Chapter 13 Spring 2010 - Chapter 13: Mid-Ocean Rifts The...

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Unformatted text preview: Chapter 13: Mid-Ocean Rifts The Mid-Ocean Ridge System Figure 13-1. After Minster et al. (1974) Geophys. J. Roy. Astr. Soc., 36, 541-576. MORB Petrogenesis q q Decompression partial melting associated with nearadiabatic rise of mantle due to plate separation. N-MORB melting initiated ~ 60-80 km depth in upper depleted mantle where it inherits depleted trace element and isotope signatures. Ridge Segments and Spreading Rates Table 13-1. Spreading Rates of Some Mid-Ocean Ridge Segments Category Ridge Fast East Pacific Rise Latitude o 21-23 N o 13 N o 11 N o 8-9 N o 2N o 20-21 S o 33 S o 54 S o 56 S SW SE Central o 85 N o 45 N o 36 N o 23 N o 48 S Rate (cm/a)* 3 5.3 5.6 6 6.3 8 5.5 4 4.6 1 3-3.7 0.9 0.6 1-3 2.2 1.3 1.8 Slow-spreading ridges: < 3 cm/a Fast-spreading ridges: > 4 cm/a Slow Indian Ocean Mid-Atlantic Ridge From Wilson (1989). Data from Hekinian (1982), Sclater et al . (1976), Jackson and Reid (1983). *half spreading Oceanic Crust and Upper Mantle Structure Typical Ophiolite Figure 13-3. Lithology and thickness of a typical ophiolite sequence, based on the Samial Ophiolite in Oman. After Boudier and Nicolas (1985) Earth Planet. Sci. Lett., 76, 84-92. Oceanic Crust and Upper Mantle Structure Layer 1 A thin layer of pelagic sediment Figure 13-4. Modified after Brown and Mussett (1993) The Inaccessible Earth: An Integrated View of Its Structure and Composition. Chapman & Hall. London. Layer 2 is basaltic Subdivided into two sub-layers Layer 2A & B = pillow basalts Layer 2C = vertical sheeted dikes Oceanic Crust and Upper Mantle Structure Figure 13-4. Modified after Brown and Mussett (1993) The Inaccessible Earth: An Integrated View of Its Structure and Composition. Chapman & Hall. London. Layer 3 more complex and controversial Believed to be mostly gabbros, crystallized from a shallow axial magma chamber (feeds the dikes and basalts) Layer 3A = upper isotropic and lower, somewhat foliated ("transitional") gabbros Layer 3B is more layered, & may exhibit cumulate textures Oceanic Crust and Upper Mantle Structure Discontinuous diorite and tonalite ("plagiogranite") bodies = late differentiated liquids Figure 13-3. Lithology and thickness of a typical ophiolite sequence, based on the Samial Ophiolite in Oman. After Boudier and Nicolas (1985) Earth Planet. Sci. Lett., 76, 84-92. Layer 4 = ultramafic rocks Ophiolites: base of 3B grades into layered cumulate wehrlite & gabbro Wehrlite intruded into layered gabbros Below cumulate dunite with harzburgite xenoliths Below this is a tectonite harzburgite and dunite (unmelted residuum of the original mantle) Petrography and Major Element Chemistry q q q A "typical" MORB is an olivine tholeiite with low K2O (< 0.2%) and low TiO2 (< 2.0%) The common crystallization sequence is: olivine ( Mg-Cr spinel), olivine + plagioclase ( Mg-Cr spinel), olivine + plagioclase + clinopyroxene Originally considered to be extremely uniform, interpreted as a simple petrogenesis 3 More extensive sampling has shown that they display a (restricted) range of compositions q q q q q Decrease in MgO and relative increase in FeO early differentiation trend of tholeiites Patterns are compatible with crystal fractionation of the observed phenocryst phases Removal of olivine can raise the FeO/ MgO ratio, and the separation of a calcic plagioclase can cause Al2O3 and CaO to decrease SiO2 is a ~ poor fractionation