Chapter 26 Ans

Chapter 26 Ans - Answers for Chapter 26: Metamorphic...

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Answers for Chapter 26: Metamorphic Reactions 1. Compare the classical notion of an isograd to treating an isograd as a reaction . An isograd in the classical sense represents the first appearance of some index mineral as one progresses up metamorphic grade in the field. Treating an isograd as a reaction is based on determining the reaction responsible for the generation of new minerals (and/or consumption of pre-existing ones for “mineral-out” isograds). 2. What are the potential advantages of treating an isograd as a reaction? Treating an isograd as a reaction allows us to explore the controls and variables associated with that reaction. We can understand why and under that P-T-X conditions a new mineral formed, why it may not appear in all rocks at the same grade, what other mineralogical changes may accompany it. 3. On what variables does the grade at which a polymorphic transformation reaction occurs depend? Only on P and T (and the attainment of equilibrium). 4. How do solid–solid net-transfer reactions differ from polymorphic transformations ? Give an example of the former. Solid–solid net-transfer reactions involve the breakdown of reacting phases and migration of ions from reacting phases to the site of growth of product phases. Polymorphic transformations are simply the rearrangement of ions virtually in place. No diffusion (transfer) is involved. An example is the albite = jadeite + quartz reaction. Why is net transfer typically involved in your example? Be as specific as you can. Albite contains more SiO 2 than jadeite for the same quantity of Na and Al. For the reaction to proceed (forward as written), an excess of SiO 2 is generated when albite breaks down to form jadeite. Either this excess must migrate to where quartz is growing or Na and Al must migrate away to form jadeite elsewhere. 1
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5. In section 26.2 the text states that “Discontinuous reactions are univariant and tend to run to completion at a single metamorphic grade…There is thus an abrupt (discontinuous) change from the reactant assemblage to the product assemblage at the reaction isograd. (The reaction behaves as though invariant in such situations, because P and T are not independent, but constrained by the P-T-t path.)” Explain this by sketching Reaction 26.2 on a hypothetical P-T phase diagram and adding a P-T-t path. Is the reaction univariant? Will it take place entirely at a single metamorphic grade? Why? The reaction is univariant as written. If we express C as the minimum number of components required to constitute all phases, C = 3 (MgSiO 3 , Al 2 SiO 5 , and CaSiO 3 ) then F = 3 – 4 + 2 = 1. It is therefore a univariant curve on the P-T diagram (I’ve drawn it quite arbitrarily). It would, however, take place at a singular P and T because the P-T-t path places a constraint on the variance (a relationship between P and T), which reduces F by one. The situation at the point of reaction is thus invariant.
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Chapter 26 Ans - Answers for Chapter 26: Metamorphic...

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