Metamorphic Petrology Session 6: Paragenetic sequence diagrams, Reaction rate, Very-Lowand Low-Grade Regional Metamorphism
Course Structure & Itinerary Session 1: Introduction to Metamorphic Petrology Session 2: Contact metamorphism and isograds Session 3: Solid solutions and reaction thermodynamics, geothermometers and barometers Session 4: PT-t paths and regional metamorphism Session 5: Metamorphism & deformation, pre-, syn- & post-kinematic mineral assemblages, brittle-ductile transition Session 6: Reaction kinetics & very-low to low grade metamorphism Session 7: Medium-, high- grade metamorphism and anatexis Session 8: Metasomatism and hydrothermal alteration If requested: Question hour in preparation for examination
Topics, Session 6 1. Review of last session 2. Rates of Metamorphic Reactions 3. Very Low Grade Metamorphism Case Study: Blueschists, HP-LT Metamorphism 4. Low Grade Metamorphism Case Study: South Island of New Zealand Practical: Interpretation of paragenetic sequence diagrams in terms of metamorphic reactions Special Interest Article Handout: Walther & Wood (1984)
1. Deformation modes Translation Rotation Co-Axial Deformation Shear Deformation Volume Change Homogeneous & Inhomogeneous uni-axial stretching ε λ From Hobbs, Means, & Williams (1963), Wiley & Sons Ltd. MP6, 4
Streching lineations Sheath fold, Scotland Gneiss, Pilbara, Australia
Review: deformation mechanisms 1 mm Intra-crystalline (within the grains) o Point defect migration o Dislocation glide o Deformation lamellae o Twinning Inter-crystalline (involving several grains) o Cataclasis = grain fragmentation o Recrystallization: dynamic & static o Grain-Boundary Migration & Pinning o Creep: dissolution precipitation
Identification criteria for pre-, syn- or post kinematic mineral growth Pre-Kinematic mineral are o Passively deformed o Preserved as orientation families o Cause later inhomogeneous deformation o Foliation drapes around them o Partially replaced by syn-kinematic minerals o Syn-Kinematic o Growth (not recrystallisation) in preferred orientation o Progressive inclusion of evolving fabric o Post-kinematic o Static overgrowth of preexisting fabric o Recrystallisation o 120 o grain-boundaries o No kinematic overprint o No undulose extinction o
2. Rates of Metamorphic Reactions Fundamental Question How much of the reaction products C and D, are created as a function of time at a given PT, by the reaction? 123 A + B C123 + D reactants products
Defining the reaction rate Reaction rate can be expressed as molar conversion rate: Rate = moles per meter cubed per second (mol m -3 s -1 ) Or relative to the oxygen equivalent (as in Walther & Wood, 1984 special interest reading). Assumptions At fixed PT the rate is constant At equilibrium, the reaction rate is zero How fast a reaction occurs is related to the degree of overstepping of equilibrium conditions
Simplest rate law: rate is a linear function of the free energy driving force reaction rate dm dt = r c ( -3 1 mol m s ) G RT r : r c = rate coefficient As suggested by Prigogine & Defay (1954)
A closer look at the rate coefficient r c : Arrhenius Law Reaction rate varies as a function of the activation energy E a, temperature, T, and surface area, A s, of the reactants according to the rate multiplier: r c = A e s E a / RT
Reaction rate via Arrhenius law dm dt = A e s E / a RT G RT r Rate is an exponential function of temperature r(t) The other key factor is surface area Rate dependence leads to a switch -like behaviour
Arrhenius plot of forward reaction rate log r c Experimental monitoring of weight changes of single crystals, lead to the fit: r c (O equiv. wt.% cm 3 s -1 ) = -2900 / T 6.85 Assumptions Rate is limited by surface area of single crystals in experiment Fluid composition is not a significant factor (in spite of known ph dependence)
