G EOL 2312 I GNEOUS AND M ETAMORPHIC P ETROLOGY Lecture 26 Metamorphic Reactions (cont.) Chemographics and Petrogenetic Grids April 8, 2009

T YPES OF M ETAMORPHIC R EACTIONS 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

C ONTINUOUS R EACTIONS I NVOLVING S OLID S OLUTION P HASES Winter (2001) Figure 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? An idealized field area of steeply dipping meta-pelitic sediments that strike directly up metamorphic grade

C ONTINUOUS R EACTIONS 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 + Qtz  Grt + Bt + H 2 O(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 + H 2 O(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

C ONTINUOUS R EACTIONS Winter (2001) Fig Isobaric T-X phase diagram at atmospheric pressure After Bowen and Shairer (1932), Amer. J. Sci. 5th Ser., 24, “Melt-in” isograd? Temperature is strongly X-dependent Recall the igneous situation with solid solution

C ONTINUOUS VS. D ISCONTINUOUS R EACTIONS Consider the reaction: Chl + Ms + Qtz  Grt + Bt + H 2 O F = C –  + 2 = 5 – = 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

C ONTINUOUS VS. D ISCONTINUOUS R EACTIONS 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 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 + H 2 O

C ONTINUOUS R EACTIONS 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.

T YPES OF M ETAMORPHIC R EACTIONS D EVOLATILIZATION 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 X CO 2 must follow the reaction equibrium curve to the max T. a – degassing of CO 2 & H 2 O drives system to b and increased T b – Trem, Cal or Qtz must be consumed before T can increase c – degassing of CO 2 causes system to move along reaction curve toward X CO 2 = 1 Fluid composition is controlled (buffered) by the progress of the reaction

R EACTIONS AND C HEMOGRAPHICS We can use chemographics to infer reactions. Consider the 2-component system MgO-SiO 2 Any two phases in a binary system can react to from a phase between them Fo + Qtz = En Mg 2 SiO 4 + SiO 2 = Mg 2 Si 2 O 6 En + Per = Fo Mg 2 Si 2 O MgO = 2 Mg 2 SiO 4 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

R EACTIONS AND C HEMOGRAPHICS 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.

R EACTIONS AND C HEMOGRAPHICS At P/T Conditions A At P/T Conditions B 2A + B + C = X X-in Isograd

R EACTIONS AND C HEMOGRAPHICS 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

R EACTIONS AND C HEMOGRAPHICS A + B = C + D 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: Increasing Grade Below the IsogradAt the IsogradAbove the Isograd

P ETROGENETIC G RIDS G RAPHICALLY P ORTRAYING S EVERAL R EACTIONS IN P RESSURE – T EMPERATURE S PACE Univariant Curves can be labelled by Absent Phase Metastable Extensions Simple One Component System

P ETROGENETIC G RIDS M ULTISYSTEMS  > 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)

P ETROGENETIC G RIDS M ULTISYSTEMS Theoretically Possible vs. Geologically Probable

Many Petrogenetic Grids will overlay grids of multiple component systems that pertain to a specific protolith type Winter (2001) Fig Simplified petrogenetic grid for metamorphosed mafic rocks showing the location of several determined univariant reactions in the CaO-MgO-Al 2 O 3 -SiO 2 -H 2 O-(Na 2 O) system (“C(N)MASH”). P ETROGENETIC G RIDS M ULTISYSTEMS Petrogenetic Grid for Mafic Rocks Actinolite Pyrophyllite Lawsonite P-T Range of Rock with Lawsonite + Actinolite + Pyrophyllite