Mantle-Derived Magmas: The Ocean Basins Pahoehoe flowing into ocean, Hawaii.

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Presentation transcript:

Mantle-Derived Magmas: The Ocean Basins Pahoehoe flowing into ocean, Hawaii

Basaltic Magmas

Thermal Divide Alkali Basalts Olivine Tholeiites Quartz Tholeiites Projected from plag

Melting of Peridotite Olivine tholeiite Quartz Tholeiite Quartz Tholeiite Alkali basalts

Mid Ocean Ridge Volcanism

Ophiolites Petrological Moho Geophysical Moho Ultrabasic cumulates Magma Chamber

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)

First partial melt Melt not Ol-rich enough

Mid Ocean Ridge Basalts (MORBS)

Can generate MORB parent Melts 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 incompatible elements 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 more than 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 Plate motion Active olivine tholeiites Recent alkaline activity (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

More alkaline at smaller partial melting (and higher P).

Magma Series of Ocean Islands: Origin