HR: 1330h
AN: V42B-0357    [PDF]
TI: Real-time Differentiation Processes at Arenal Volcano, Costa Rica, 1968-2003
AU: * Ryder, C H
EM: cryder@es.ucsc.edu
AU: Gill, J
EM: jgill@es.ucsc.edu
AU: Tepley, F J
EM: ftepley@es.ucsc.edu
AB: The current eruption of Arenal volcano, Costa Rica (1968-present) provides opportunity to study magma differentiation in real-time. Fifty lavas were analyzed by XRF and ICPMS for major and trace element concentrations. The data are consistent with previous work and suggest that the current eruption has three stages. During the first stage (1968-early 1970), a zoned magma chamber was emptied after being intruded by an unseen new magma. The most differentiated magma was erupted first as the zoned chamber erupted from the top down. Mixing of the two magmas occurred in the second stage (1970-early 1974). The minimum concentration of incompatible elements (e.g. K) occurs earlier in the eruption than the maximum concentration of compatible elements (e.g. Ni, Cr, V). The third stage (early 1974-present) is characterized by fractionation and recharge of mafic magma. Two different differentiation processes have caused the geochemical variations observed during this eruption. The first operated pre-1968, and the second has been operating since 1974. During the first differentiation, compatible elements decreased by up to 75% and incompatible elements rose by 25%. During the second differentiation, compatible element concentrations remained constant within 10-30%, and incompatible elements (Ba, Zr, and Sr) increased by 30% while others remained constant. Lavas erupted at Arenal now are more differentiated in some respects than in 1968. For example, Ba, Rb, and Sr concentrations are higher now than they were at the beginning. Since 1974, rare earth element patterns are steeper, lavas are richer in incompatible elements (Ba, Rb, Sr, and Zr), U/Th ratio is higher, and Fe$_{2}$O$_{3}$, CaO, MgO, TiO$_{2}$, Ni, Cr, Sc, and V concentrations have remained constant, whereas they decreased with differentiation initially. Sr, Nd, Pb, and Hf isotopic ratios will be presented.
DE: 1010 Chemical evolution
DE: 1040 Isotopic composition/chemistry
DE: 1065 Trace elements (3670)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting