Composition and Significance of Mariana Trough Basalts Julian Pearce (Cardiff)

Composition and Significance of Mariana

Trough Basalts

Julian Pearce (Cardiff)

Composition and Significance of Mariana

Trough Basalts

with contributions from:

Bob Stern (Dallas)Sherman Bloomer (Corvallis)

Patty Fryer (Honolulu)Jon Woodhead (Melbourne)

The Mariana Trough Setting

24

22

20

18

16

14

12

140 142 144 146 148

arc crus t

rifted c rust

ocean crust

NM T

M arian a Arc-Basin S ystem

CM T

S M T

ºN

ºE

Unzipping of island arc with maximum extension in the centre and a N-S trend of rifting to drifting

Mariana Trough: A Key Locality in BABB Studies

Hart et al. (1972): First analyses of back-arc basalts

Gill (1976): First documentation of the differences between back-arc basin basalts and MORB

Hawkins (1977, 1978): First systematic sampling program for back-arc basins

Tarney et al. (1977): First discovery of island arc basalt from a back-arc basin

Garcia et al. (1979): First analysis of water in a back-arc basin glass

Fryer et al. (1981): First treatment of back-arc basin lavas as a distinct magma type

Sinton and Fryer (1987): First systematic petrogenetic interpretation of BABB

Stolper and Newman (1994): First use of BABB glasses to estimate the composition of the subduction component

Mariana Trough: Type Locality of Back-arc Basin Basalts (BABB)

High• Volatile Elements notably

H (water)• Vesicularity

• Large Ion Lithophile Elements (Ba, Sr, La, Th

etc.)• Al2O3

Low• High Field Strength Elements (Nb, Zr, Y etc.)

• FeO

How Distinctive are Mariana BABB?

Regional MORB A

rray

1000

10

100

10

Ba/Yb

Mariana IA

B

Nb/Yb0.11

1

SZ

24

22

20

18

16

14

12140 142 144 146 148

arc crus t

rifted c rust

ocean c rust

NM T


CM T

S M T

ºN

ºE


Regional MORB A

rray

1000

10

100

10

Ba/Yb

Mariana IA

B

Mariana

BABB

Nb/Yb0.11

1

N

C

S

24

22

20

18

16

14

12140 142 144 146 148

arc crus t

rifted c rust

ocean crust

NM T


CM T

S M T

ºN

ºE


14% of Mariana Trough lavas are MORB

6% of Mariana Trough lavas are IAB

80% of Mariana Trough lavas are BABB

But it is not obvious whether BABB are simply transitional

between MORB and IAB or a truly distinct

magma typeRegional M

ORB Arra

y

1000

10

100

10

Ba/Yb

Mariana IA

B

Mariana

BABB

Nb/Yb0.11

1

N

C

S

What can the Mariana Trough Basalts tell us?

rear-arcseamount

intraplatevolcano

volcanicarc

asthenosphere

lithosphere

back-arcridge

mantle depletion by episodicmelt extraction towards arc front

A’

B

B ’

C

F

A

1. Mantle Input

2. Subduction Input

3. Mantle-Subduction interaction

4. Hydrous Ridge Crest Processes

(melting, reaction & crystallization)

2 1

3

4

Mantle Input

Mantle Fertility

Mantle Flow

Mantle Provenance

rear-arcseamount

intraplatevolcano

volcanicarc

asthenosphere

lithosphere

back-arcridge


A’

B

B ’

C

F

A

1

Geochemical Tracing of Mantle Flow

rear-arcseamount

intraplatevolcano

volcanicarc

asthenosphere

lithosphere

back-arcridge


A’

B

B ’

C

F

A

1

Loss of melt fractions during flow to the arc front causes decrease in ratios of highly to moderately incompatible elements – as shown by McCulloch, Gamble, Woodhead and others in the early 1990s.

Thus Nb/Yb is a good tracer for mantle flow (provided degrees of melting are high) –

Pearce (2005)

Nb/Yb decreases

Geochemical Tracing of Mantle Flow

So

ut

h

Sa

nd

wic

h

Tr

en

ch

So

ut h

S

an

dw

ich

A n ta rct ic P l ate

E a s t S c o ti a

S e a S p r e a d ing

C e n t r e

3 0 Wo2 5 W

o

6 0 S

o

5 8 S

o

5 6 S

o

Arc

24

N

E

22

20

18

16

14

12

Mariana

140 142 144 146 148

32N

34N

Japan

Izu arc

138E 142E

Izu: corner flow

Lau: one-way ‘sideways’

flow

Scotia: two-way

sideways flow

followed by corner flow

Mariana: multi-centre

upwelling followed by along-axis and corner

flow

Red – high Nb/YbBlue – low Nb/Yb

Pearce & Stern, 2006

Mariana Trough Mantle Flow

24

N

E

22

20

18

16

14

12

M ariana

140 142 144 146 148

<0.4

0.4-0.8

0.8-1.6

1.6-3.2

>3.2

N b/Yb

enriched

dep le ted

ave . N -M O R B

Consistent with mantle upwelling in several centres

within the basin and complex flow pattern

Pearce et al., 2005

Mariana Trough Mantle Flow

24

N

E

22

20

18

16

14

12

M ariana

140 142 144 146 148

Pozgay et al. (2007)

Mantle Provenance

Isotopes can be used to fingerprint the mantle domain that feeds the arc-basin systems. The Mariana system is dominated by

‘Indian’ mantle sources. But are these from a closing Indian Ocean, from contamination by lithosphere, or from a sub-Pacific plume of

‘Indian’ character?

I

I

I

PP

P

37383940

17 18 19 20

N. Fiji BasinMariana Trough

Mariana ArcC. Tonga Arc

N. Lau BasinC-E Lau Basin

N. Tonga Arc

Havre Trough

Indian

Pacif

ic

(a)

(b)

206 204Pb/ Pb

208

204

Pb/

Pb

Indian

Pacific

Hf

Nd05

20

1015

0 5 10 15Kermadec ArcValu Fa Ridge

Manus Basin

MORB

SSZ

Key Mantle Questions

What is the origin of the ‘Indian’ component of the Mariana Trough lavas?

How does the fertile mantle enter the Mariana Trough?

How has the mantle evolved with time since subduction started?

What can the Mariana Trough Basalts tell us?

rear-arcseamount

intraplatevolcano

volcanicarc

asthenosphere

lithosphere

back-arcridge


A’

B

B ’

C

F

A

1. Mantle Input

2. Subduction Input


4. Ridge Crest Processes


2 1

3

4

Subduction Input

Spatial Variations in the Subduction Component

Composition of the Subduction Components

Origin of the Subduction Components

rear-arcseamount

intraplatevolcano

volcanicarc

asthenosphere

lithosphere

back-arcridge


A’

B

B ’

C

F

A

2

Subduction Component MappingI: Yb Normalization

ZrBa U Ta La Pb Sr Nd Eu Gd Dy Ho Tm

Rb Th Nb K Ce Pr P SmHf

Ti Tb Y Er Yb

100

10

1

0.1

rock

/MO

RB

URA 5 (6.1% MgO)

(a)

URA 6 (2.3% MgO: andesite)URA 7 (2.7% MgO: plag.- )



Ti Tb Y Er Yb

(b)

100

10

1

0.1

rock

/MOR

B

URA 5 (6.1% MgO)URA 6 (2.3% MgO: andesite)URA 7 (2.7% MgO: plag.- )

Yb-normal ized

Normalizing to Yb, i.e. using ratios such as Nb/Yb, greatly

reduces the effect of fractional crystallization and crystal

cumulation

Subduction Component MappingII: Splitting Patterns into Components



Ti Tb Y Er Yb

(b)

100

10

1

0.1

rock

/MO

RB

URA 5 (6.1% MgO)URA 6 (2.3% MgO: andesite)URA 7 (2.7% MgO: plag.- )

Yb-normal ized



Ti Tb Y Er Yb

100

10

1

0.1

rock

/MO

RB

a

a: Nb/Yb (mantle fertility and % melting)

f

b

b: Ba/Nb (total subduction component)

c

c: Th/Nb (deep subduction component)d d: Ba/Th (shallow subduction component)

e: (U/Th) and f (Nb/Ta) have complexproperties as discussed in the text e

proxies:

mantle componentfocus of this paper

deep subduction component

shallow subduction component

(c)

The patterns can be broken up into

components and ratios used as proxies of

subduction processes

Mapping Total

Subduction Input

Each system has a different

pattern with clear relationships to subduction zone proximity and mantle flow

pattern

Pearce & Stern (2006)

Mapping Total Subduction Input

24

N

E

22

20

18

16

14

12Ba/Nb

140 142 144 146 148

8-16

<8

16-32

32-64

64-128

>128

The subduction zone input in the back-arc

basin is highest at the margins as the basin

converges with the arc.

Within the central part of the basin, there are

three regions where the mantle is unaffected by

subduction with subduction enrichments

between them

can also break this down into components

Pearce et al., 2005

The Lithospheric Subduction Component

100

10

1

0.112 16

s z2

sz1

s z3

20 24Nb/Ta

(a)

Th/Ta

MO RB-OIB array

arc

B asin(B ABB)Basin

(MORB)

B asin(rifting)

Seg. 2-3

NSP

VTZ(S)VTZ(N)CIP

SS P

Seg. 17-18

The Mariana region is complicated by the presence of a high Nb/Ta component that cannot easily be explained by subduction. They are globally characteristic of small degree melts, but here the degree of melting is high. We tentatively

explain them in terms of enrichment of the lithosphere

by small-degree melts. The lithosphere is reactivated by arc

rifting.

Where present, this component has to be numerically

subtracted before other components can be studied

Mapping the Lithosphere Subduction Component

24

22

20

18

16

14

12

Nb-Ta

lithosphere

140 142 144 146 148

<20%

20-30%

30-40%

>4 0%

ºE

ºN(b)

Nb/Yb

(b)

sz3

arc

Basin(spreading)

Basin(rifting)

MT(C)

NSP VTZ(S)VTZ(N)

CIPSSP

MT(S)

0.1Ta/Yb

0.5

5

0.01

1

man

tle +

sz 1-

2

arc

basin

50%

25%

Possible Origin of the Lithosphere Component

Pre -riftin g: small melt fr actio nsenr ich th e su b-arc litho sphe re

T h-Ba c omponentBa c omponen t

T h-Ba component

Th-B

a com

pone

ntBa

compone

nt

Ba componen

t

Th-

Ba

com

pone

ntB

a co

mp

onen

t

Nb- Th-Ba com pone nt

R ifting : rem obili zation of enric hed lithos pheregives s hosh onitic lava s

Diffuse s preading: a stho sphere me lts b ut

incorpo rates a small litho-sphe ric c ompo nent

Spreading : asthenospheric sou rce w ith a subd uctio n

component n ear the tren ch

Pre -riftin g: small melt fr actio nsenr ich th e su b-arc litho sphe re

T h-Ba c omponentBa c omponen t

T h-Ba component

Th-B

a com

pone

ntBa

compone

nt

Ba componen

t

Th-

Ba

com

pone

ntB

a co

mp

onen

t

Nb- Th-Ba com pone nt

R ifting : rem obili zation of enric hed lithos pheregives s hosh onitic lava s

Diffuse s preading: a stho sphere me lts b ut

incorpo rates a small litho-sphe ric c ompo nent

Spreading : asthenospheric sou rce w ith a subd uctio n

component n ear the tren ch

Mapping the Deep Subduction Component

24

22

20

18

16

14

12

deepsubduction

Th-Nb

140 142 144 146 148

<3 0%

30-60%

60-75%

>75%

ºE

ºN(c)

10

1

1

0.1

Th/Ybsz2

arc rift

man

tle

basin

Ta/Yb0.010.01

0.1

90%

75%

30%

Mapping the Shallow Subduction Component

24

22

20

18

16

14

12

shallowsubduction

Ba-Th

140 142 144 146 148

<30%

30-60%

60-75%

>75%

ºE

ºN (d)

sz1

man

tle +

sz 2

1000

10

100

10

Ba/Yb

Th/Yb0.01

110.1

arc

rift

basin

50%

25%

Summary of Geochemical Mapping

24

22

20

18

16

14

12

140 142 144 146 148

l ithosphe re

MORB mant le

highBa-rich

sz comp .

ma ntle flow

h =100k mS Z

200km

NS P

CIP

SS P

ºN

ºE

Subduction Component: HFSE Mobility?