index Na2O K2O TiO2 and P2O5 are all conserved and the concentration of each triples over FX range 3 This implies that the parental magma undergoes 67% fractionation in a magma chamber somewhere beneath the ridge to reduce the original mass by 1/3 MORBS MORBS Conclusions about MORBs, and the processes beneath mid-ocean ridges 3 MORBs are not the completely uniform magmas that they were once considered to be v They show chemical trends consistent with fractional crystallization of olivine, plagioclase, and perhaps clinopyroxene 3 MORBs cannot be primary magmas, but are derivative magmas resulting from fractional crystallization (~ 60%) MORBS Incompatible-rich and incompatible-poor mantle source regions for MORB magmas 3 N-MORB (normal MORB) taps the depleted upper mantle source v Mg# > 65: K O < 0.10 TiO < 1.0 2 2 3 E-MORB (enriched MORB, also called P-MORB for plume) taps the (deeper) fertile mantle v Mg# > 65: K O > 0.10 TiO > 1.0 2 2 v Mg # = 100Mg/(Mg+Fe) q REE diagram for MORBs Trace Element and Isotope Chemistry Figure 13-10. Data from Schilling et al. (1983) Amer. J. Sci., 283, 510-586. MORBS Conclusions: q MORBs have > 1 source region q The mantle beneath the ocean basins is not homogeneous 3 3 3 N-MORBs tap an upper, depleted mantle E-MORBs tap a deeper enriched source T-MORBs = mixing of N- and E- magmas during ascent and/or in shallow chambers Generation q q MORB Petrogenesis q q Separation of the plates Upward motion of mantle material into extended zone Decompression partial melting associated with near-adiabatic rise N-MORB melting initiated ~ 60-80 km depth in upper depleted mantle where it inherits depleted trace element and isotopic char. Figure 13-13. After Zindler et al. (1984) Earth Planet. Sci. Lett., 70, 175-195. and Wilson (1989) Igneous Petrogenesis, Kluwer. Generation q q Region of melting Melt blobs separate at about 25-35 km Figure 13-13. After Zindler et al. (1984) Earth Planet. Sci. Lett., 70, 175-195. and Wilson (1989) Igneous Petrogenesis, Kluwer. q Lower enriched mantle reservoir may also be drawn upward and an EMORB plume initiated Figure 13-13. After Zindler et al. (1984) Earth Planet. Sci. Lett., 70, 175195. and Wilson (1989) Igneous Petrogenesis, Kluwer. The Axial Magma Chamber Original Model q q q q Figure 13-14. From Byran and Moore (1977) Geol. Soc. Amer. Bull., 88, 556-570. Semi-permanent Fractional crystallization derivative MORB magmas Periodic reinjection of fresh, primitive MORB from below Dikes upward through the extending and faulting roof q q q Crystallization near top and along the sides successive layers of gabbro (layer 3) Dense olivine and pyroxene crystals ultramafic cumulates (layer 4) Layering in lower gabbros (layer 3B) from density currents flowing down the sloping walls and floor? Figure 13-14. From Byran and Moore (1977) Geol. Soc. Amer. Bull., 88, 556-570. A modern concept of the axial magma chamber beneath a fastspreading ridge Figure 13-15. After Perfit et al. (1994) Geology, 22, 375-379. Model for magma chamber beneath a slow-spreading ridge, such as the Mid-Atlantic Ridge 3 MORBS 3 Dike-like mush zone and a smaller transition zone beneath well-developed rift valley Most of body well below the liquidus temperature, so convection and mixing is far less likely than at fast ridges 2 Depth (km) 4 6 Moho Gabbro Mush Rift Valley Transition zone Figure 13-16 After Sinton and Detrick (1992) J. Geophys. Res., 97, 197-216. 8 10 5 0 Distance (km) 5 10 ...
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