temperature increase

Contact Metamorphism: Magma “contacts” solid-rock Heat is transferred to the overlying rocks through conduction, which chance the pre-existing rock mineral composition Involves: High heat/ low pressure Localized geographically Fast acting process

Shock metamorphism Regional metamorphism High-pressure metamorphism Contact metamorphism Depth, km Oceanic crust 35 Continental crust Oceanic lithosphere 75 Continental mantle lithosphere Asthenosphere Burial metamorphism Affect any sedimentary rocks at depth due to increase in pressure and temperature

Hydrothermal metamorphism Shock metamorphism Regional metamorphism High-pressure metamorphism Contact metamorphism Depth, km Oceanic crust 35 Continental crust Oceanic lithosphere 75 Continental mantle lithosphere Asthenosphere Burial metamorphism Seafloor Metamorphism / Hydrothermal metamorphism At mid oceanic ridges due to circulation of seawater in the produced basalts (oceanic crust)

Hydrothermal Metamorphism: Chemical alteration of pre-existing rocks from the action of hot water. Usually the hot water is due to the presence of magma and is iron rich Involves: Medium heat/ low pressure + Hot water Localized geographically Fast acting process

What do all metamorphic processes have in common? Heat Provides the energy to cause recrystallization of preexisting minerals to new minerals Examples of heat sources: contact with hot magma (conduction); geothermal gradient (rocks are hotter when buried); contact with hot fluids

How do we know how much a rock has been metamorphosed? The rock texture and composition reflects the degree of metamorphism HP/HT – High grade (regional metamorphism): Rocks show a foliation (texture), a preferred mineral orientation, and signs of high pressure (deformation) LP/HT – Low grade (contact metamorphism): Rocks show a semi-parallel orientation of minerals and minimal signs of deformation

The metamorphic facies –mineral assemblages – reflects the temperature and pressure experienced by a rock

● metamorphism occurs in or near ● plate interiors 5. Plate Tectonics and Metamorphism ● metamorphism occurs in or near ● plate interiors ● divergent plate margins ● convergent plate margins ● transform plate margins

Tectonic transport moves rocks through different pressure-temperature zones … Low P, Low T High P, High T

Tectonic transport moves rocks through different pressure-temperature zones … Low P, Low T High P, High T …and then transports them back to the shallow crust or the surface.

● metamorphic pressure-temperature paths 5. Plate Tectonics and Metamorphism ● metamorphic pressure-temperature paths ● history of burial and exhumation ● prograde and retrograde paths

The garnet crystal initially grows in a schist but ends up growing in a gneiss. Low Grade Slate RETROGRADE PATH Intermediate Grade Phyllite PROGRADE PATH Depth (km) Pressure (kilobars) Schist Gneiss High Grade Temperature (°C)

Pressure (kilobars) Depth (km) Temperature (°C) Low Grade RETROGRADE PATH RETROGRADE PATH Pressure (kilobars) Depth (km) PROGRADE PATH High Grade PROGRADE PATH Peak metamorphism Temperature (°C) Low temperature– high-pressure metamorphism within a subduction zone Deep-ocean sediment Trench Continental crust Shelf sediment Oceanic crust Mélange ophiolites Continental crust Prograde path Peak metamorphism Retrograde path

Pressure (kilobars) Depth (km) Temperature (°C) Low Grade RETROGRADE PATH RETROGRADE PATH Pressure (kilobars) Depth (km) PROGRADE PATH High Grade PROGRADE PATH Peak metamorphism Temperature (°C) Low temperature– high-pressure metamorphism within a subduction zone High temperature– high-pressure metamorphism within a mountain belt Suture Deep-ocean sediment Trench Continental crust Deformed and metamorphosed shallow- and deep- ocean sediments Shelf sediment Continental crust Oceanic crust Multiple thrusts Mélange ophiolites Continental crust Continental crust Prograde path Prograde path Peak metamorphism Retrograde path Peak metamorphism Retrograde path

● rapid erosion (exhumation) rates of mountain ranges show a 5. Plate Tectonics and Metamorphism ● rapid erosion (exhumation) rates of mountain ranges show a relationship between ● tectonics (orogeny) ● climate ● controls the flow of metamorphic rocks to the surface

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