HR: 17:15h
AN: V44C-06 INVITED     [Abstracts]
TI: Stability of arc lower crust: Insights from the Talkeetna Arc section, Alaska, and the seismic structure of modern arcs
AU: * Behn, M D
EM: mbehn@whoi.edu
AF: Dept. Geology and Geophysics, WHOI, Woods Hole, MA 02543 United States
AU: Kelemen, P B
EM: peterk@ldeo.columbia.edu
AF: LDEO, Columbia University, Palisades, NY 10964 United States
AB: One process for formation of continental crust is accretion of arc terranes at continental margins. A longstanding problem with this model is that while the composition of the continental crust is andesitic [e.g.1], the majority of arc lavas are basaltic [e.g.2]. Also, andesitic arc lavas tend to be evolved (lower Mg#) compared to continental crust. Continental crust can be produced through mixing of basaltic and silicic arc lava compositions, assuming that mafic cumulates formed during generation of the silicic component are removed. If these cumulates are denser than the underlying mantle, differentiation can occur via foundering of lower arc crust [e.g.3]. Indeed, field observations of the Talkeetna arc section in south-central Alaska, combined with modeling of fractionation of primitive arc magmas, suggest that large amounts of primitive gabbronorite and pyroxenite are missing from the lower crust [2,4]. Using rock compositions from the Talkeetna section and PerplE_x [5], we calculate equilibrium mineral assemblages for a range of gabbroic and ultramafic compositions at P, T, oxygen fugacity (fO2), and H₂O contents appropriate for arc lower crust. The quartz-olivine-garnet-free assemblage in Talkeetna gabbronorites (and in the similar Kohistan section in Pakistan) defines a narrow range of fO2 centered on NNO+2 (±1 log unit). Predicted mineral assemblages are used to calculate the density and seismic structure of the lower arc crust. The missing gabbroic and ultramafic rocks from the Talkeetna section were likely denser than the underlying mantle, while the gabbronorites that remain are approximately neutrally buoyant. Generalizing, we show that crustal Vp > 7.3 km/s in modern arcs indicates lower crust that may be convectively unstable relative to the underlying mantle. However, most lower crust in modern arcs has Vp < 7.3 km/s, implying that gravitationally unstable material must founder rapidly on geologic time-scales, or the missing high Vp plutonic rocks crystallize beneath the Moho. 1. Rudnick & Fountain JGR95; 2. Kelemen et al TOG03; 3. Jull & Kelemen JGR01; 4. Greene et al., J Pet05; 5. Connolly EPSL05
DE: 8104 Continental margins: convergent
DE: 8410 Geochemical modeling (1009, 3610)
DE: 8411 Thermodynamics (0766, 1011, 3611)
DE: 8413 Subduction zone processes (1031, 3060, 3613, 8170)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005