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