Isograds for a single shale unit in southern Vermont
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Transcript of 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
• An open system can lose stuff – liquids, gases especially
Hunk o’ rock
Outsideworld
Phase diagrams• Tool for ‘seeing’ phase transitions
• H2Oice H2Oliquid
• Reaction (line) governed by G=H – TS
• 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??
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
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
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.)
The Phase Rule in Metamorphic SystemsConsider the following three scenarios:
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)
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.
Representing Mineral Reactions
• albite jadeite + quartz
From Hacker, B.R.,
Let’s put it all together…
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)
• What if we had staurolite and andalusite together? What conditions would that indicate?