Geol 2312 Igneous and Metamorphic Petrology

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

• EVIDENCE
• Seismic Velocities
• Deep Sea Drilling Program
• Ophiolites
• Dredging of Fracture Zone Scarps

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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
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Oceanic Crust and Upper Mantle formation
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Composition of MORB
Fractional Crystallization
Primitive Magma
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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 gt 65 K2O lt 0.10 TiO2 lt 1.0
• E-MORB (enriched MORB) taps undepleted (deeper?)
  mantle
• Mg gt 65 K2O gt 0.10 TiO2 gt 1.0

FC
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Origin of NMORB and EMORB
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Distribution of NMORB EMORB at Fast Spreading
Ridges Promotes development of larger magma
chambers ?more fractionated basalt compositions
X-Sectional View
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.
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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.
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Global Variability of MORBThermal Structure of
the Mantle
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Ocean Island Basalts (OIB) Plume-influenced
Volcanism
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Hawaiian VolcanismStaging of Alkaline and
Tholeiitic Magma Series
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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.
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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
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Trace Elements OIB / MORB
Winter (2001) Figure 14-3. An Introduction to
Igneous and Metamorphic Petrology. Prentice Hall.
Data from Sun and McDonough (1989).
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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