HR: 0830h
AN: V31C-0944    [PDF]
TI: Helium Variations in Mineral Separates from Cerro Negro Volcano, Nicaragua: Assessing Short Time-Scale Variations
AU: * Shaw, A M
EM: amshaw@dtm.ciw.edu
AF: Department of Terrestrial Magnetism, Carnegie Institution of Washington,, 5241 Broad Branch Rd. NW, Washington, DC 20015 United States
AU: Hilton, D
EM: drhilton@ucsd.edu
AF: Scripps Institution of Oceanography, UCSD, LaJolla, CA 92093 United States
AU: Fischer, T
EM: fischer@unm.edu
AF: University of New Mexico, Northrop Hall, Albuquerque, NM 87131 United States
AU: Walker, J
EM: jwalker@niu.edu
AF: Department of Geological Sciences, Northern Illinois University, DeKalb, IL 60115 United States
AB: Cerro Negro volcano is a young basaltic volcano located in northwestern Nicaragua. It has undergone at least 23 eruptive phases over its 150-year history. We report new He abundance and isotope results in phenocryst phases from recent volcanic ashes and lavas. Analyses were carried out on both olivine and pyroxene phases for the 1992, 1995 and 1999 eruptions, allowing He variations over relatively short time scales to be assessed. He abundance and isotope ratios results show the following: 1) co-genetic olivine-pyroxene pairs show a consistent trend of higher $^{3}$He/$^{4}$He ratios in the olivine phase (6.1-6.6 R$_{A}$) relative to the pyroxenes (4.6-5.5 R$_{A}$), 2) He concentrations are lower in the olivine phase (0.59-2.2 x 10$^{-9}$ cm$^{3}$/g) than the pyroxene phase, (2.2-5.4 x 10$^{-9}$ cm$^{3}$/g), 3) the $^{3}$He/$^{4}$He ratios in the olivines are similar, yet slightly lower than values measured in geothermal fluid samples (6.99 +/- 0.07 R$_{A}$), and 4) no significant difference is found in values measured in the three eruptive phases. The first two observations are consistent with a scenario where the olivines crystallized out of a magma before the pyroxenes, preserving a more pristine He isotope signature. The lower $^{3}$He/$^{4}$He ratios and higher concentrations found in the pyroxenes indicate radiogenic He additions, likely due to crustal assimilation occurring after olivine crystallization. This hypothesis will be tested by electron microprobe analyses of co-genetic pairs, evaluating whether olivines are in fact more primitive (higher Mg values). The third observation shows that although both phenocryst and geothermal fluids preserve magmatic volatiles, geothermal fluids may be less influenced by contamination effects, which act to lower $^{3}$He/$^{4}$He ratios. The final observation suggests that the volatile composition of the magma chamber feeding the eruptions has been relatively homogeneous over decadal time scales.
DE: 1010 Chemical evolution
DE: 1020 Composition of the crust
DE: 1025 Composition of the mantle
DE: 1040 Isotopic composition/chemistry
DE: 8434 Magma migration
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting