Download MORB AND OPHIOLITES Mid-Ocean Ridge system = 97,000 miles

MORB AND OPHIOLITES
Mid-Ocean Ridge system = 97,000 miles (60,000 km) Long.
13000 ft (4 km) relief average (= Sierra Nevada Mountains).
Morpphology varies with spreading rate:
Fast (10-17cm/yr half rate) :
Smooth profiles no axial valley low relief relatively few transforms with
long offset.
Slow (1-3cm/yr half rate) :
High relief, rugged profile, with abundant ridge-parallel faults.
Deep narrow axial valley.
Short offset transforms common.
Axial valley 3-10km wide 100-500 in deep.
Structure of Ocean Crust
Spreading Center
Seismic
Stratigraphy
Continuous,
episodic influx
of primitive magma
Seismic velocity correlated with specific rock types:
Layer 1
Layer 2
Layer 3
0-1 km thick,
2 km thick
6-8 km thick
Ophiolite
Stratigraphy
Vp= 2.5 Km/s, sediments, pelagic ooze,
Vp= 4.5 km/s, pillow basalt, diabase
Vp= 6-7 km/s, gabbro, diorite
Rocks based on ophiolite model of oceanic crust and on Deep Sea Drilling
Project results.
What is ophiolite ? ÒOn-land Oceanic CrustÓ -- Exposed in Mountain belts
Ophiolites common in Alps: Separate basement massifs+cover sequences of
different provenance from one another.
Typical Ophiolite Sequence:
1. Abyssal sediments
2. Pillow lava
3. Sheeted dikes
4. Layered Gabbro-Peridotite
5. Mantle Tectonite
(deep sea provenance)
(underwater eruptions)
(extensional environment)
(Magma chamber)
(Oceanic Lithosphere)
Moores and Vine (1971) Ophiolites represent ocean crust emplaced on land,
mark suture zones between collided plates.
Evidence:
1. Sediments indicate deep sea environment.
2. Lavas formed by submarine eruptions.
3. Sheeted dikes have no wall rock -- essentially 100% extension.
4. Magma chmaber = Mafic cumulates of MORB (?).
5. Mantle lithosphere = Depleted harzburgite (MORB extraction?).
Most ophiolites now thought of as not true ocean crust
= Fore-arc rifts, early island arc volcanism.
ORIGIN OF Mid-Ocean Ridge Basalts (MORB)
MORB - Limited Compositional Range:
MORB
Primitive
Evolved
SiO2
49.0
52.0
TiO2
0.6
2.5
Al2O3
16.5
14.5
FeO
8.2
12.0
MgO
10.0
6.5
CaO
14.0
12.0
Na2O
1.6
2.7
K2O
0.05
0.35
P2O5
0.05
0.4
Average
51.0
1.5
15.3
9.2
7.5
13.0
2.1
0.20
0.25
Continuous, episodic influx of primitive magma --> MAGMA MIXING:
>> Phenocrysts zoning patterns.
>> Restricted range in liquid (glass) compositions.
>> Many Phenocrysts not in Equilibrium with Glass.
>> Elevated Incompatible, Compatible element concentrations
>> Compositions lie on Ol-Plag-Cpx-Liquid Cotectic
PRIMITIVE MORB -- PICRITE OR BASALT ??
1. Picrite with 15-20% MgO -- CanÕt erupt because of high density.
2. Normal High-MgO MORB (10% MgO) -- Melting on the CUSP.
Plag -> Spinel Cusp
Spinel -> Garnet Cusp
Melting where
Geotherm
Intersects Cusp
Mantle Geotherm