Ch 24 Mineral Assemblages - Chapter 24. Stable Mineral...

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Unformatted text preview: Chapter 24. Stable Mineral Assemblages in Metamorphic Rocks Equilibrium Mineral Assemblages At equilibrium, the mineralogy (and the composition of each mineral) is determined by T, P, and X "Mineral paragenesis" refers to such an equilibrium mineral assemblage Relict minerals or later alteration products are excluded unless specifically stated The Phase Rule in Metamorphic Systems Phase rule, as applied to systems at equilibrium: F=C- +2 the phase rule (6-1) = the number of phases in the system C = the number of components: the minimum number of chemical constituents required to specify every phase in the system F = the number of degrees of freedom: the number of independently variable intensive parameters of state (such as temperature, pressure, the composition of each phase, etc.) The Phase Rule in Metamorphic Systems If F 2 is the most common situation, then the phase rule may be adjusted accordingly: F=C- +2 2 C (24-1) Goldschmidt's mineralogical phase rule, or simply the mineralogical phase rule The Phase Rule in Metamorphic Systems Suppose we have determined C for a rock Consider the following three scenarios: a) = C 3 The standard divariant situation The rock probably represents an equilibrium mineral assemblage from within a metamorphic zone 3 The Phase Rule in Metamorphic Systems b) 3 <C Common with mineral systems that exhibit solid solution Liquid Plagioclase plus Liquid Plagioclase The Phase Rule in Metamorphic Systems c) > C A more interesting situation, and at least one of three situations must be responsible: 1) F < 2 v The sample is collected from a location right on a univariant reaction curve (isograd) or invariant point 3 The Phase Rule in Metamorphic Systems Consider the following three scenarios: C=1 v = 1 common v = 2 rare v = 3 only at the specific P-T conditions of the invariant point (~ 0.37 GPa and 500oC) Figure 21-9. The P-T phase diagram for the system Al2SiO5 calculated using the program TWQ (Berman, 1988, 1990, 1991). Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. The Phase Rule in Metamorphic Systems 2) Equilibrium has not been attained v The phase rule applies only to systems at equilibrium, and there could be any number of minerals coexisting if equilibrium is not attained The Phase Rule in Metamorphic Systems 3) We didn't choose the # of components correctly Some guidelines for an appropriate choice of C 3 Begin with a 1-component system, such as CaAl2Si2O8 (anorthite), there are 3 common types of major/minor components that we can add a) Components that generate a new phase v Adding a component such as CaMgSi O (diopside), results 2 6 in an additional phase: in the binary Di-An system diopside coexists with anorthite below the solidus The Phase Rule in Metamorphic Systems 3) We didn't choose the # of components correctly b) Components that substitute for other Adding a component such as NaAlSi3O8 (albite) to the 1-C components v v v v anorthite system would dissolve in the anorthite structure, resulting in a single solid-solution mineral (plagioclase) below the solidus Fe and Mn commonly substitute for Mg Al may substitute for Si Na may substitute for K The Phase Rule in Metamorphic Systems 3) We didn't choose the # of components correctly c) "Perfectly mobile" components v v v Mobile components are either a freely mobile fluid component or a component that dissolves readily in a fluid phase and can be transported easily The chemical activity of such components is commonly controlled by factors external to the local rock system They are commonly ignored in deriving C for metamorphic systems The Phase Rule in Metamorphic Systems Consider the very simple metamorphic system, MgO-H2O 3 Possible natural phases in this system are periclase (MgO), aqueous fluid (H2O), and brucite (Mg(OH)2) How we deal with H2O depends upon whether water is perfectly mobile or not A reaction can occur between the potential phases in this system: MgO + H O Mg(OH) Per + Fluid = Bru 3 3 Figure 24-1. P-T diagram for the reaction brucite = periclase + water. From Winter (2001). An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Figure 24-1. P-T diagram for the reaction brucite = periclase + water. From Winter (2001). An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Figure 24-1. P-T diagram for the reaction brucite = periclase + water. From Winter (2001). An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. The Phase Rule in Metamorphic Systems How do you know which way is correct? The rocks should tell you 3 3 The phase rule is an interpretive tool, not a predictive tool, and does not tell the rocks how to behave If you only see low- assemblages (e.g. Per or Bru in the MgO-H2O system), then some components may be mobile If assemblages have many phases in an area it is unlikely that so much of the area is right on a univariant curve, and may require the number of components to include otherwise mobile phases, such as H2O or CO2, in order to apply the phase rule correctly 3 Chemographic Diagrams Chemographics refers to the graphical representation of the chemistry of mineral assemblages A simple example: the plagioclase system as a linear C = 2 plot: = 100 An/(An+Ab) Chemographic Diagrams 3-C mineral compositions are plotted on a triangular chemographic diagram as shown in Fig. 24-2 x, y, z, xz, xyz, and yz Suppose that the rocks in our area have the following 5 assemblages: x-xy-x z Figure 24-2. Hypothetical three-component chemographic compatibility diagram illustrating the positions of various stable minerals. Minerals that coexist compatibly under the range of P-T conditions specific to the diagram are connected by tie-lines. After Best (1982) Igneous and Metamorphic Petrology. W. H. Freeman. Petrology. 3 Note that this subdivides the chemographic diagram into 5 sub-triangles, labeled (A)-(E) x-xy-x z Common point corresponds to 3 phases, thus = C Figure 24-2. Hypothetical three-component chemographic compatibility diagram illustrating the positions of various stable minerals. Minerals that coexist compatibly under the range of P-T conditions specific to the diagram are connected by tie-lines. After Best (1982) Igneous and Metamorphic Petrology. W. H. Freeman. Petrology. What happens if you pick a composition that falls directly on a tie-line, such as point (f)? Figure 24-2. Hypothetical three-component chemographic compatibility diagram illustrating the positions of various stable minerals. Minerals that coexist compatibly under the range of P-T conditions specific to the diagram are connected by tie-lines. After Best (1982) Igneous and Metamorphic Petrology. W. H. Freeman. Petrology. In the unlikely event that the bulk composition equals that of a single mineral, such as xyz, then = 1, but C = 1 as well "compositionally degenerate" Chemographic Diagrams Valid compatibility diagram must be referenced to a specific range of P-T conditions, such as a zone in some metamorphic terrane, because the stability of the minerals and their groupings vary as P and T vary Previous diagram refers to a P-T range in which the fictitious minerals x, y, z, xy, xyz, and x z are A diagram in which some minerals exhibit solid solution Figure 24-2. Hypothetical three-component chemographic compatibility diagram illustrating the positions of various stable minerals, many of which exhibit solid solution. After Best (1982) Igneous and Metamorphic Petrology. W. H. Petrology. Freeman. If Xbulk on a tie-line Figure 24-2. Hypothetical three-component chemographic compatibility diagram illustrating the positions of various stable minerals, many of which exhibit solid solution. After Best (1982) Igneous and Metamorphic Petrology. W. H. Petrology. Freeman. Xbulk in 3-phase triangles F = 2 (P & T) so Xmin fixed Figure 24-2. Hypothetical three-component chemographic compatibility diagram illustrating the positions of various stable minerals, many of which exhibit solid solution. After Best (1982) Igneous and Metamorphic Petrology. W. H. Petrology. Freeman. Chemographic Diagrams for Metamorphic Rocks Most common natural rocks contain the major elements: SiO2, Al2O3, K2O, CaO, Na2O, FeO, MgO, MnO and H2O such that C = 9 Three components is the maximum number that we can easily deal with in two dimensions What is the "right" choice of components? Some simplifying methods: 1) Simply "ignore" components 3 Trace elements 3 Elements that enter only a single phase (we can drop both the component and the phase without violating the phase rule) 3 Perfectly mobile components 2) Combine components 3 Components that substitute for one another in a solid solution: (Fe + Mg) 3) Limit the types of rocks to be shown 3 Only deal with a sub-set of rock types for which a simplified system works 4) Use projections 3 I'll explain this shortly The phase rule and compatibility diagrams are rigorously correct when applied to complete systems A triangular diagram thus applies rigorously only to true (but rare) 3-component systems If drop components and phases, combine components, or project from phases, we face the same dilemma we faced using simplified systems in Chapters 6 and 7 3 Gain by being able to graphically display the simplified system, and many aspects of the system's behavior become apparent 3 Lose a rigorous correlation between the behavior of the simplified system and reality The ACF Diagram Illustrate metamorphic mineral assemblages in mafic rocks on a simplified 3-C triangular diagram Concentrate only on the minerals that appeared or disappeared during metamorphism, thus acting as indicators of metamorphic grade Figure 24-4. After Ehlers and Blatt (1982). Petrology. Freeman. And Miyashiro (1994) Metamorphic Petrology. Oxford. The ACF Diagram The three pseudo-components are all calculated on an atomic basis: A = Al O + Fe O - Na O - K O The ACF Diagram A = Al O + Fe O - Na O - K O Na and K in the average mafic rock are typically combined with Al to produce Kfs and Albite In the ACF diagram, we are interested only in the other Kbearing metamorphic minerals, and thus only in the amount of Al2O3 that occurs in excess of that combined with Na2O and K2O (in albite and K-feldspar) Because the ratio of Al2O3 to Na2O or K2O in feldspars is 1:1, we subtract from Al2O3 an amount equivalent to Na O and K O in the same 1:1 ratio The ACF Diagram C = CaO - 3.3 P O The ACF Diagram By creating these three pseudo-components, Eskola reduced the number of components in mafic rocks from 8 to 3 Water is omitted under the assumption that it is perfectly mobile Note that SiO2 is simply ignored 3 We shall see that this is equivalent to projecting from quartz In order for a projected phase diagram to be truly valid, the phase from which it is projected must be present in the mineral assemblages represented The ACF Diagram An example: Anorthite CaAl2Si2O8 A = 1 + 0 - 0 - 0 = 1, C = 1 - 0 = 1, and F = 0 Provisional values sum to 2, so we can normalize to 1.0 by multiplying each value by , resulting in A = 0.5 C = 0.5 F=0 Where does Ab plot? Plagioclase? Figure 24-4. After Ehlers and Blatt (1982). Petrology. Freeman. And Miyashiro (1994) Metamorphic Petrology. Oxford. A typical ACF compatibility diagram, referring to a specific range of P and T (the kyanite zone in the Scottish Highlands) Figure 24-5. After Turner (1981). Metamorphic Petrology. McGraw Hill. The AKF Diagram Because pelitic sediments are high in Al2O3 and K2O, and low in CaO, Eskola proposed a different diagram that included K2O to depict the mineral assemblages that develop in them In the AKF diagram, the pseudo-components are: A = Al O + Fe O - Na O - K O - CaO Figure 24-6. After Ehlers and Blatt (1982). Petrology. Freeman. AKF compatibility diagram (Eskola, 1915) illustrating paragenesis of pelitic hornfelses, Orijrvi region Finland Figure 24-7. After Eskola (1915) and Turner (1981) Metamorphic Petrology. McGraw Hill. Three of the most common minerals in metapelites: andalusite, muscovite, and microcline, all plot as distinct points in the AKF diagram And & Ms plot as the same point in the ACF diagram, and Micr doesn't plot at all, so the ACF diagram is much less useful for pelitic rocks (rich in K and Al) Figure 24-7. After Ehlers and Blatt (1982). Petrology. Freeman. Projections in Chemographic Diagrams When we explore the methods of chemographic projection we will discover: Why we ignored SiO2 in the ACF and AKF diagrams What that subtraction was all about in calculating A and C It will also help you to better understand the AFM diagram and some of the shortcomings of projected metamorphic phase diagrams Projection from Apical Phases Example- the ternary system: CaO-MgO-SiO2 ("CMS") Straightforward: C = CaO, M = MgO, and S = SiO2... none of that fancy subtracting business! Let's plot the following minerals: Fo - Mg SiO Per - MgO En - MgSiO Di - CaMgSi O Qtz - SiO Cc - CaCO Projection from Apical Phases Fo - Mg SiO Per - MgO Qtz - SiO En - MgSiO Di - CaMgSi O Cc - CaCO The line intersects the M-S the side at a point equivalent to 33% MgO 67% SiO2 Figure 24-8. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Note that any point on the dashed line from C through Di to the M-S side has a constant ratio of Mg:Si = 1:2 Projection from Apical Phases Fo - Mg SiO Per - MgO Qtz - SiO En - MgSiO Di - CaMgSi O Cc - CaCO Pseudo-binary Mg-Si diagram in which Di is projected to a 33 Mg - 66 Si + Cal MgO Per Fo En Di' Q SiO 2 Projection from Apical Phases Could project Di from SiO2 and get C = 0.5, M = 0.5 + Qtz MgO Per, Fo, En Di' Cal CaO Projection from Apical Phases MgO SiO2 Per Fo En Di' Q In accordance with the mineralogical phase rule ( = C) get any of the following 2-phase mineral assemblages in our 2-component system: Per + Fo Fo + En En + Di Di + Q Projection from Apical Phases What's wrong? Figure 24-11. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Better to have projected from Diopside + Cal MgO Per Fo En Di' Q SiO2 Projection from Apical Phases What's wrong? Figure 24-11. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Better to have projected from Diopside + Di MgO Per Fo En Q SiO2 Projection from Apical Phases ACF and AKF diagrams eliminate SiO2 by projecting from quartz Math is easy: projecting from an apex component is like ignoring the component in formulas The shortcoming is that these projections compress the true relationships as a dimension is lost Projection from Apical Phases Two compounds plot within the ABCQ compositional tetrahedron, 3 x (formula ABCQ) 3 y (formula A2B2CQ) Figure 24-12. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Projection from Apical Phases x = ABCQ) y = A2B2CQ) Figure 24-12. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Projection from Apical Phases x = ABCQ) y = A2B2CQ) Figure 24-12. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Projection from Apical Phases x plots as x' since A:B:C = 1:1:1 = 33:33:33 y plots as y' since A:B:C = 2:2:1 = 40:40:20 x = ABCQ) y = A2B2CQ) Figure 24-13. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Projection from Apical Phases If we remember our projection point (q), we conclude from this diagram that the following assemblages are possible: (q)-b-x-c (q)-a-x-y (q)-b-x-y (q)-a-b-y (q)-a-x-c The assemblage a-b-c appears to be impossible Fig. 24-13 Projection from Apical Phases Figure 24-12. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Projection from Apical Phases Fig. 24-13 J.B. Thompson's A(K)FM Diagram An alternative to the AKF diagram for metamorphosed pelitic rocks Although the AKF is useful in this capacity, J.B. Thompson (1957) noted that Fe and Mg do not partition themselves equally between the various mafic minerals in most rocks J.B. Thompson's A(K)FM Diagram Figure 24-17. Partitioning of Mg/Fe in minerals in ultramafic rocks, Bergell aureole, Italy After Trommsdorff and Evans (1972). A J Sci 272, 423-437. J.B. Thompson's A(K)FM Diagram A = Al O J.B. Thompson's A(K)FM Diagram Project from a phase that is present in the mineral assemblages to be studied Figure 24-18. AKFM Projection from Mu. After Thompson (1957). Am. Min. 22, 842-858. J.B. Thompson's A(K)FM Diagram At high grades muscovite dehydrates to K-feldspar as the common high-K phase Then the AFM diagram should be projected from K-feldspar When projected from Kfs, biotite projects within the F-M base of the AFM triangle Figure 24-18. AKFM Projection from Kfs. After Thompson (1957). Am. Min. 22, 842-858. J.B. Thompson's A(K)FM Diagram A = Al O - 3K O (if projected from Ms) J.B. Thompson's A(K)FM Diagram Biotite (from Ms): KMg2FeSi3AlO10(OH)2 A = 0.5 - 3 (0.5) = - 1 F =1 M =2 To normalize we multiply each by 1.0/(2 + 1 - 1) = 1.0/2 = 0.5 Thus A = -0.5 F = 0.5 M=1 J.B. Thompson's A(K)FM Diagram Figure 24-20. AFM Projection from Ms for mineral assemblages developed in metapelitic rocks in the lower sillimanite zone, New Hampshire After Thompson (1957). Am. Min. 22, 842-858. Mg-enrichment typically in the order: cordierite > chlorite > biotite > staurolite > garnet Choosing the Appropriate Chemographic Diagram Example, suppose we have a series of pelitic rocks in an area. The pelitic system consists of the 9 principal components: SiO2, Al2O3, FeO, MgO, MnO, CaO, Na2O, K2O, and H2O How do we lump those 9 components to get a meaningful and useful diagram? Choosing the Appropriate Chemographic Diagram Each simplifying step makes the resulting system easier to visualize, but may overlook some aspect of the rocks in question 3 MnO is commonly lumped with FeO + MgO, or ignored, as it usually occurs in low concentrations and enters solid solutions along with FeO and MgO 3 In metapelites Na O is usually significant only in 2 plagioclase, so we may often ignore it, or project from albite 3 As a rule, H O is sufficiently mobile to be ignored as 2 well Choosing the Appropriate Chemographic Diagram Common high-grade mineral assemblage: Sil-St-Mu-Bt-Qtz-Plag Figure 24-20. AFM Projection from Ms for mineral assemblages developed in metapelitic rocks in the lower sillimanite zone, New Hampshire After Thompson (1957). Am. Min. 22, 842-858. Choosing the Appropriate Chemographic Diagram Figure 24-21. After Ehlers and Blatt (1982). Petrology. Freeman. Choosing the Appropriate Chemographic Diagram 3 We don't 3 3 have equilibrium There is a reaction taking place (F = 1) We haven't chosen our components correctly and we do not really have 3 components in terms of AKF Figure 24-21. After Ehlers and Blatt (1982). Petrology. Freeman. Choosing the Appropriate Chemographic Diagram Figure 24-21. After Ehlers and Blatt (1982). Petrology. Freeman. Choosing the Appropriate Chemographic Diagram Myriad chemographic diagrams have been proposed to analyze paragenetic relationships in various metamorphic rock types Most are triangular: the maximum number that can be represented easily and accurately in two dimensions Some natural systems may conform to a simple 3component system, and the resulting metamorphic phase diagram is rigorous in terms of the mineral assemblages that develop Other diagrams are simplified by combining components or projecting Choosing the Appropriate Chemographic Diagram Variations in metamorphic mineral assemblages result from: 1) Differences in bulk chemistry 2) differences in intensive variables, such as T, P, PH2O, etc (metamorphic grade) A good chemographic diagram permits easy visualization of the first situation The second can be determined by a balanced reaction in which one rock's mineral assemblage contains the reactants and another the products These differences can often be visualized by comparing separate chemographic diagrams, one for each grade ...
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This note was uploaded on 07/21/2009 for the course GEOPHYSICS metamorphi taught by Professor Dr. during the Spring '09 term at Ain Shams University.

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