Download LECTURE 21 - Carbonates and Calc

Introduction to the Metamorphism of
Carbonate Rocks
IN THIS LECTURE
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Calcite marbles
Decarbonation
Dolomitic marbles
Calc-silicate rocks
Fluid composition in marbles
Marbles
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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
Calcite
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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 rotation
nw = 1.658
ne = 1.486
Cleavage
– perfect rhombohedral cleavage, angle between cleavages 74°57‘
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
Calcite
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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
Calcite
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Calcite vs Dolomite
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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
We’ll come back to this
Calcite Marbles
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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 -> CaSiO3 + CO2
Calcite + quartz -> wollastonite + fluid
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However, at pressures of more than a couple of kilobars the
temperature required to form wollastonite is beyond the range of
normal regional metamorphism
Wollastonite
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Formula
– CaSiO3 Pyroxenoid group.
– Usually pure, but Mn and Fe2+ can substitute for Ca
Crystal System
– Triclinic -> 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
Wollastonite
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Refractive Indices
a: 1.616-1.645
b: 1.628-1.652
g: 1.631-1.656
d: 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.
Wollastonite
P-T Stability of Calcite + Quartz
The Role of Fluid Composition
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How then do we explain the presence of wollastonite in marbles that
have not been to such high temperatures?
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Reduce the pressure of the CO2 phase.
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At temperatures of the greenschist facies and above, H2O and CO2
supercritical fluids are completely miscible
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Hence the partial pressure of CO2 in a mixed H2O-CO2 fluid may be
much less than the total fluid pressure.
P-T Stability of Calcite + Quartz
Phase Rule Constraints
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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.
Effect of Fluid Composition
T-XCO2 diagrams
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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.
Dolomitic Marbles
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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 mineralappearance 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.
Dolomitic Marbles and the Phase Rule
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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.
Calc-Silicates
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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
Kyanite
Anorthite-hornblende
Sillimanite
Anorthite-pyroxene
Calc-Silicates
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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.
Diopside
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Formula
CaMgSi2O6
Crystal System
Monoclinic -> Biaxial
Crystal Habit
Short, stubby, prismatic crystals with square, rectangular, or eight
sided cross section
Granular, lamellar, or columnar masses
Anhedral grains
Cleavage
Fair to good cleavage on (110), Partings on (100) and (001)
Imperfect cleavage intersecting at 87º and 93º ie typical pyroxene
cleavage
Color/Pleochroism
No pleochroism
Colorless to pale green in thin section
Diopside
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Refractive Indices
a = 1.664-1.745
b = 1.672-1.753
g = 1.694-1.771
d = 0.018-0.034
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Extinction
inclined in (010) sections
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Distinguishing Features
light green color, cleavage
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Occurrence
Commonly found in metamorphosed carbonate rocks like skarns and
marbles. Found with: tremolite, actinolite, grossular garnet, epidote,
wollastonite, forsterite, calcite and dolomite
Diopside
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Epidote
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Formula
– Ca2(Al,Fe)Al2O(SiO4)(Si2O7)(OH)
– Complete solid solution from clinozoisite (Al: Fe 3+ = 3:0) to epidote
(Al:Fe 3+ = 2:1)
Crystal System
– Monoclinic -> 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
Epidote
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Clinozoisite
epidote
a = 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.
Epidote
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Actinolite-Tremolite
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Formula
– Ca2(Mg,Fe2+)5Si8O22(OH)2
Crystal System
– Monoclinic -> 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
Actinolite-Tremolite
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Refractive Indices
a = 1.599-1.688
b = 1.612-1.697
g = 1.622-1.705
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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
ppl
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