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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
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