Lecture part 60% Tests: 1st: Topic 1-3 (20%) 2nd: Topic 4-9 (20%) 3rd: Topic (20%) Final: all

Transcription

1 Igneous and metamorphic petrology 1. Fundamentals 2. Classification 3. Thermodynamics and kinetics Igneous 4. Silicate melts and fluids 5. Crystal melt equilibria 6. Chemical dynamics of melts and crystals 7. Magma ascent, emplacement and eruption 8. Generation of magma and differentiation 9. Magmatism and tectonics Metamorphic 10. Fabric, composition and classification 11. Mineral reactions and equilibria 12. Processes and kinetics 13. P-T-t paths, facies and zones Lecture part 60% Tests: 1st: Topic 1-3 (20%) 2nd: Topic 4-9 (20%) 3rd: Topic (20%) Final: all Lab: 1. Identification of rocks Hand specimen and microscope 2. CIPW norm calculation 3. Thermodynamics problemset 4. Petrological databases 5. MELTS Lab part 40% Identification: igneous (15%), metamorphic (9%); exercises (16%)

2 Magmatic rocks: Formed by cooling of magma ( o C at the surface) Concentrated in regions in the Earth Both igneous and metamorphic processes require thermal energy Energy: capacity to do work (w), product of force (F) and displacement (d). W=Fd Work in geological systems related to pressure and volume (PV). Pressure: force over area P=F/area, volume V=area x d. PV=Fd=w. Kinetic energy: F=1/2 mv 2 Potential energy: related to position. Gravitational potential energy: E= mgz Thermal energy: internal, transferred as heat

3 Energy transfer and heat Thermal energy and work (PV) are convertible and transformation is conservative No loss of energy or mass: first law of thermodynamics Heat flow: quantity of heat ( q) transferred to a body results in a rise in temperature ( T): q=c p T. c p is heat capacity (J/molK). Heat can be transferred through: 1. Radiation 2. Advection 3. Conduction 4. Convection Radiation insignificant for Earth s heat budget, because Cool rocks are opaque Advection, where? Fluid flow through rocks, cracks. Hydrothermal systems Conduction: transfer of kinetic energy by vibrating atoms. No conduction in perfect vacuum. Difference in T between two locations: thermal gradient Rate at which heat is conducted from a unit surface area: heat flux or heat flow heatflow= thermal conductivity x thermal gradient Geothermal gradient or geotherm T/ z

4 Geotherm and convection At the surface thermal gradient is 20K/km. Convecting mantle results in a less steep geotherm with depth Three pieces of evidence for convection and the existence of a viscous mantle: Viscosity: measure of resistance to flow Mantle is times more viscous than tar There is a pressure dependence on the viscosity 1. Mid-ocean ridge volcanism 2. Subducting slabs 3. Mantle plumes

5 Igneous activity has petrotectonic association: Certain rocktypes are found together. Energy sources: Accretion Core formation Radioactive decay Inner core growth Pressure: Geobaric gradient P/ z 1bar=10 5 Pa= atm, 1000bar = 1 kbar=0.1 GPa. Lithostatic load is confining pressure P=F/A=mg/A, m is mass and g is acceleration of gravity or P/ z= ρg, where ρ is density Rock forming processes: Changes in states of a system. System is user defined. State of the system: conditions that define its properties or energy. Equilibrium, stable-metastable

6 Rock properties 1) Composition a) -Chemical b) -Mineralogical c) -Modal 2) Field relation 3) Fabric

7 What does petrology want to answer When and how did a particular magma originate How was the magma transported from dource to emplacement What physical, chemical and thermal processes operated on the system during crystallization What was the nature of the rock prior to metamorphism and its history of deformation and recrystallization How do petrologic processes control evolution of the crust and relate to global tectonics How can the modern petrotectonic associations by used to infer tectonic regimes in ancient rocks How did the planet originate and evolve What is the effect of petrological processes on society and life

8 Composition and classification Analytical procedures: -Sampling controlled by factors like: grainsize, alteration, weathering -Accuracy and precision. Precision: how well can you reproduce the number Accuracy: how close to the true value. -Modal analysis often done by point counting -Chemical analyses Major elements content reported in wt% Trace element content in ppm or ppb Instruments: XRF, ICP, electron probe Volatiles are driven off: Loss On Ignition

9 Mineral composition Mineral association: There are a limited number of combinations: For example: quartz and magnesian olivine do do co-exist Other examples: leucite and orthopyroxene

