igneous and metamorphic petrology - libvolume3.xyzlibvolume3.xyz/.../petrologypresentation1.pdf ·...
TRANSCRIPT
Igneous and metamorphic petrology 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 10-13 (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%)
Formed by cooling of magma (700-1200oC 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 mv2
Potential energy: related to position. Gravitational potential energy: E=∆mgz Thermal energy: internal, transferred as heat
Magmatic rocks: Magmatic rocks:
Energy transfer and heat 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=cp∆T. cp 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, becauseBB
Advection, where?
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
Cool rocks are opaque
Fluid flow through rocks, cracks. Hydrothermal systems
Geotherm and convection Geotherm and convection
At the surface thermal gradient is 20K/km.
Convecting mantle results in a less steep geotherm with depth
1. Mid-ocean ridge volcanism Three pieces of evidence for convection
and the existence of a viscous mantle:
2. Subducting slabs
3. Mantle plumes
Viscosity: measure of resistance to flow Mantle is 1018 times more viscous than tar
There is a pressure dependence on the viscosity
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=105 Pa=0.9896 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
Rock properties Rock properties 1) Composition
a) -Chemical
b) -Mineralogical c) -Modal
2) Field relation
3) Fabric
What does petrology want to answer 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
Composition and classification 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
Mineral composition 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
Major minerals and their composition Major minerals and their composition
Major mineral Simple formula Compatible trace elements Olivine (Mg,Fe)2SiO4 Ni, Cr, Co
Orthopyroxene (Mg,Fe)2Si2O6 Ni, Cr, Co Clinopyroxene Ca(Mg,Fe)(Si,Al)2O6 Cr, Sc
Hornblende (Ca,Na)2-3(Mg,Fe,Al)5 Ni,Cr,Co,Sc
(Si,Al)8O22(OH,F)2
Biotite K2(Mg,Fe,Al,Ti)6 Ni,Cr,Co,Sc,Ba,Rb
(Si,Al)8O20(OH,F)4 Muscovite K2Al4(Si,Al)8O20(OH,F)4 Rb,Ba
Plagioclase (Na,Ca)(Si,Al)4O8 Sr,Eu
K-feldspar KAlSi3O8
Accessory minerals
Magnetite Fe3O4 V,Sc Ilmenite FeTiO3 V,Sc
Sulfides Cu,Au,Ag,Ni,PGE
Zircon ZrSiO4 Hf,U,Th, heavy REE Apatite Ca5(PO4)3(OH,F,Cl) U, middle REE
Allanite Ca2(Fe,Ti,Al)3(O,OH) Light REE, Y, Th, U (Si2O7)(SiO4)
Xenotime YPO4 Heavy REE
Monazite (Ce,La,Th)PO Y, light REE Titanite (Sphene) CaTiSiO5 U,Th,Nb,Ta, middle REE
Chemical composition Chemical composition
Cartesian or triangular variation diagrams Diagrams are designed to highlight process,
Chemical composition II Chemical composition II
Modal composition Sierra Nevada batholith
Classification based on fabric 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
Mafic and ultramafic Mafic and ultramafic
Apanitic and Glassy Rocks Apanitic and Glassy Rocks
CIPW Normative composition CIPW Normative composition
Hypothetical mineral assembledge based on the whole rock composition
1. Molecular ratio of Fe2O3/FeO=0.15 2. 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 P2O5 6. Il, allocate FeO equal to the proportion f
TiO2
7. If there is excess TiO2 allocate amount of
CaO equal to the excess TiO2 to make
titanite, but only after An allocation 8. If there is still excess TiO2 allocate it to
rutile 9. Allocate Al2O3 for Or
10. If there is excess K2O make Ks, peralkaline
11. Allocate excess Al2O3 to make provisional Ab,
12. If there is excess Na2O allocate Fe2O3 to make Ac.
CIPW Normative composition cont’d CIPW Normative composition cont’d 13. If there is excess Na2O make Ns.
14. If there is excess Al2O3 make An
15. If there is excess Al2O3 make C.
16. Allocate equal amount of FeO to Fe2O3 to make Mt.
17. If there is excess Fe2O3 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 SiO2 to the normative minerals.
23. If there is excess SiO2 make Qz.
24. If there is a deficit of SiO2 an additional 10 steps
CIPW Normative composition cont’d CIPW Normative composition cont’d
Why? Silica saturation (Mg,Fe)2SiO4 + SiO2 = 2(Mg,Fe)SiO3 and
NaAlSiO4 + 2SiO2 = NaAlSi3O8
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: