Op ch08 lecture_earth3, metamorphic minerals

Chapter 8: Metamorphism: A Process of Change Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak

Chapter 8Metamorphism: A Process of Change

©2008 W. W. Norton & Company, Inc.

Portrait of a PlanetThird Edition

earth

LECTURE OUTLINE

Metamorphism: A Process of Change

Prepared by

Ron Parker Earlham College Department of Geosciences

Richmond, Indiana


IntroductionIntroduction Metamorphic – Changed from an original “parent.” Metamorphic – Changed from an original “parent.”

Meta = Change.Meta = Change. Morph = Form or shape.Morph = Form or shape.

Parent rocks are called “protoliths.”Parent rocks are called “protoliths.” Metamorphism can Metamorphism can

occur to any protolith. occur to any protolith.


IntroductionIntroduction Protoliths undergo pronounced changes in…Protoliths undergo pronounced changes in…

Texture. Texture. Mineralogy.Mineralogy.

Due to change in physical or chemical conditions.Due to change in physical or chemical conditions. Burial.Burial. Tectonic stresses.Tectonic stresses. Heating by magma.Heating by magma. Fluid alteration.Fluid alteration.


MetamorphismMetamorphism Metamorphism occurs in the Metamorphism occurs in the solidsolid state. state. It doesn’t include weathering, diagenesis, melting.It doesn’t include weathering, diagenesis, melting. Metamorphics often look totally unlike protoliths.Metamorphics often look totally unlike protoliths.


Metamorphic CharacterMetamorphic Character Metamorphic rocks have distinctive properties. Metamorphic rocks have distinctive properties.

Unique texture – Intergrown and interlocking grains.Unique texture – Intergrown and interlocking grains. Unique minerals – Some that are Unique minerals – Some that are onlyonly metamorphic. metamorphic.

Staurolite, Kyanite, Sillimanite, etc.Staurolite, Kyanite, Sillimanite, etc. Unique foliation – A planar fabric from aligned minerals.Unique foliation – A planar fabric from aligned minerals.

These transformations can change the rock utterly. These transformations can change the rock utterly.

Red mudstoneRed mudstone

Garnet gneissGarnet gneiss

Fossiliferous limestoneFossiliferous limestone

MarbleMarble


Metamorphic ProcessesMetamorphic Processes Metamorphic change is slow and in the solid state.Metamorphic change is slow and in the solid state. Several processes are at work.Several processes are at work.

Recrystallization – Minerals change size and shape.Recrystallization – Minerals change size and shape.

Phase change – New minerals form with…Phase change – New minerals form with…Same chemical formula.Same chemical formula.Different crystal structure.Different crystal structure.

Kyanite


Metamorphic ProcessesMetamorphic Processes Several processes may operate at the same time.Several processes may operate at the same time.

Neocrystallization – New minerals with P-T changes. Neocrystallization – New minerals with P-T changes. Initial minerals become unstable; change to new minerals.Initial minerals become unstable; change to new minerals.

Decomposition of the original minerals in the protolith.Decomposition of the original minerals in the protolith.

Chemical reaction of the elements to form new minerals.Chemical reaction of the elements to form new minerals.

In this way, a shale can transform into a garnet mica schist.In this way, a shale can transform into a garnet mica schist.


Metamorphic ProcessesMetamorphic Processes Several processes may operate at the same time.Several processes may operate at the same time.

Pressure solution – Mineral grains partially dissolve.Pressure solution – Mineral grains partially dissolve.

Plastic deformation – Mineral grains soften and deform.Plastic deformation – Mineral grains soften and deform.


Causes of MetamorphismCauses of Metamorphism The agents of metamorphism are… The agents of metamorphism are…

Heat (Temperature – T).Heat (Temperature – T). Pressure (P).Pressure (P). Differential stress.Differential stress. Hydrothermal fluids.Hydrothermal fluids.

Not all agents are required; they often do co-occur. Not all agents are required; they often do co-occur. Rocks may be overprinted by multiple events.Rocks may be overprinted by multiple events.


