Mantle-Derived Magmas on the Continents Gelia Shield Volcano, East African Rift

Oxygen Fugacity 3Fe 2+ SiO 4 + O 2- 2 = 2(Fe 2+ O.Fe 3+ 2 O 3 ) + SiO 2 Fayalite + free oxygen = magnetite + quartz Oxidation (increase in valence state) Reduction (decrease in valence state) Redox Reactions

Oxygen Fugacity

buffer

Metal-bearing Basalts C (wood) + O 2 (melt) = CO 2 (gas)

Disko Island Assimilation of coal reduces basalts

Disko Island Iron metal droplets in basalt

Disko Island

Continental Magmatism (Mantle-Derived) Usually associated with continental rifting Some in stable cratonic areas Magma-types Qtz Tholeiites, Tholeiites + Alkali Basalts Alkaline (Na 2 O+K 2 O+CaO>SiO 2 ), SiO 2 - undersaturated. Basanites, phonolites, tephrites. Peralkaline (Na 2 O+K 2 O>Al 2 O 3 ). These can be SiO 2 - undersaturated (nephelinites, leucitites, phonolites) or oversaturated (e.g. Peralkaline rhyolites, alkali granites) Ultrabasic (e.g. kimberlites, lamproites)

Continental Magmatism (Mantle-Derived) Doming and lithospheric thinning by ductile flow Large scale melting of lithosphere producing flood basalts

Flood Basalts Mg-contents 5-8% are too-low to be directly derived from the mantle Probably stored in magma chambers Also contain few phenocrysts

Flood Basalts Extrusion rates of up to 1 M m 3 s -1

Flood Basalts Columnar jointing in flood basalt

Flood Basalts Enrichments similar to E-morb Mainly fertile mantle. Some distinct depletions due to crystal fractionation. Not Mg-rich enough to be direct melts of the mantle. Accumulation in large magma chambers.

Flood Basalts Flood basalts

Continental Magmatism: Rift Stage

Extends 3,700 km Rifting began in the early Tertiary around the Ethiopian dome. Rifting in the Miocene in Kenya and Tanzania.

Lies on a series of crustal domes. Rift triple junctions occur on the domes rrr-junction

Continental Magmatism: Rift Stage Gelia Shield Volcano (alkali basalt) Shombole Nephelinite/Carbonatite Motnick Nephelinite/Carbonatite Oldoinyo Lengai (Nephelinite/Carbonatite)

Continental Magmatism: Rift Stage

Continental Magmatism: Alkaline Series in Rift Stage

Continental Magmatism: Rift Stage Two main alkaline series: Silica-undersaturated Silica-saturated

Carbonatite-Nephelinite Association Most commonly in rift settings but occasionally cratonic Carbonatites have the highest REE concentrations of any rock type.

Carbonatite-Nephelinite Association Fenite generated by metasomatism and contains aegirine-augite Intrusive carbonatites are always emplaced after alkaline silicate magmas.

Carbonatite-Nephelinite Association Kerimasi Oldoinyo Lengai Ca-carbonatite lava Natrocarbonatite lavas

Carbonatites plot on the mantle array. Carbon isotopes are also mantle values.

Continental Magmatism: Rift Stage Enriched in incompatibles relative to E-MORB. Similar enrichment to OIB = plume like enrichment.

Continental Magmatism: Rift Stage Some melting in lithosphere Some contamination by crust

Continental Magmatism: Afar Stage Qtz tholeiites, alkali basalts further from plume. N-MORB-like signatures

Extends 3,700 km Rifting began in the early Tertiary around the Ethiopian dome. Rifting in the Miocene in Kenya and Tanzania.

Pre-rifting Flood basalts Rifting-stage Alkaline volcanics Afar Alkali basalts and quartz tholeiites

Cratons: Kimberlites Potassic ultrabasic rocks dominated by xenocrysts of olivine, Cr-diopside, phlogopite etc. Matrices consist of serpentine, phlogopite(mica), carbonates, perovskite. More a clan of related rocks rather than specific rock types. Carbonate rich – basaltic kimberlites Mica rich - Orangeites

Cratons: Kimberlites

Basaltic kimberlites CO 2 -rich Micaceous Kimberlites (Orangeites)

Cratons: Kimberlites Increasing enrichment in incompatibles: Basaltic kimberlites< Micaceous kimberlites< Lamproites Suggests plume involvement. Depletions in HREE due to garnet in source region.

Small degrees of partial-melting of CO 2 -H 2 O bearing mantle

Cratons: Kimberlites Group 2 micaceous Group 1 basaltic

Kimberlites and Mantle Metasomatism?