HR: 17:45h
AN: V32H-08    [PDF]
TI: Volatile, Trace Element and Isotopic Variations of Mafic Arc Volcanic Rocks from Nicaragua and Costa Rica
AU: * Hoernle, K
EM: khoernle@geomar.de
AF: GEOMAR Research Center, Wischhofstr. 1-3, Kiel, 24148 Germany
AU: * Hoernle, K
EM: khoernle@geomar.de
AF: SFB 574, University of Kiel, Kiel, 24148 Germany
AU: Sadofsky, S
AF: SFB 574, University of Kiel, Kiel, 24148 Germany
AU: Nichols, H
AF: SFB 574, University of Kiel, Kiel, 24148 Germany
AU: Portnyagin, M
AF: GEOMAR Research Center, Wischhofstr. 1-3, Kiel, 24148 Germany
AU: van den Bogaard, P
AF: GEOMAR Research Center, Wischhofstr. 1-3, Kiel, 24148 Germany
AU: van den Bogaard, P
AF: SFB 574, University of Kiel, Kiel, 24148 Germany
AU: Alvarado, G
AF: ICE, Apartado 10032-1000, San Jose, 10000 Costa Rica
AB: Quaternary volcanic rocks from the Central American Volcanic Arc in central Nicaragua and central Costa Rica exhibit major differences in their volatile, trace element and isotopic compositions. Olivine-hosted melt inclusions in Nicaraguan volcanic rocks with high Fo contents ($>$73) extend to high H$_{2}$O (up to 5.3%), S (10-6860 ppm) and Cl (490-2340 ppm) contents. The volcanic rocks have high ratios of fluid mobile to fluid immobile elements such as Ba/La (65-122), Ba/Th (484-1304) and U/La (0.08-0.17). Additionally, they have $^{143}$Nd/$^{144}$Nd (0.51300-0.51307) similar to normal mid-ocean-ridge basalts (N-MORB) from the East Pacific Rise (EPR), but $^{87}$Sr/$^{86}$Sr (0.7035-0.7042) ratios are much higher than those found in fresh EPR glasses. Pb isotopic compositions of the samples (e.g. $^{206}$Pb/$^{204}$Pb = 18.5-19.0, $^{207}$Pb/$^{204}$Pb = 15.52-15.58) form an array between EPR basalts and subducted sediments. The volatile, trace element and isotope data are consistent with mixing of fluids highly enriched in fluid-mobile elements from subducted sediments with a N-MORB-type mantle wedge to produce the Nicaraguan volcanic rocks. In contrast, olivine-hosted melt inclusions (Fo $>$82) in Costa Rican volcanic rocks show a similar range in H$_{2}$O (up to 5.1%) to Nicaraguan inclusions but overall have lower S (0-1340 ppm) and Cl (10-790 ppm) contents. Costa Rican lavas also have lower Ba/La (7-35), Ba/Th (55-338), U/La (0.02-0.12), $^{87}$Sr/$^{86}$Sr (0.7035-0.7038) and $^{143}$Nd/$^{144}$Nd (0.51292-0.51301) than Nicaraguan lavas, but $^{87}$Sr/$^{86}$Sr and Pb isotope ratios (e.g. $^{206}$Pb/$^{204}$Pb = 19.02-19.32) are more radiogenic than in Nicaragua and than usually found in fresh EPR MORB. Our data are consistent with the presence of Galapagos Hotspot-type components in the source of the central Costa Rican volcanic rocks, derived from the subducting Galapagos Hotspot Track and from Galapagos-type material entering the mantle wedge through a slab tear or window (Abratis and Worner, 2000; Geology). The estimated volume of volcanic rocks erupted in the last 100,000 years (Carr et al., 1990, Contrib. Min. Pet.; in press, AGU Spec. Pub.) are substantially higher in central Costa Rica than in Nicaragua, suggesting greater productivity of melting beneath Costa Rica. Since the flux of hydrous fluids appears to be similar beneath both arc segments, higher melt productivity beneath Costa Rica could reflect the presence of larger volumes of more fertile, hotter Galapagos-type mantle upwelling through a slab tear or window into the Costa Rican mantle wedge.
DE: 1000 GEOCHEMISTRY (New field, replaces Rock Chemistry)
DE: 1010 Chemical evolution
DE: 1025 Composition of the mantle
DE: 1040 Isotopic composition/chemistry
DE: 1065 Trace elements (3670)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting