Metamorphic Petrology Lecture 1: Metamorphic phenomena and their characterization: An introduction by Stephan K Matthäi MP-SKM, slide 1
I will try to teach you: Course Objectives To identify common metamorphic rocks in the field and infer their protoliths (original rock types and composition), Understand how they formed, Get broad estimates of the pressure and temperature conditions under which the rocks were metamorphosed, How to use overprinting relationships and deformation structures to determine the geological / metamorphic history of the rocks, Infer the burial depth and thermal history of the metamorphic pile, Make PTt-path diagrams Interpret the plate-tectonic setting of metamorphism, Quantify the chemical changes that the rock underwent during metamorphism (gains & losses), Get you ready for independent field work. MP-SKM, slide 2
ES4.08 Prerequisites Geology: plate-tectonic settings, basics of sedimentary and igneous rocks, magmatism and volcanism (Internal Processes, Dynamic Earth) Mineralogy: ability to determine the main rock-forming minerals in hand specimen and thin section; ideally, a knowledge of the chemical composition of minerals (Minerals & Rocks, Optical Mineralogy & Petrography) Chemistry: stochiometry (balancing reactions), possible valency states of cations, law of mass action, equilibrium constants (Geochemistry 1) Thermodynamics: absolute basics Gibbs free energy, heat capacity, entropy, enthalpy, work etc. (Thermodynamics is desirable but not essential) Mathematics: basic algebra and elementary calculus (Basic Math, Introduction to Calculus). MP-SKM, slide 3
Outcomes By the end of this course you should be able to: Identify the most important metamorphic minerals and know their approximate chemical composition Make inferences on the protolith of metamorphic rocks on the basis of their mineralogy and chemical composition Know the key metamorphic reactions and parageneses that define the boundaries of metamorphic grades Have a basic knowledge of how you can use crystalline solid solutions as geo-thermometers and barometers Distinguish metamorphic fabrics and pre-, syn and post-kinematic mineral growth as well as pro- and retrograde assemblages Use petrogenetic grids Interpret metamorphic history in terms of PTt paths based on field and laboratory observations MP-SKM, slide 4
Course Structure & Itinerary Session 1: Introduction to Metamorphic Petrology Session 2: Solid solutions and continuous/discontinuous reaction thermodynamics Session 3: Contact metamorphism, isograds, geothermometers and Barometers Session 4: Metamorphism & deformation, pre-, syn- & post-kinematic mineral assemblages, brittle-ductile transition Revision Break Session 5: Regional metamorphism: facies concept surpassed by metamorphic grades, petrogenetic grids. Session 6: Incipient metamorphism and reaction kinetics Session 7: Medium-, high- grade metamorphism and anatexis Session 8: Metasomatism and hydrothermal alteration Extra: Question hour in preparation for examination MP-SKM, slide 5
Course work You will have to prepare 2 assignments: A1: following lecture 4, due for lecture 6: Exercise on the mineralogy of common metamorphic minerals A2: following lecture 7, due April 1: Case study: reconstruction of the metamorphic history of a Greek island MP-SKM, slide 6
Recommended Reading Bucher, K. & Frey, M., 2002, Petrogenesis of metamorphic rocks (7 th ed.), Springer, Berlin, 341 p. Best, M.G., 2002, Igneous and Metamorphic Petrology (extended version of Best, M.G. & Christiansen, E.H. Igneous Petrology. Blackwell Science, ISBN 0-86542-541-8, 458 p.). Bucher, K., 1997, Petrogenesis of metamorphic rocks. Based on Winkler (5 th ed., Springer, 348 p.). You may also be interested in Pichler, H. & Schmitt-Riegraf, C., 1989, Rock-Forming Minerals in Thinsection. Chapmann & Hall, 230 p. Spear, S. & Peacock, S.M. 1989, Metamorphic Pressure-Temperature- Time Paths, AGU Short Course in Geology 7, 102 p. Wood, B. J. & Fraser, D. G. 1977, Elementary Thermodynamics for Geologists. Oxford University Press, 303 p. Yardley, B., 2001, Introduction to metamorphic petrology (2nd ed.). Blackwell Scientific Communications. MP-SKM, slide 7
