Serpentine Seamounts in the Mariana Forearc: Shallow Material Releases from Downgoing Plates
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This is a resource of published images and diagrams, and GeoMapApp/Google Earth captures of geophysical survey results for use in describing the unique phenomenon of active serpentinite mud volcanism observed in the shallow forearc region of the Mariana subduction system. Students should be familiar with the concept of subduction and one of its primary petrologic implications - that materials (specifically sediments, ocean crust, and H2O-rich fluids) long residing at the Earth's surface are transported down deep sea trenches, with the release of bound fluids (and fluid-soluble chemical species from the downgoing plate) occurring progressively with increasing depth. These materials are intended to be used as aids in lecture or discussions of this phenomenon in the context of instruction on the process of subduction.t
The primary concept being addressed is the idea that fluids are released from subducting plates due to the prograde metamorphism of ocean crust and sediments. These fluids metasomatize the overlying Mariana forearc mantle wedge, producing serpentine, which rises diapirically along zones of structural weakness. The magnitude of these fluid releases is large (i.e., significant fractions of the forearc mantle are serpentinized), and these releases are shallow (at 20-30 km depth to slab) relative to the depths at which arc magmatism is believed to occur. As well, a very select menu of trace elements are mobilized from the slab during the fluid releases that lead to serpentine formation.
This resource is intended to be used in a lecture/demonstration setting, as setup materials for more involved exercises examining material fluxes through subduction systems and/or the physical and tectonic implications of the development of mantle serpentinites.
Context for Use
This resource is probably best used in courses on igneous/metamorphic petrology or structural geology/tectonics as a means of providing an information-rich description of physical and geochemical processes in forearc settings. Parts of this resource may be useful in introductory geoscience or oceanography courses.
Description and Teaching Materials
Included is a hyperlinked pdf file of annotated images, and an instructor's description for each. The file can be used audio-visual aids for discussion in class, and can also be provided to students for more detailed examination and inquiry. Links to the source literature are provide for each diagram and image.
Linked PDF file of Mariana forearc seamount pictures and data (Acrobat (PDF) 122.9MB May28 09)
Instructor's notes (Microsoft Word 33kB May28 09)
Teaching Notes and Tips
I've tried to set up a variety of link trajectories through this imagery to facilitate both a short lecture and/or discussion and question/answer sessions with students. As it's a linked pdf document, it can be easily edited and added to via Adobe Acrobat.
Assessment will necessarily be varied, depending on how one uses these materials. My approach is to follow on with context-appropriate questions as a short homework assignment, or to have them delve into some of the background literature for further information. One option is to have students drill into the Savov et al and Fryer et al papers on the Mariana serpentinites, either to compare elemental abundance patterns in the rocks with those in associated porefluids, or to answer questions about some of the physical implications of wedge serpentinite formation (i.e., how much serpentinization is required to produced the forearc rise topographic highs).
References and Resources
Mottl, Michael J; Wheat, C Geoffrey; Fryer, Patricia; Gharib, Jim; Martin, Jonathan B., 2004, Chemistry of springs across the Mariana forearc shows progressive devolatilization of the subducting plate. Geochimica et Cosmochimica Acta, vol.68, no.23, pp.4915-4933
Oakley, A.J.; Taylor, B.; Fryer, P.; Moore, G.F.; Goodliffe, A M; Morgan, J.K., 2007, Emplacement, growth, and gravitational deformation of serpentinite seamounts on the Mariana forearc. Geophysical Journal International, vol. 170, no. 2, pp.615-634.
Fryer, Patricia B; Salisbury, Matthew H. (2006) Leg 195 synthesis; Site 1200; serpentinite seamounts of the Izu-Bonin/Mariana convergent plate margin, ODP Leg 125 and 195 drilling results. Proceedings of the Ocean Drilling Program, Scientific Results (CD ROM), vol. 195, 30 pp.
Savov, I.P., Ryan, J.G., D'Antonio, M. and P. Fryer, 2007, Petrology and geochemistry of serpentinized peridotites from Mariana Forearc, South Chamorro Seamount, ODP Leg 195: Implications for the elemental recycling across and along the Mariana arc-basin system. Journal of Geophysical Research, v. 112, doi:10.1029/2006JB004749