ce que nous apprennent les roches* du manteau sur la migration des magmas dans le manteau peter...
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![Page 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](https://reader035.vdocuments.pub/reader035/viewer/2022062315/56649d435503460f94a1eeee/html5/thumbnails/1.jpg)
Ce que nous apprennent les roches* du manteau sur la
migration des magmas dans le manteau
Peter Kelemen
* Roches experimentales, volcaniques et du manteau
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Minerals in the mantle and lower crust
Olivine Mg2SiO4 - Fe2SiO4
Orthopyroxene Mg2Si2O6 - Fe2Si2O6, etcClinopyroxene CaMgSi2O6 - CaFeSi2O6, etc Spinel (Mg,Fe)(Cr,Al)2O4 , etcGarnet (Mg,Fe,Ca)3Al2Si3O10, etcPlagioclase CaAl2Si2O8 - NaAlSi3O8
Melting reactions
P > 20 kilobars (2 Gpa)Ol + Opx + Cpx + Gnt = melt8 kb < P < 20 kbOpx + Cpx + Sp = Ol + meltP < 8 kbOpx + Cpx + Plag = Ol + melt if fertileOpx + Cpx + Sp = Ol + melt if depleted
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really lowF
~3 to 20%melting
reallyhigh F
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mantle solidus
liquidadiabat
olivine saturation
pyroxene saturation D
epth
Temperature
{
peridotite dissolves(even olivine), MgO up{pyroxenes dissolve
olivine precipitates, SiO2 up
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0 .0 0 10 .0 10 .111 01 0 01 0 0 01 04C sR bB aT hUKT aN bL aC eP bP rS rN dZ rH fS mE uG dT bT iD yH oE rT mY bL uYN iN -M O R Bc o n c e n tra tio n , p p m
0.001
0.01
0.1
1
10
100
1000
104
Cs Rb Ba Th U K Ta Nb La Ce Pb Pr Sr Nd Zr Hf Sm Eu Gd Tb Ti Dy Ho Er Tm Yb Lu Y Ni
N-MORB
concentration, ppm
0.1
1
10
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
N-MORB REE
concentration, ppm
Rare Earth Elementsin order of increasing Z
periodic tablein approximate order of crystal/liquid partitioning
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0.1
1
10
Cs Rb Ba Th U K Ta Nb La Ce Pb Pr Sr Nd Zr Hf Sm Eu Gd Tb Ti Dy Ho Er Tm Yb Lu Y Ni
N-MORB/Primitive Mantle
N-MORB/C1 chondrite
Primitive Mantle/C1 chondrite
normalized concentration
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Bottom up:
Diffuse porous flow Melting & diapirsMagma fractureFocused porous flowSills & lenses at “top”
Top down:
MORB compositionMORB focusingMORB ascent rate
Arc compositionArc focusingHotspot flux, comp,
focusing
= WFρs/(wρf) STEADY STATE!
( = 1)
w = kΔρg/(φμf) “DARCY’S LAW”
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w = kΔρg/(φμf)
k = d2φ3/c
Von Bargen & WaffWark, Watson, et al. k = d2φ3/270
Faul et al.
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Von Bargen & Waff
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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 edgesIf rock is banded in grain size, low permeability to banding
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hzol+spol
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HARZBURGITE (+) OL + SP ()
OL only ()
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hzol+spol
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ul e
t al.
Von Bargen & WaffWark, Watson, et al.
quartzite
marble
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compositional variation across a large dunite in the Josephine peridotite
0
0.05
0.1
0.15
0.2
12.5 17.5 22.5 27.5 32.5
Position (m)
w.t. %CaO in Olivine
0.00
0.05
0.10
0.15
0.20
12.5 17.5 22.5 27.5 32.5
Position (m)
w.t. %CaO in Whole Rock
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upper bound estimate of “permeability threshold”
based on upper bound estimate of “trapped melt”,
based on CaO in whole rock - olivine
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w = kΔρg/(φμf)
k = d2φ3/c
Wark, Watson, et al. k = d2φ3/270
Von Bargen & WaffWark, Watson, et al.
Faul et al.X
X
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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”
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ol + melt ol + meltol + melt ol + melt
ol + opx + melt
ol ± opx NO initial melt
6h
ol + opx + melt
ol ± opx NO initial melt
6h
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3 = 270μWFρs/(d2Δρgρf)
from
= WFρs/(wρf)
STEADY STATE!
( = 1)
w = kΔρg/(φμf)
“DARCY’S LAW”
k = d2φ3/270
Wark et al.
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Bottom up:
Diffuse porous flow OK, prefer Wark et al.
(for now) field evidence? Melting & diapirsMagma fractureFocused porous flowSills & lenses at “top”
Top down:
MORB compositionMORB focusingMORB ascent rate
Arc compositionArc focusingHotspot flux, comp,
focusing
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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
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-12,000
-10,000
-8,000
-6,000
-4,000
-2,000
0
89.5 90 90.5 91 91.5 92 92.5
harzburgite
dunite
meters below crust/mantle transition
forsterite content in olivine
molar Mg/(Mg+Fe)
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melt outresidual porosity
nothing coming in
melt outmelt coming in
residual porosity
nothing outMORB coming in
nothing outlocal melt coming in
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ickTim
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light REE “enriched”
light REE depleted
low Allow Al
high Al
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coarse,granular(high T)
Porphyroclastic(low T)
light REEdepleted(“MORB source”)
Light REEEnriched
(addition of low degree melts)