Sediment Recycling during Plate Subduction in Central America
Robyn is investigating the Middle America Trench where the Cocos
Plate is being subducted beneath the Caribbean Plate. She is trying
to understand the relation between the sediments on the downgoing
plate and the nature of the volcanic activity in the Central American
Volcanic Arc between Guatemala and Costa Rica. This is important
in determining a mass balance for sediment input and output at subduction
zones, and understanding how subduction processes work.
To do this, Robyn is applying both geophysical and geochemical techniques. She is using an isotope of beryllium (10Be) as a tracer. 10Be is generated from cosmic ray spallation reactions of oxygen and nitrogen in the atmosphere. Because it has a relatively short half-life, it is only found in significant concentrations in surficial oceanic sediments. Hence its occurrence in volcanic rocks on Central America suggests that young oceanic sediments are being incorporated into the magmas generated in association with subduction.
The Central American Volcanic Arc exhibits a wide range in 10Be contents with enrichment that progresses from moderate in Guatemala to high in Nicaragua, and then rapidly diminishes towards Costa Rica, where little enrichment in 10Be is observed. Working with Julie Morris (Washington University, St. Louis), Robyn has analyzed 10Be in 35 piston core sediment samples seaward of the Middle America Trench. She has shown that the small variability of 10Be content in the cores could not have caused the pattern of enrichment observed in the Central American Volcanic Arc. Hence, other processes must be important.
Working with Kirk McIntosh (University of Texas Institute of Geophysics), Eli Silver (University of California, Santa Cruz), Neal Driscoll (Scripps Institute of Oceanography), and Julie Morris, Robyn collected geophysical (bathymetry and seismic reflection) data across the down-going Cocos Plate. She has demonstrated a first-order correlation between basement fault structure and variation of 10Be in the Central American Volcanic Arc. Robyn is now analyzing and interpreting her data, and expects to complete her thesis in spring 2003.