Pearce et al (1999): no significant Hf mobility

+

+

+

+

m

m

s z

m 1

2

3

PacificMarginalBasins

Nd/Hf) =20sz

Nd/Hf) =sz

Nd/Hf) =40sz

Hf=0.56

Woodhead et al (2001): Hf and Nd both mobile

HFSE immobility an important assumption for computing subduction components: so important to resolve

Subduction Component: HFSE Mobility?

Pearce et al (1999): no significant Hf mobility

+

+

+

+

m

m

s z

m 1

2

3

PacificMarginalBasins

Nd/Hf) =20sz

Nd/Hf) =sz

Nd/Hf) =40sz

Hf=0.56

Woodhead et al (2001): Hf and Nd both mobile

Green circles = new data from Woodhead et al. (in prep.)

Subduction Component: HFSE mobility?

0-5

10

20

5

15

0 5 10 15

Hf

Nd/Hf

sediments

MO

RB

Sediment

to Nd/Hf = c. 30

Mariana Trough

Mariana Trough

Central Mariana Arc

BABBMORB

Woodhead et al. (in prep.):still debated

but Hf mobility likely limited.

Subduction Component: HFSE mobility?

0-5

10

20

5

15

0 5 10 15

Hf

Nd/Hf

sedimentsM

OR

B

Sediment

Les kov (rear-arc)

Nelson (sl ab edge)

to Nd/Hf = c. 30

East Scotia Sea

E. Scotia S ea

BABBMORB

0-5

10

20

5

15

0 5 10 15

Hf

Nd/Hf

sediments

MO

RB

Sediment

to Nd/Hf = c. 30

Mariana Trough

Mariana Trough

Central Mariana Arc

BABBMORB

The Scotia system (Barry et al., 2006) is similar, but this time rear-arc and arc edge volcanoes demonstrate clear Hf mobility by addition of an inferred melt component with low Nd/Hf. This is not evident in the Mariana system – except for the high Nb/Ta samples (not shown)

Subduction Component: The Stolper and Newman Conundrum

0-5

10

20

5

15

0 5 10 15

Hf

Nd/Hf

sediments

MO

RB

Sediment

to Nd/Hf = c. 30

Mariana Trough

Mariana Trough

Central Mariana Arc

BABBMORB

Stolper and Newman’s (1994) innovative use of water in Mariana Trough glasses to

predict the subduction

component, gives a component with

Nd/Hf of c. 6 and Nb of 40-50 ppm, as well as high Sc. This does

not match observation. The question is why?

Subduction Component: The Stolper and Newman Conundrum

10

10

1

0.1

HO

/Yb

2

Nb/Yb

0.0110.1

enrichmentMORB

array

depletion

IndianMORB

Mariana BABB

x

1 2

2

Mariana

Arc

U sing c orre lations with water to evaluate N b mo bility is m odel-d epen dent: mo del 1 indica tes m obil it y, 2 indic ates immobility

One explanation is that least-squares

methods break down when there

are >2 components, some with variable

composition.

Key Subduction Questions

Is the ‘lithosphere’ component really derived from the sub-arc lithsophere and, if so, when and how was it introduced?

Why does the Stolper and Newman study give a major HFSE component in the aqueous component: what do the data really show?

When and how were the subduction components added to the Mariana Trough mantle source?

