Fifteen-Twenty Fifteen-Twenty Fracture ZoneFracture Zone

“MAGMA STARVEDMAGMA STARVED” region in the Mid-Atlantic ridge exposes the Earth’s Mantle

“Magma Starved”???

MAGMA = derived from partial melting of pre-existing rocks in Earth’s mantle or crust

At Mid Ocean Ridges, plates diverge, solidmagma decompresses, gets squeezed = ADIABATIC MELTING…leaving some elements behind and

cooling to form BASALT

Slow-Spreading: • 0.6-2 cm/yr • Deep rift valley• Highly variable relief (400-2500 m)Intermediate-Spreading• 5–7 cm/yr• Long, alternating spreading ratesFast-spreading• 8-18 cm/yr• Low axial highs• Minimal rift mountain topography

MORPHOLOGIES

Slow-Spreading

Much thinner oceanic crust

Absence of volcanic activity on long segments of the ridge

Abundant mantle peridotite

Gakkel = Ultraslow, 6 – 1.7 mm/yr, lies beneath the Arctic Ocean

Southwest Indian Ridge = Very slow, 14-16 mm/yr, lies beneath the southern oceans

Mid-Atlantic Ridge = Slow, 25 mm/year, FTFZ

Fifteen-Twenty Fracture Zone15º20N, 45ºW

FTFZ

North American

African

Is there a HOLE in the Oceanic crust??!!

Basalts which form the crust are thin/absent

• Mantle is NOT partially melting

Slow-spreading Minor crust = Faults

EXPOSED MANTLE MATERIAL

Olivine + Seawater = Serpentine + Magnetite + H2

H2 + seawater carbonate = CH4

CRUISES:

• 1988 - “Ridelente”• 1992 - “Faranaut”• 1998 - “MODE 98, Leg 1”• 2001 - “RRS James Clark Ross JR63”• 2003 - “ODP Leg 209”• 2007 - “IFREMER Pourquoi Pas”• 2007 - “RRS James Cook JC007”

UNDERLYING EVIDENCE

A. Bathymetry

Transform fault

Smooth

Rugged

A. Gravity= Bouguer and mantle anomalies

+ Thin basaltic crust,

Rugged topography

Serpentinized peridotites

Transform walls

- Abyssal hill topography

Shallower avg. topography

Transform valley

B. Rocks

• Thin crust, rugged terrain

• Within/near discontinuous ridge segments

= SERPENTINIZED PERIDOTITES

Equipment = Robotic rock drill

Dredging

UNSOLVED MYSTERIES…

1) Why is the mantle not melting in this area (low magma supply)?

Mantle is too cold Mantle is infertile from previous partial melting Melt is trapped in deep axial lithosphere

2) Why is the oceanic crust so thin? Basaltic crust was never formed Crust was removed by tectonic