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What Goes into Making Volcanic Arc Magmas, and How Do We Know It?

Jeffrey G. Ryan, University of South Florida
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Summary

One of the challenges in teaching undergraduate petrology students about the origins of subduction-related magmas and igneous activity at volcanic arcs is that this system is much more complex than that encountered at mid-ocean or intraplate settings: the sources of magmas are variable (i.e., the mantle, the downgoing plate, and/or the overriding crust may provide part or all of the magmatic source materials), and their histories from their point of origin to eruption are complex. An additional complication is that much of what we know about the material contributors to subduction-zone magmas has been learned through arcane trace element and isotopic studies of arc lavas and associated subduction zone metamorphic rocks, a body of literature (and geologic subdiscipline) with which undergraduates are likely to have little experience.

This activity is intended as a directed reading exercise for undergraduates in a junior- or senior-level petrology course aimed at leading them through the how and why of our current views on arc petrogenesis, while along the way familiarizing them with key aspects of trace element and isotope geochemistry: specifically, the concept of trace element solid/melt and solid/fluid partitioning, and its use in defining how a particular elemental signature in a rock can originate. The readings are key geochemical papers that track the development of our current views on the petrogenesis of arc lavas, and some associated controversies (i.e., melting of the subducting slab, and the role of subduction-related serpentinites).

Learning Goals

Key learning goals are:
1) Understanding the role of trace element data in helping us understand the development of magmas, especially in subduction zone settings
2) A basic understanding of elemental partitioning between solid, melt, and hydrous fluids
3) The ability to "read for information" in the scientific literature, and to distinguish between data presentation and argument in scientific papers

Context for Use

This activity can be used in whole or in part as a homework/discussion activity in presenting content on the origins of subduction zone igneous rocks. In whole it becomes a multi-day research exercise that may also be suitable for graduate teaching, but in part it offers a way of framing a discussion of key questions about arc petrogenesis (i.e., what is the role of subducted sediments? Of subducted volatiles? Do slabs melt?, etc.)

Description and Teaching Materials

The activity sheet is simply a set of questions on key papers. One can provide these papers to the students, or (better) have the students download them from online journals and/or other library references. Also included in the activity sheet are references for some key papers on elemental partitioning, and the web reference for the Geochemical Earth Reference Model, which includes a current compendium of elemental partitioning data and of elemental abundances in key earth reservoirs.

Activity Worksheet (Microsoft Word 50kB Sep29 09)

An "answer key" (or at least my view of good answers to these questions) is available on request.

Teaching Notes and Tips

The whole concept of element partitioning may be new to students in a petrology course. As it is related in relatively direct ways to crystal chemistry (i.e., valence and ionic radius determine how compatible a trace element may be in a given mineral), some discussion of this concept will certainly be necessary. Chapter 4 in Rollinson (1998) is a good synopsis of the principles of trace element variation in igneous systems.

Assessment

One can have students hand in answers to the questions posed in the activity, and/or one can include questions related to the readings and to elemental geochemistry on petrology examinations. I personally like the model where students complete the reading and answer questions as homework, followed by a classroom discussion of student responses and any problems or questions that the readings elicited.

References and Resources

All the relevant references are included in the worksheet. Additional text readings on elemental and isotopic geochemistry may be drawn from:

Faure, G., Principles and Applications of Inorganic Geochemistry (most recent edition): Chapters 6,7,8,10,17

Rollinson, H., (1998) Using Geochemical Data: Evaluation, Presentation, Interpretation Chapter 4.

or like sections of other introductory geochemistry textbooks.

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