Lecture 14: A brief review

Transcription

1 Lecture 14: A brief review A few updates for the remainder of the course Report for the lab on pelite metamorphism - Lab 3 Needs to be handed in before Tuesday the 14 th of March at 17:00. My most important advice based on the lab reports for Lab 2: ask more questions! Subsequent lab will be on subduction zone metamorphism - Lab 4 The lab will be the afternoons of the 20 th and 27 th of March and 3 rd of April. The hand-in date for this lab is Tuesday the 4 th of April by 17:00. Final lab will showcase the effect of protolith on metamorphism - Lab 5 The lab will be one afternoon (the 10 th of April) and no lab report will need to be handed in for this lab. Final exam will likely be on the 27 th of April at 9:00 The exam is a standard, 3-hour, closed-book written exam. You will be allowed to use a calculator, but nothing else. The final exam will take place in the department. 1 Course practicalities - subjects covered causes of metamorphism: effects of metamorphism: metamorphic facies: mineral stability: geothermobarometry: rock metamorphism: effects of temperature, pressure and strain; contact, regional and subduction style metamorphism; heating from pluton intrusion development of fabric and texture; changes in mineral assemblage; metasomatism and devolatilisation; overgrowths and relicts characteristic mineral assemblages for different P-T domains and P-T-t paths; low to high grade rocks; prograde and retrograde paths phase diagrams; terminal and sliding reactions; effect of bulk composition; solvi and exsolution; pseudo-section calculations constraining P-T conditions from mineral equilibria and assemblages; garnet-biotite thermometer; GASP barometer; solubility thermometers characteristics of metamorphism of pelites, mafic rocks, carbonates and calc-silicates, and granites 2

2 A short review Temperature ( o C) Pressure (kbar) Dora Maira fluid-saturated granite melting Kokchetav Napier, Antarctica Scourian, Scotland depth (km) burial metamorphism regional orogenic metamorphism conditions not found on Earth Su-Lu fluid-absent granite melting modified after Bucher and Frey regional subduction metamorphism contact or pyrometamorphism 3 Cause and effect in metamorphism temperature: pressure: stress: leads to different minerals, larger minerals, and eventually melting of the rock preference for lower volume minerals, compaction of the rock and loss of volatiles (mostly H2O and CO2) makes textures and fabrics in rocks as minerals align themselves Pressure distribution in the Earth: simple loading by the rock column P = ρ g z Temperature distribution in the Earth: steady-state and disturbed geotherms -A z 2 Q A D T = + 2 k + k z + 20 o C k Combined, this produces the various Earth geotherms 4

3 A short review - Lab 2 P(kbar) Chl Amp prl ab q ru cc Chl prl ab q ru cc Chl pump ab q ru cc MH8 MH11 MH12 Chl Ep Amp sph ab q Chl Ep Amp Pl sph q 1 1 Ep Amp MVD12Amp Pl Gt Cpx q ru Ep Amp Pl Gt Cpx q ru Pl Gt Cpx Opx Ilm q Pl Gt q ru Amp Pl Gt Cpx Ilm q ru Pl Gt Cpx Opx Ilm Chl Ep Amp Pl ilm q ru Amp Pl Gt Amp Pl Gt Cpx Opx Ilm q Ep Amp Pl Gt Ilm q ru Cpx Ilm q ML25 Chl MVD9 Ep Amp Pl Gt Ilm q MVD7 Amp Pl Gt Cpx Opx Ilm Ep Amp Pl Gt Ilm q Amp Pl Gt Ilm q MD13 MA13 Chl Ep Amp Pl Ilm q Ep Amp Pl Ilm q Amp Pl Ilm q Amp Pl Amp Pl Gt Ilm Gt Opx Ilm Chl Amp Pl Ilm q Amp Pl Cpx Opx Ilm Chl Ep Amp Pl Ilm MD12 MH12 MVD4 Pl Cpx Opx Ilm 3 Chl Amp MA1s Pl Ilm Chl Ep sph ab q cc Chl Ep pump sph ab q cc Chl pump sph ab q cc Amp Pl Opx Ilm MA15 Chl Ep Amp Pl sph Chl Amp Pl sph T( C) 5 A short review - Lab 2 Pressure (kbar) lab 2 3 Temperature ( o C) 4 Dora Maira fluid-saturated granite melting Kokchetav 6 5 fluid-absent granite melting Napier, Antarctica fluid-absent basalt melting Scourian, Scotland depth (km) conditions not found on Earth Su-Lu

4 P-T paths, geotherms and field gradients P-T path geotherm field gradient 7 Quantifying metamorphism The goal of metamorphism is to quantify processes in the Earth Need to know temperature, pressure, stress, which tells you depth, heat flow, material flow which in turn tells you how plate tectonics works: how deep can we subduct materials, how fast do orogens form and collapse, how fast is material transported in the Earth, etc And not just plate tectonics: at what P-T did an ore deposit develop; what was the T of my molten slag in steel making, etc Need to quantify the observations we make in metamorphic rocks use textures, mineral parageneses and mineral composition to derive these 8

5 Stress: alignment, subgrains, pressure shadows σ1 σ3 σ2 σ2 σ3 σ1 9 Quantifying metamorphism - petrogenetic grids 10

6 Petrogenetic grid anchored to invariant points invariant point: degrees of freedom = 0 df = c - f + T + P + i c: Al2O3 + SiO2 + MgO f: En, Crd, Cor, An, Sp T, P, but no i 11 Continuous and discontinuous reactions terminal reaction continuous (sliding) reactions tie-line switch reaction 12

7 Projections, assemblages and parageneses fixed a(h2o) + Qtz + Ms A ky, sill, and gt chl F M bt Bt + Chl typically low-grade; Gt + Bt high-grade... maybe not 13 GeoThermoBarometry There are three several types of GeothermoBarometers: Exchange reactions (e.g. Fe Mg and Fe Mn between Bt and Gt) Net transfer reactions (e.g. GASP: An = Gt + Al-sil + Qtz) Solubility thermobarometers (e.g. Zr in rutile; Ti in quartz) Solvus thermometry (e.g. the two-pyroxene and two-feldspar thermometers) Inter-sector thermometry (at present only calibrated for tourmaline) pseudosection-based thermobarometry: contours 14

8 Sections in the book The following chapters in Philpotts & Ague - Principles of Igneous and Metamorphic Petrology are covered in this course. Not all sections of each chapter are included. chapter 7; Thermodynamics intro chapter 8; Phase rule and Schreinemakers analysis chapter 9; Thermodynamics of solid solutions chapter 16; Cause and effect in metamorphism, Metamorphic facies chapter 17; Textures of metamorphic rocks chapter 18; Projections chapter 19; Geothermobarometry, Metamorphic reactions chapter 20; Reactions involving CO2 chapter 21; Metasomatism chapter 22; Pressure-Temperature-time paths 15 Sections in the book The following chapters in Winter - Principles of Igneous and Metamorphic Petrology are covered in this course. Not all sections of each chapter are included. chapter 5; Thermodynamics chapter 6; The phase rule chapter 9; Isotopes and dating chapter 21; Cause and effect in metamorphism chapter 22; Metamorphic rock nomenclature chapter 23; Textures of metamorphic rocks chapter 24; Projections chapter 25; Metamorphic facies chapter 26; Metamorphic reactions chapter 27; Thermodynamics in metamorphic reactions chapter 28; Pelite metamorphism chapter 29; Metamorphism of impure limestones and UMs chapter 30; Metasomatism 16