1. Ch 25 Metamorphic Facies

2. V.M. Goldschmidt (1911, 1912a) studied contact metamorphosed pelitic (mudrocks) , calcareous (limestone and dolostone) , and psammitic (sandstone) hornfels near Oslo s. Norway
Fewer than six major minerals in the aureoles around granitoid intrusives
First to note that the equilibrium mineral assemblage of a metamorphic rock could be related to Xbulk
Aluminous pelites contained Al-rich minerals, such as cordierite, plagioclase, garnet, and/or an Al2SiO5 polymorph
Calcareous rocks contained Ca-rich and Al-poor minerals such as Diopside, Wollastonite, and/or amphibole

3. Victor Moritz Goldschmidt- Oslo
Certain mineral pairs (e.g. anorthite + enstatite) were consistently present in rocks of appropriate composition, whereas the compositionally equivalent pair (diopside + andalusite) was not
If two alternative assemblages are X-equivalent, we must be able to relate them by a reaction
In this case the reaction is simple:
MgSiO3 + CaAl2Si2O8 = CaMgSi2O6 + Al2SiO5
En An Di And

4. Metamorphic Facies Pentii Eskola (1914, 1915) Orijärvi, S. Finland
Rocks with K-feldspar + cordierite at Oslo contained the compositionally equivalent pair biotite + muscovite at Orijärvi
Eskola: difference must reflect differing physical conditions
His Finnish rocks (more hydrous and lower volume assemblage) equilibrated at lower temperatures and higher pressures than the Norwegian ones of Goldschmidt.

5. Metamorphic Facies Oslo: Kfs + Mg-Crd Orijärvi: Biot + Ms + Qtz
Reaction:
2 KMg3AlSi3O10(OH)2 + 6 KAl2AlSi3O10(OH) SiO2
Biot Ms Qtz
= 3 Mg2Al4Si5O KAlSi3O8 + 8 H2O
Mg-Crd K-spar water

6. Metamorphic Facies
Pentii Eskola (1915) developed the concept of metamorphic facies:
“In any rock or metamorphic formation which has arrived at a chemical equilibrium through metamorphism at constant temperature and pressure conditions, the mineral composition is controlled only by the chemical composition. We are led to a general conception which the writer proposes to call metamorphic facies.”
Eskola ‘s dual basis facies : Xbulk & mineralogy
A metamorphic facies is a set of repeatedly associated metamorphic mineral assemblages

7. Metamorphic Facies
Eskola was aware of the P-T implications and correctly deduced the relative temperatures and pressures of facies he proposed
Modern lab results: can now assign relatively accurate temperature and pressure limits to individual facies

8. Metamorphic Facies Eskola (1920) proposed 5 original rock facies:
Greenschist
Amphibolite
Hornfels (this term has too many conflicting definitions)
Sanidinite
Eclogite
Easily defined on the basis of mineral assemblages that develop in mafic rocks

9. Metamorphic Facies In his final account, Eskola (1939) added:
Granulite
Epidote-amphibolite
Glaucophane-schist (now called Blueschist)
... and changed the name of the hornfels facies to the pyroxene hornfels facies

10. Mafic Metamorphic Facies
Fig Temperature- pressure diagram showing the generally accepted limits of the various facies used in this text.
Mafic Metamorphic Facies
The limits are approximate and gradational, because the reactions vary with rock composition and the nature and composition of the fluid phase
The 30oC/km geothermal gradient is an example of an elevated orogenic geothermal gradient.
The boundaries between metamorphic facies represent T-P conditions in which key minerals in mafic rocks are either added or removed, thus changing the mineral assemblages observed
They are thus separated by mineral reaction isograds

11. Metamorphic Facies
Table The definitive mineral assemblages that characterize each facies (for mafic rocks).
The definitive mineral assemblages that characterize each facies (for mafic rocks) are listed in Table (details and variations later)

12. Miyashiro (1961,1973) extended the facies concept to encompass broader progressive sequences: facies series
A traverse up grade through a metamorphic terrane should follow one of several possible metamorphic field gradients (Fig. 21-1, next slide), and, if extensive enough, cross through a sequence of facies
Facies Series
How to traverse up grade? Walk toward the migmatite.

