Course: Depth (km) based on geothermal gradient Depth (km) based on geobaric gradient P (kbar) Rock Polymorph A Kyanite Aluminosiat 450°C Andawsite Awminasiiante 580°C 830°C. 14:3km 27,7km 33 B. 66km (b) What does the discrepancy between depth estimates based on geothermal and geobaric gradients suggest about metamorphism? (c) What mineral changes would occur, and at what temperature and pressure, if • Rock A was heated to 800°C without a change in pressure? • Rock A experienced a drop in pressure to 1.5 kbar without a change in temperature? • Rock B was heated to 700°C and pressure on it was increased to 10 kbar? Rock C was cooled to 500°C without a change in pressure? (d) In some cases, it is possible to detect how rocks have changed position in the crust during metamorphism. Suppose that Rock X has experienced a complex tectonic history in which T-P conditions changed over time as shown by the dashed arow. • Describe the sequence of aluminosilicate phase changes in Rock X and the T-P conditions and depths at which they occurred. 7.3 AGENTS OF METAMORPHISM deeper in the Earth, it must adjust to progressively increasing temperatures and pre sures. An assembiage of metamorphic minerals that formed at 10 km would have to change to adjust to more intense conditions if it were buried more deeply at 25 km. EXERCISE 7.1 Responses to Changing Metamorphic Conditions Name: Section: Course: Date: The minerals andalusite, sillimanite, and kyanite are polymorphs of aluminosilicate (AI, SIO) that differ in their internal structures and in the conditions un- der which they are stable. The aluminosilicate phase diagram on the right, which uses different colors to delineate the stability zones of these polymorphs, pro- vides insight into how metamorphic minerals respond to changes in temperature and pressure conditions. Examine the phase diagram and answer the following questions: 10 Kyanite (A Salimanite (a) Indicate the temperature-pressure (T-P) conditions and estimated depths for rocks A, B, and Cin the following table. Assume average geothermal (30°C per kilometer) and geobaric gradients (0.33 kbar per kilometer). Andalusite 400 500 600 700 800 900 1,000 Temperature (C) Phase diagram showing stability zones for three aluminosilicate (AI SIO) polymorphs. (continued) 178 CHAPTER 7 INTERD DETING METAM ORPHIC POCKe Pressure (kbar)