Small pyroclastic eruption at Arenal Volcano (left), Floor of Poas Crater (right)

Magma genesis and differentiation in Central America

Over the past several years, my students and I have studied the recent eruptive histories of Central American volcanoes, the sources of basaltic magmas in these volcanoes, and the processes and time scales required to produce intermediate and silicic magmas. Ongoing projects include a collaborative effort with Craig Lundstrom at the University of Illinois and Frank Tepley at UCSC to investigate the ages of crystals and the effects of degassing events in young lavas from Arenal Volcano. Another collaborative effort with Rebecca Thomas, Marc Hirschmann and others at the University of Minnesota has led to the development of a new model for arc magma genesis (Thomas et al., in press). Future plans include an investigation of the time scales of rhyolite genesis in El Salvador and Nicaragua, and participation in studies of glass inclusions in phenocrysts in basalts from Nicaragua.

 

 

Eocene pillow lavas on the southwest coast of Guam

Petrological consequences of Mariana arc Inititation

Our group is presently funded together with Rosemary Hickey-Vargas and Barry Hanan by the MARGINS initiative of NSF to investigate of the earliest stages of volcanism in the Mariana arc. The primary goals of this study are to determine how subduction starts in an oceanic setting and how magma generation processes change as arcs evolve from infancy to maturity. Field work for the first stage of the project was completed in 2002. Major element, trace element, and isotopic analyses of the samples collected form this trip as well as from DSDP cores are presently being analyzed.

Crater Lake

Time-scales of rhyolite genesis

Our group is nearling completion of an NSF-sponsored project in collaboration with Ken Sims at WHOI to use U-series techniques to determine the time needed to produce rhyolites in continental arcs.  Young rhyolites and associated lavas and magmatic enclaves from the Katmai-Novarupta, Crater Lake and Medicine Lake volcanic systems have been analyzed for U and Th isotope abundances, as well as major and trace element concentrations in order to investigate the time-scales of the processes that lead to rhyolite generation in continental arcs. Results (Reagan et al., submitted to Journal of Petrology) suggest that basalts and basaltic andesites typically migrate from the mantle to the surface within several thousand years. Variations in (²³⁰Th)/(²³²Th) and (²³⁸U)/(²³²Th) ratios with SiO₂ concentrations in intermediate lavas appear to result from crystal fractionation combined with assimilation of recently crystallized magmas. U-Th data suggest that ca. 10⁴ - 10⁵ y of mafic magmatism are required at a volcanic center to generate silicic andesites and dacites. The near-equilibrium (²³⁸U)/(²³⁰Th) ratios for all of the rhyolites suggest an average crustal residence time for the constituents making up these rhyolites of > 10⁵ years.