Introduction to the Metamorphism of Carbonate Rocks

1
Introduction to the Metamorphism of Carbonate
Rocks

• IN THIS LECTURE
• Calcite marbles
• Decarbonation
• Dolomitic marbles
• Calc-silicate rocks
• Fluid composition in marbles

2
Marbles

• The term marble is used for metamorphosed
  calcareous rocks in which carbonate minerals
  dominate.
• This represents essentially two end-member
  compositions
• Very pure calcite limestones
• Impure calcite or dolomitic limestones
• Metamorphism of these two end-member compositions
  produces two different rock types
• Pure calcite marbles which are petrologically not
  very interesting
• Dolomitic marbles which are petrologically
  interesting

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Calcite

• Colour
• colourless
• Pleochroism
• non pelochroic
• Form
• variety of habits, but usually coinsist of
  scalenohedron and rhombohedron combinations. In
  most rocks calcite forms anhedral grains or grain
  aggregates
• Relief
• moderate negative to high positive, marked change
  with stage rotationnw 1.658ne 1.486
• Cleavage
• perfect rhombohedral cleavage, angle between
  cleavages 7457
• Birefringence
• 0.172, extreme, creamy high order colours
• Twinning
• lamellar twins parallel to one edge of the
  cleavage rhomb or along the long diagonal of the
  rhomb

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Calcite

• Optic Character
• uniaxial
• Extinction
• extinction is inclined or symmetrical to cleavage
  traces
• Composition
• dominantly CaCO3, but substitution of Mg, Fe, Mn,
  or Zn and minor Sr and Ba
• Alteration
• altered to dolomite during diagenesis, calcite is
  soluble in natural waters and may be removed by
  solution
• Occurrence
• common and widespread as a major mineral in
  limestones, and an accessory in igneous,
  metamorphic and sedimentary rocks
• Distinguishing Features
• cleavage, variable relief, extreme interference
  colours

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Calcite
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Calcite vs Dolomite

• Distinguishing calcite, dolomite and other
  rhombohedral carbonates from each other can be
  very difficult without obtaining chemical
  analysis or using chemical stains.
• Often can use associated mineralogy to help
  decide
• Well come back to this

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Calcite Marbles

• In general, metamorphism of a pure calcite
  limestone simply produces a pure calcite marble.
• Petrologically not very interesting since calcite
  is stable to very high pressures and
  temperatures.
• Relatively pure limestones that contain a small
  amount of quartz are more interesting as they
  show one of the simplest examples of the most
  common reaction type in carbonate rocks,
  decarbonation reactions.
• CaCO3 SiO2 -gt CaSiO3 CO2
• Calcite quartz -gt wollastonite fluid
• However, at pressures of more than a couple of
  kilobars the temperature required to form
  wollastonite is beyond the range of normal
  regional metamorphism

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Wollastonite

• Formula
• CaSiO3 Pyroxenoid group.
• Usually pure, but Mn and Fe2 can substitute for
  Ca
• Crystal System
• Triclinic -gt Biaxial
• Crystal Habit
• Columnar and fibrous elongate grains, often with
  twinning 
• Cleavage
• Perfect cleavage on 100, good cleavages on
  001 and -102 Splitery cleavage fragments.
  Angles of cleavage 84.5 degrees, and 70 degrees
• Color/Pleochroism
• Colorless, white, greyish, often with yellowish
  or brownish tint. Vitreous. No pleochroism

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Wollastonite

• Refractive Indices a 1.616-1.645b
  1.628-1.652g 1.631-1.656d 0.013-0.017
• Increase with Fe and Mn content. Wollastonite
  resembles tremolite and pectolite, but both have
  a higher birefringence.
• Extinction
• Parallel  Elongate crystals display parallel
  extinction.
• Distinguishing Features
• Colorless to grey in thin sectionwith moderate to
  moderatly high relief. First order interference
  color yellow-orange. One perfect cleavage and two
  good cleavages producing splintery cleavage
  fragments. H 4.5-5. G 2.86-3.09. Streak is
  colorless or white. 
• Occurrence
• Occurs commonly as a product of contact and/or
  regional metamorphism in limestone and dolomite.
  Associated minerals include calcite, and
  grossular in hornfels, tremolite, epidote group
  members, diopside, and other Ca-Mg silicates.