Remember the equilibrium constant: As explained in Session 3, the Gibbs free energy of a reaction is the sum over the products of reactants and reaction products in moles, ν i, and their Gibbs free energy at PT of interest relative to their Gibbs free energy of formation from the elements reactants & products G r = ν G i= 0 i o f, i And from G r we arrived at the equilibrium constant, K: log K = G RT r
How can K be related to the backward reaction rate? 123 A + B C12 + 23 D reactants forward rate, k [ C][ D] [ A][ B] 2 products backward rate, k K = k k f b = K at equilibrium f r k and [ A][ B] = k [ C][ D] da = k dt since k f b = k f K f r [ A][ B] + k 2 at equilibrium f [ C][ D] K 2
Example: computation of calcite dissolution in HCL Molality 1.E+00 1.E-01 1.E-02 1.E-03 1.E-04 1.E-05 1.E-06 1.E-07 OH- 1.E-08 CO2_aq 1.E-09 1.E-10 calcite 1.E-11 H+ 1.E-12 HCO3-1.E-13 Cl- 1.E-14 Rate-Controlled Calcite Dissolution HCl 1.E-15 Ca+2 1.E-11 1.E-09 1.E-07 1.E-05 1.E-03 1.E-01 1.E+01 1.E+03 1.E+05 Time (s)
Components and rate-defining reactions (last slide) doh_1m/dt = -KF H_1p OH_1m + KB1 H2O_aq dco2_aq/dt = -KF CO2_aq H2O_aq + KB2 HCO3_1m H_1p dcc/dt = -KF CC H_1p + KB3 HCO3_1m Ca_2p dh_1p/dt = -KF H_1p OH_1m + KB1 H2O_aq + KF CO2_aq H2O_aq + -KB2 HCO3_1m H_1p + -KF CC H_1p + KB3 HCO3_1m Ca_2p + KF HCl_aq + -KB4 Cl_1m H_1p dh2o_aq/dt = KF H_1p OH_1m + -KB1 H2O_aq + -KF CO2_aq H2O_aq + KB2 HCO3_1m H_1p dhco3_1m/dt = KF CO2_aq H2O_aq + -KB2 HCO3_1m H_1p + KF CC H_1p + -KB3 HCO3_1m Ca_2p dca_2p/dt = KF CC H_1p + -KB3 HCO3_1m Ca_2p dhcl_aq/dt = -KF HCl_aq + KB4 Cl_1m H_1p dcl_1m/dt = KF HCl_aq + -KB4 Cl_1m H_1p
The four metamorphic grades Pressure [MPa] Depth [km] nonexistant conditions Temperature [ o C]
3. Very Low Grade Metamorphism P [MPa] 300 500 diagenesis Calcite Aragonite 200 320 T [ o C] laumontite lawsonite very low grade pm + chl + qtz = zo + act + H 2 O Pressure (MPa) indicators Aragonite (>300, but T-dep.) Laumontite < 300 Lawsonite > 300 Glaucophane > 600 Jadeite > 800 6 o C km -1 prehnite + pumpellyite no more lawsonite clinozoisite
Selected metamorphic reactions Pumpellyite + Quartz = Clinozoisite + Prehnite + Chlorite + H 2 O Pumpellyite + Chl + Quartz = Clinozoisite + Actinolite + H 2 O Prehnite + Chl + Qtz = Clinozoisite + Actinolite + H 2 O Lawsonite + Chlorite (Al-poor) = Zo or clinozoisite + Chl (Al-rich) + Quartz + H 2 O Lawsonite + Quartz = Zoisite + Pyrophyllite + H 2 O Albite (NaAl[Si 3 O 8 ]) = Jadeite (NaAl[Si 2 O 6 ]) + SiO 2 Calcite = Aragonite
Prehnite Ca 2 Al 2 Si 3 O 10 (OH) 2 Orthorhombic (+) Birefringence 0.022-0.035 27 x
Pumpellyite Ca 4 (Mg,Fe)Al 5 Si 6 O 23 (OH) 3 x 2 H 2 O Monoclinic (+) Birefringence 0.012-0.022 44 x
Lawsonite CaAl 2 Si 2 O 7 (OH) 2 x H 2 O Orthorhombic (+) Birefringence 0.02 27 x
Zoisite Ca 2 Al 3 Si 3 O 12 (OH) Orthorhombic (+) Birefringence 0.004-0.008 60 x
Epidote Ca 2 (Al,Fe) 3 Si 3 O 12 (OH) Monocline (+) Birefringence 0.004-0.008 1 cm
Hydrothermal in the presence of CO 2 from W. Giggenbach in Barnes (1997)
Glaucophane Na 2 Mg 3 Al 2 Si 8 O 22 (OH) 2 Monoclinic (+) 27 x
Case Study: Blueschists and HP-LT metamorphism
The Franciscan Accretionary Complex, CA, USA
Franciscan melange at Big Sur
Franciscan knockers : blueschist and eclogite blocks in serpentinite matrix
Franciscan melange outcrops blueschist sheared serpentinite eclogite
Minerals in Franciscan Complex (1)
Minerals in Franciscan Complex (2)
Paragenetic zones in Franciscan melange complex
Tectonic problem: exhumation of blueschists, but how? T P?When subduction ended in Cretaceous times the hydrated (serpentinized) mantle wedge was rapidly extruded in the transition to the transpressional regime that persists today?
