Chapter 21 Ans

Chapter 21 Ans - Answers for Chapter 21: An Introduction to...


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Answers for Chapter 21: An Introduction to Metamorphism 1. What defining processes may occur during metamorphism? Briefly describe each. Recrystallization: the rearrangement and grain size or shape readjustment of existing minerals. Neocrystallization: the nucleation and growth of new mineral types. Mineral reactions: the formation and/or loss of some mineral species to conform to new stability conditions. Deformation: microstructural changes that may or may not include recrystallization, cataclasis, etc. Metasomatism: gain and/or loss of chemical constituents. Volatile transfer: devolatilization of addition of volatiles (hydration, carbonation…). 2. Into what does metamorphism grade at the low-grade end? Diagenesis and weathering How do we define the low-grade boundary of metamorphism? There is a general consensus that metamorphism begins in the range of 100 to 150 o C for the more unstable types of protolith, and may be marked by the formation of minerals such as laumontite, analcime, heulandite, carpholite, paragonite, prehnite, pumpellyite, lawsonite, glaucophane or stilpnomelane. Some zeolites have been considered diagenetic and others metamorphic. Why is this a rather difficult distinction to make? The processes of weathering and diagenesis are essentially metamorphic in nature. The distinction is thus quite arbitrary. 3. Metamorphism grades into what at the high-grade end? Melting and igneous processes. How do we define the high-grade boundary of metamorphism? We don’t really. Metamorphism grades entirely into melting. The melts themselves are igneous, but the solid restites in equilibrium with those melts, or even the minor melt stringers in migmatites are generally considered metamorphic. Why is this a rather difficult distinction to make? Again this is rather arbitrary as melts coexist with metamorphic rocks between the solidus and liquidus temperatures and it may be difficult to distinguish the products of locally-derived melt, precipitates from fluids, or fluid-enhanced recrystallization along fluid-filled fractures. 1
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4. In what ways may changing temperature affect metamorphic mineral assemblages and textures? Increasing temperature may 1) overcome low-T kinetic barriers to metamorphic processes and promote the attainment of equilibrium, 2) enhance recrystallization and increase grain size, 3) drive metamorphic reactions as mineral assemblage stability conditions change (higher grade mineral assemblages replace lower grade ones). 5. What is a metamorphic field gradient , and how do they differ from a geothermal gradient (at the time of metamorphism)? A metamorphic field gradient is a series of P-T estimates for rocks exposed at the surface in metamorphic terranes along a traverse from lowest to highest metamorphic conditions. A geothermal gradient is the variation of temperature with increasing pressure along a vertical section at any place or time. Describe a situation in which a
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