Links between melting, rheology and the composition and structure of the upper mantle

Greg Hirth, Brown Univeristy

Marc Parmentier & Alberto Saal

Mark Behn, Jim Gaherty, John Collins, Dan Lizarralde

Kiyoshi Baba, Rob Evans, Alan Chave, Don Forsyth

Joerg Renner, Karen Viskupic, Brian deMartin & Brian Evans

Cagnioncle et al. 2007

Hirth & Kohlstedt, 2003

Zhao et al. 2007

Gaherty et al., 1999

Mei & Kohlstedt, 2000; Jung & Karato, 2001; Borch & Green, 1989;Chopra & Paterson, 1981; Karato et al., 1986

Hirth & Kohlstedt, 2003

Influence of water of rheology (Strain Rate [Viscosity]-1)

Evans et al., 2005

Resistive layer consistent withdry olivine

Conductive “asthenosphere” consistent with damp olivine

Magnitude of anisotropyconsistent with crystallographicdependence of H diffusion inOlivine

Anisotropy hard to preserve ifconductive layer controlled bypresence of melt

0.000 0.005 0.010 0.015 0.020

Melt Fraction

10-4

10-3

10-2

10-1

100

101

Bul

kC

ondu

ctiv

ity

(Sm

)

[100][010][001]

0.000 0.005 0.010 0.015 0.020

Melt Fraction

10-4

10-3

10-2

10-1

100

101

Bul

kC

ondu

ctiv

ity

(Sm

)[100][010][001]

Basaltic Melt: 10 S/m

Fictive Melt: 100 S/m

(S/m

)(S

/m)

0.000 0.005 0.010 0.015 0.020

melt fraction

0

500

1000

1500

2000

2500

3000

oliv

ine

wat

erco

nten

t(p

pmH

/Si)

Enriched (650 ppm)

Depleted (85 ppm)

Two Component Mantle with 125 ppm Bulk Water

Yang, Weeraratne & Forsyth

dehydratation

dehydratation

Faul & Jackson, 2005

0

100

200

300

400

Vs

Behn et al., 2009

Behn et al., 2009

Need constraints on permeability (k)and bulk viscosity ()

0.0 0.1 0.2 0.3 0.4

melt fraction

10-8

10-7

10-6

10-5

10-4

10-3

com

pact

ion

rate

(1/s

)Olivine + Melt1200oC, P = 10 MPagrain size = 20 m

rate 1

rate 2

Renner et al., 2003

Predicts > 1022 Pas

10-1 100 101 102 103

grain size (mm)

10-2

10-1

100

101

102

103

104

com

pact

ion

leng

th(k

m)

n =

1

n = 3.5

“shear”

= 1%

Compaction Rate Pn = P/Compaction RateP = g

g 7000 (SI)

= 300 km gives

P = 2.1 GPa!!NO WAY

deMartin, Hirth & Evans

S = Carbonate melt fluxS = Fcarb*Upwelling fluxUpwelling flux = volume/area/timeUpwelling flux = Slab Flux/Earth surface areaUpwelling Flux = 1km/yrFcarb = 10-4 to 10-3

0 20 40 60 80 100

time (My)

0.00

0.02

0.04

0.06

0.08

0.10

mel

tfr

acti

on

F = 0.01%

F = 0.1%

-2-1

Darcy Flux = 0.01%

Pond

ed M

elt F

ract

ion

t = (pond /3) ( )/S

Dasgupta et al., 2009

“Ponded” 2%

Crystallize

H20 400 ppm