HR: 17:45h
AN: T44C-08    [Abstracts]
TI: Oxygen Isotopes of Silicic Volcanic Deposits of NW Costa Rica: A Key to Understanding Crustal Evolution in Central America?
AU: * Vogel, T A
EM: vogel@msu.edu
AF: Michigan State University, Department of Geological Sciences, East Lansing, MI 48824, United States
AU: Valley, J W
AF: University of Wisconsin, Department of Geology and Geophysics, Madison, WI 53706, United States
AU: Patino, L C
AF: Michigan State University, Department of Geological Sciences, East Lansing, MI 48824, United States
AU: Alvarado, G E
AF: University of Costa Rica, Escuala Centroamericana de Geologia, Apdo. 35, San Jose, 00000, Costa Rica
AU: Szymanski, D W
AF: Michigan State University, Department of Geological Sciences, East Lansing, MI 48824, United States
AU: Deering, C D
AF: Canterbury University, Department of Geological Sciences, Christchurch, 00000, New Zealand
AB: NW Costa Rica has a long history of silicic volcanism, from > 6 my to 0.66 my recorded in silicic ignimbrite deposits. We have determined a preliminary stratigraphic sequence containing at least 23 eruptive units, which allows us to evaluate chemical variations with stratigraphic position. This stratigraphy is based on recently determined ⁴⁰Ar/³⁹Ar dating, supplemented by new stratigraphic field constraints. The regional chemical variations (including oxygen isotopes) of the silicic deposits in NW Costa Rica were discussed in a recent report (Vogel et al., 2006) on the silicic ignimbrites in Nicaragua and Costa Rica. In Nicaragua and Costa Rica the overall chemical trends of selected trace elements and oxygen isotopes in these silicic deposits are similar to the trends in the basaltic lavas from the active volcanic front. These observations led us to conclude that the silicic deposits were genetically related to the lavas. Oxygen isotopes of phenocrysts (clinopyroxene, orthopyroxene and magnetite) from Nicaraguan and Costa Rican pyroclastic deposits show an increase in δ¹⁸O from northwest to southeast of 1.5‰, except for a low δ¹⁸O excursion in NW Costa Rica, which is the subject of this report. We showed that the oxygen isotopes of the coexisting clinopyroxene, orthopyroxene and magnetite in these silicic deposits (glassy pumice samples) were in equilibrium and therefore we could use the ubiquitous magnetite as a proxy for the overall variation of oxygen isotopes of whole rock (magma). The δ¹⁸O (Mt) variation is independent of silica content of the samples and thus is not due to crystal fractionation. A better understanding of the stratigraphy in NW Costa Rica (Guanacaste province) allows us to evaluate the variation of δ¹⁸O of magnetite with time. Values of δ¹⁸O (Mt) decrease with time (the low δ¹⁸O excursion reported previously). The δ¹⁸O (Mt) of the oldest deposit are 3.49 ‰ and it decreases to 1.29 ‰ in the youngest deposits. The youngest ignimbrites were erupted from a nested caldera complex. However the source of the older deposits is not known because the younger deposits covered earlier calderas and continuous outcrops are absent. It is possible that all of these ignimbrites erupted from the same caldera complex. Interpretation of this remarkable oxygen isotopic trend is still in progress. One possibility is that the low δ¹⁸O magmas represent melting of previously emplaced calc-alkaline plutons that have interacted with hydrothermal systems (possibly meteoric). With continued evolution of the crust, each successive melting event produced silicic magmas from a source that was more hydrothermally altered. Thus the young silicic deposits represent a source that was more altered than the source for the older silicic deposits. The decreasing δ¹⁸O (Mt) values may represent a source area that is continually being reprocessed and hydrothermally altered leading to the development of a "continental- like" crust.
DE: 1020 Composition of the continental crust
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 1037 Magma genesis and partial melting (3619)
DE: 1041 Stable isotope geochemistry (0454, 4870)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting