Download Oceanic Crust: The Ophiolite Model

Mid-Oceanic Ridge Basalt
Ridge Segments and Spreading
Rates
The Mid-Ocean Ridge System
•
•
•
Slow-spreading
ridges:
< 3 cm/year
Fast-spreading
ridges:
> 4 cm/year
All Half Rates
Oceanic Crust: The
Ophiolite Model
Seismic Velocity
Structure of
Oceanic Crust (Vp)
From McBirney, 1993
Minster et al. (1974) Geophys. J. Roy. Astr. Soc., 36, 541-576.
Oceanic Crust and Upper Mantle Structure
Typical Ophiolite:
1. Radiolarian chert
on top of…
2. Pillow lavas
3. Sheeted Complex
4 layers distinguished from Geophysics:
(dikes and sills)
Typical Ophiolite
4. Isotropic Gabbro with
diorite and tonalite,
aka “plagiogranite”
5. Cumulate Gabbro
6. Cumulate Ultramafics
7. Ultramafic tectonite
Sediments (layer 1)
Volcanic Crust (layer 2)
Plutonic Crust (layer 3)
Mantle Harzburgite (layer 4)
From McBirney, 1993
Inferred Rock
Types Based on
Ophiolites, Ocean
Drilling
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.
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.
Typical Ophiolite
Oceanic Crust and Upper Mantle Structure
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%
Wehrlite intrudes
into layered gabbros
Below: harzburgite
and dunite
(=refractory residuum
of the original
mantle)
Glass in pillow rims is represents liquid
compositions -- no phenocryst
accumulation.
Modified after Brown and Mussett (1993) The Inaccessible Earth: An Integrated View of Its Structure and Composition. Chapman & Hall. London.
MgO and FeO
Al2O3 and CaO
The major
element
chemistry of
MORBs
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.
OLivine+
OLivine Plagioclase
Phenocrytsts: Olivine, Plagioclase,
±Diopside
Crystallization
Sequence:
• Olivine (±Cr-Spinel)
• Olivine + Plagioclase
(±Cr-Spinel)
• Plagioclase + Augite
After Bowen (1915), A. J. Sci., and Morse (1994),
Basalts and Phase Diagrams. Krieger Publishers.
N-MORB vs E-MORB
Incompatible-rich and incompatible-poor
mantle source regions for MORB magmas:
!
!
N-MORB (normal MORB) taps the
depleted upper mantle source
" Mg# > 65: K O < 0.10
TiO2 < 2.0
2
E-MORB (enriched MORB, aka P-MORB
for plume) taps the (deeper) fertile mantle
" Mg# > 65: K O > 0.10
TiO2 > 1.5
2
E-MORBs (squares) enriched over N-MORBs (red
triangles): regardless of Mg#
N-MORB Mid-Atlantic Ridge
1000.00
N-MORB
La/Lu < 1
!
E-MORBs La/Sm > 1.8
!
N-MORBs La/Sm < 0.7
!
T-MORBs (transitional) intermediate values
100.00
10.00
E-MORB Mid-Atlantic Ridge
1000.00
E-MORB
1.00
La
Ce
La
Ce
Nd
Nd
Sm
Sm
Eu
Gd
Eu
Tb
Gd
Tb
Dy
Dy
Er
Er
Tm
Tm
Yb
Yb
100.00
10.00
La/Lu > 1
1.00
La
La
Ce
Ce
Nd
Nd
Sm
Sm
Eu
Eu
Gd
Gd
Tb
Tb
Dy
Dy
Er
Er
Tm
Tm
Yb
Yb
Lu
Lu
Fractionation
won’t change
incompatible
element ratios -La/Sm variation
inherited from
source.
Lu
Lu
Data from Schilling et al. (1983) Amer. J. Sci., 283, 510-586.
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
MORB Petrogenesis
Generation
Separation of the plates
Upward motion of mantle
material into extended zone
!
N-MORBs tap an upper, depleted
mantle
Decompression partial
melting associated with nearadiabatic rise
!
E-MORBs tap a deeper enriched
source
!
T-MORBs = mixing of N- and Emagmas during ascent and/or in
shallow chambers
N-MORB melting initiated ~
60-80 km depth in upper
depleted mantle where it
inherits depleted trace
element and isotopic char.
Zindler et al. (1984) Earth Planet. Sci. Lett.,
70, 175-195. and Wilson (1989) Igneous
Petrogenesis.