Mantle-Derived Magmas II Skaergaard Intrusion, Greenland.

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Presentation transcript:

Mantle-Derived Magmas II Skaergaard Intrusion, Greenland

Enrichment of OIB and E-MORB Mantle

Depleted N-MORBS 20-40% 1% 10% 5% Incompatible elements are more enriched in a magma generated by smaller amounts of partial melting

Incompatibles versus Compatibles O% melting Rock: red/blue = 1 15% melting Rock: red/blue = 0.6 Melt: red/blue = 7 50% melting Rock: red/blue = 0.31 Melt: red/blue = 1.6

Enrichment of OIB and E-MORB Mantle Partial melting alone cannot explain the enrichment of E-MORB and N-MORB. Need metasomatism of source.

Isotopes Isotope systems are used to investigate the contribution of different geochemical reservoirs Two nuclides of the same heavy element have the same charge, size and electronic structure and behave in the same way during melting and crystallisation. They cannot be separated (i.e. fractionated). Daughter nuclides produced by decay are usually different elements and are fractionated by igneous processes.

Rb-Sr Isotopes 87 Rb decays to 87 Sr in 4.88x10 10 yrs. 86 Sr is a stable isotope and its abundance doesn’t change. 87 Rb 86 Sr 87 Sr Time 87 Sr 86 Sr= Sr 86 Sr= 2.5

Rb-Sr Isotopes: Change with Time The abundance of 87 Sr increases with time. The abundance of 86 Sr stays the same. Therefore, the 87 Sr/ 86 Sr ratio increases with time. Larger initial Rb/Sr = larger slope

Changes with Melting 87 Rb 86 Sr 87 Sr Melting increases Rb but doesn’t change Sr Crystallisation + time Rb is more incompatible than Sr

Changes with Melting

Rb-Sr of MORB

Rb-Sr of OIB Need to add significant amounts of Rb to generate OIB

Rb-Sr Isochrons Depleted rocks Enriched rocks

Nd-Sm Isotopes 147 Sm decays to 143 Nd with a half life of 1.06x10 11 yr 144 Nd is the stable isotope Nd is more incompatible than Sm (this is the opposite of Rb-Sr). 143 Nd/ 144 Nd are higher in depleted sources and lower in enriched sources.

Nd-Sm and Rb-Sr Values measured today!

Nd-Sm and Rb-Sr Enriched melts

Mantle Plumes

Diversification of Basaltic Magmas How Do We Make All Those Different Rocks? AFM diagram

Liquid Line of Descent Compositions of Lavas Crystal Fractionation (removal of crystals from a magma).

Layered Intrusions - Fossil Magma Chambers Lopolith (funnel shaped intrusions). (Some layered intrusions are sills) Skaergaard Intrusion, Greenland (500 km 3 )

Layered Series Rhythmic layering mafics plag

Layered Series Cross-bedding

Layered Series Imbrication of Gneiss Xenoliths (flow direction towards centre).

Layered Series Chilled Margins around Gneiss Xenoliths.

Cryptic Layering Cryptic layering is the change in compositions of the cumulate Minerals through the intrusion

Cryptic Layering Mg-rich olivine and pyroxene Ca-rich plag

Cryptic Layering More Fe-rich px and ol More Na-rich plag

Cryptic Layering Very iron-rich ol and px (ferrodiorite) Na-rich plag

Dunite & Peridotite as Cumulates Dunite overlain by Gabbro In some intrusions olivine crystallises first and settles to produce a dunite cumulate.

Cryptic Layering

LZ and MZ increase in Fe, Na, Si

Cryptic Layering UZ Fe-oxides crystallise rapid increases in Si, Na

Acid Granophyres (Pegmatites)

Cryptic Layering Tholeiitic fractionation trend

Phase Layering No olivine! Olivine disappears in the middle zone!

Phase Layering No olivine! Olivine disappears in the middle zone! Olivine reacts with the liquid to generate low-Ca px!

Rhythmic Layering

Densest crystals sink faster!

Rhythmic Layering But plag floats!

Rhythmic Layering Rafts of plag + mafics will sink

Rhythmic Layering Nuclei form More nuclei form Due to supercooling Sudden pulse of crystallisation Nuclei depleted Crystal melt slush sinks

Rhythmic Layering Turbidity currents of melt+crystals in magma chamber Normal grading + cross-bedding

Cumulates

Cumulates: Lamination

Cumulates: Filtration Intercumulus liquid can be squeezed out. On a larger scale, liquid squeezed from the lunar mantle may explain KREEP basic magmas erupted in the lunar highlands.