4. Low Grade Metamorphism
Average metapelite: very low- to low grade transition Vol.% VL -> L Grade Reaction Index Fig.1 from Haack et al. (1984)
Very-Low- to Low Grade boundary reactions Lawsonite + Quartz = Zoisite + Pyrophyllite + H 2 O Law + Chl (Al-poor) = Zo or clinozoisite + Chl (Al-rich) + Quartz + H 2 O Pumpellyite + Quartz Prehnite + Chl + Qtz = Zo or clinozoisite + Prehnite + Chl + H 2 O = Zo or clinozoisite + actinolite + H 2 O Pumpellyite + Chl + Quartz = Zo or clinozoisite + actinolite + H 2 O
Low grade field of metamorphism P [MPa] 320 very low grade 500 T [ o C] cordierite staurolite chlorite + muscovite actinolite out, hornblende in 300 clinozoisite pm + chl + qtz = zo + act + H 2 O 500
Typical low-grade mineral assemblages Overlap with greenschist and lower amphibolite facies Plagioclase jump occurs: Albite (An 05 -> An 20 ) Basalts -> Greenschists (clinozoisite, albite, chlorite & actinolite, titanite; no muscovite unless rocks experienced K-metasomatism) Mudstones -> Phyllites (phengite - sericite, quartz, albite, chlorite sometimes biotite, apatite, pyrophyllite, chloritoid, garnet, apatite, tourmaline, pyrite) Dirty limestones & marls -> Finely crystalline marbles & calc-silicate schists (talc, magnesite, recrystallised calcite & dolomite, grossular & Mn-almandine garnet, actinolite, chlorite) Chloritoid appears in Fe-rich assemblages At upper temperature limit: Clinozoisite + Chl + Ab + Actinolite = Plagioclase + Hornblende + H 2 O
Metamorphic Plagioclase Below 450-500 o C, there is a solubility gap in the solidsolution series between albite and anorthite Rule of Thumb The An(Ca)-content of plagioclase increases with increasing temperature unless plagioclase is involved in Caexchange reactions very-low grade assemblages gap low grade assemblages (for instance in amphibolites)
Typical reactions in the Low-Grade field Chloritoid, Ctd (Ino-Silicate: Fe 2 AlAl 3 [(OH) 4 O 2 (SiO 4 ) 2 ]) Clinozoisite + Chl + Ab + Actinolite = Plag + Hornblende + H 2 O Pyrophyllite + Chl (Fe-rich) = Chloritoid + Chl (Fe-poor) + H 2 O Ph + Chl (Al-poor) = Bt + Chl (Al-rich) + H 2 O Pyrophyllite = Andalusite + 3 Quartz + H 2 O Chl + Ph + Qtz = Ga + Bt + H 2 O (at elevated pressure) And = Ky (at elevated pressure) Transition to Medium Grade Phengite + Chl + Qtz = Crd + Bt + H 2 O (at low pressure) Chl + Mu = St + Bt + Quartz (Fe-rich protolith) Ctd + AS = St + Qtz + H 2 O (Fe-rich protolith) Ctd + Qtz = St + almandine + H 2 O (green = continuous reactions, blue = discontinuous reactions)
Actinolite vs. Hornblende Actinolite Ca 2 (Mg,Fe) 5 [(OH,F)Si 4 O 11 ] 2 Monoclinic, Light green, yellow-green, bluegreen pleochroism Intermediate extinction angle: 10-15 o Hornblende NaCa 2 (Mg,Fe,Al) 5 [(OH,F) 2 (Si,Al) 2 Si 6 O 22 ] Monoclinic, Green-brown-yellow pleochroism Intermediate extinction angle: 15-20 o
Actinolite Ca 2 Fe 5 [(OH,F) 2 Si 8 O 22 ] Monoclinic (-) Birefringence 0.017-0.027 20 x
Hornblende (NaCa 2 (Mg,Fe,Al) 5 [(OH,F) 2 (Si,Al) 2 Si 6 O 22 ]) Monoclinic (- or +) Birefringence 0.014-0.026 20 x
Chloritoid (Mg,Fe) 2 AlSi 4 O 10 (OH) 4 Monoclinic (+ or -) Birefringence 0.006-0.022 43 x
Stilpnomelane (K(Fe,Mg,Al) 3 [(OH) 2 OSi 4 O 10 ] x 3 H 2 O, simplified) Monoclinic (-) Birefringence 0.030-0.110 brown-green pleochroism, low relief, a bit like biotite, but poor cleavage 32 x
Case Study: South Island of New Zealand
Transform fault at plate boundary
Geological history This and following illustrations from Glen Coates (2002), Canterbury University Press, NZ.
Geologic structure / metamorphism
Metamorphic grade & age of metamorphism
The Alpine Fault Zone
Present-day reverse displacement on Alpine Fault
Uplift rates over time
Practical 1 1. Explain the appearance and disappearance of the minerals in the paragenetic sequence diagram on the right with the prograde reactions from the lecture notes 2. Try to stochiometrically balance as many of the prograde reactions listed for the boundary between very-low and low grade given the data in todays notes. 3. Name components for a petrogenetic grid suitable for these rocks.
Practical 2 With the aid of the combined binoculars and microscopes, investigate the supplied handspecimens of very-low to low grade metamorphic rocks. In your characterisation of the sample, proceed as follows: 1. 1. Describe the fabric. 2. 2. Identify main and accessory minerals and their relative proportions (vol.%). 3. 3. Classify the identified minerals in terms of pre- syn- and post-kinematic mineral growth. 4. 4. Define parageneses and paragenetic sequences as applicable. 5. 5. Name the rock. 6. 6. Identify (using the handouts) what chemical category the rock belongs to (i.e. what its protolith was). Use this information in conjunction with your observations of the mineral assemblages to select geothermometers and-or barometers that could be applied to the rock. 7. Interpret the conditions of rock formation or transformation of a protolith on the basis of your observations and in terms of the phase diagrams we studied in class. and interpret the geological history of the sample from protolith to exhumation drawing a tentative PTt-path.