Heat (Temperature)Heat (Temperature) Metamorphism occurs as the result of heat.Metamorphism occurs as the result of heat.

Temperature (T) ranges between 200Temperature (T) ranges between 200ooC and 850C and 850ooC.C.

The upper T limit is…melting. It varies based upon The upper T limit is…melting. It varies based upon rock mineral composition and water content.rock mineral composition and water content.

Heat energy breaks and reforms atomic bonds. Heat energy breaks and reforms atomic bonds. Sources of heat.Sources of heat.

The geothermal gradient. The geothermal gradient. Magmatic intrusions.Magmatic intrusions. Compression.Compression.


Pressure (P)Pressure (P) P increases with depth in the crust.P increases with depth in the crust.

270 to 300 bars per km (1 bar is almost 1 atm = 14.7 psi).270 to 300 bars per km (1 bar is almost 1 atm = 14.7 psi). Metamorphism occurs mostly in 2 to 12 kbar range. Metamorphism occurs mostly in 2 to 12 kbar range.

T and P T and P bothboth change with depth. change with depth. Mineral stability is highly dependent upon T and P. Mineral stability is highly dependent upon T and P.

This stability can be graphed on a “phase diagram.”This stability can be graphed on a “phase diagram.” Changes in T and P lead Changes in T and P lead

to changes in minerals.to changes in minerals.


Pressure that is greater in one orientation. Pressure that is greater in one orientation. A common place result of tectonic forces.A common place result of tectonic forces. Two kinds of differential stress: Normal and shear.Two kinds of differential stress: Normal and shear.

Normal stress – Operates perpendicular to a surface. Normal stress – Operates perpendicular to a surface. Tension – Pull-apart normal stress.Tension – Pull-apart normal stress.Compression – Push-together normal stress.Compression – Push-together normal stress.

Differential StressDifferential Stress


Differential StressDifferential Stress Two kinds of differential stress: Normal and shear.Two kinds of differential stress: Normal and shear.

Shear stress – Operates sideways across a surface.Shear stress – Operates sideways across a surface.Causes material to be “smeared out.”Causes material to be “smeared out.”


At higher T and P, differential stress deforms rock.At higher T and P, differential stress deforms rock. Rocks change shape slowly without breaking.Rocks change shape slowly without breaking.

Differential StressDifferential Stress


Differential StressDifferential Stress Deformation acts on minerals with specific shapes.Deformation acts on minerals with specific shapes.

Equant – Roughly equal in all dimensions.Equant – Roughly equal in all dimensions. Inequant – Dimensions not the same. Inequant – Dimensions not the same.

Platy (pancake-like) – 1 dimension shorter.Platy (pancake-like) – 1 dimension shorter.Elongate (cigar-shaped) – 1 dimension longer.Elongate (cigar-shaped) – 1 dimension longer.

Differential stress causes these minerals to align.Differential stress causes these minerals to align. Alignment fabric records stress trajectory.Alignment fabric records stress trajectory.


Differential Stress Differential Stress Preferred platy mineral alignment is called foliation. Preferred platy mineral alignment is called foliation.

Foliation imparts a layered or banded appearance.Foliation imparts a layered or banded appearance. Rocks commonly break parallel to foliation planes.Rocks commonly break parallel to foliation planes.

Foliation develops perpendicular to compression.Foliation develops perpendicular to compression. Minerals flatten, recrystallize, and rotate.Minerals flatten, recrystallize, and rotate.

Inequant grains align by rotation and new growth.Inequant grains align by rotation and new growth.


Hydrothermal FluidsHydrothermal Fluids Hot water with dissolved ions and volatiles.Hot water with dissolved ions and volatiles. Hydrothermal fluids facilitate metamorphism. Hydrothermal fluids facilitate metamorphism.

Accelerate chemical reactions.Accelerate chemical reactions. Alter rocks by adding or subtracting elements. Alter rocks by adding or subtracting elements.

Hydrothermal alteration is called metasomatism.Hydrothermal alteration is called metasomatism.


Metamorphic Rock TypesMetamorphic Rock Types Two major subdivisions of metamorphic rocks.Two major subdivisions of metamorphic rocks.

