Fifteen-Twenty Fracture Zone “MAGMA 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, solid magma decompresses, gets squeezed = ADIABATIC MELTING …leaving some elements behind and cooling to form BASALT

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

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 Zone 15º20N, 45ºW FTFZ African North American

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 Bathymetry Transform fault Rugged Smooth

- 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

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

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