I. Structure

A. Seismic Data

1. Seismic Refraction (Pioneered by Ewing in late 30's)

a. Showed that ocean crust was much thinner than continental crust

b. Results

2. Seismic Reflection (Multichannel) Can Image:

a. the base of the ocean crust (MOHO)

b. the top of the magma chamber underlying the crest of the MOR

c. the boundary between Layers 2 and 3 (rarely)

B. Deep Sea Drilling Project Data

1. 600 m into young & old crust (beneath sediments) in both the Pacific and Atlantic

2. Results

a. interbedded pillow basalts and massive basalts

b. rarer dikes (increasing abundance with depth) and sediments

C. Submersible Data

1. Project FAMOUS

a. south of the Azores triple junction

2. Eastern Pacific projects

a. Galapagos Ridge

b. 21 degrees N - off mouth of Gulf of California

c. 13 degrees N

3. Results

a. Geomorphology is shown in handouts from MORPHOLOGY notes

b. 2 types of volcanic activity

i. early, very fluid sheet flows from fissures

ii. late stage pillows forming axial volcanoes

D. Ophiolites (Pieces of Oceanic Crust and Mantle Incorporated into Collisional Mountain Belts)

1. show nature of lower crust and upper mantle at MOHO

a. see figure 1.8b on Page 23 of your textbook

II. Composition of the Ocean Crust

A. Basaltic

1. Mixture of Plagioclase Feldspar and Clinopyroxene ± Accessories

a. accessories include olivine or quartz, magnetite, spinel, amphibole

2. Textural terms

a. extrusive - basalt (or glass)

b. intrusive - sills and dikes: dolerite or diabase

plutons: gabbro

B. Tholeiitic vs. Alkalic Basalt (these are 2 different series)

1. Tholeiites are saturated with silica, depleted in the relative abundances of the alkalies (Sodium + Potassium) and are the dominant basalts of the ocean crust

a. Alkalic basalts are undersaturated with silica, enriched in the relative abundances of the alkalies and are found on oceanic islands

b. In normative calculations:

i. oceanic tholeiites contain olivine and hence are classified as olivine tholeiites

ii. continental tholeiites contain quartz and hence are classified as quartz tholeiites

iii. alkali basalts contain nepheline

2. Detailed comparison

	Oceanic tholeiites	Alkali basalts
SiO2	~50%	~47.5%
Na2O+K2O	~3%	~5.5%
TiO2, Fe2O3 (& total Fe), P2O5, and lithophiles (Na, K, Rb, Cs, Sr, Ba, Zr, Nb, U, Th, Pb)	greatly depleted	greatly enriched
Al2O3, Li, Y, La, Ga	depleted	enriched
MgO, CaO, Cr2O3, Sc, Ni, Cu	greatly enriched	greatly depleted
FeO, MnO, V, Co	enriched	depleted

3. These data indicate 2 different source areas in the mantle that generate 2 different parent magmas

III. Magma Generation and Evolution

A. Partial Melting and Fractional Crystallization

1. Basaltic magma is generated by partial melting of upper mantle rocks

a. upper mantle = peridotite (non-feldspathic mafic rock consisting of olivine ± other mafic minerals) or eclogite

2. Primary basaltic magma then rises bouyantly through the lithosphere

3. Fractional crystallization occurs because different minerals of different density crystallize at different temperatures

a. these different mineral crystals are segregated by gravity

b. because each mineral differs in composition from the parent magma, as each mineral is segregated, the composition of the residual magma changes

c. early crystallizing minerals are magnesium-rich (mafic) or calcium-rich (felsic) and generally depleted in SiO2

i. thus residual magma compositions become more silicic, more iron-rich and more sodium-rich

d. little fractional crystallization occurs in magmas that rise rapidly and erupt quickly

significant fractional crystallization occurs in magmas that rise slowly because they stop periodically in the upper mantle

B. Multiple Mantle Sources

1. As noted above, the detailed geochemical differences between tholeiitic and alkalic basalts implies that there are at least 2 mantle source areas generating parent magmas

a. Mid-Ocean Ridge Basalts (MORBS) from depleted aesthenosphere (low velocity zone)

i. probably homogeneous globally

b. Hot Spot Basalts from deeper than low velocity zone

i. probably heterogeneous

2. Other geochemical data also indicate multiple source areas

a. Isotopic ratios

i. 87Sr/86Sr ratios (87Sr is radiogenic from decay of 87Rb)

ii. Pb isotopes

iii. Nd/Sm ratios

iv. Ni compositions

IV. Diagenesis and Metamorphism

A. Results from interaction of seawater with ocean crust

1. Seawater circulates through crust because it is highly fractured and because of hydrothermal convection induced by the temperature differential between surface and deep crust

B. Low Temperature (<20 degrees C)

1. Influx of cold seawater into new crust

2. Depth of penetration is 2-5 km (upper part of Layer 3)

3. Results in hydration of crust by alteration of original minerals to smectite,.zeolite and chlorite (greenschist metamorphism)

a. Mg+2 and SO4-2 extracted from seawater and emplaced in crust

C. High Temperature (>20 degrees C to at least 350 degrees C)

1. Exit of hot seawater from new crust

2. Results in deposition of metallic sulfides, sulphates and oxides within fractures and vugs

a. Ca+2 extracted from rock and carried out with seawater

3. Abundant H2S serves as energy source for sulfide oxidizing bacteria, which form the base of the food chain for vent communities

V. Magnetism

A. Early Idea

1. Magnetic layer - upper 500 m

a. fresh basalts have a sufficient magnetic susceptibility

B. Now

1. Entire crust responsible

2. Pillows have mixed magnetization because of post-cooling rolling, tumbling and rotation

3. Layer 3 less strongly magnetized, but larger volume and not chemically altered

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