Foliated – Has a through-going planar fabric.Foliated – Has a through-going planar fabric.Subjected to differential stress.Subjected to differential stress.Has a significant component of platy minerals. Has a significant component of platy minerals. Classified by composition, grain size, and foliation type.Classified by composition, grain size, and foliation type.


Metamorphic RocksMetamorphic Rocks Non-foliated rocks lack a planar fabric.Non-foliated rocks lack a planar fabric.

Absence of foliation possible for several reasons.Absence of foliation possible for several reasons.Rock not subjected to differential stress.Rock not subjected to differential stress.Dominance of equant minerals.Dominance of equant minerals.Absence of platy minerals.Absence of platy minerals.


Metamorphic RocksMetamorphic Rocks Quartzite – Almost pure quartz in composition. Quartzite – Almost pure quartz in composition.

Forms by alteration of quartz sandstone. Forms by alteration of quartz sandstone. Sand grains in the protolith recrystallize and fuse. Sand grains in the protolith recrystallize and fuse. Like quartz, it is hard, glassy, and resistant. Like quartz, it is hard, glassy, and resistant.

Metamorphic Alteration


Metamorphic RocksMetamorphic Rocks Marble - Coarsely crystalline calcite or dolomite.Marble - Coarsely crystalline calcite or dolomite.

Forms from a limestone or dolostone protolith. Forms from a limestone or dolostone protolith. Extensive recrystallization completely changes the rock.Extensive recrystallization completely changes the rock. Original textures and fossils in the parent are obliterated. Original textures and fossils in the parent are obliterated. Used as a decorative and monument stone.Used as a decorative and monument stone. Exhibits a variety of colors.Exhibits a variety of colors.

Metamorphic Alteration


Metamorphic RocksMetamorphic Rocks Type depends on protolith. Type depends on protolith.

Minerals contribute elements.Minerals contribute elements. Some protoliths yield specific rocks.Some protoliths yield specific rocks.

Broad compositional classes.Broad compositional classes. Pelitic.Pelitic. Basic (or Mafic).Basic (or Mafic). Calcareous.Calcareous. Quartzo-feldspathic.Quartzo-feldspathic.


Metamorphic ClassesMetamorphic Classes Pelitic – Shale protoliths.Pelitic – Shale protoliths.

Al-rich clay minerals yield micas. Al-rich clay minerals yield micas. Rock type depends on grade.Rock type depends on grade.

Slate.Slate.Phyllite.Phyllite.Schist.Schist.Gneiss.Gneiss.


Metamorphic ClassesMetamorphic Classes Mafic – Basalt or gabbro protolith.Mafic – Basalt or gabbro protolith.

Rich in Fe and Mg and poor in Si, Al, Na, and K.Rich in Fe and Mg and poor in Si, Al, Na, and K. Turn into biotite- and amphibole-dominated rocks.Turn into biotite- and amphibole-dominated rocks.


Metamorphic ClassesMetamorphic Classes Calcareous – Carbonate protolith.Calcareous – Carbonate protolith.

Rich in Ca (limestones) and Ca and Mg (dolostones). Rich in Ca (limestones) and Ca and Mg (dolostones). Recrystallize into calcite and dolomite marbles.Recrystallize into calcite and dolomite marbles.


Metamorphic GradeMetamorphic Grade Example: Pelitic rock.Example: Pelitic rock.

Low grade – Shale protolith.Low grade – Shale protolith.Clays recrystallize into larger, aligned clays to yield a slate.Clays recrystallize into larger, aligned clays to yield a slate.Clays neocrystallize into tiny, aligned micas in a phyllite. Clays neocrystallize into tiny, aligned micas in a phyllite.

Intermediate grade – Intermediate grade – Micas recrystallize and grow large to form a schist.Micas recrystallize and grow large to form a schist.New minerals grow in the schist.New minerals grow in the schist.

High grade – High grade – Micas decompose; elements recombine into new minerals.Micas decompose; elements recombine into new minerals.Neocrystallization yields quartz and feldspars in a gneiss. Neocrystallization yields quartz and feldspars in a gneiss.


