composition and significance of mariana trough basalts julian pearce (cardiff)
TRANSCRIPT
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
M arian a Arc-Basin S ystem
CM T
S M T
ºN
ºE
How Distinctive are Mariana BABB?
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
M arian a Arc-Basin S ystem
CM T
S M T
ºN
ºE
How Distinctive are Mariana BABB?
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
mantle depletion by episodicmelt extraction towards arc front
A’
B
B ’
C
F
A
1
Geochemical Tracing of Mantle Flow
rear-arcseamount
intraplatevolcano
volcanicarc
asthenosphere
lithosphere
back-arcridge
mantle depletion by episodicmelt extraction towards arc front
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
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. Ridge Crest Processes
(melting, reaction & crystallization)
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
mantle depletion by episodicmelt extraction towards arc front
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.- )
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
(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
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
(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
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
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
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. Ridge Crest Processes
(melting, reaction & crystallization)
2 1
3
4
Mantle-Subduction Interaction
rear-arcseamount
intraplatevolcano
volcanicarc
asthenosphere
lithosphere
back-arcridge
mantle depletion by episodicmelt extraction towards arc front
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
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. Ridge Crest Processes
(melting, reaction & crystallization)
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
mantle depletion by episodicmelt extraction towards arc front
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