1; movies

Topography of a fast spreading ridge (EPR)

Topography of a slow spreading ridge (south atlantic)

2; topography

Melt beneath a fast-spreadingridge (East Pacific Rise)

Ophiolites

3; classic ophiolites

Oman ophiolite

Pillow lavas

Sheeted Dikes

Layered Gabbros

Vs is the particles' settling velocity (vertically downwards if ρp > ρf, upwards if ρp < ρf) g is the acceleration due to gravity, ρp is the density of the particles, and ρf is the density of the fluid

Stokes law

4; settling

Massive gabbro

Impregnated dunnite

Banded harzburgite

Hot spot volcanism: a global phenomenon

5; Hawaii-emperor chain

The origin of hot spot volcanoes from melting of

plumes

Dynamic models of mantle convection

Plates going down Plumes coming up

Rapid, small-cell convection on Io

Why does the mantle melt to produce hot spot magmas?

• Isentropic decompression melting

• Fluxing by volatiles• Heating of the

lithosphere by a hot plume

• Unconventional heat sources

Simple variations on the decompression melting theme

• Variations in potential temperature -- hotter mantle produces deeper melting, more magma

• Variations in the thickness of the lithosphere -- controls the depth at which melting terminates

• Fractional vs. batch melting• All of these can vary from

hot spot to hot spot and within a single volcano, producing distinctive chemical signatures

Temperature variations near head of plume

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3D Model by Ribe and Christensen

Why does the mantle melt to produce hot spot magmas?

• Isentropic decompression melting

• Fluxing by volatiles• Heating of the

lithosphere by a hot plume

• Unconventional heat sources

Why does the mantle melt to produce hot spot magmas?

• Isentropic decompression melting

• Fluxing by volatiles• Heating of the

lithosphere by a hot plume

• Unconventional heat sources

p l u m e

h o t

c o l d

Hawaii (topography/bathymetry)

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Geological map of the big island of Hawaii

0 20 km

N

Loihi

"Loa" trend

"Kea" trend

Hilo

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HSDP drilling in 1993 and 1999 into the flank of Mauna Kea volcano

• >95% recovery, to a total depth of 3.1 km below sea level

• Penetration through ~1 km of subaerial lavas, ~2 km of submarine deposits, both hyaloclastites and pillows

HSDP drill siteMauna Kea

.

100 150 200 250 300 350 400 450 500

200

400

600

800

1000

1200

1400

Mauna Kea

Model (subaerial)Ar-Ar ageModel (submar.)

Age (Ka)

pilot hole data HSDP 1999: subaerial-submarine transition,1082 mbsl

estimate of average subsidence rate

ML

MK

vesicles - MK

vesicles - ML

vesicles - intrusives

vesicle abundances

rotary drilledintervals

depth (mbsl)

0

1,000

2,000

3,000

500

1,500

2,500

first pillow lavafirst intrusive

subaerial

submarine

0 0.1 0.2 0.3 0.4

modal abundance (volume fraction)

hyaloclastite formation -- “prograding delta” volcano growth

volcanic samplingof a zoned plume

a

b

c

d

e

Trace elements and isotopic ratios are generally correlated with variations in SiO2 content (Kurz et al,

2003)

What if the length scales of compositional heterogeneities are small?