13. Metamorphism of Mafic Rocks
Mineral changes and associations along T-P gradients characteristic of the three facies series
Hydration of original mafic minerals generally required
If water unavailable, mafic igneous rocks will remain largely unaffected, even as associated sediments are completely re- equilibrated
Coarse-grained intrusives are the least permeable and likely to resist metamorphic changes
Tuffs and graywackes are the most permeable and so subject to metamorphic changes.
Most of the common minerals in mafic rocks exhibit extensive solid solution, so that metabasites tend to have fewer phases than pelites
This results in fewer reactions and isograds, which is one of the reasons that Eskola chose to define his facies on these rocks, since it resulted in broader subdivisions, and made it easier to categorize and correlate different areas

14. Greenschist, Amphibolite, and Granulite Facies
The greenschist, amphibolite and granulite facies constitute the most common facies series of regional metamorphism on Mafic rocks

15. Metamorphism Of A Basaltic Rock Example:
Progressive Thermal Metamorphism of a Diabase (usually a shallow dike, basaltic in composition)
Each successive case is hotter: closer to the magma

16. HOT GREENSCHIST 1 200C Sphene is CaTiSiO5
Progressive thermal metamorphism of a Diabase (coarse basaltic)
Stage 1: lots of relict textures from Diabase Augite and Plagioclase
Plag becomes Albite NaAlSi3O8, so it loses Ca and (Ca -> calcite)
Augite
(Ca,Na)(Mg,Fe,Al,Ti)(Si,Al)2O6
hydrates -> Chlorite
(Mg,Fe)3(Si,Al)4O10(OH)2.(Mg,Fe)3(OH)6
And Actinolite
Ca2(Mg,Fe)5Si8O22(OH)2.
Color is greener
HOT
GREENSCHIST C
Sphene is CaTiSiO5

17. Progressive thermal metamorphism of a diabase (coarse basalt)
Stage 2- closer to igneous body, thoroughly recrystallized
Epidote forms as more Ca released from Calcite
Even greener
EVEN HOTTER
Greenschist 2
Epidote is Ca2Al2(Fe+3Al)(SiO4)(Si2O7)O(OH)

18. Progressive thermal metamorphism of a diabase
Stage 3: Warmer yet, Actinolite, Chlorite, Epidote, Calcite become unstable as Hornblende and more calcic plagioclase become stable “Amphibolite”
Note coarser grain size as well
REALLY HOT 500C
Amphibolite

19. Progressive thermal metamorphism of a diabase (coarse basalt)
Stage 4: Right up against a magma (hot), dehydration at high grade -> Cpx + Plagioclase rock
Note that mineralogy is now ~ same as original diabase, but the granoblastic texture is new
VERY HOT> 700C
Granulite

20. Granulite Facies
Origin of granulite facies: general agreement on two points
1) Granulites represent unusually hot conditions
Temperatures > 700oC (geothermometry has yielded some very high temperatures, even in excess of 1000oC)
Granoblastic: Phaneritic, equidimensional, not foliated, no porphyroblasts

21. Granulite Facies 2) Granulites are dry, no significant water
Rocks don’t melt despite high Temps due to lack of available water
Granulite facies represent deeply buried and dehydrated roots of the continental crust
Fluid inclusions in granulite facies rocks of S. Norway are CO2-rich, whereas those in the amphibolite facies rocks are H2O-rich
It is a long-held tenet that granulite facies terranes represent the deeply buried and dehydrated roots of the continental crust
Most exposed granulite facies rocks are found in the deeply eroded areas of the Precambrian continental shields
Touret showed that the fluid inclusions in granulite facies rocks of S. Norway are CO2-rich, while those in the amphibolite facies rocks are more H2O-rich
This may imply that dehydration of at least some granulite facies terranes may be due to the infiltration of CO2 replacing H2O, rather than the elimination of fluids altogether

22. Fig Temperature- pressure diagram showing the generally accepted limits of the various facies used in this text. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

23. Ophiolite suite = Ocean Lithosphere

24. Mafic Assemblages of the High P/T Series: Blueschist and Eclogite Facies
Mafic rocks (not pelites) develop definitive mineral assemblages under high P/T conditions
High P/T geothermal gradients characterize subduction zones
Mafic blueschists are easily recognizable by their color, and are useful indicators of ancient subduction zones. Formation requires abundant water present near subducted ophiolites.
The great density of eclogites: subducted basaltic oceanic crust becomes more dense than the surrounding mantle
Begins to sink , extended necks thin and break off pieces
The implications of eclogite density on subduction processes was discussed in Chapter 16, and on mantle dynamics and isotopes in Chapter 14

25. Pressure-Temperature-Time (P-T-t) Paths
Staurolite (St) as relict within poikiloblastic andalusite (And).
Pressure-Temperature-Time (P-T-t) Paths
Proposed metamorphic P-T-t paths may be tested by:
1) Checking for partial overprints of one mineral assemblage upon another
The relict minerals may indicate a portion of either the prograde or retrograde path (or both) depending upon when they were created
Muscovite + chlorite + quartz + staurolite = andalusite / sillimanite + biotite + H20
We learned that thermodynamic equilibrium is generally maintained during prograde metamorphism, so that relict minerals are more common during the retrograde path, but both types have been observed
Staurolite (St) as relict within poikiloblastic andalusite (And).

26. Pressure-Temperature-Time (P-T-t) Paths
Proposed metamorphic P-T-t paths may be tested by:
2) Applying geothermometers and geobarometers to the core vs. rim compositions of chemically zoned minerals to document the changing P-T conditions experienced by a rock during their growth

27. Pressure-Temperature-Time (P-T-t) Paths
Classic view: regional metamorphism is a result of deep burial or intrusion of hot magmas
Plate tectonics: regional metamorphism is a result of crustal thickening and heat input during orogeny at convergent plate boundaries (not simple burial)
Heat flow higher than the normal continental geotherm is required for typical greenschist to amphibolite medium P/T reaction.
Nearly all models indicate that heat flow higher than the normal continental geotherm is required for typical greenschist-amphibolite medium P/T facies series, and that uplift and erosion has a fundamental effect on the geotherm and must be considered in any complete model of metamorphism

29. Miyashiro’s (1961,1973) facies series don’t look correct for a subduction zone.
We think that the sequence of facies from basaltic is Greenschist, then Amphibolite, then Eclogite, as if PT conditions follow something like the red line.

30. Chapter 26: Metamorphic Reactions
Yellow Isograds are reaction lines

31. 1. Phase Transformations
Isochemical phase transformations (the polymorphs of SiO2 or Al2SiO5 or graphite-diamond or calcite-aragonite SINGLE COMPONENT
The transformations depend on temperature and pressure only
Aragonite is the stable CaCO3 polymorph commonly found in blueschist facies terranes

32. 1. Phase Transformations
Independent of other minerals, fluids, etc., present.
Example:
Andalusite -> Sill as T and P increase
regardless of other phases Stau, Mus, Qtz
We used the presence of andalusite to indicate low-pressure metamorphic conditions, and by referring to Fig we can effectively limit the pressure of andalusite-bearing rocks to values below about 0.38 GPa
The presence of two coexisting polymorphs in a single rock has often been taken to indicate that the metamorphic peak corresponded to equilibrium conditions along the univariant boundary curve separating the pair
If an independent estimate of either pressure or temperature is available, the other parameter may then be estimated from the location of the equilibrium curve
Thus if kyanite and sillimanite were to be observed together, for example, and the pressure were estimated via geobarometry to be 0.5 GPa, then the temperature of equilibration could be determined from Fig to be approximately 560oC
If all three Al2SiO5 polymorphs were to be found in stable coexistence, the assemblage would indicate conditions at the invariant point (ca. 500oC and 3.8 GPa)

33. 1. Phase Transformations
Small DS for most polymorphic transformations
 small DG between two alternative polymorphs, even several tens of degrees from the equilibrium boundary
 little driving force for the reaction to proceed ® common metastable relics in the stability field of other
Coexisting polymorphs may therefore represent non-equilibrium states (overstepped equilibrium curves)
Often by carefully observing the textures one can distinguish partial replacement and metastable coexistence from true stable equilibrium grain boundaries
Hietanen (1956) reported all three Al2SiO5 polymorphs in northern Idaho, and proposed that metamorphic events in the area occurred near the invariant point
More likely that kyanite is partially replaced by sillimanite during a prograde event near the kyanite-sillimanite boundary, and that andalusite replaces kyanite during a later event at lower pressure
Staurolite poikiloblast

34. 2. Exsolution
Some solid solutions are unstable at lower T, and exsolve as T falls.
Classic example K-spar KAlSi3O8 and Albite NaAlSi3O8 form a solid solution at 1000C but separate into Microcline + Albite -> Perthitic Texture
Figure T-X phase diagram of the system albite-orthoclase at 0.2 GPa H2O pressure. After Bowen and Tuttle (1950). J. Geology, 58, Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

35. 3a. Solid-Solid Net-Transfer Reactions
Differ from polymorphic transformations: involve solids of differing composition
Material must diffuse from one site to another for the reaction to proceed
NaAlSi2O6 + SiO2 = NaAlSi3O8
Jd Qtz Ab
MgSiO3 + CaAl2Si2O8 = CaMgSi2O6 + Al2SiO5
En An Di And
4 (Mg,Fe)SiO3 + CaAl2Si2O8 =
Opx An Plag
(Mg,Fe)3Al2Si3O12 + Ca(Mg,Fe)Si2O6 + SiO2
Gnt Cpx Qtz
Reaction curves typically pretty straight
DS and DV change little

37. 3b. Solid-Solid Net-Transfer Reactions with conserved water
If minerals contain volatiles, but the volatiles are conserved in the reaction so that no fluid phase is generated or consumed
For example, the reaction:
Mg3Si4O10(OH)2 + 4 MgSiO3 = Mg7Si8O22(OH)2
Talc Enstatite Anthophyllite
involves hydrous phases, but conserves H2O
It may therefore be treated as a solid-solid net-transfer reaction

38. 4. Devolatilization Reactions
For example the location of the reaction line on a
P-T phase diagram of the dehydration reaction:
KAl2Si3AlO10(OH)2 + SiO2 = KAlSi3O8 + Al2SiO5 + H2O
Musc Qtz K-spar Sill Water
depends upon the partial pressure of H2O (pH2O)

39. 4. Devolatilization Reactions
Here the equilibrium curve represents equilibrium between the reactants and products under water-saturated conditions (pH2O = PLithostatic)
Fig is a P-T phase diagram showing the equilibrium reaction curve for reaction (26-6)
The hydrous assemblage is always on the low-temperature side of the curve, and the evolved fluid phase is liberated as temperature increases
The concave upward shape is characteristic of all devolatilization equilibrium curves at low pressure because the slope, as determined by the Clapyron equation
is low at low pressures due to the high volume of the gas phase, but steepens quickly at higher pressures because the gas is most easily compressed
P-T phase diagram for the reaction Ms + Qtz = Kfs + Al2SiO5 + H2O showing the shift in equilibrium conditions as pH2O varies (assuming ideal H2O-CO2 mixing). Calculated using the program TWQ by Berman (1988, 1990, 1991). After Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

40. KAl2Si3AlO10(OH)2 + SiO2 = KAlSi3O8 + Al2SiO5 + H2O Ms. Qtz. K-spar
KAl2Si3AlO10(OH)2 + SiO2 = KAlSi3O8 + Al2SiO5 + H2O Ms Qtz K-spar Sill water
Suppose H2O is withdrawn from the system at some point on the water-saturated equilibrium curve: pH2O < Plithostatic
According to Le Châtelier’s Principle, removing water at equilibrium will be compensated by the reaction running to the right, thereby producing more water
This has the effect of stabilizing the right side of the reaction at the expense of the left side
So as water is withdrawn the Kfs + Sill + H2O field expands slightly at the expense of the Ms + Qtz field, and the reaction curve shifts toward lower temperature
Pfluid < PLith by drying out the rock and reducing the fluid content
Pfluid = PLith, but the water in the fluid can become diluted by adding another fluid component, such as CO2 or some other volatile phase

41. 4. Decarbonization Reactions
CaCO3 + SiO2 = CaSiO3 + CO2 (26-6)
Cal Qtz Wollastonite + CO2
Excess CO2 drives reaction to Cal + Qtz
CaCO3 + SiO2 <= CaSiO3 + CO2
The temperature of a wollastonite isograd based on this reaction is obviously dependent upon pCO2
Figure T-XCO2 phase diagram for the reaction Cal + Qtz = Wo + CO2 at 0.5 GPa assuming ideal H2O-CO2 mixing, calculated using the program TWQ by Berman (1988, 1990, 1991). Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
Figure A portion of the equilibrium boundary for the calcite-aragonite phase transformation in the CaCO3 system. After Johannes and Puhan (1971), Contrib. Mineral. Petrol., 31, Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

42. 5. Continuous Reactions
Occur when F  1, and the reactants and products coexist over a temperature (or grade) interval
Fig Schematic isobaric T-XMg diagram representing the simplified metamorphic reaction Chl + Qtz  Grt + H2O. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

43. 6. Ion Exchange Reactions
Reciprocal exchange of components between 2 or more minerals
MgSiO3 + CaFeSi2O6 = FeSiO3 + CaMgSi2O6
Enstatite + Hedenbergite = Ferrosilite + Diopside
Expressed as pure end-members, but really involves Mg-Fe (or other) exchange between intermediate solutions
Basis for many geothermobarometers