10 Major minerals and their composition Major mineral Simple formula Compatible trace elements Olivine (Mg,Fe) 2 SiO 4 Ni, Cr, Co Orthopyroxene (Mg,Fe) 2 Si 2 O 6 Ni, Cr, Co Clinopyroxene Ca(Mg,Fe)(Si,Al) 2 O 6 Cr, Sc Hornblende (Ca,Na) 2-3 (Mg,Fe,Al) 5 Ni,Cr,Co,Sc (Si,Al) 8 O 22 (OH,F) 2 Biotite K 2 (Mg,Fe,Al,Ti) 6 Ni,Cr,Co,Sc,Ba,Rb (Si,Al) 8 O 20 (OH,F) 4 Muscovite K 2 Al 4 (Si,Al) 8 O 20 (OH,F) 4 Rb,Ba Plagioclase (Na,Ca)(Si,Al) 4 O 8 Sr,Eu K-feldspar KAlSi 3 O 8 Accessory minerals Magnetite Fe 3 O 4 V,Sc Ilmenite FeTiO 3 V,Sc Sulfides Cu,Au,Ag,Ni,PGE Zircon ZrSiO 4 Hf,U,Th, heavy REE Apatite Ca 5 (PO 4 ) 3 (OH,F,Cl) U, middle REE Allanite Ca 2 (Fe,Ti,Al) 3 (O,OH) Light REE, Y, Th, U (Si 2 O 7 )(SiO 4 ) Xenotime YPO 4 Heavy REE Monazite (Ce,La,Th)PO Y, light REE Titanite (Sphene) CaTiSiO 5 U,Th,Nb,Ta, middle REE

11 Chemical composition Cartesian or triangular variation diagrams Diagrams are designed to highlight process,

12 Chemical composition II Modal composition Sierra Nevada batholith

13 Classification based on fabric Phaneritic: contains grains large enough to identify by eye Aphanitic: grains are too small to be identified by eye Porphyritic: Large grain size (phenocysts) and small grain size (matric) Aphyric: contains no crystals Sparsely phyric: contains less then 5% crystals Phyric: contain more then 5% crystals Holocrystalline: made entirely of crystals Felsic: contains large amount of feldspars Mafic: Fe-rich Ultramafic: Fe and Mg-rich Granite Aplite Pegmatite

14 Mafic and ultramafic

15 Apanitic and Glassy Rocks

16 CIPW Normative composition Hypothetical mineral assembledge based on the whole rock composition 1. Molecular ratio of Fe 2 O 3 /FeO= Calculate molar proportions of the oxides 3. Add MnO and NiO to FeO 4. Add SrO and BaO to CaO 5. Normative apatite, Ap, allocate CaO equal to 3.3 times P 2 O 5 6. Il, allocate FeO equal to the proportion f TiO 2 7. If there is excess TiO 2 allocate amount of CaO equal to the excess TiO 2 to make titanite, but only after An allocation 8. If there is still excess TiO 2 allocate it to rutile 9. Allocate Al 2 O 3 for Or 10. If there is excess K 2 O make Ks, peralkaline 11. Allocate excess Al 2 O 3 to make provisional Ab, 12. If there is excess Na 2 O allocate Fe 2 O 3 to make Ac.

17 CIPW Normative composition cont d CIPW Normative composition cont d 13. If there is excess Na2O make Ns. 14. If there is excess Al 2 O 3 make An 15. If there is excess Al 2 O 3 make C. 16. Allocate equal amount of FeO to Fe 2 O 3 to make Mt. 17. If there is excess Fe 2 O 3 make Hm. 18. Calculate FeO/MgO ratio. 19. Allocate (FeO+MgO) equal to CaO with FeO/MgO ratio to Di. 20. If there is excess CaO allocate it to Wo. 21. If there is excess (FeO+MgO) make Hy. 22. Assign SiO 2 to the normative minerals. 23. If there is excess SiO 2 make Qz. 24. If there is a deficit of SiO 2 an additional 10 steps

18 CIPW Normative composition cont d Why? Silica saturation (Mg,Fe) 2 SiO 4 + SiO 2 = 2(Mg,Fe)SiO 3 and NaAlSiO 4 + 2SiO 2 = NaAlSi 3 O 8 Modest silica deficiencies are shown by normative Ol, while strong undersaturation is shown by normative Ne and Lc. Silica oversaturated: Qz; silica saturated: Hy; silica undersaturated: Ol. Different saturation levels lead to different pathways during melting and crystallization. Alumina saturation: