Download Chemical Inputs and Outputs at Subduction Zones: A

Chemical Inputs and Outputs at Subduction Zones: A MARGINS Mini-lesson
Karin A. Block Lamont-Doherty Earth Observatory of Columbia University 61 Route 9W Palisades, NY 10964
Contact: [email protected]
Chemistry of Volcanic Rocks at Plate Margins
Summary: In this activity the student analyzes global
geochemistry data to create hypotheses regarding
possible geochemical inputs at subduction zones,
and the range of compositions associated with this
type of convergent boundary. The exercise utilizes
the EarthChem federation of online geochemical
databases, GeoMapApp and Excel. The activity
focuses on Central America, an NSF-MARGINS
Focus Site, and Cascadia, an NSF-MARGINS allied
study site.
Learning Goals: Students will examine the basic
chemical inputs and outputs at subduction zones
by comparing and contrasting major element
chemistry of MORB and arc magmas. The data
used in this activity is obtained directly from the
literature via online databases and will thus contain
the full and messy range of possible compositions
associated with this particular convergent margin.
The intent is introduce to students to the chemical
similarities and differences between MORB and
arc volcanism and begin to explore ways in which
magmatic sources can be traced.
Materials and Tools: The activity includes an
assignment sheet, Excel spreadsheets containing the
East Pacific Rise/Central America and Juan De Fuca/
Cascadia datasets for download if the instructor prefers
to provide the datasets rather than have the students
download them. Computers should be equipped with
Excel and GeoMapApp (www.geomapapp.org). The
instructor should be comfortable with query and
download procedures on the databases and with use
of the software applications.
Students are instructed on how to:
Adding the sedimentary component:
SedDB
1.  Create geochemical datasets from online
resources
2.  Generate a map in GeoMapApp
Define a Query
Geochemistry of Marine Sediments:
Trace element and isotopic composition of marine
sediments provide powerful tracers of chemical
fluxes at subduction zones. The sedimentary
component at SubFac focus sites can be added to
the mini-lesson for an advanced undergraduate
course. Instructors can download data from
SedDB for a variety of applications, for example:
3.  Manipulate GeoMapApp tools to better
visualize data
4.  Work with color histograms to highlight
geochemical variations within a
compositional range
5.  Plot their data to deduce the relative roles of
oceanic crust, continental crust and mantle
components in MORB and arc volcanism.
Teaching Level: This activity is intended for use in an
introductory geology course and can be used as an inclass discussion tool or as part of a laboratory
assignment that could complement the topics of plate
tectonics and identification of rocks and minerals.
Therefore, students should be familiar with tectonic plate
interactions, the major oceanic and continental rock
types, and with the basic geometry of subduction zones.
The assignment can be modified for an upper-level
undergraduate petrology class by including additional
sites and having the students create hypotheses
regarding magma source components by examining
trace element patterns, incompatible element ratios, and
isotopic variation.
Select Chemical Parameters
1.  Reconstruct the composition of Global Subducting
Sediment (GLOSS) of Plank and Langmuir (1998)
or come up with an average sediment composition
specific to the location, for example, Central
America. Students can then discuss the influence
of biogenic and terrigenous input in relation to a
particular location and how the composition may
be different at other subduction zones.
2.  Compare important trace element signature ratios
such as Ba/La and U/Th in ocean floor basalts,
volcanic arcs, and subducting sediments.
View and Download Data
Assessment: Assessment will depend on whether the
activity is used as an in-class example or as a lab
activity. If used as part of a lab, students will be
asked to discuss their answers in small groups where
they will be required to produce a report synthesizing
their hypotheses on inputs and outputs at subduction
zones.
Above left: SedDB Query interface: access to major and trace element composition, radiogenic and stable
isotope ratios, in inorganic and organic matrices for diverse suites of samples. Right: MARGINS Central
America Focus Site as shown in GeoMapApp. Red circles are stations containing data from more than 1500
samples obtained through SedDB. All others are PetDB and GEOROC stations downloaded from
EarthChem.
For more information on education resources
using online geochemical databases visit:
www.earthchem.org
www.seddb.org
Left: Data output from EarthChem for the Costa Rica/Nicaragua and Cascadia subduction zones. Download is
available in Excel format. Right: Cascadia data displayed in GeoMapApp. Students can plot data to see spacial
relations between tectonic setting and silica content in the rocks. Datapoint colors are scaled to MgO content.
www.petdb.org
EarthChem, PetDB, and SedDB are supported
by the U.S. National Science Foundation