Chapter 28 Ans

Chapter 28 Ans - Answers for Chapter 28: Metamorphism of...

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Answers for Chapter 28: Metamorphism of Pelitic Sediments 1. What are pelites (the protolith) and where are they typically deposited? Pelites are shales or mudstones and are typically deposited in low energy environments due to their fine grain size. A typical example of this environment is the distal portion of a sedimentary wedge off a stable or active continental margin 2. What are their principal chemical characteristics of pelites, and how does that affect the predominant minerals that develop upon metamorphism? Pelites are characterized by clay minerals with high concentrations of Al, K, and Si. The predominant metamorphic minerals are thus micas (particularly muscovite) and quartz, but also (depending on composition and grade) garnet, staurolite, cordierite, and/or an Al 2 SiO 5 polymorph. 3. What is meant by “in excess” when referring to a phase? If a reaction A + B + C D + E occurs with increasing grade, how would A being either in excess or not for all natural rocks, affect the mineral assemblages that develop above the isograd? “In excess” means that the phase is present in sufficient quantity that it will never be consumed by a metamorphic reaction, so it will never limit the progress of that reaction and therefore also never affect the stability of another mineral or mineral assemblage. For example, if A were not present in excess it might be consumed before B and C by the reaction above, and B + C would remain stable above the reaction isograd. If A were present in excess B + C can never be found together above the isograd. 1
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4. Using the fictitious phases A, B, C, and D, propose a reaction that (a) represents the absolute stability limit of phase A. A = B + C + D Next propose a reaction (b) that involves all four phases but does not absolutely limit the stability of any. A + B = C + D Illustrate the topology changes of both reactions using an imaginary triangular compatibility diagram within which these phases plot. Show the topology both above and below the reaction conditions. Describe which phases appear and/or disappear at the reaction isograds for various bulk compositions within your diagrams. 2
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5. Use the phase rule to determine the variance of Reaction (28.17). St (+Ms Qtz) = Grt Bt Als + H O 2 + + + (28.17) This is in the system KFMASH so F = 5 - 6 + 2 = 1 (with H 2 O mobile) Why is it considered discontinuous? Because F = 1 above but P/T is constrained by the P-T-t path so F is reduced by one so F 0. Why is it considered “terminal” for staurolite? Because H 2 O and quartz are considered in excess, so they cannot run out and limit the reaction. Staurolite is then the only limiting phase, so it must disappear as the reaction runs to completion. How does the geometry of the chemographic topology change differ from most non-terminal
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Chapter 28 Ans - Answers for Chapter 28: Metamorphism of...

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