Lecture 1: Topics A overview of Metamorphic Petrology: 1. The main driver: heat 2. Exercise: common metamorphic rocks 3. Metamorphic reactions 4. Mineral paragenesis 5. An overview of metamorphic settings 6. Classification of metamorphism: index minerals, metamorphic facies and metamorphic grades MP-SKM, slide 8
Definition of Metamorphism The change of the mineral assemblage (and composition of a rock) in response to changes in temperature, pressure, or volatile content. Mineralogical and usually structural transformation of a rock in the solid state, as a consequence of physical/chemical conditions which differ from those under which the protolith was formed (Schreiner, Mehnert & Winkler). Distinctions: Gradual transition to metasomatism where changes are not isochemical Gradual transition to hydrothermal alteration which, in some cases, may be referred to as metamorphism in response to changes in temperature and volatile content. Gradual transition to diagenesis, poorly defined as the transformation of a rock between sedimentation and metamorphism (Correns). NB: The transition between diagenesis and metamorphism can be defined as the boundary above which mineralogical changes can be clearly related to elevated temperature and-or pressure. MP-SKM, slide 9
Where does metamorphism occur? MP-SKM, slide 10
1. The main driver: Heating in the Crust & Mantle MP-SKM, slide 11
Geotherms & gradients Put values on the axis, Mark the boundary of the continental and the oceanic crust? Estimate some typical geothermal gradients. MP-SKM, slide 12
2. Metamorphic rocks and their protoliths Increasing temperature and pressure Granite Basalt Dunite Shale Sandstone Limestone Write the names of corresponding metamorphic rocks into the empty fields in the columns; circle those rocks which we had a chance to look at in handspecimen. MP-SKM, slide 13
Assigning names to metamorphic rocks Names often contain the word schist (puff-pastry like), fels (massive, fine-crystalline), marble (predominantly carbonate), or gneiss Determine relative volumetric proportions of minerals, for example: garnet biotite schist with 30 vol.% gnt & 25 vol.% biotite Start with a generic name and become more specific: metapelite -> quartz phyllite MP-SKM, slide 14
3. Metamorphic Reactions Solid-solid (a) and solid-fluid (b = dissolution precipitation) reactions, e.g. A + B = C + D; classification: Continuous reactions (over PT range) chlorite (Fe-rich) + phengite biotite + chlorite (Fe-poor) + quartz + H 2 O Discontinuous reactions (at fixed PT conditions) andalusite sillimanite Recrystallization versus. grainsize reduction during deformation. b) devolatilization: Dehydration loss of H 2 O Decarbonation loss of CO 2 Pyrolysis liquification of C MP-SKM, slide 15
Discontinuous reactions: Al 2 SiO 5 ppolymorphs Pressure [GPa] MP-SKM, slide 16
Continuous reactions are exchange reactions Garnet (Fe,Mg) 3 Al 2 Si 3 O 12 and Biotite K(Fe,Mg) 3 AlSi 3 O 10 (OH) 2 FeMg (garnet) = FeMg (biotite) (Fe,Mg) 3 Al 2 Si 3 O 12 + KAlSi 3 O 8 + H 2 O = Al 2 SiO 5 + K(Fe,Mg) 3 AlSi 3 O 10 (OH) 2 + 2 SiO 2 With increasing temperature andradite garnet becomes more Mg-rich and Fe-poor while biotite does the opposite. MP-SKM, slide 17
4. Mineral paragenesis Paragenesis = A group of minerals which formed contemporaneousl y and in contact with one-another, at the same PT conditions in the rock, implying that these minerals were in chemical equilibrium when they formed. crossed polarizers, 6mm MP-SKM, slide 18
