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Geol 2312 Igneous and Metamorphic Petrology

Geol 2312 Igneous and Metamorphic Petrology. Lecture 13 Mid-Ocean Ridge Basalts (MORB) and Ocean Island Basalts (OIB). February 23, 2009. Present-Day Mid-Ocean Ridges Birthplaces of MORB. Spreading Rates. Oceanic Crust and Upper Mantle Structure. EVIDENCE Seismic Velocities

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Geol 2312 Igneous and Metamorphic Petrology

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  1. Geol 2312 Igneous and Metamorphic Petrology Lecture 13 Mid-Ocean Ridge Basalts (MORB) and Ocean Island Basalts (OIB) February 23, 2009

  2. Present-Day Mid-Ocean RidgesBirthplaces of MORB

  3. Spreading Rates

  4. Oceanic Crust and Upper Mantle Structure EVIDENCE • Seismic Velocities • Deep Sea Drilling Program • Ophiolites • Dredging of Fracture Zone Scarps

  5. Oceanic Crust and Upper Mantle Structure Layer 3A = upper isotropic and lower, somewhat foliated (“transitional”) gabbros Layer 3B is more layered, & may exhibit cumulate textures

  6. Oceanic Crust and Upper Mantle formation

  7. Composition of MORB Fractional Crystallization Primitive Magma

  8. Compositional Variability in MORB NOT related to Fractional Crystallizaiton Incompatible-rich and incompatible-poor mantle source regions for MORB magmas • N-MORB (normal MORB) taps the depleted upper mantle source • Mg# > 65: K2O < 0.10 TiO2 < 1.0 • E-MORB (enriched MORB) taps undepleted (deeper?) mantle • Mg# > 65: K2O > 0.10 TiO2 > 1.0 FC

  9. Origin of NMORB and EMORB

  10. X-Sectional View Distribution of NMORB & EMORB at Fast Spreading RidgesPromotes development of larger magma chambers more fractionated basalt compositions Winter (2001) Figure 13-15, After Perfit et al. (1994) Geology, 22, 375-379. Longitudinal View Winter (2001) Figure 13-16 ; After Sinton and Detrick (1992) J. Geophys. Res., 97, 197-216.

  11. 2 Rift Valley 4 Depth (km) Gabbro 6 Transition zone Moho Mush 8 10 10 5 0 5 Distance (km) Slow-spreading Ridges Smaller, crystal-rich, dike-like magma bodies results is less fractionation  less evolved MORB Winter (2001) Figure 13-16 After Sinton and Detrick (1992) J. Geophys. Res., 97, 197-216.

  12. Global Variability of MORBThermal Structure of the Mantle

  13. Ocean Island Basalts (OIB) Plume-influenced Volcanism

  14. Hawaiian VolcanismStaging of Alkaline and Tholeiitic Magma Series

  15. Major Element Chemistry of OIB Variable alkalinity likely reflects variable depths and degrees of partial melting in the plume and variable degrees of mixing and re-equilbration as magma rises through the mantle plume to the ocean crust. Winter (2001) Figure 14-2. After Wilson (1989) Igneous Petrogenesis. Kluwer.

  16. REE Compositions of OIB Alkali basalts strongly LREE-enriched  low degrees of partial melting Similar to E-MORB, but stronger depletion of HREE for both tholeiitic and alkali magmas series  deep garnet-bearing source for both with re-equilibration at shallower depths Lack of Eu anomaly indicates no significant fractionation of plagioclase

  17. Trace Elements OIB / MORB Winter (2001) Figure 14-3. An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Data from Sun and McDonough (1989).

  18. A Model for Ocean Magmatism Chondritic (undepleted) mantle From recycled ocean crust Low Sr87/Sr86 mantle Mantle Reservoirs (based on radiogenic isotopes) Low Nd143/Nd144 mantle Previously melted mantle Winter (2001) Figure 14-10. Nomenclature from Zindler and Hart (1986). After Wilson (1989) and Rollinson (1993). U-enriched mantle Chondritic (undepleted) mantle

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