Subduction Metamorphism
Summary
This activity uses images of rocks and thin sections, including photomicrographs (still images, videos), thin section scans, element maps, BSE images, and hand sample photos, to guide students in an examination of 4 metamorphic rocks from an oceanic subduction complex: two blueschists, one eclogite, and a metachert. All contain the hydrous index mineral lawsonite, which is used to introduce concepts related to water and element cycling during subduction.
Context
Audience
The activity can be used in an undergraduate Petrology or Earth Materials course and is adaptable for other courses that involve concepts related to metamorphic rocks and/or tectonics (subduction).
Skills and concepts that students must have mastered
The activity includes images from thin sections, so students should have some exposure to viewing minerals/rocks with a petrographic microscope or images of thin sections seen in plane light and cross polarized light. It is helpful if students have basic knowledge of subduction zones and metamorphic rocks, but this is not necessary. The activity includes editable Powerpoint presentations that instructors can use to introduce subduction metamorphism and related topics.
How the activity is situated in the course
A sample-based version of this activity comprises one lab in a Petrology course for undergraduate students. A shorter version of this activity focused on oceanic subduction has also been used, along with a short version of an activity on ultrahigh-pressure metamorphism (continental subduction).
Goals
Content/concepts goals for this activity
main content/concepts:
- recognizing index minerals of 'cold' subduction metamorphism (glaucophane, omphacite, garnet, lawsonite)
- identifying and interpreting evidence for progressive metamorphism (protolith -> prograde -> peak -> retrograde)
- understanding the importance of hydrous minerals for element cycling
Higher order thinking skills goals for this activity
After establishing knowledge about subduction index minerals / textures, students will:
- infer the pre-subduction parent rock (protolith) from metamorphic mineral assemblages.
- develop hypotheses about geologic processes based on the presence or absence of key minerals in high-pressure - low-temperature (subduction) metamorphic rocks.
- interpret different stages of metamorphic history based on a synthesis of observations and interpretations about minerals and their textures.
Other skills goals for this activity
This activity involves some writing and interpretation of various types of images (including those obtained with an SEM) and could involve group work.
Description and Teaching Materials
The activity includes the following documents:
3 Word documents that can be edited as needed for time constraints and context:
Subduction metamorphism - Word docs.zip (Zip Archive 3.5MB Nov22 24)
- CORE QUESTIONS (the activity),
- Question Bank (optional additional activities and data),
- a Readme file
Core questions instructor KEY -- educator-only file
Hide3 Slide decks on subduction metamorphism, lawsonite, and regional geology:
A folder containing 3 slide decks for instructors: these Powerpoint presentations are editable and cover (1) subduction metamorphism, (2) lawsonite, and (3) the field site where the samples were collected. The presentation files have been vetted by SERC for copyright compliance; all images are either original to the activity contributors or represent substantially modified figures from references cited.
Slide decks for instructors-20241120T014710Z-001.zip (Zip Archive 96.6MB Nov19 24)
Folders with images and other supporting materials:
5 folders with images for each of the 4 samples (SV23-4, -5, -6, -20); SV23-05 is separated into two zip files. Each folder contains videos showing the optical properties of key minerals in plane light and crossed polarized light (video shows stage rotation)
SV23-04-images.zip (Zip Archive 1722.8MB Nov20 24)
SV23-06-images.zip (Zip Archive 1054.2MB Nov20 24)
SV23-05-images.zip (Zip Archive 1077.3MB Nov21 24)
SV23-05-videos questions 3_4 3_5.zip (Zip Archive 1343.9MB Nov21 24)
SV23-20-images.zip (Zip Archive 1910.3MB Nov21 24)
Folders with supplementary data and images and key mineral videos
Supplementary data and images-20241120T014552Z-001.zip (Zip Archive 75.2MB Nov19 24)
Mineral Example_Videos-20241120T014840Z-001.zip (Zip Archive 1900.9MB Nov19 24)
Folders with high-res BSE images
Two folders containing high-resolution thin-section-scale backscattered electron (BSE) images for each of the 4 samples; lower resolution versions of these images are included in the main image folders labeled by sample numbers. The high-resolution image files are very large.
Hi-res thin-section-scale BSE SV23-04_05.zip (Zip Archive 1350.9MB Nov21 24)
Hi-res thin-section-scale BSE SV23-06_20.zip (Zip Archive 1483.6MB Nov21 24)
Teaching Notes and Tips
Note that some files are very large, even in compressed form. In some contexts, it may be advisable to make even lower resolution versions, such as using screen capture images.
In a pilot test at UMN, students who have had prior experience with petrography completed the activity in 2 - 2.5 hours.
There is a Readme file that provides additional information. This will be updated based on feedback from instructors.
Assessment
A sample-based version of this activity was piloted in undergraduate courses at 21 different institutions in the US (representing both Petrology and Earth Materials courses). This version was assessed by SERC using results from 149 students from 15 institutions (surveys, instructor stories, evaluation of completed activities). See Toraman et al. (2024) abstract for the GSA annual meeting for additional information about the sample-based version and assessment. The online-only version has not been formally assessed.
References and Resources
Resources related to this activity are provided and cited in the three presentation files that are included with the other materials.