Types of events recorded by mineral parageneses One classifies metamorphic paragenetic mineral assemblages as prograde, peak-metamorphic and retrograde. This interpretation is based on the PT conditions defined by petrogenetic stability fields and mutual overprinting relationships. NB: This interpretation of sequential equilibrium assemblages is in conflict with genuine equilibration which is not obtained as is indicated by presence of remnant minerals from the previous assemblage. The mitigating factor is the reaction rate. Reaction kinetics influenced by factors such as temperature, grain-to-grain contact, deformation rate, presence of fluids, grain-surface area, mineral zonations etc. control whether a mineral is preserved or replaced. Thus - it is not always easy to determine which part of a PT path a mineral assemblage is related to. Sometimes prograde assemblages have vanished, often there is no retrograde assemblage. MP-SKM, slide 19
5. Metamorphic Settings from geoscience flyer (1998) Univ. Minnesota, St Paul MP-SKM, slide 20
Island Arcs & Active Continental Margins MP-SKM, slide 21
Metamorphism of a Subducting Slab MP-SKM, slide 22
Deep Intrusions (>10 km) MP-SKM, slide 23
Metamorphism of the Oceanic Crust MP-SKM, slide 24
Metamorphism & Rifting MP-SKM, slide 25
Extension: metamorphism & detachment faults MP-SKM, slide 26
6. Classification of Metamorphism Historical evolution: 1. Index minerals (=Barrowian zones, Barrow, 1893) 2. Phenomenological classification based on common transformations of specific rock types: Metamorphic Facies, Eskola (1915) 3. Classification in terms of the peak pressure (P) and temperature (T) which the rock experienced, grouped into 4 Metamorphic Grades, Winkler (1976) 4. Petrogenetic grids MP-SKM, slide 27
6.1: Index minerals Developed for medium pressure rocks in Scottish Highlands by Barrow (1893). Simple: The occurrence of index minerals is mapped, e.g. Chlorite biotite almandine staurolite kyanite sillimanite Temperature conditions are inferred Problems Specific to rock type Ignores pressure dependence of mineral stability (different sequences are possible) Imprecise MP-SKM, slide 28
6.2: Eskola s (1920) facies classification for basaltic rocks MP-SKM, slide 29
Facies definition & its shortcomings Each facies comprises a set of minerals which formed over a poorly defined range of PT conditions from a specific protolith. The common facies names refer to mineral assemblages common in basaltic rocks. Applying this strictly one would classify a staurolite micaschist as part of the amphibolite facies The introduction of sub-facies (e.g., Turner 1960) led to a proliferation of facies names It follows that metamorphic facies is not indicative of specific PT conditions It is purely descriptive. MP-SKM, slide 30
6.3: Metamorphic grade (Winkler 1976) Winkler replaced the facies classification by 4 metamorphic grades as defined by largely protolith-independent univariant mineral reactions: very-low low Winkler Metamorphic Grade Eskola and later. Facies Equivalent zeolite facies greenschist facies epidote-amphibolite facies medium amphibolite facies pyroxene fels facies high - anatexis Example: Reaction boundary between medium & high grade: muscovite + quartz = K-feldspar + sillimanite + H 2 O MP-SKM, slide 31
The four metamorphic grades Pressure [MPa] diagenesis Depth [km] nonexistent conditions Temperature [ o C] MP-SKM, slide 32
Solution exercise 1 (slide 12) We had a look at hand specimens of the following rocks: 1. slate - phyllite - mica-schist gneiss, granulite - hornfels 2. basalt - greenschist / blueschist - amphibolite / eclogite 3. quartzite 4. limestone - marble - calc-silicate rock (marble with olivine or diopside, tremolite, wollastonite etc.) 5. dunite - serpentinite - soapstone MP-SKM, slide 33
Revision questions for Session 1 1. Try to memorize the new terminology 2. Make a list of metamorphic minerals noting which are hydrous and which are not 3. Calculate by how much the temperature of the granite from slide 11 (radiogenic heat production) should rise by radioactive decay of unstable elements within a years time, assuming that the sample is thermally insulated from its surroundings. Hint: Revise what the heat capacity is. MP-SKM, slide 34