Factors Controlling the Composition of Back-arc Basin Basalts

rear-arcseamount

intraplatevolcano

volcanicarc

asthenosphere

lithosphere

back-arcridge


A’

B

B ’

C

F

A

1. Mantle Input

2. Subduction Input




2 1

3

4

Mantle-Subduction Interaction

rear-arcseamount

intraplatevolcano

volcanicarc

asthenosphere

lithosphere

back-arcridge


A’

B

B ’

C

F

A

The back-arc subduction

component could be by:

• Incorporation of previously-enriched

lithosphere.

• Mixing of sub-arc and incoming

mantle

• Direct addition from the subduction

zone

• Via subduction-modified mantle

melts

2 1

3

Subduction Zone Input: TimingS

ou

t h

Sa

nd

wic

h

Tr

en

ch

30Wo 2 5 Wo

6 0 So

5 8 S

o

5 6 So

Arc

So

ut h

S

an

dw

ich

equilin

e100 ka

E8E4

0.4 0.8 1.2 1.6

1.6

1.2

0.8

0.4

2 38 23 2U/ Th

230 232Th/ Th

Fretzdorff et al., 2003: Subduction component must have been added to the back-arc within 350ka. Isochron may be real but may also represent mixing of fluid and melt components

E4

E8

fluid

(same result as Peate et al. (2001) for Valu Fa Ridge, Lau Basin

A similar study is needed on the Mariana Trough

Subduction Zone Input: Role of Mixing

Mariana

1000

10

100

10

Ba/Nb

Th/Nb0.01

110.1

deep

com

pone

nt

PhilippineSea

MORB

MarianaArc (CIP & SNSP)

ShallowComponent

MarianaArc (SSP)

If the back-arc gets its

subduction component by

mixing with sub-arc mantle (the Martinez-Taylor model), why is it

not simply a diluted version of

the arc?

It does, however, explain why

back-arc basins are not more

shallow

Factors Controlling the Composition of Back-arc Basin Basalts

rear-arcseamount

intraplatevolcano

volcanicarc

asthenosphere

lithosphere

back-arcridge


A’

B

B ’

C

F

A

1. Mantle Input

2. Subduction Input




2 1

3

4

Trace Element Data Indicate Shallow Melting, and F increasing with XH20

Mariana Trough(Gribble et al., 1998)

5

spinellherzolite

Yb ppm

TiO

wt.

%2

% m

elt in

g PU

M

% m

eltin

g P

UM

garnetlh erzolite

5

1010

1515

20

00

1

0.5

1.5

1 2 3 4

20

3030

40

4 0

DMMPUM

preconditioning & mixi ng

Mariana Trough glasses plot on a spinel lherzolite melting trend,

indicating shallow melting processes.

The degree of melting varies (as

Stolper and Newman first showed) as a

function of water content.

Exact calibration for F is difficult without

knowing source composition,

however

And an Increase in F from MORB through BABB to IAT

1

10

0.110.1 10

Nb/

Yb)

P

DMM

PUM

Mariana Trough Mariana ArcMORB

BABB

IAB

IAB

10%20%

30%

Yb) ppmP

melting

mixing

pre

cond

itio

ning

6km

50km

100km

25%Fm ax

fluxmelting

Moho

drysolidus

H O2

am

gt

decompressionmelting

MO

RB

BAB

B

IAB

Use of Nb/Yb (the flow tracer) as well as Yb deals with variations in

source compositionPearce & Stern (2006)

Summary

rear-arcseamount

intraplatevolcano

volcanicarc

asthenosphere

lithosphere

back-arcridge


A’

B

B ’

C

F

A

1. Mantle InputMantle rises diapirically at

points within the basin; ‘Indian’ provenance, but

cause not known

2. Subduction InputLithospheric signals during

arc rifting; deep subduction signals between ‘diapirs’; shallow subduction

signal at centre and south of arc; cool

subduction (no HFSE enrichment)

3. Mantle-subduction Interaction

No evidence yet for mixing of incoming mantle with

sub-arc mantle

4. Ridge Crest ProcessesDegree of melting increases

from MORB through BABB to IAT (5-30%)

2 1

3

4

Composition and Significance of Mariana Trough Basalts Julian Pearce (Cardiff)

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Transcript of Composition and Significance of Mariana Trough Basalts Julian Pearce (Cardiff)