1. Fifteen-Twenty Fracture Zone
“MAGMA STARVED” region in the Mid-Atlantic ridge exposes the Earth’s Mantle

2. “Magma Starved”???
MAGMA = derived from partial melting of pre-existing rocks in Earth’s mantle or crust
At Mid Ocean Ridges, plates diverge, solid
magma decompresses, gets squeezed
= ADIABATIC MELTING
…leaving some elements behind and cooling to form BASALT

3. MORPHOLOGIES Slow-Spreading: Intermediate-Spreading Fast-spreading
0.6-2 cm/yr
Deep rift valley
Highly variable relief ( m)
Intermediate-Spreading
5–7 cm/yr
Long, alternating spreading rates
Fast-spreading
8-18 cm/yr
Low axial highs
Minimal rift mountain topography

4. Slow-Spreading Much thinner oceanic crust
Absence of volcanic activity on long segments of the ridge
Abundant mantle peridotite

5. Gakkel = Ultraslow, 6 – 1.7 mm/yr, lies beneath the Arctic Ocean
Southwest Indian Ridge = Very slow, mm/yr, lies beneath the southern oceans
Mid-Atlantic Ridge = Slow, 25 mm/year, FTFZ

6. Fifteen-Twenty Fracture Zone 15º20N, 45ºW
FTFZ
African
North American

7. 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

8. CRUISES: 1988 - “Ridelente” 1992 - “Faranaut” 1998 - “MODE 98, Leg 1”
“RRS James Clark Ross JR63”
“ODP Leg 209”
“IFREMER Pourquoi Pas”
“RRS James Cook JC007”

9. UNDERLYING EVIDENCE
Bathymetry
Transform fault
Rugged
Smooth

11. - Abyssal hill topography
Gravity
= Bouguer and mantle anomalies
+ Thin basaltic crust,
Rugged topography
Serpentinized peridotites
Transform walls
- Abyssal hill topography
Shallower avg. topography
Transform valley

12. B. Rocks Thin crust, rugged terrain
Within/near discontinuous ridge segments
= SERPENTINIZED PERIDOTITES
Equipment = Robotic rock drill
Dredging

13. UNSOLVED MYSTERIES…
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
Why is the oceanic crust so thin?
Basaltic crust was never formed
Crust was removed by tectonic