Earth Materials Final

Role of protolith composition vs geologic conditions in producing equilibrium metamorphic mineral assemblages

Rocks develop stable mineral assemblages under specific P, T, X at thermodynamic/chemical equilibrium

Relict (pre-metamorphic) or alteration minerals NOT included

Mineral Paragenesis

equilibrium mineral assemblage

Textural means for assessing equilibrium in metamorphic systems

• No obvious signs of reaction or disequilibrium textures 

• All mineral phases present are in physical contact 

  • inclusions likely not in equilibrium with matrix phases

• Lack of layering

• Textural equilibrium attained - 

  • Protolith textures completely replaced by metamorphic textures

The aluminosilicate (Al2SiO5) triple point diagram

At 0.4 Gba and 500 degrees celsius

Kyanite in right left corner, sillimanite in other, and andalusite on the bottom

Goldschmidts phase rule

P<C- for equilibrium to be true

System displays solid solution

P > C

F < 2 – systems represents an univariant curve (F = 1) or invariant point (F = 0)

Equilibrium has not been attained

Number of components not chosen correctly

Strategies for selecting the adequate number of chemical components to plot graphically

for a given metamorphic rock

• Need to choose enough components to represent all major minerals present

  • individual elements or oxides

• Ignore components that are a) only present in trace amounts or components that only enter a single phase, or  mobile components (e.g., volatiles)

• Combine components that behave similarly (e.g., Mg and Fe in solid solution)

• Limit rock types being shown (less chemical variability shown, the better)

• Projections 

Eskola

Concluded different mineral assemblages corresponded to different P-T regimes called facies

different mineral assemblages in chemically identical metamafic rocks

descriptive meaning vs interpretive meaning

Descriptive - metamorphic facies corresponds to a set of repeatable mineral associations/assemblages

Interpretive - metamorphic facies corresponds to different relative P-T conditions

Low P/T facies

• Zeolite Facies, Albite-Epidote Hornfels Facies, Hornblende Hornfels Facies, Pyroxene Hornfels Facies. Sanidinite Facies

• Common in Contact Metamorphic Settings (static P, med-hi T)

Medium P/T facies

• Common in Orogenic Belts (lo-med P, lo-hi T)

  • Zeolite Facies, Prehnite-Pumpellyite Facies,  Greenschist Facies, Amphibolite Facies, Granulite Facies

  • gneiss development and partial melting (i.e., migmatization) common

High P/T facies

• Common in Subduction Zones (med-hi P, lo-med T)

  • (Zeolite Facies),  Blueschist Facies,  Eclogite Facies

Metamorphic processes at subduction zones

increasing pressure and temperature, which causes the minerals in the rocks to change into new forms - vary with depth, leading to distinct metamorphic processes

production of paired metamorphic belts

• direct consequence of the varying pressure-temperature conditions at different depths and regions of the subduction zone

• Low-Temperature, High-Pressure Belt (Blueschist Facies) 

• High-Pressure, High-Temperature Belt (Eclogite Facies)

igneous processes at subduction zones

• Subducted plate shoved under overriding plate - Melting in overlying mantle wedge due to rising fluids from slab

• Generated melts rise, fractionate, interact with overriding plate, and may eventually erupt

• Oceanic SZ - Mostly produced volcanic rocks and rocks with mafic to intermediate compositions

• Continent SZ - intermediate to felsic extrusive and intrusive igneous rocks

Textural and mineralogical reactivity of basaltic protoliths

Texture - Fine grains become coarse and vesicles collapse

Mineralogical - volcanic glasses breakdown, any H2O from hydration is released

Tectonic implications of P-T(-t) paths of different shapes

Clockwise - common in regional settings - subduction and collision

Boomerang - common in contact settings - plutons

Counterclockwise - common in select granulite facies settings - proximal to intruding plutons 

Polymorphic mineral reactions

Chemistry of phases doesn’t change, but structure does

Coexisting polymorphs represent incomplete transformations

Exsolution mineral reactions

Unmixing of two phases (typically in the same mineral family) upon cooling or decompression

Commonly results in exsolution lamellae

Solid-Solid Net Transfer mineral reactions

mineral products are chemically different from mineral reactants

Devolatilization mineral reactions

Influenced not only by P-T conditions, but also by composition of fluid (Xfluid) present

Continuous mineral reactions

occur over a range of temperatures depending on composition of phases present with respect to their chemical endmembers

Most common in Fe-Mg silicates

Ion exchange mineral reactions

involving the exchange of select ions between components

Redox mineral reactions

Reactions involving changing oxidation state of a particular ion

Influenced by oxygen content

multiple mineral polymorphs in the same metamorphic rock

varied P-T conditions the rock has experienced during metamorphism

common in tectonically active regions, such as subduction zones

Aluminosilicate most common

Geologic fluid composition (i.e., H2O-CO2 content) impact on (P-)T of devolatilization

plays a crucial role in determining the P-T conditions at which devolatilization occurs

Water systems generally lead to devolatilization at lower temperatures and pressures

CO₂ systems promote devolatilization at higher temperatures and pressures

Importance of calculating P-T conditions of metamorphism

• interpreting the geological history of rocks, understanding tectonic processes, and predicting the stability and transformation of minerals

• framework for reconstructing the thermal and pressure evolution of metamorphic terrains

Geobarometry vs geothermometry

Geobarometry 

• Geobarometers- geologic pressure constraints

• GASP’ Barometer: Ca-garnet + kyanite + quartz → Ca-plagioclase

Geothermometry 

• Geothermometers - geologic T constraints)

• Garnet-Biotite Thermometer:

  • Annite (Fe-biotite) + Pyrope (Mg-garnet) → Phlogopite (Mg-biotite) + Almandine (Fe-garnet

Pelite

mudstone or shale - mostly clay and micas

Along continental margins

Textural and mineralogical reactivity of pelitic protoliths

• Fine-grain sizes coarsen in response to increasing metamorphic grade

•  Original clays and micas readily breakdown and dehydrate in response to increasing metamorphic grade

• Results in extensive changes to mineralogy in response to metamorphism

KFMASH

K2O-FeO-MgO-Al2O3-SiO2-H2O

KFMASH strengths and weaknesses

Strengths - includes all major oxides

micas, garnet, staurolite, kyanite, sillimanite, chlorite, and feldspar

Weaknesses - may not represents all minerals - no calcium or sodium - cannot show plag

General temperature conditions and mineral reactions associated with anatexis of

metapelites

between 650°C to 800°C

5 to 10 kbar

role of anatexis in producing migmatite

Migmatites typically occur at conditions where the temperature is high enough for partial melting to occur but not high enough to fully melt the rock. This means that the rock undergoes partial anatexis, where some minerals melt while others remain solid

Igneous processes associated with anatexis

Partial melting of crust - need either thickened crust or increased heat flow

Caused by H2O liberation from breakdown of micas in metasedimentary rocks