steven b shirey department of terrestrial magnetism carnegie institution of washington

Archean Plate Tectonics: Isotopic Evidence from Samples of the

Lithospheric Mantle to the Upper Crust

Steven B Shirey

Department of Terrestrial Magnetism

Carnegie Institution of Washington

Pb-Pb Array for MORB-OIB

Sialic rocks with juvenile isotopes

Enriched Os in wet komatiites

O isotopes in magmatic zircon

Low-T O isotopes in eclogites

C & N isotopes in diamond

+ Os isotopes in sulfide inclusions

MIF sulfur in sulfide inclusions

CRUSTALEVIDENCE

MANTLELITHOSPHEREEVIDENCE

OCEANIC SLABRECYCLING

OCEANIC ARCSWITH START OFCONTINENTAL ARCS

EARLY RELATIVELYCOOL MAGMATISM

SUBDUCTION IMPRINT EVIDENT

ISOTOPIC DATA INFERENCEDEPTH

(Davies, JGR 1984) (Hart & Gaetani, CMP 2006)

Current mantle heterogeneity,

especially the Pb-Pb array for MORB-OIB

supports Paleoarchean

mantle recycling.

(Davies, EPSL 2002)

QuickTime™ and aGraphics decompressor

are needed to see this picture.

(Davies, EPSL 2002)

Nd and Hf isotope data on Archean

crustal rocks used to require separation on an early crustal

component. This is no longer the

case.

(Vervoort & Blichert-Toft, GCA 2001)

(Boyet & Carlson, Science, 2005)

An Early Depleted Reservoir (EDR) has been proposed for

the Earth. All Archean crust would

come from this reservoir.

(Bennett, Treatise on Geochemistry, 2004; with additions)

EDR of Boyet & Carlson

(Bennett, Treatise on Geochemistry, 2004)

EDR of Boyet & Carlson

(Bennett, Treatise on Geochemistry, 2004; with additions)

EDR of Boyet & Carlson

(Harrison et al, Science, 2004; with additions)

CRUSTAL EVOLUTION

Isotopic heterogeneity

4.3 Ga reservoir

mantle

(Kamber et al, CMP 2003; with additions)

(Valley et al, CMP, 2005)

(modified from Stachel et al, Elements, 2005)

(Carlson et al, 1998-2002; James and Fouch, 2002)

0.00

2.00

4.00

6.00

8.00

0 100 200 300 400

JwanengKimberley

OrapaKoffiefontein

187Re/188Os

MEDIAN PERIDOTITETma = 3.1 Ga

MEDIAN PERIDOTITETrd = 2.7 Ga

187Os/188Os

(Shirey et al., GRL, 2001)

Most sulfide inclusions are late

Archean

Peridotite Re-Os shows the cratonic lithosphere is

Archean

Re, trace elements and oxygen correlate with eclogite groups

Oxygen, Re-Os and REE allow eclogites to be placed into an

Archean oceanic lithosphere

section

Re-Os whole rock systematics give a scattered Archean

age

Same for other localities and some

other isotope systems (e.g. Nd,

Pb)

U-Pb age on zircon supports Re-Os age

(modified from original fig by Schmitz et al, 2004)

DiamondStable

Isotopes

(separate symbols are E-

Types &fibrous

diamonds)

(Farquhar et al., Science, 2000)

(Mojsis et al., GCA, 2003)

MIF Sulfur

An Archean tracer of surficial sulfur

J. Farquhar et al., Science 298, 2369 -2372 (2002)

Sulfide inclusions in

Orapa diamonds have surface S

Eclogitic Sulfides

20 μm

Peridotitic Sulfides Sulfides

6a

6b

incident light incident light

transmittedCLSESE

Panda pipe, Slave Craton

Age = 3520±170 Ma (Isoplot model 1 MSWD = 0.46)

OsIR = 0.1093±0.0001

(Westerlund, Shirey, Richardson, Carlson, Harris, Gurney, CMP, 2006 in press)

Mixing of subduction fluids with peridotite in the mantle

wedge(Westerlund, Shirey, Richardson, Carlson,

Harris, Gurney, CMP in press)

(Widom, Kepezhinskas, and Defant, Chemical Geology, 2003)

Archean (3.5 Ga) example Cenozoic example

Pb-Pb Array for MORB-OIB

Sialic rocks with juvenile isotopes

Enriched Os in wet komatiites

O isotopes in magmatic zircon

Low-T O isotopes in eclogites

C & N isotopes in diamond

+ Os isotopes in sulfide inclusions

MIF sulfur in sulfide inclusions

CRUSTALEVIDENCE

MANTLELITHOSPHEREEVIDENCE

OCEANIC SLABRECYCLING

OCEANIC ARCSWITH START OFCONTINENTAL ARCS

EARLY RELATIVELYCOOL MAGMATISM

SUBDUCTION IMPRINT EVIDENT

ISOTOPIC DATA INFERENCEDEPTH