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Wollastonite
11
P-T Stability of Calcite Quartz
12
The Role of Fluid Composition

• How then do we explain the presence of
  wollastonite in marbles that have not been to
  such high temperatures?
• Reduce the pressure of the CO2 phase.
• At temperatures of the greenschist facies and
  above, H2O and CO2 supercritical fluids are
  completely miscible
• Hence the partial pressure of CO2 in a mixed
  H2O-CO2 fluid may be much less than the total
  fluid pressure.

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P-T Stability of Calcite Quartz
14
Phase Rule Constraints

• The observed effect of adding H2O to the calcite
  quartz wollastonite CO2 equilibria accords
  with the phase rule.
• Recalling that F C P 2
• In the H2O-absent system there are four phases
  and three components (CaO, SiO2 and CO2) giving
  one degree of freedom.
• This means that the full assemblage can only
  exist along a univariant curve.
• If H2O is added to the system then the number of
  components is increased by one but the number of
  phases stays the same since H2O is miscible with
  CO2.
• Hence there are two degrees of freedom
• Therefore fluid composition is a variable in
  addition to T and P and by specifying one of
  these three variables the equilibrium conditions
  can be represented by a univariant curve on a
  plot with the other two variables as axes.

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Effect of Fluid Composition
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T-XCO2 diagrams

• These types of plots are known as isobaric T-XCO2
  diagrams
• On these types of plots divariant equilibria plot
  as a line known as an isobaric univariant curve.
• Therefore if the P is specified there is still
  one degree of freedom within the system.

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Dolomitic Marbles

• Limestones that contain dolomite provide much
  more useful indicators of metamorphic grade
  because of a range of Ca-Mg silicates can form in
  the more usual P-T conditions of metamorphism,
  such as talc, tremolite and diopside.
• With prograde metamorphism there is a zonal
  sequence of mineral-appearance isograds similar
  to what we saw with pelites.
• This zonal sequence in regionally metamorphosed
  dolomitic limestones appears to be
• Talc (not always present)
• Tremolite
• Diopside or forsterite
• Diopside forsterite
• This zonal scheme was first identified by Eskola,
  one of the fathers of metamorphic petrology in
  1922.

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Dolomitic Marbles and the Phase Rule

• The zonal scheme identified by Eskola, although
  applying generally, is actually much more complex
  in natural systems.
• Why is this?
• Again look at the phase rule.
• Dolomitic marbles can be described by five
  components
• CaO, MgO, SiO2, H2O and CO2
• No assemblages have more than five phases,
  normally four minerals and a mixed fluid phase.
• Therefore according to the phase rule, there
  should be two degrees of freedom in most systems
  and thus most mineral assemblages will occur over
  a wide range of pressures and temperatures
  depending on what the composition of the fluid
  phase is.

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Calc-Silicates

• Calc-silicates are rocks rich in Ca-Mg silicate
  minerals but with only minor amounts of carbonate
  present.
• Like dolomitic marbles, calc-silicates are useful
  indicators of metamorphic grade.
• They can be correlated with the pelite zones in
  the following manner
• Pelite zone Calc-silicate zone
• Garnet Zoisite-calcite-biotite
• Zoisite-hornblende
• Staurolite Anorthite-hornblende
• Kyanite
• Sillimanite Anorthite-pyroxene

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Calc-Silicates

• Calc-silicates contain significant amounts of
  other chemical components especially Al, K and
  Fe.
• Therefore their mineralogy is more complex than
  that of dolomite marbles and additional phases
  include
• Zoisite
• Garnet
• Hornblende
• Ca-pyroxene like diopside
• Calcic-plagioclase
• K-feldspar
• Phlogopite and vesuvianite
• In general zoisite and grossular garnet are only
  stable if the fluid phase is rich in water, while
  calcic-plagioclase is favoured by CO2 dominated
  fluids.

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Diopside

• FormulaCaMgSi2O6
• Crystal SystemMonoclinic -gt Biaxial
• Crystal HabitShort, stubby, prismatic crystals
  with square, rectangular, or eight sided cross
  sectionGranular, lamellar, or columnar masses
  Anhedral grains
• CleavageFair to good cleavage on (110), Partings
  on (100) and (001)Imperfect cleavage
  intersecting at 87º and 93º ie typical pyroxene
  cleavage
• Color/PleochroismNo pleochroism Colorless to
  pale green in thin section

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Diopside

• Refractive Indicesa 1.664-1.745b
  1.672-1.753g 1.694-1.771d 0.018-0.034
• Extinctioninclined in (010) sections
• Distinguishing Featureslight green color,
  cleavage
• OccurrenceCommonly found in metamorphosed
  carbonate rocks like skarns and marbles. Found
  with tremolite, actinolite, grossular garnet,
  epidote, wollastonite, forsterite, calcite and
  dolomite

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Diopside
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Epidote

• Formula
• Ca2(Al,Fe)Al2O(SiO4)(Si2O7)(OH)
• Complete solid solution from clinozoisite (Al Fe
  3 30) to epidote (AlFe 3 21)
• Crystal System
• Monoclinic -gt Biaxial (ep ve, czo ve)
• Crystal Habit
• coarse to fine granular also fibrous
• Cleavage
• 001 perfect, 100 imperfect perfect cleavage
  in one direction
• Color/Pleochroism
• clinozoisite pale green to gray epidote
  pistachio-green to yellowish-green to black

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Epidote

• Clinozoisite epidotea 1.670-1.715 1.715-1.751
  b   1.674-1.725 1.725-1.784 g  1.690-1.734
  1.734-1.797
• Max Birefringence
• 0.004 - 0.049 Refractive indices and
  birefringence increase with iron content
• Extinction
• Parallel to length of elongate crystals and to
  the trace of cleavage.
• Distinguishing Features
• Epidote is characterized by its green color and
  one perfect cleavage. H 6-7. G 3.25 to 4.45.
  Streak is white to gray. Clinozoisite and epidote
  are distinguised from eachother by optic sign,
  birefringence, and color
• Occurrence
• Occurs in areas of regional metamorphism forms
  during retrograde metamorphism and forms as a
  reaction product of plagioclase, pyroxene, and
  amphibole. Common in metamorphosed limestones
  with calcium rich garnets, diopside, vesuvianite,
  and calcite.

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Epidote
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Actinolite-Tremolite

• Formula
• Ca2(Mg,Fe2)5Si8O22(OH)2
• Crystal System
• Monoclinic -gt Biaxial
• Crystal Habit
• occurs as columnar, bladed or acicular grains,
  elongated parallel to c axis, may be fibrous,
  basal sections are diamond shaped, with typical
  amphibole cleavage
• Cleavage
• two amphibole cleavages on 110, intersect at 56
  and 124
• Colour/Pleochroism
• colourless to pale green to dark green, darker
  colours and stronger pleochroism associated with
  high Fe contents

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Actinolite-Tremolite

• Refractive Indices
• a 1.599-1.688b 1.612-1.697g 1.622-1.705
• Birefringence
• 0.017-0.027
• maximum interference colours are upper 1st to mid
  2nd order
• Extinction
• Inclined extinction greater for tremolite than
  actinolite
• Distinguishing Features
• Can exhibit simple and lamellar twins
• Alters to talc, chlorite and carbonates
• Resembles hornblende but often has lower
  extinction angle
• Occurrence
• common occurrence is in contact and regional
  metamorphosed limestone and dolomite. Also found
  in metamoprhosed mafic and ultramafic rocks. It
  is the common fine-grained alteration product of
  pyroxenes.

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Actinolite-Tremolite
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