Subduction zone metamorphism

1. Students should be able to describe what minerals and rocks are characteristic of subduction-related metamorphism.
2. Students should be able to sketch a cross-section of a subduction zone.
3. Students should be able to explain the thermal structure of a subduction zone.
4. Students should be able to explain how metamorphic rocks change during metamorphism
5. Students should be able to interpret how metamorphic rocks record the P-T-t history they have experienced.
6. Students should be able to describe factors that affect P-T conditions of subduction zone metamorphism.

This activity is designed for use in an upper-level undergraduate petrology course. It could be included within a metamorphic petrology course or the metamorphic petrology section of a general petrology course. This lesson might also be incorporated as part of a course on tectonics or structural geology.

Instructors can take portions of the presentations to incorporate into single lectures or use all materials which will last 1-2 weeks of classroom time (lab and lecture). The color-coded flow chart (to the right on this page) is designed to suggest some possible sequences of materials for use.

Student background: For the three introductory exercises and powerpoint slides a background in general geology with introductory mineralogy is required. For the lab and rocks vs. models exercises, students will need more background in petrology and for the lab they will need to have the ability to recognize different minerals in thin section.

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Flow chart of mini lesson

Provenance: Sarah Penniston-Dorland, University of Maryland-College Park
Reuse: This item is offered under a Creative Commons Attribution-NonCommercial-ShareAlike license http://creativecommons.org/licenses/by-nc-sa/3.0/ You may reuse this item for non-commercial purposes as long as you provide attribution and offer any derivative works under a similar license.

This mini lesson consists of the following components:

Introductory Exercises: These three different exercises can be used independently or in concert to motivate students and get them thinking about the concepts introduced in this mini-lesson.

1) Introduction/conclusion exercise (Acrobat (PDF) 61kB Sep18 14): Draw a cross-section of a subduction zone - This exercise can be given to students in the classroom as both an introduction and a conclusion to the topic. It may take about a half hour, depending on how much time the instructor allows the students to work on it. It is designed to be motivating to make students to start asking questions about subduction zones.

2) Introductory exercise 2: Rock comparison (Acrobat (PDF) 57kB Sep30 14) - Have rocks in the classroom and have the students make first-order physical observations about the different types of rocks and compare them. Designed to be motivational to students to understand why metamorphic rocks of the subduction zone are the way they are.

• Rock comparison powerpoint (PowerPoint 2007 (.pptx) 3MB Jul20 15)
• Rock comparison table (Acrobat (PDF) 32kB Aug9 14)

3) Suitcase exercise (Acrobat (PDF) 84kB Aug9 14) aimed at helping students understand how rocks respond to increase in temperature and pressure. Introduction powerpoint - Suitcase exercise (PowerPoint 2007 (.pptx) 440kB Aug9 14)

Powerpoint slides: There is a series of powerpoint slides that can be used as a resource for instructors to share with students in class as needed. They are designed to address the questions that are posed in the cross-section exercise.

4) Subduction zone geometry (PowerPoint 483kB Sep28 15):
This short powerpoint (3 slides) addresses questions #3 and #4 on the cross-section exercise. This powerpoint demonstrates some of the cutting-edge MARGINS research.

5) Subduction thermal structure (PowerPoint 492kB Dec8 14):
This powerpoint (7 slides) addresses question #2 on the cross-section exercise. This powerpoint demonstrates some of the cutting-edge MARGINS research.

6) Slab-mantle interface (PowerPoint 1.8MB Aug17 15):
This short powerpoint (3 slides) addresses question #5 on the cross-section exercise.

7) Subduction rocks and minerals (PowerPoint 16.7MB Dec8 14):
This powerpoint (~26 slides) addresses questions #6 and #7 on the cross-section exercise. It is really the meat of the metamorphic petrology part of this module.

8) Reactions (PowerPoint 2007 (.pptx) 131kB Jul20 15):
This powerpoint (4 slides) is designed to complement the powerpoint on subduction rocks and minerals to present additional information that students may have already learned about metamorphic reactions and geothermobarometry.

For more information on mineral reactions, thermobarometry and P-T-t paths see Phase equilibria

9) Subduction volatile loss (PowerPoint 2007 (.pptx) 187kB Dec3 14):
This powerpoint addresses questions raised by the suitcase exercise about H2O loss during subduction. The powerpoint can be followed up by the Changes in rock density exercise below.

Assignments for students: These assignments are designed for a petrology class in which students have had background on metamorphic minerals and rocks, mineral reactions, and are able to identify metamorphic minerals in thin section.

10) Rocks vs. Models homework assignment (Acrobat (PDF) 4MB Sep30 14):
This exercise should be given to students to work on by themselves or in groups. It requires the students to read two papers (Page et al., 2007 and Syracuse et al., 2010) and answer questions based on those two papers. Ideally, the instructor will be able to discuss the exercise in class, eliciting answers from the students. This exercise showcases some of the cutting-edge MARGINS research.

Rocks vs. Models powerpoint (PowerPoint 2007 (.pptx) 525kB Aug9 14):
This powerpoint shows a comparison of data from Page et al., 2007 and model results of Syracuse et al., 2010

11) Subduction zone metamorphism lab (Acrobat (PDF) 382kB Sep30 14):
This exercise makes use of a suite of rocks available for instructors to use in a lab associated with the class. The instructor will need to make thin sections of the rocks in order to make full use of this lab. Students are asked to identify minerals, name rocks, and assign metamorphic grade to samples.

Request rocks for this lab. Send an email to Sarah Penniston-Dorland at sarahpd "at" umd.edu. For the subject line type in "Subduction zone lab rock request". Indicate in your email which rocks you are requesting: MORB, greenstone, blueschist, eclogite. Also include the shipping address. The rocks will be sent to you as hand samples, not thin sections. You will need to have your own thin sections made. If your department does not have the resources to create thin sections or for students to observe thin sections, thin sections for each of these rocks have been scanned and are available online at the Virtual Petrographic Microscope. The Virtual Petrographic Microscope has online thin section scans of the rocks for this lab under the heading "UNE SERC samples".

Minerals students will observe in lab (PowerPoint 2007 (.pptx) 4.7MB Sep28 15):
This powerpoint shows photomicrographs demonstrating the minerals and textures in the samples that students will see in the lab.

12) Changes in rock density (Acrobat (PDF) 72kB Sep30 14):
This exercise can be used in class or as a homework assignment. It requires students to look up mineral densities and calculate the overall density of subduction-related metamorphic rocks. The main points of this exercise are that the density increases during subduction and that rocks lose H2O during subduction metamorphism.

1)

Introductory/conclusion exercise rubric -- private instructor-only file

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Answers to the introductory/concluding exercise (pre-test and post-test) that can be done by students at both the beginning and end of the unit. This assessment addresses learning goal #2 (see above).

10)

Rocks vs. Models homework assignment rubric -- private instructor-only file

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Answers and guidelines to the homework assignment asking students to compare P-T estimates from subduction-related metamorphic rocks to predictions of P-T paths from thermal models of subduction zones. This exercise addresses learning goals # 3, 4, 5 and 6.

11)

Lab rubric -- private instructor-only file

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Answers and guidelines for how to assess the lab assignment in which students examine rocks and minerals in thin section. This exercise addresses learning goals # 1, 4 and 5.

12)

Changes in rock density rubric -- private instructor-only file

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Answers and guidelines for how to assess the assignment on changes in rock density. This exercise addresses learning goal #4.

• Bebout, G.E., 2007, Metamorphic chemical geodynamics of subduction zones, Earth and Planetary Science Letters, 260, 373-393.doi:10.1016/j.epsl.2007.05.050
• Carlson, W.D., 1983, The polymorphs of CaCO3 and the aragonite-calcite transformation, in Carbonates: mineralogy and chemistry, edited by R.J. Reeder, Reviews in Mineralogy and Geochemistry, 11, 191-225.
• Catlos, E.J., and Sorensen, S.S., 2003, Phengite-based chronology of K- and Ba-rich fluid flow in two paleosubduction zones, Science, 299, 92-95. doi:10.1126/science.1076977
• Hayes, G.P., Wald, D.J., and Johnson, R.L., 2012, Slab1.0: A three-dimensional model of global subduction, Journal of Geophysical Research, 117, B01302. DOI: 10.1029/2011JB008524
• Hacker, B., 2008, H2O Subduction beyond arcs, Geochemisty, Geophysics, Geosystems. Volume 9, Issue 3. doi/10.1029/2007GC001707
• Krogh Ravna, E., 2000, The garnet-clinopyroxene Fe²⁺-Mg geothermometer: an updated calibration, Journal of Metamorphic Geology, 18, 211-219.
• Liou, J.G., 1971, P-T stabilities of laumontite, wairakite, lawsonite, and related minerals in the system CaAl2Si2O8-SiO2-H2O, Journal of Petrology, 12, 379-411. doi: 10.1093/petrology/12.2.379
• Liou, J.G., Maruyama, S., and Cho, M., 1987, Very low-grade metamorphism of volcanic and volcaniclastic rocks-mineral assemblages and mineral facies, in Low Temperature Metamorphism, edited by M. Frey, p. 59-113, Blackie, Glasgow.
• Page, F.Z., Armstrong, L.S., Essene, E.J., and Mukasa, S.B., 2007, Prograde and retrograde history of the Junction School eclogite, California, and an evaluation of garnet-phengite-clinopyroxene thermobarometry, Contributions to Mineralogy and Petrology, 153: 533-555.
• Schmidt, M.W. and Poli, S., 1998, Experimentally based water budgets for dehydrating slabs and consequences for arc magma generation, Earth and Planetary Science Letters, 163, 361-379.
• Spear, F., 1993, Metamorphic Phase Equilibria and Pressure-Temperature-Time Paths.
• Syracuse, E.M. and Abers, G.A., 2006, Global compilations of variations in slab depth beneath arc volcanoes and implications, Geochemistry, Geophysics, Geosystems, 7, 5. DOI: 10.1029/2005GC001045
• Syracuse, E.M., van Keken, P.E., and Abers, G.A., 2010, The global range of subduction zone thermal models, Physics of the Earth and Planetary Interiors, 183: 73-90. doi:10.1016/j.pepi.2010.02.004
• van Keken, P.E., 2003, The structure and dynamics of the mantle wedge, Earth and Planetary Letters, 215, 323-338. doi:10.1016/S0012-821X(03)00460-6
• van Keken, P.E., Hacker, B.R., Syracuse, E.M., and Abers, G.A., 2011, Subduction factory: 4. Depth-dependent flux of H₂O from subducting slabs worldwide, Journal of Geophysical Research, 116, B01401.

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