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Isograds for a single shale unit in southern Vermont

Isograds for a single shale unit in southern Vermont Which side reflects a higher grade, or higher P/T environment?. Systems. Rock made of different minerals Metamorphic agents of change beat on it  metamorphic reactions occur A closed system does not gain or lose material of any kind

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Isograds for a single shale unit in southern Vermont

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  1. Isograds for a single shale unit in southern Vermont • Which side reflects a higher grade, or higher P/T environment?

  2. Systems • Rock made of different minerals • Metamorphic agents of change beat on it  metamorphic reactions occur • A closed system does not gain or lose material of any kind • An open system can lose stuff – liquids, gases especially Outside world Hunk o’ rock

  3. Phase diagrams • Tool for ‘seeing’ phase transitions • H2Oice H2Oliquid • Reaction (line) governed by DG=DH – TDS • Phase Rule: • P+F=C+2 • Phases coexisting + degrees of freedom = number of components + 2 • Degree of freedom  2= either axis can change and the phase stays the same  where??

  4. Phase diagrams • Let’s think about what happens to water as conditions change… • P+F=C+2 • Point A? • Point B? • Point C? A B C

  5. 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 • Relict minerals or later alteration products are thereby excluded from consideration unless specifically stated

  6. The Phase Rule in Metamorphic Systems • Phase rule, as applied to systems at equilibrium: F = C - P + 2 the phase rule P is the number of phases in the system C is the number of components: the minimum number of chemical constituents required to specify every phase in the system F is 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.)

  7. The Phase Rule in Metamorphic Systems C = 1 (Al2SiO5) • F = 1 common • F = 2 rare • F = 3 only at the specific P-T conditions of the invariant point (~ 0.37 GPa and 500oC) Consider the following three scenarios: 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.

  8. Representing Mineral Reactions • albite  jadeite + quartz

  9. From Hacker, B.R.,

  10. Let’s put it all together…

  11. Metamorphic facies • P-T conditions, presence of fluids induces different metamorphic mineral assemblages (governed by thermodynamics/ kinetics) • These assemblages are lumped into metamorphic facies (or grades)

  12. What if we had staurolite and andalusite together? What conditions would that indicate?

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