HR: 0800h
AN: V41C-1454    [Abstracts]
TI: Origin of the Grande Ronde Basalts, Columbia River Basalt Group
AU: * Durand, S R
EM: sduran06@fiu.edu
AF: Florida International University Earth Science Department, 11200 SW 8th Street, PC 344, Miami, FL 33199 United States
AU: * Durand, S R
EM: sduran06@fiu.edu
AF: Florida Center for Analytical Electron Microscopy, 11200 SW 8th Street, PC 50, Miami, FL 33199 United States
AU: Sen, G
EM: seng@fiu.edu
AF: Florida International University Earth Science Department, 11200 SW 8th Street, PC 344, Miami, FL 33199 United States
AU: Sen, G
EM: seng@fiu.edu
AF: Florida Center for Analytical Electron Microscopy, 11200 SW 8th Street, PC 50, Miami, FL 33199 United States
AU: Reidel, S P
EM: sp.reidel@pnl.gov
AF: Pacific Northwest National Laboratory and Battelle-Pacific Northwest Division, MS K6-75; PO Box 999, Richland, WA 99352 United States
AB: The Columbia River basalts are generally thought to have formed by plume melting. Takahashi et al. (1998) suggested that the near-aphyric Grande Ronde Basalts (GR), which comprise ~63% of the CRBG, are essentially primary melts formed by nearly complete fusion of eclogite source rock in the plume and that such melting took place ~2.0 GPa. Durand and Sen (2002) examined phenocrysts and whole rock analyses and concluded that all the basalts are non-primary and, more importantly, that they underwent significant "processing" in shallow crustal magma chambers which erased their higher pressure geochemical signal, thus casting doubt on the validity of the eclogitic plume melting model. Here we report the results of our efforts to simulate the higher pressure histories of GR basalts using COMAGMAT and MELTS software. Our intent was to evaluate (1) whether such melts could be derived from primary melts formed by partial melting of a peridotite source as an alternative to the eclogite model, or if bulk melting of eclogite is required; and (2) at what pressure such primary melts could have been in equilibrium with the mantle. We carried out both forward and inverse modeling. In the forward models we chose different starting melt compositions, all produced in laboratory experiments, from peridotite vs. eclogitic sources. Our starting melts were produced by 6-17% partial melting of the peridotite KLB-1 (Hirose and Kushiro, 1993) and 18-40% melting of eclogites (77SL-582; CRB72-31; Keshav et al., 2004; Takahashi et al., 1998) at 1-3.0 GPa. In a second model, our starting melt composition was the most primitive GR lava with 6.5 wt. % MgO. We extrapolated a linear regression through the GR data to 8 wt. % MgO. We then assumed that such a melt was only olivine-equilibrated, and incrementally added olivine while maintaining equilibrium between olivine and melt using a Kd of 0.3, until a melt in equilibrium with the mantle olivine (Fo₈₉) was found. This composition was fractionated at 0.2, 1.0 and 1.5 GPa. Because of the pressure limits with the COMAGMAT software, we could not model this composition at higher pressures. Therefore, we searched for pressures at which our calculated mantle-equilibrated melt would be multiply saturated with mantle minerals using the MELTS software. The best fit forward model converges with the best plausible inverse model in that both indicate that most primitive parent melts related to GR could have been multiply saturated at ~1.5-2.0 GPa. We interpret this result to indicate that the parental melts last equilibrated with a peridotitic mantle at 1.5-2.0 GPa and such partial melts rose to 0.2 GPa where they underwent efficient mixing and fractionation before erupting. Our models suggest that the source rock was not eclogitic but a typical upper mantle peridotite, and that the melts had ~0.5% water. We suggest that the plume that generated the GR basalts intruded and displaced much of the lower lithosphere at ~16.5 Ma, perhaps aided by back-arc extension due to subduction of the Farallon plate. Although the plume may have begun melting at a deeper level, the bulk of the melting (which perhaps overwhelmed the earlier melts) did not occur until the plume reached ~60-45 km.
DE: 3610 Geochemical modeling (1009, 8410)
DE: 3619 Magma genesis and partial melting (1037)
DE: 3640 Igneous petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005