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Lecture 7: Chemography II Chemography: Graphical representation of mineral and bulk compositions to aid in determining parageneses, mineral reactions and the impacts of protolith composition C2O min-3 P = 2 kbar T = 400oC We have 7 possible minerals; 4 minerals of fixed composition and 3 solid solutions + H2O B A min-7 min-6 min-4 C min-5 D AO2 min-1 BO min-2 At this P + T, min-4 is not stable and the solid solutions have only a limited compositional range So what are the mineral parageneses for bulk compositions A → D ? 1 A system AO2-BO-C2O + H2O at 500oC At 500oC, min-4 has become stable and the compositional ranges have changed: C2O min-3 P = 2 kbar T = 400oC + H2O P = 2 kbar T = 500oC B A + H2O min-7 min-6 B A min-4 C min-7 min-5 min-6 D AO2 min-1 C2O min-3 min-4 C BO min-2 min-5 D AO2 min-1 BO min-2 2 A system AO2-BO-C2O + H2O at 500oC The appearance of min-4 means that there is no longer a unique solution to the tielines between the minerals: P = 2 kbar T = 500oC Such an intersecting tieline points to a possible reaction in the rock; C2O min-3 + H2O min-5 + min-7 → B A min-4 + min-6 min-7 min-6 min-4 C This is called a crossing tie-line reaction min-5 D AO2 min-1 BO min-2 3 A system AO2-BO-C2O + H2O at 525oC The appearance of min-4 means that there is no longer a unique solution to the tielines between the minerals: P = 2 kbar T = 525oC Such an intersecting tieline points to a possible reaction in the rock; C2O min-3 + H2O min-5 + min-7 → B A min-4 + min-6 min-7 min-6 min-4 C This is called a crossing tie-line reaction min-5 D AO2 min-1 BO min-2 4 A system AO2-BO-C2O + H2O at 550oC At even higher temperature, min-6 is no longer stable and breaks down P = 2 kbar T = 550oC This type of reaction is called a terminal stability reaction; C2O min-3 + H2O min-6 → B A min-3 + min-1 + min-5 min-7 min-6 min-4 C min-5 D AO2 min-1 BO min-2 5 Terminal and crossing tie-lines: reaction-isograds Reactions in compositional space result in changes in mineralogy and these can be recognized in the field. This allows these reactions to be mapped out and these reaction lines in the field are called reaction-isograds Because reactions are mostly related to a change in grade, this allows you to map out metamorphic grade in the field (assuming that the bulk composition remains constant) A+B=C+D below the isograd at the isograd above the isograd 6 Crossing tie-line reaction in AKF Metamorphic rocks around Oslo contain Crd + Kfs, but at Orijärvi Ms + Bt + Qtz 2 Bt + 6 Ms + 15 Qtz = 3 Crd + 8 Kfs + 8 H2O 7 Rules for choosing the projection variables Metamorphic rocks commonly contain 9 major components and many more minor components, so how do you choose what to project from ? Philpotts specified 5 rules that you can use as a guide: 1. Components that occur as pure phases can be ignored (= projected from) 2. Components whose chemical potential is externally controlled can be ignored (for example H2O and CO2) 3. Components occurring in just one phase can be ignored if we also ignore that phase (for example ZrO2 if it is only present in zircon) 4. Components that are not sufficiently abundant to stabilize their own phase can be ignored. This gets rid of most of the minor and trace elements 5. If a component occurs in two or more phases it can’t be ignored. However, if one of these phases is always present; use it to project from Let’s apply this to the pelites 8 Meta-pelitic rocks: the KFMASH system Meta-pelitic rocks are predominantly composed of: SiO2 - Al2O3 - MgO - FeO - K2O and H2O. We call this the KFMASH compositional system. To work with this system, need to reduce the number of components from 6 to 3 1. Project from H2O, because there’s always a fluid present 2. Project from SiO2, because will always have quartz which buffers the activity of SiO2 3. Meta-pelitic rocks contain either muscovite or K-feldspar, depending on grade: qtz + ms = Kfs + Al-sil + H2O → can project from muscovite at low grade and from K-feldspar at high grade 9 KFMASH projections onto the AFM plane 10 The AFM diagram changes with grade As metamorphic grade changes the mineralogy changes, as well as the composition of solid solutions: minerals will disappear and new minerals appear, and the field for solid solutions will change shape and size y At this grade, bulk composition z x: sill + bt + st y: st + gt + bt z: bt only different bulk compo = different minerals at the same grade! 11 Continuous reactions Systematic variations in mineral composition as grade changes because of exchange of Fe and Mg among the phases: low grade gt A A medium grade chl gt F M bt chl F M bt 12