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Fifteen-Twenty Fracture Zone

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

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Fifteen-Twenty Fracture Zone

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  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: • 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

  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, 14-16 mm/yr, lies beneath the southern oceans Mid-Atlantic Ridge = Slow, 25 mm/year, FTFZ

  6. Fifteen-Twenty Fracture Zone15º20N, 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” • 2001 - “RRS James Clark Ross JR63” • 2003 - “ODP Leg 209” • 2007 - “IFREMER Pourquoi Pas” • 2007 - “RRS James Cook JC007”

  9. UNDERLYING EVIDENCE • Bathymetry Transform fault Rugged Smooth

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

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

  12. 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

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