Metamorphic EnvironmentsMetamorphic Environments The types (and settings) of metamorphism are...The types (and settings) of metamorphism are...

Thermal – Heating by a plutonic intrusion.Thermal – Heating by a plutonic intrusion. Burial – Increases in P and T by deep burial in a basin.Burial – Increases in P and T by deep burial in a basin. Dynamic – Shearing in a fault zone. Dynamic – Shearing in a fault zone. Regional – P and T alteration due to orogenesis. Regional – P and T alteration due to orogenesis. Hydrothermal – Alteration by hot water leaching.Hydrothermal – Alteration by hot water leaching. Subduction – High P–Low T alteration.Subduction – High P–Low T alteration. Shock – Extreme high P attending a bolide impact. Shock – Extreme high P attending a bolide impact. Mantle – Extreme high mantle P causes phase changes.Mantle – Extreme high mantle P causes phase changes.


Contact MetamorphismContact Metamorphism Due to heat from magma invading host rock.Due to heat from magma invading host rock. Creates zoned bands of alteration in host rock.Creates zoned bands of alteration in host rock.

Called a contact (or metamorphic) aureole.Called a contact (or metamorphic) aureole. The aureole surrounds the plutonic intrusion. The aureole surrounds the plutonic intrusion.

Zoned from high (near pluton) to low grade (far from pluton).Zoned from high (near pluton) to low grade (far from pluton).


Burial MetamorphismBurial Metamorphism As sediments are buried in a sedimentary basin…As sediments are buried in a sedimentary basin…

P increases because of the weight of the overburden.P increases because of the weight of the overburden. T increases because of the geothermal gradient. T increases because of the geothermal gradient.

Requires burial below diagenetic effects. Requires burial below diagenetic effects. This is ~ 8–15 km depending on the geothermal gradient.This is ~ 8–15 km depending on the geothermal gradient.


Regional MetamorphismRegional Metamorphism Tectonic collisions deform huge “mobile belts.” Tectonic collisions deform huge “mobile belts.” Directed compression thickens mountains.Directed compression thickens mountains.

Rocks caught up in mountain building are…Rocks caught up in mountain building are…Heated via the geothermal gradient and plutonic intrusions.Heated via the geothermal gradient and plutonic intrusions.Squeezed and heated by deep burial. Squeezed and heated by deep burial. Smashed and sheared by differential stresses.Smashed and sheared by differential stresses.


Regional MetamorphismRegional Metamorphism Regional metamorphism creates foliated rocks.Regional metamorphism creates foliated rocks. This type of metamorphism is, by far, the most This type of metamorphism is, by far, the most

important in terms of the amount of rock altered.important in terms of the amount of rock altered. Collisional belts are often… Collisional belts are often…

1000s of km long. 1000s of km long. 100s of km wide.100s of km wide.


Hydrothermal MetamorphismHydrothermal Metamorphism Alteration by hot, chemically aggressive water.Alteration by hot, chemically aggressive water. A dominant process near mid-ocean ridge magma.A dominant process near mid-ocean ridge magma.

Cold ocean water seeps into fractured crust.Cold ocean water seeps into fractured crust. Heated by magma, this water then reacts with mafic rock.Heated by magma, this water then reacts with mafic rock. The hot water rises and is ejected via black smokers.The hot water rises and is ejected via black smokers.


Subduction MetamorphismSubduction Metamorphism Trenches and accretionary prisms have…Trenches and accretionary prisms have…

A low geothermal gradient – low temperature.A low geothermal gradient – low temperature. High pressures. High pressures.

High P – Low T favor High P – Low T favor

glaucophane, a blue glaucophane, a blue

amphibole mineral.amphibole mineral.


Chapter 8Metamorphism: A Process of Change

©2008 W. W. Norton & Company, Inc.

Portrait of a PlanetThird Edition

earth

LECTURE OUTLINE

This concludes the

Op ch08 lecture_earth3, metamorphic minerals

Education

process of change earth

unstable change

unique minerals

original minerals

aligned minerals

recrystallization minerals

initial minerals

heat temperature t