HR: 11:20h
AN: V11F-05    [PDF]
TI: Mineral-Scale Sr and Pb Isotopic Variations as Recorders of Magma Differentiation Processes in the Fish Canyon Magmatic System, San Juan Volcanic Field, U.S.A.
AU: * Charlier, B L
EM: b.l.a.charlier@durham.ac.uk
AF: Dept. Earth Sciences, South Road, Durham, DH1 3LE United Kingdom
AU: Davidson, J P
AF: Dept. Earth Sciences, South Road, Durham, DH1 3LE United Kingdom
AU: Bachmann, O
AF: Dept. of Earth and Space Sciences, Mailstop 351310, Seattle, WA 98195-1310 United States
AU: Dungan, M A
AF: Universit\'{e} de Geneve, 13, rue des Mara\^{i}chers, Geneve 4, 1211 Switzerland
AB: The use of crystal isotope microstratigraphy, through microanalysis for Sr and more recently Pb isotopes, shows that inter- and intra-crystalline isotopic and compositional heterogeneities exist within many volcanic rocks. Here we report preliminary Sr and Pb isotope data for sanidine, plagioclase and biotite (Sr only) crystals separated from representative samples of the 5000km$^{3}$, 28Ma Fish Canyon Tuff and the pre-caldera Pagosa Peak Dacite, from the La Garita Caldera, San Juan Volcanic Field, U.S.A. Age-corrected whole-rock $^{87}$Sr/$^{86}$Sr values define a small range (0.7063 to 0.7065), whereas plagioclase values range from 0.7063 to 0.7072 and sanidines define a more limited range 0.7063 to 0.7067. These ranges in $^{87}$Sr/$^{86}$Sr cannot be solely attributed to radiogenic ingrowth during residence in the Fish Canyon magma reservoir, as the $^{87}$Rb/$^{86}$Sr values (plagioclase; 0.003 to 0.011, sanidine; 0.30 to 0.73) are too low to significantly affect $^{87}$Sr/$^{86}$Sr over magmatic timescales. Biotites exhibit a much greater range in initial Sr isotope ratios (0.7202 to 0.7295), but with even higher $^{87}$Rb/$^{86}$Sr ratios of 8 to 12, more than 50 Myrs would be needed to evolve such ratios from the whole-rock ratio. Similarly, large ranges of Pb isotope ratios in sanidines and plagioclase, cannot be produced given the U/Pb ratios of these phases on any geologically reasonable timescale. We interpret the isotopic variations to represent open system processes in the generation of the Fish Canyon magma either by 1) crystallisation from heterogeneous isotopically modified (ultimately mantle-derived) magmas during interaction with old, heterogeneous crust, and/or 2) the direct incorporation of xenocrystic phases from the crust to produce an isotopically heterogeneous magma (and rock) at the mineral scale. Small but significant variations in $^{39}$Ar/$^{40}$Ar total fusion ages for each of the studied phases, are consistent with the latter interpretation, suggesting that the crystal population is a mixture of newly-crystallised and old xenocrystic, incompletely degassed crystalline products.
DE: 1040 Isotopic composition/chemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting