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Oceanic Crust: the Ophiolite Model

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Oceanic Crust: the Ophiolite Model Mid-Oceanic Ridge Basalt Ridge Segments and Spreading Rates Oceanic Crust: The The Mid-Ocean Ridge System Ophiolite Model • Slow-spreading ridges: < 3 cm/year • Fast-spreading ridges: > 4 cm/year Inferred Rock Seismic Velocity • All Half Rates Types Based on Structure of Ophiolites, Ocean Oceanic Crust (Vp) Drilling From McBirney, 1993 Minster et al. (1974) Geophys. J. Roy. Astr. Soc., 36, 541-576. Oceanic Crust and Upper Mantle Structure Typical Ophiolite: Typical Ophiolite 1. Radiolarian chert on top of… 2. Pillow lavas 4. Isotropic Gabbro with diorite and tonalite, 3. Sheeted Complex aka “plagiogranite” (dikes and sills) 5. Cumulate Gabbro 4 layers distinguished from Geophysics: 6. Cumulate Ultramafics 7. Ultramafic tectonite Sediments (layer 1) Volcanic Crust (layer 2) Plutonic Crust (layer 3) Lithology and thickness of a typical ophiolite Lithology and thickness of a typical ophiolite sequence, based on the Samial Ophiolite in sequence, based on the Samial Ophiolite in Oman. Mantle Harzburgite (layer 4) Oman. After Boudier and Nicolas (1985) Earth After Boudier and Nicolas (1985) Earth Planet. Sci. Planet. Sci. Lett., 76, 84-92. Lett., 76, 84-92. From McBirney, 1993 Oceanic Crust and Upper Mantle Structure Petrography and Major Typical Ophiolite Element Chemistry A “typical” MORB is an olivine tholeiite with low K2O (< 0.2 wt%) and moderate TiO2 (!1.0 to 2.0 wt%). MgO from !10 wt% to 6 wt% Wehrlite intrudes into layered gabbros Glass in pillow rims is represents liquid Below: harzburgite compositions -- no phenocryst and dunite accumulation. (=refractory residuum of the original mantle) Phenocrytsts: Olivine, Plagioclase, Modified after Brown and Mussett (1993) The Inaccessible Earth: An Integrated View of Its Structure and Composition. Chapman & Hall. London. ±Diopside After Bowen (1915), A. J. Sci., and Morse (1994), Crystallization Basalts and Phase Diagrams. Krieger Publishers. OLivine+ OLivine Plagioclase Sequence: MgO and FeO • Olivine (±Cr-Spinel) Al O and CaO 2 3 • Olivine + Plagioclase The major SiO2 (±Cr-Spinel) element • Plagioclase + Augite chemistry of Na2O, K2O, TiO2, P2O5 MORBs MgO variation diagrams for basaltic glasses from the AMAR region of the MAR. Note different ordinate scales. From Stakes, Shervais & Hopson, (1984) Journal of Geophys. Res., 89, 6995-7028. N-MORB vs E-MORB E-MORBs (squares) enriched over N-MORBs (red Incompatible-rich and incompatible-poor triangles): regardless of Mg# mantle source regions for MORB magmas: N-MORB Mid-Atlantic Ridge 1000.00 ! E-MORBs La/Sm > 1.8 N-MORB La/Lu < 1 ! N-MORBs La/Sm < 0.7 ! N-MORB (normal MORB) taps the 100.00 ! T-MORBs (transitional) intermediate values depleted upper mantle source 10.00 " Mg# > 65: K O < 0.10 TiO < 2.0 E-MORB Mid-Atlantic Ridge 2 2 1000.00 Fractionation ! E-MORB (enriched MORB, aka P-MORB 1.00 won’t change E-MORB LaLa CeCe NdNd SmSm EuEu GdGd TbTb DyDy ErEr TmTm YbYb LuLu for plume) taps the (deeper) fertile mantle 100.00 incompatible element ratios -- " Mg# > 65: K2O > 0.10 TiO2 > 1.5 La/Sm variation 10.00 inherited from La/Lu > 1 source. 1.00 La CeCe NdNd SmSm EuEu GdGd TbTb DyDy ErEr TmTm YbYb Lu Data from Schilling et al. (1983) Amer. J. Sci., 283, 510-586. Conclusions: N-MORBs: 87Sr/86Sr < 0.7035 and 143Nd/144Nd > 0.5030, = MORB Petrogenesis depleted mantle source # MORBs have > 1 source region Generation E-MORBs extend to more enriched values ® stronger support Separation of the plates # The mantle beneath the ocean basins is distinct mantle reservoirs for N-type and E-type MORBs Upward motion of mantle not homogeneous material into extended zone ! N-MORBs tap an upper, depleted Decompression partial melting associated with near- mantle adiabatic rise ! E-MORBs tap a deeper enriched N-MORB melting initiated ~ 60-80 km depth in upper source depleted mantle where it inherits depleted trace ! T-MORBs = mixing of N- and E- element and isotopic char. magmas during ascent and/or in Zindler et al. (1984) Earth Planet. Sci. Lett., shallow chambers 70, 175-195. and Wilson (1989) Igneous Figure 13-12. Data from Ito et al. (1987) Chemical Geology, 62, 157-176; and LeRoex et al. (1983) J. Petrol., 24, 267-318. Petrogenesis. .
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