Mid-Oceanic Ridge BasaltThe Mid-Ocean Ridge System

Minster et al. (1974) Geophys. J. Roy. Astr. Soc., 36, 541-576.

Ridge Segments and Spreading Rates

• Slow-spreading ridges:

< 3 cm/year

• Fast-spreading ridges:

> 4 cm/year

• All Half Rates

Oceanic Crust: The Ophiolite Model

Inferred Rock Types Based on

Ophiolites, Ocean Drilling

From McBirney, 1993

Seismic Velocity Structure of Oceanic Crust (Vp)

From McBirney, 1993

Oceanic Crust and Upper Mantle Structure

Sediments (layer 1)

Volcanic Crust (layer 2)

Plutonic Crust (layer 3)

Mantle Harzburgite (layer 4)

4 layers distinguished from Geophysics:

Typical Ophiolite:

1. Radiolarian chert on top of…

2. Pillow lavas

3. Sheeted Complex

(dikes and sills)

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.

4. Isotropic Gabbro with diorite and tonalite, aka “plagiogranite”5. Cumulate Gabbro6. Cumulate Ultramafics7. Ultramafic tectonite

Typical Ophiolite

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.

Wehrlite intrudes into layered gabbros

Below: harzburgite and dunite

(=refractory residuum of the original mantle)

Typical OphioliteOceanic Crust and Upper Mantle Structure

Modified after Brown and Mussett (1993) The Inaccessible Earth: An Integrated View of Its Structure and Composition. Chapman & Hall. London.

Petrography and Major 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%

Glass in pillow rims is represents liquid compositions -- no phenocryst accumulation.

Phenocrytsts: Olivine, Plagioclase, ±Diopside

The major element

chemistry of MORBs

MgO and FeO

Al2O3 and CaO

SiO2

Na2O, K2O, TiO2, P2O5

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.

OLivineOLivine+

Plagioclase

After Bowen (1915), A. J. Sci., and Morse (1994), Basalts and Phase Diagrams. Krieger Publishers.Crystallization

Sequence:

• Olivine (±Cr-Spinel)• Olivine + Plagioclase

(±Cr-Spinel)• Plagioclase + Augite

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 < 2.0

! E-MORB (enriched MORB, aka P-MORB for plume) taps the (deeper) fertile mantle"Mg# > 65: K2O > 0.10 TiO2 > 1.5

N-MORB vs E-MORB

E-MORB Mid-Atlantic Ridge

1.00

10.00

100.00

1000.00

La Ce Nd Sm Eu Gd Tb Dy Er Tm Yb Lu

N-MORB Mid-Atlantic Ridge

1.00

10.00

100.00

1000.00

La Ce Nd Sm Eu Gd Tb Dy Er Tm Yb Lu

La Ce Nd Sm Eu Gd Tb Dy Er Tm Yb Lu

La Ce Nd Sm Eu Gd Tb Dy Er Tm Yb Lu

La/Lu < 1

La Ce Nd Sm Eu Gd Tb Dy Er Tm Yb Lu

La/Lu > 1

N-MORB

E-MORB

E-MORBs (squares) enriched over N-MORBs (red triangles): regardless of Mg# ! E-MORBs La/Sm > 1.8! N-MORBs La/Sm < 0.7! T-MORBs (transitional) intermediate values

Data from Schilling et al. (1983) Amer. J. Sci., 283, 510-586.

Fractionation won’t change incompatible

element ratios -- La/Sm variation inherited from

source.

N-MORBs: 87Sr/86Sr < 0.7035 and 143Nd/144Nd > 0.5030, = depleted mantle source

E-MORBs extend to more enriched values ® stronger support distinct mantle reservoirs for N-type and E-type MORBs

Figure 13-12. Data from Ito et al. (1987) Chemical Geology, 62, 157-176; and LeRoex et al. (1983) J. Petrol., 24, 267-318.

Conclusions:# MORBs have > 1 source region# The mantle beneath the ocean basins is

not homogeneous! 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

MORB PetrogenesisSeparation of the platesUpward motion of mantle material into extended zoneDecompression 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.

Generation

Zindler et al. (1984) Earth Planet. Sci. Lett., 70, 175-195. and Wilson (1989) Igneous

Petrogenesis.