Lecture 7 Metamorphism Alteration of Rocks by Temperature

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.

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

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

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

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

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 5 km below the surface (and up to ~400 C) 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 > 400 C which is typically > 15 km below the surface At this temperature melting of mineral occurs

1. Causes of Metamorphism

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 0 Con tine nta 35 l cr Con ust tin ent al m 75 ant le l ith osp As her the e no sp he re Oceanic crust Oceanic lithosphere

Shock metamorphism Depth, km 0 Con tine nta 35 l cr Con ust tin ent al m 75 ant le l ith osp As her the e no sp he re Heat and shock wave from impact transform rocks at impact site Oceanic crust Oceanic lithosphere

Shock metamorphism Regional metamorphism Depth, km 0 Con tine nta 35 l cr Con ust tin ent al m 75 ant le l ith osp As her the e no sp he re At convergent plate boundaries, occurs du to pressure and temperature increase Oceanic crust Oceanic lithosphere

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 Depth, km 0 Con tine nta 35 l cr Con ust tin ent al m 75 ant le l ith osp As her the e no sp he re At convergent plate boundaries, occurs due to very high pressure at depth High-pressure metamorphism Oceanic crust Oceanic lithosphere

Shock metamorphism Regional metamorphism Depth, km 0 Con tine nta 35 l cr Con ust tin ent al m 75 ant le l ith osp As her the e no sp he re High-pressure metamorphism Contact metamorphism Oceanic crust Oceanic lithosphere 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 preexisting rock mineral composition Involves: High heat/ low pressure Localized geographically Fast acting process

Shock metamorphism Regional metamorphism Depth, km 0 Con tine nta 35 l cr Con ust tin ent al m 75 ant le l ith osp As her the e no sp he re Burial metamorphism High-pressure metamorphism Contact metamorphism Oceanic crust Oceanic lithosphere Affect any sedimentary rocks at depth due to increase in pressure and temperature

Shock metamorphism Regional metamorphism Depth, km 0 Con tine nta 35 l cr Con ust tin ent al m 75 ant le l ith osp As her the e no sp he re Burial metamorphism At mid oceanic ridges due to circulation of seawater in the produced basalts (oceanic crust) High-pressure metamorphism Contact metamorphism Oceanic crust Oceanic lithosphere Seafloor Metamorphism / Hydrothermal metamorphism

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

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.

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. RE TR PR O e lit yl Ph Pressure (kilobars) te Low Grade OG RA DE PA G Intermediate Grade R A TH Depth (km) Sla D E PA TH Sch ist Temperature (°C) Gneiss High Grade

PR OG RA DE PA TH High Grade PA TH Peak metamorphism Temperature (°C) Low temperature– high-pressure metamorphism within a subduction zone Continental crust Deep-ocean Shelf sediment Oceanic crust Trench Mélange ophiolites Continental crust Prograde path Peak metamorphism Retrograde path Depth (km) Pressure (kilobars) TH PA TH DE PA E AD R OG PR RA OG TR RE Low RE TR Grade OG RA DE

PR OG RA DE PA TH High Grade Depth (km) Pressure (kilobars) TH PA TH DE PA E AD R OG PR RA OG TR RE Low RE TR Grade OG RA DE PA TH Peak metamorphism Temperature (°C) Low temperature– high-pressure metamorphism within a subduction zone Continental crust Deep-ocean Shelf sediment Oceanic crust Trench Mélange ophiolites High temperature– high-pressure metamorphism within a mountain belt Suture Continental crust Multiple thrusts Deformed and metamorphosed shallow- and deepocean sediments Continental crust Prograde path Peak metamorphism Prograde path Retrograde path Peak metamorphism Retrograde path

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

This lecture Previous lecture