HR: 1330h
AN: V42D-0387    [PDF]
TI: Pb Isotopic Compositions of Olivine-Hosted Melt Inclusions in Continental Basalt, Eastern Snake River Plain
AU: * Cooper, L B
EM: lcooper@ess.ucla.edu
AF: Dept. of Earth and Space Sciences, UCLA, Los Angeles, CA 90095
AU: Reid, M R
EM: mary.reid@nau.edu
AF: Dept. of Earth and Space Sciences, UCLA, Los Angeles, CA 90095
AU: Reid, M R
EM: mary.reid@nau.edu
AF: Dept. of Geology, Northern Arizona University, Flagstaff, AZ 86011
AU: Bryce, J G
EM: julie.bryce@unh.edu
AF: Dept. of Earth Sciences, University of New Hampshire, Durham, NH 03824
AB: Previous studies of melt and mineral inclusions from oceanic island basalts found Pb isotopic variability more than five times than that exhibited by the basalts themselves and values that encompass much of the variability exhibited in mantle products erupted worldwide. In our study, {\it in situ} Pb isotopic analyses of olivine-hosted melt inclusions from an intracontinental basalt from the eastern Snake River Plain (SRP) of southern Idaho were obtained using a Cameca IMS 1270 ion microprobe in order to investigate the role of diverse magma sources in the genesis of these magmas. Analyses of 14 melt inclusions in zoned olivines (Fo = 78-83) in a magnesian Pleistocene SRP basalt yield an average isotopic composition of $^{208}$Pb/$^{206}$Pb = 2.121 and $^{207}$Pb/$^{206}$Pb = 0.8671, essentially identical to that of the whole rock. However, duplicate analyses of one melt inclusion yield distinctly lower ratios ($^{208}$Pb/$^{206}$Pb = 2.052; $^{207}$Pb/$^{206}$Pb = 0.8424), outside all published SRP fields. The dominant phases contained within the melt inclusions are silicate glass, clinopyroxene (cpx) and trace amounts of plagioclase and Fe-Ti oxides. Most cpx associated with these melt inclusions are more calcic (Wo = 48-53) than other SRP cpx (Wo = 35-45, including both phenocrystic and groundmass cpx). The Mg\# of the less radiogenic melt inclusion is higher (37-48) than that of the other included cpx (23-38). Our preliminary results for an intracontinental basalt find less heterogeneity than that obtained in previous studies which focused on oceanic basalts, even though the basalts are equally magnesian. Either the source is more uniform in composition or homogenization of disparate melt fractions occurs at an earlier stage in the melt aggregation process. The less radiogenic isotopic compositions of one inclusion may be explained by entrapment of melt that was affected by incipient crustal contamination, possibly at the margins of the magma chamber, or may be evidence of small-scale mantle source variability. Further study, including analyses of additional basalts, is being conducted to distinguish between these two possibilities.
DE: 1020 Composition of the crust
DE: 1025 Composition of the mantle
DE: 1040 Isotopic composition/chemistry
DE: 3655 Major element composition
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting