Serpentinite in Subduction Zones: How do we find it, and how common is it?
One of the interesting discoveries arising out of the MARGINS Subduction Factory Initiative is the recognition that serpentinites - metamorphically hydrated products of ultramafic rocks that are rich in serpentine group minerals - are significant constituents of both the mantle wedge and downgoing plate. Serpentine minerals are interesting mineralogically because of their distinctive physical properties, habits and appearance; and for their commonly close affinities for olivine and Mg-rich pyroxenes. As well, serpentine group minerals are interesting rheologically because, as sheet silicates, they can behave in a plastic fashion in rocks that are undergoing deformation.
This activity leads students through a range of literature sources and activities, including GeoMapApp manipulations and some "back of the envelope" calculations about isostatic compensation and volumetric expansion during serpentinization to help them appreciate the different ways that the presence of serpentinite can be observed or inferred in subduction zone settings. A second but equally important goal is to familiarize students with some of the basic kinds of data and interpretive approaches used by geochemists and geophysicists in studying modern subduction zone environments.
This activity is targeted for use in Junior- or Senior-level mineralogy and/or petrology courses. It makes use of published data and information from both of the MARGINS Subduction Factory focus sites (Izu-Bonin-Mariana and Central America), as well as information from meeting presentations posted on the NSF-MARGINS homepage site. It presumes a basic familiarity with examining geospatial data (primarily comparative elevations) using GeoMapApp. It is suggested that students be provided with thin sections or hand samples of actual serpentinites to give them a better understanding of what these materials are, and how they compare to olivine-rich ultramafic rock samples more typical of 'fresh' mantle.
Context for Use
Description and Teaching Materials
Teaching Notes and Tips
What these problems are likely to be will vary with student group: my students, as an example, are comfortable with the mineralogical and petrologic aspects of the activity, but have difficulty conceptualizing the seismological aspects and with doing the suggested calculations. Also, in general, getting students to read technical papers for information (as opposed to the way one reads a novel) is challenging.
References and Resources
Students will need access to GeoMapApp (http://www.geomapapp.org/) for one part of the activity.