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Ce que nous apprennent les roches* du manteau sur la migration des magmas dans le manteau

Ce que nous apprennent les roches* du manteau sur la migration des magmas dans le manteau. Peter Kelemen * Roches experimentales, volcaniques et du manteau. Minerals in the mantle and lower crust Olivine Mg 2 Si O 4 - Fe 2 SiO 4 Orthopyroxene Mg 2 Si 2 O 6 - Fe 2 Si 2 O 6 , etc

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Ce que nous apprennent les roches* du manteau sur la migration des magmas dans le manteau

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  1. Ce que nous apprennent les roches* du manteau sur la migration des magmas dans le manteau Peter Kelemen * Roches experimentales, volcaniques et du manteau

  2. Minerals in the mantle and lower crust Olivine Mg2SiO4 - Fe2SiO4 Orthopyroxene Mg2Si2O6 - Fe2Si2O6, etc Clinopyroxene CaMgSi2O6 - CaFeSi2O6, etc Spinel (Mg,Fe)(Cr,Al)2O4 , etc Garnet (Mg,Fe,Ca)3Al2Si3O10, etc Plagioclase CaAl2Si2O8 - NaAlSi3O8 Melting reactions P > 20 kilobars (2 Gpa) Ol + Opx + Cpx + Gnt = melt 8 kb < P < 20 kb Opx + Cpx + Sp = Ol + melt P < 8 kb Opx + Cpx + Plag = Ol + melt if fertile Opx + Cpx + Sp = Ol + melt if depleted

  3. really low F really high F ~3 to 20% melting

  4. { pyroxenes dissolve olivine precipitates, SiO2 up peridotite dissolves (even olivine), MgO up { liquid adiabat olivine saturation pyroxene saturation  Depth mantle solidus Temperature 

  5. periodic table in approximate order of crystal/liquid partitioning Rare Earth Elements in order of increasing Z

  6. Top down: MORB composition MORB focusing MORB ascent rate Arc composition Arc focusing Hotspot flux, comp, focusing Bottom up: Diffuse porous flow Melting & diapirs Magma fracture Focused porous flow Sills & lenses at “top” • = WFρs/(wρf) STEADY STATE! ( = 1) w = kΔρg/(φμf) “DARCY’S LAW”

  7. Von Bargen & Waff Wark, Watson, et al. k = d2φ3/270 Faul et al. w = kΔρg/(φμf) k = d2φ3/c

  8. Von Bargen & Waff

  9. Grain size variation: some grains smaller, more melt on triple grain boundaries (= grain edges) At low melt fraction, little or no melt on large grain edges If rock is banded in grain size,  low permeability  to banding

  10. HARZBURGITE (+) OL + SP () hz ol+sp ol OL only ()

  11. quartzite marble hz ol+sp ol Faul et al. Von Bargen & Waff Wark, Watson, et al.

  12. compositional variation across a large dunite in the Josephine peridotite

  13. upper bound estimate of “permeability threshold” based on upper bound estimate of “trapped melt”, based on CaO in whole rock - olivine

  14. X Von Bargen & Waff Wark, Watson, et al. X Faul et al. w = kΔρg/(φμf) k = d2φ3/c Wark, Watson, et al. k = d2φ3/270

  15. Wetting angles may vary depending on crystallographic orientation and mineral At low melt fractions, “unfavorable” grain edges have no melt at all Positive or negative feedback on permeability? k = d2φ3/c c is a “geometric factor”

  16. ol + melt ol + melt ol + melt ol + melt 6h 6h ol ± opx NO initial melt ol ± opx NO initial melt ol + opx + melt ol + opx + melt

  17. f3 = 270mWFrs/(d2Drgrf) from • = WFρs/(wρf) STEADY STATE! ( = 1) w = kΔρg/(φμf) “DARCY’S LAW” k = d2φ3/270 Wark et al.

  18. Top down: MORB composition MORB focusing MORB ascent rate Arc composition Arc focusing Hotspot flux, comp, focusing Bottom up: Diffuse porous flow OK, prefer Wark et al. (for now) field evidence? Melting & diapirs Magma fracture Focused porous flow Sills & lenses at “top”

  19. Models of regional pervasive porous flow conflict with structural and seismic evidence that fractures control fluid transportation in the upper mantle. Effects of porous-medium flow have been inferred in studies of mantle peridotite … but are well documented only on scales of centimeters or decimeters. In all these [cases], porous flow is fundamentally controlled by proximity to magma-filled fractures. Nielsen & Wilshire, 1993

  20. melt out residual porosity nothing coming in melt out melt coming in residual porosity nothing out local melt coming in nothing out MORB coming in

  21. light REE “enriched” high Al light REE depleted low Al low Al

  22. Light REE Enriched (addition of low degree melts) Porphyroclastic (low T) coarse, granular (high T) light REE depleted (“MORB source”)

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