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Mantle-Derived Magmas: The Ocean Basins
Pahoehoe flowing into ocean, Hawaii
Basaltic Magmas
Thermal Divide
Alkali Basalts
Olivine Tholeiites
QuartzTholeiites
Projected from plag
Melting of Peridotite
Olivine tholeiite
Olivine tholeiiteQuartzTholeiite
QuartzTholeiite
Alkali basalts
Mid Ocean Ridge Volcanism
Ophiolites
Petrological Moho
Geophysical Moho
Ultrabasic cumulates
MagmaChamber
Structure of the Mid Oceanic Ridge
Mid Ocean Ridge Basalts (MORBS)
Olivine and Quartz Tholeiites
Mid Ocean Ridge Basalts (MORBS)
Fractionation of Ol + Cpx by crystal settling at low pressure
Mid Ocean Ridge Basalts (MORBS)
What causes the extension towards more olivine-rich compositions?
Mid Ocean Ridge Basalts (MORBS)
Mid Ocean Ridge Basalts (MORBS)
First partial melt
Melt not Ol-richenough
Mid Ocean Ridge Basalts (MORBS)
Mid Ocean Ridge Basalts (MORBS)
Can generate MORB parentMelts by 20-40% melting at>15 kbar
Depleted N-MORBS
Normal MORBs (N-MORBS) have a depleted signature similar to depleted lithosphere.
Depleted N-MORBS
20-40%
1%
10%
5%
Incompatible elements are more enriched in a magma generated by smaller amounts of partial melting
Enriched E-MORBS
E-MORBS are enriched in incompatibleelements in particular LREE
Iceland
Iceland includes intermediate icelandite lavas and rhyolites.
Small amounts of alkali basalt are erupted away from the rift. Explosive volcanism in Iceland (e.g. Hekla) is caused by rhyolite magmas.
Hekla
The 1104 AD eruption of Hekla may have caused the climatic downturn of the “dark ages”.
Ocean Islands
40 Ma
60 Ma
Ocean Island Basalts (OIBs)
OIBs are enriched morethan E-MORBs
Other Ocean Islands: Tristan Du Cuhna, The Azores, Ascession Island.
Ocean island volcanism includes the eruption of alkaline magmas (e.g. nephelinites).
Magma Series of Ocean Islands: Hawaii
Platemotion
Active olivine tholeiites
Recent alkalineactivity (alkali basalts-trachytes)
Recent highly alkaline activity (Nephelinites-basanites)
Magma Series of Ocean Islands: Hawaii
Olivine Tholeiite
Alkali Basalt + Trachtyes
Crater of Haleakala, Maui
Magma Series of Ocean Islands: Hawaii
Magma Series of Ocean Islands: Hawaii
More alkaline at smaller partial melting (and higher P).
Magma Series of Ocean Islands: Origin