Geol 2312 Igneous and Metamorphic Petrology Lecture 26 Metamorphic Reactions (cont.) Chemographics and Petrogenetic Grids April 8, 2009
Types of Metamorphic Reactions • Phase Transformations • Exsolution Reactions • Solid-Solid Net Transfer Reactions • Devolatilization Reactions • Continuous Reactions (THIS LECTURE) • Ion Exchange Reactions • Oxidation/Reduction (redox) Reactions • Reactions involving Dissolved Species
Continuous ReactionsInvolving Solid Solution Phases An idealized field area of steeply dipping meta-pelitic sediments that strike directly up metamorphic grade The bulk chemistry of each unit is homogeneous, but differs somewhat from the other units in the area The garnet-in field isograd varies from unit to unit, occurring at different grades. WHY? Winter (2001) Figure 26-8.
Continuous Reactions Two possible reasons: 1. Each unit has contrasting compositions such that the garnet reaction is different Example: garnet in some pelites may be created by the (unbalanced) reaction: Chl + Ms + QtzGrt + Bt + H2O (26-11) Whereas in more Fe-rich and K-poor pelites, garnet might be generated by an (unbalanced) reaction involving chloritoid: Chl + Cld + QtzGrt + H2O (26-12) • 2. The reaction on which the isograd is based is the same in each unit, but it is a continuous reaction, and its location is sensitive to the composition of the solutions (either solid or fluid) involved • The offsets this creates in an isograd are usually more subtle than for reason #1, but in some cases they can be substantial • Let’s evaluate the second situation
Continuous Reactions Recall the igneous situation with solid solution “Melt-in” isograd? Temperature is strongly X-dependent Winter (2001) Fig. 6-10. Isobaric T-X phase diagram at atmospheric pressure After Bowen and Shairer (1932), Amer. J. Sci. 5th Ser., 24, 177-213
Continuous vs. Discontinuous Reactions • Consider the reaction: Chl + Ms + QtzGrt + Bt + H2O • F = C – f + 2 = 5 – 4 + 2 = 1 (univariant) • But effectively F=0 since T • and P are controlled by • moving along a geothermal • or field gradient • Therefore, the reaction occurs • at a particular T for a given • gradient and would be considered • DISCONTINUOUS
Continuous vs. Discontinuous Reactions However, when considering the solid solution of Mg and Fe in garnet, biotite, and chlorite, the general reaction is (Fe,Mg)Chl + Ms + Qtz (Fe,Mg)Grt + (Fe,Mg)Bt + H2O This is a continuous reaction and we expect to find chlorite, muscovite, quartz, biotite, and garnet all together in the same rock over an interval of metamorphic grade above the garnet-in isograd The composition of solid solution phases will vary across the interval, and the proportions of the minerals will change until one of the reactants disappears with increasing grade
Continuous Reactions Perhaps a more realistic way to portray this continuous reaction (minus the K component) is this. Thus, the offsets in the idealized field area may be due to differences in the Mg/Fe ratios of the different rock layers.
Types of Metamorphic ReactionsDevolatilization Another example of a continuous reaction over a range in T due the potential for the fluid composition to change CLOSED (buffered) Volatiles are trapped in the rock with degassing XCO2 must follow the reaction equibrium curve to the max T. a – degassing of CO2 & H2O drives system to b and increased T b – Trem, Cal or Qtz must be consumed before T can increase c – degassing of CO2 causes system to move along reaction curve toward XCO2 = 1 Fluid composition is controlled (buffered) by the progress of the reaction
Reactions and Chemographics We can use chemographics to infer reactions. Consider the 2-component system MgO-SiO2 Any two phases in a binary system can react to from a phase between them Fo + Qtz = En Mg2SiO4 + SiO2 = Mg2Si2O6 En + Per = Fo Mg2Si2O6 + 2 MgO = 2 Mg2SiO4 Per + Qtz = Fo or En If we know the chemographics we can determine that a reaction is possible (and can dispense with balancing it) However, thermodynamics determines whether such a reaction is probable
Reactions and Chemographics For a ternary system, any phase that is coplanar with three other phases can be related by a chemical reaction 2A + B + C = X Again, whether this reaction is probable under natural conditions of P & T depends on Thermodynamics. If the diagram represents a projection from another phase or component, then that phase is implied in the reaction. For component D, the reaction might be: 2A + B + C + #D = X However, the amount of D in the reaction would have to be figured out by balancing the reaction.
Reactions and Chemographics 2A + B + C = X At P/T Conditions B At P/T Conditions A X-in Isograd
Reactions and Chemographics Another Possibility If phase X can be defined by two different reactions: 2A+B+C = X D+E = X Then, the reaction: 2A+B+C = D+E is also true
Reactions and Chemographics A Tie-line Flip - results in new groupings in the next metamorphic zone Because A+B and C+ D share a common tie-line, they can be related by the reaction: A + B = C + D Below the Isograd At the Isograd Above the Isograd Increasing Grade
Petrogenetic GridsGraphically Portraying Several Reactions in Pressure – Temperature Space Simple One Component System Univariant Curves can be labelled by Absent Phase Metastable Extensions
Petrogenetic GridsMultisystems> C +2 One-Component System Four Phases (ABDE) Four invariant points (labelled by the absent phase) Seven univariant lines (labelled by the absent phases) Four divariant fields (labelled by stable phase)
Petrogenetic GridsMultisystems Theoretically Possible vs. Geologically Probable
Petrogenetic GridsMultisystems Many Petrogenetic Grids will overlay grids of multiple component systems that pertain to a specific protolith type Petrogenetic Grid for Mafic Rocks Lawsonite Actinolite P-T Range of Rock with Lawsonite + Actinolite + Pyrophyllite Pyrophyllite Winter (2001) Fig. 26-19.Simplified petrogenetic grid for metamorphosed mafic rocks showing the location of several determined univariant reactions in the CaO-MgO-Al2O3-SiO2-H2O-(Na2O) system (“C(N)MASH”).