HR: 0830h
AN: V31C-0938    [PDF]
TI: Evidence of Magma Mingling in the Tirib\'{i} Tuff, Valle Central Costa Rica
AU: * Eaton, J K
EM: eaton@geology.wisc.edu
AF: Department of Geology and Geophysics, University of Wisconsin-Madison 1215 W. Dayton St., Madison, WI 53706
AU: Valley, J
EM: valley@geology.wisc.edu
AF: Department of Geology and Geophysics, University of Wisconsin-Madison 1215 W. Dayton St., Madison, WI 53706
AU: Vogel, T A
EM: vogel@msu.edu
AF: Department of Geological Sciences, Michigan State University 206 Natural Science Building, East Lansing, MI 48828-1115
AU: Patino, L C
EM: patinol@msu.edu
AF: Department of Geological Sciences, Michigan State University 206 Natural Science Building, East Lansing, MI 48828-1115
AB: This study is part of a larger oxygen isotope investigation to sort out the role of assimilation and crustal melting in the formation of abundant, large-volume silicic ignimbrites in Central America. The 0.33 Ma Tirib\'{i} Tuff of Valle Central Costa Rica is a large-volume, compositionally heterogeneous ash flow sheet (25 km$^{3}$ dense rock equivalent) that did not form in the presence of evolved continental crust. Glassy pumices, from the dominant ash flow sheet within the Tirib\'{i}, have been sub-divided into three groups based on their textures and whole rock compositions: a low-silica group (55.1-65.6 wt% SiO$_{2}$), a mingled group (58.6-67.7 wt% SiO$_{2}$), and a silicic group (66.2-69.2 wt% SiO$_{2}$). Variations in major and trace elements within the low-silica group are best explained by fractional crystallization. However, differences in trace element data between the low and silicic groups cannot be explained by this same process. The mingled group has been interpreted as mingling of the low-silica and silicic magmas based on petrographic evidence and trace element data. Oxygen isotopes can provide an excellent test for the proposed origin of the magma groups in the Tirib\'{i} Tuff. If crustal material has been incorporated into a primitive magma, the $\delta$$^{18}$O (WR) of the whole rock will typically vary with increasing SiO$_{2}$(WR). The $\delta$$^{18}$O of 10 Tirib\'{i} samples range from 3.27$^{o}$/$_{oo}$ to 3.90$^{o}$/$_{oo}$ for magnetite and 5.15$^{o}$/$_{oo}$ to 5.63$^{o}$/$_{oo}$ for clinopyroxene. These data suggest $\delta$$^{18}$O (WR) values consistent with derivation from mantle melts or melting of primitive crustal rocks. Likewise, fractional crystallization in the absence of contamination causes a predictable correlation of $\delta$$^{18}$O to wt% SiO$_{2}$. Oxygen isotope thermometry of magnetite and clinopyroxene demonstrates that these phenocrysts have preserved magmatic $\delta$$^{18}$O values and further divides the low-silica from the silicic group, suggesting formation of these magmas at $1200\deg$C and below $1100\deg$C, respectively. These $\delta$$^{18}$O data of magnetite and clinopyroxene are not consistent with fractional crystallization of the low-silica magma to produce the silicic magma. The $\delta$$^{18}$O data better support a model where the low-silica and silicic magmas were two separately generated magma batches that, in part, mingled shortly before or during eruption.
DE: 1000 GEOCHEMISTRY (New field, replaces Rock Chemistry)
DE: 1010 Chemical evolution
DE: 1040 Isotopic composition/chemistry
DE: 1094 Instruments and techniques
DE: 3640 Igneous petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting