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