Research Training Program

Tabitha Hensley
University of New Orleans
New Orleans, Louisiana

Jim Luhr, Ph.D.
Supervising Scientist
Department of Mineral Sciences

"This experience has not only taught me more about geochemistry than I knew before, but more importantly, it has demonstrated to me how to interact within a scientific community as a member of it"

Volcanoes represent a chance to study the chemistry of subsurface regions of the earth through lavas erupted onto the surface. The geochemistry of magmas does not remain the same, but varies globally. This project concerns the Central American Volcanic Arc (CAVA), which extends 1,500 km along the Pacific Coast of Central America. Arcs are linear arrangements of volcanoes that are created when two plates collide and one dives underneath the other; magma is generated at these boundaries and erupts through volcanoes. The CAVA provides a unique opportunity to study the geochemical variation of an arc as different physical parameters, such as crustal thickness, change. This project utilizes a database of 1,829 geochemical analyses compiled from previous databases and professional papers by the Global Volcanism Program at the Smithsonian. It is part of a larger effort to combine these analyses with other information about the volcanic arc in a CD-ROM format, which will ultimately be made available to the scientific community. This study focused on several problems related to subduction zone processes of magma generation. Major and trace element geochemistry of volcanic rocks illustrate along-arc and across-arc variation of magma evolution. Indicators of magma genesis include K2O, Ba/La, La/Yb, Rb/Sr, 87Sr/86Sr, 143Nd/144Nd and other element and isotopic concentrations. These constituents as well as normative and modal mineralogy were examined graphically. Notable conclusions include a slight increase of K2O from volcanic arc lavas to back arc (behind the front) lavas in Guatemala but only in the least differentiated samples and more obviously in Honduran samples. 87Sr/86Sr vs. 143Nd/144Nd graphs for along arc variation reflect crustal thickness, as has been shown by previous research, with the highest values plotting in Nicaragua, which has the thinnest crust, and lower values plotting in Guatemala, which has the thickest crust.