geol 2312 igneous and metamorphic petrology
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DESCRIPTIONGeol 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 - PowerPoint PPT Presentation
GEOL 2312 IGNEOUS AND METAMORPHIC PETROLOGYLecture 26Metamorphic Reactions (cont.)Chemographics and Petrogenetic Grids
April 8, 2009
TYPES OF METAMORPHIC REACTIONS Phase Transformations Exsolution ReactionsSolid-Solid Net Transfer Reactions Devolatilization Reactions Continuous Reactions (THIS LECTURE) Ion Exchange Reactions Oxidation/Reduction (redox) ReactionsReactions involving Dissolved Species
CONTINUOUS REACTIONSINVOLVING SOLID SOLUTION PHASESWinter (2001) Figure 26-8. The bulk chemistry of each unit is homogeneous, but differs somewhat from the other units in the areaThe garnet-in field isograd varies from unit to unit, occurring at different grades.
WHY?An idealized field area of steeply dipping meta-pelitic sediments that strike directly up metamorphic grade
CONTINUOUS REACTIONSTwo possible reasons:1. Each unit has contrasting compositions such that the garnet reaction is differentExample: garnet in some pelites may be created by the (unbalanced) reaction: Chl + Ms + Qtz Grt + 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 + Qtz Grt + 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) involvedThe offsets this creates in an isograd are usually more subtle than for reason #1, but in some cases they can be substantialLets evaluate the second situation
CONTINUOUS REACTIONSWinter (2001) Fig. 6-10. Isobaric T-X phase diagram at atmospheric pressure After Bowen and Shairer (1932), Amer. J. Sci. 5th Ser., 24, 177-213Melt-in isograd?
Temperature is strongly X-dependentRecall the igneous situation with solid solution
CONTINUOUS VS. DISCONTINUOUS REACTIONS Consider the reaction: Chl + Ms + Qtz Grt + Bt + H2OF = C f + 2 = 5 4 + 2 = 1 (univariant)
But effectively F=0 since T and P are controlled by moving along a geothermalor field gradient
Therefore, the reaction occurs at a particular T for a givengradient and would be consideredDISCONTINUOUS
CONTINUOUS VS. DISCONTINUOUS REACTIONSThis 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 isogradThe 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 gradeHowever, 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
CONTINUOUS REACTIONSPerhaps 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 REACTIONSDEVOLATILIZATIONAnother example of a continuous reaction over a range in T due the potential for the fluid composition to changeCLOSED (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 Tb 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 CHEMOGRAPHICSWe 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 themFo + Qtz = En Mg2SiO4 + SiO2 = Mg2Si2O6En + Per = Fo Mg2Si2O6 + 2 MgO = 2 Mg2SiO4Per + Qtz = Fo or EnIf 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 CHEMOGRAPHICSFor 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 = XHowever, the amount of D in the reaction would have to be figured out by balancing the reaction.
REACTIONS AND CHEMOGRAPHICSAt P/T Conditions AAt P/T Conditions B2A + B + C = XX-inIsograd
REACTIONS AND CHEMOGRAPHICSAnother 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 CHEMOGRAPHICSA + B = C + DA Tie-line Flip - results in new groupings in the next metamorphic zoneBecause A+B and C+ D share a common tie-line, they can be related by the reaction: Increasing GradeBelow the IsogradAt the IsogradAbove the Isograd
PETROGENETIC GRIDSGRAPHICALLY PORTRAYING SEVERAL REACTIONS IN PRESSURE TEMPERATURE SPACEUnivariant Curves can be labelled by Absent PhaseMetastableExtensionsSimple One Component System
PETROGENETIC GRIDSMULTISYSTEMS > C +2One-Component SystemFour 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
Many Petrogenetic Grids will overlay grids of multiplecomponent systems that pertain to a specific protolith typeWinter (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). PETROGENETIC GRIDSMULTISYSTEMS Petrogenetic Grid for Mafic RocksActinolitePyrophylliteLawsoniteP-T Range of Rock with Lawsonite + Actinolite + Pyrophyllite