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Magmatic differentiation

Differentiation: crystallization of a magma Determine cogenetic relationships between magmas (lavas) Determine least fractionated, parental magma. Fractionation trends gives clues about P, T of magma chamber Differentiation processes: I: Closed system A. Crystal-melt fractionation

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Magmatic differentiation

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  1. Differentiation: crystallization of a magma Determine cogenetic relationships between magmas (lavas) Determine least fractionated, parental magma. Fractionation trends gives clues about P, T of magma chamber Differentiation processes: I: Closed system A. Crystal-melt fractionation 1. Gravitational segregation 2. Flow segregation 3. Filter pressing 4. Convective fractionation B. Separation of immiscible melts C. Melt fluid separation II: Open system: A. Assimilation of a solid B. Mixing of different magmas Magmatic differentiation

  2. Variation diagrams Harker diagrams: Oxide-oxide variation diagrams Lever rule applies Liquid moves away from the composition of the crystallizing assemblage

  3. Variation diagrams cont’d Resorbed olivines, constant composition of phenocrysts

  4. Trace element modeling Compatible element concentrations decrease quickly; Incompatible element concentrations increase slowly m is melt and p is parent

  5. Assimilation Assimilation of crustal material often better recognized with isotopes Assimilation of mafic material hard to recognize. Data often permissive, but not conclusive. Evidence: crustal xenoliths, resorbed qtz, trace elements, isotopes.

  6. Palisades sill: example of gravitational settling Basaltic intrusions Evidence: Olivine rich layer at the bottom changes in thickness based on the underlying topography Sharp change in olivine abundance going upwards Olivines are more Fe-rich than what is expected based on the chilled margin Local internal chilled contacts: new influx

  7. Layered intrusions

  8. Cumulus fabric Muskox

  9. Skaergaard Extreme Fe-enrichment Phase layering: changes in mineralogy Cryptic layering: changes in chemical composition of the minerals

  10. MORB fractionation trends Fractionation trend toward Fe-enrichment. Where is the primary magma? High P melts to low P: olivine fractionation

  11. Fe-enrichment Tholeiitic trend shows Fe-enrichment. Lack of enrichment in calc-alkaline trend Higher oxygen fugacity Fe-oxide stable at higher temperature i.e. fractionates earlier in the sequence

  12. Tonga-Kermadec-New Zealand Arc Ocean-ocean in North, ocean-continent at New Zealand Often a bimodal distribution in silica Taupo rhyolite field in New Zealand, too large a volume for simple fractionation Large addition of crustal melts. “Complicating factors at continental arcs: Sediment from continent gets subducted enhancing felsic magma Subcontinental lithosphere has been metasomatically enriched over time The thicker continental crust results in more opportunity for assimilation.

  13. Assimilation Combined crystallization and assimilation

  14. Medicine Lake Incompatible and compatible element concentrations to high for fractional XX AFC more likely, but component of mixing required.

  15. Magmatic petrotectonic associations Spreading centers

  16. Spreading center cont’d

  17. Plume at the ridge

  18. Plumes

  19. Hawaii cont’d

  20. Plumes cont’d

  21. Flood basalts

  22. Flood basalts cont’d

  23. Island arcs Trace elements in island arc rock distinct: Depletion in high field strength elements (Ti, Zr, Hf, Nb, Ta) In oceanic settings the HREE can be more depleted then MORB

  24. Rift volcanics Carbonatite: >50% carbonate minerals; alkali carbonatite <0.2wt% SiO2+Al2O3. Related to strongly Si-undersaturated rocks: phonolite, nephelinite, melilitite, hawaiite. Strongly enriched in LIL: large ion lithophile elements. Alkalic rarities Lamprophyres, lamproites, orangeites and kimberlites Potassic, volatile rich, mafic to ultramafic

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