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

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

really lowF

~3 to 20%melting

reallyhigh F

mantle solidus

liquidadiabat

olivine saturation

pyroxene saturation D

epth

Temperature

{

peridotite dissolves(even olivine), MgO up{pyroxenes dissolve

olivine precipitates, SiO2 up

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

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

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”



QuickTime™ and aTIFF (LZW) decompressor


w = kΔρg/(φμf)

k = d2φ3/c

Von Bargen & WaffWark, Watson, et al. k = d2φ3/270

Faul et al.





Von Bargen & Waff



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

QuickTime™ and a

TIFF (Uncompressed) decompressorare needed to see this picture.



hzol+spol





HARZBURGITE (+) OL + SP ()

OL only ()





hzol+spol


are needed to see this picture.Fa

ul e

t al.

Von Bargen & WaffWark, Watson, et al.

quartzite

marble

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

upper bound estimate of “permeability threshold”

based on upper bound estimate of “trapped melt”,

based on CaO in whole rock - olivine



QuickTime™ and aTIFF (LZW) decompressor


w = kΔρg/(φμf)

k = d2φ3/c

Wark, Watson, et al. k = d2φ3/270

Von Bargen & WaffWark, Watson, et al.

Faul et al.X

X



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”









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

3 = 270μWFρs/(d2Δρgρf)

from

= WFρs/(wρf)

STEADY STATE!

( = 1)

w = kΔρg/(φμf)

“DARCY’S LAW”

k = d2φ3/270

Wark et al.

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

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











-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)



melt outresidual porosity

nothing coming in

melt outmelt coming in

residual porosity

nothing outMORB coming in

nothing outlocal melt coming in







Qu

ickTim

e™

and

aTIF

F (

Un

com

pre

sse

d)

de

com

pre

sso

rare

nee

de

d t

o s

ee t

his

pic

ture

.

light REE “enriched”

light REE depleted

low Allow Al

high Al





coarse,granular(high T)

Porphyroclastic(low T)

light REEdepleted(“MORB source”)

Light REEEnriched

(addition of low degree melts)

Documents

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