用岩石地球化学方法探讨大洋岩浆活动和地幔动力学过程

Magmatism in Ocean Basins and Mantle Dynamics

– A Petrologic and Geochemical Approach

Yaoling Niu (牛耀龄）

1. Introduction – Basic Concept of Magma Generation and Evolution

2. Ocean Ridge Magmatism – New Perspectives from Global MORB Data

3. Ocean Island magmatism – New Perspectives from Global OIB Data

4. Plume-Ridge Interactions – A petrologic/geochemical perspective

6. Mesozoic/Cenozoic lithosphere thinning & volcanism in eastern China: A

special consequence of plate tectonics

5. MORB Geochemistry and Continental Crust Growth

2011.10 西安“全国岩石学与地球动力学年会会前讲座”

Niu, Y.L. & R. Hékinian, 2004. Ridge suction drives plume-ridge interactions

(Chapter 9), In Oceanic Hotspots, edited by R. Hékinian and P. Stoffers,

Springer-Verlag, New York, p. 285-307.

Niu, Y.L., M. Regelous, J.I. Wendt, R. Batiza & M.J. O’Hara, 2002.

Geochemistry of near-EPR seamounts: Importance of source vs. process and

the origin of enriched mantle component, Earth and Planetary Science

Letters, 199, 327-345.

Niu, Y.L., D. Bideau, R. Hékinian & R. Batiza, 2001. Mantle compositional

control on the extent of melting, crust production, gravity anomaly, ridge

morphology, and ridge segmentation: a case study at the Mid-Atlantic Ridge 33

- 35°N, Earth and Planetary Science Letters, 186, 383-399.

Niu, Y.L., K.D. Collerson, R. Batiza, I. Wendt & M. Regelous, 1999. The

origin of E-type MORB at ridges far from mantle plumes: The East Pacific Rise

at 11°20’N,Journal of Geophysical Research, 104, 7067-7087.

References:

What drives

plume-ridge interactions?

From Davidson, J., W. E. Reed & P. M. Davis, Exploring Earth, Prentice Hall, 1997]

Core

Plate tectonics is driven by the

cold thermal boundary layer atop

the mantle - cooling plates

Mantle plumes are derived from the

hot Earth’s interiors – perhaps at the

basal thermal boundary layer (CMB?)

After Davidson, J., W. E. Reed & P. M. Davis, Exploring Earth, Prentice Hall, 1997]

Core

Plate tectonics is driven by the

cold thermal boundary layer atop

the mantle - cooling plates

Mantle plumes are derived from the

hot Earth’s interiors – perhaps at the

basal thermal boundary layer (CMB?)

Ba Th U La Pr Pb Sm Hf Gd Tb Ho Er YbRb Nb Ta Ce Sr Nd Zr Eu Ti Dy Y Tm Lu

Prim

itiv

e M

an

tle N

orm

ali

zed OIB

MORB

1

10

100

• Incompatible element-depleted MORB

source must be shallow.

• Incompatible element-enriched plume

sources for OIB must come from hot

deep interiors.

Such distinctions

Plumes–hot–deep-enriched OIB

vs.

Ridges–cold–shallow–depleted MORB

“Plume-ridge interaction” - as a concept and a process has received

much attention of the solid Earth geochemists and geophysicists in the last ~ 30 years! [e.g., Sun et al., 1975; Schilling et al., 1983, 1994, 1995, 1996, 1999; Schilling, 1991; Feighner &

Richards, 1995; Ito & Lin, 1995a,b; Ito et al., 1996; Kincaid et al., 1995, 1996; Ribe, 1996; Sleep,

1996; Haase & Devey, 1996; Hékinian et al., 1996, 1997, 1999; Pan & Batiza, 1998; Niu et al., 1999;

Graham et al., 1999; Georgen et al., 2001; Haase, 2002] … .

From Jian Lin [Hitting the hotspots,

Oceanus, 41, 34-37, 1998]

“Normal ridges” become anomalous when spatially close to “hotspots” in both

topography (thermal?) and MORB composition.

Surface expression of

Mantle Plumes

Our view: Plume materials passively flow

to ridges due to Ridge Suction

Because of the passive nature of ocean ridges:

All existing models simply assume or accept that Plumes materials actively

flow to and affect ridges [e.g., Ito & Lin, Geology, 23, 657-660, 1995].

Ridge Suction

Ridge Suction comes from material needs at ocean ridges to

form ocean crust and much of the oceanic lithosphere!!!

0

10

20

30

40

50

0 2 4 6 8 10 12 14 16 18

% o

f fu

ll th

ick

nes

of

oce

anic

lith

osp

heri

c m

antl

e

~ 50% of the lithosphere thickness is

achieved in the first ~ 17.5 Myr (i.e., t1/2 =

[0.5*701/2]2). This, plus crust formation at

ridges, requires ridge-ward material supply.

tTPLATE

Lithospheric Age (Ma)

Distance from ridge axis (km)

Lit

hosp

her

ic m

antl

e th

icknes

s (k

m)

due

to c

onduct

ive c

ooling

R1/2 = 60 mm/yr

440 km3 per Myr per km ridge length

R1/2 = 10 mm/yr

73.33 km3 per Myr per km ridge length

Half spreading rate R1/2 (mm/yr)

0

2

4

6

8

10

0 10 20 30 40 50 60

0 10 20 30 40 50 60

Material needs for the 1st Myr of lithosphere

as a function of spreading rate Note the linearity: 440/73.33 = 60/10 = 6

b

adxxc

where x is the distance from ridge

axis a = 0 to b of interest (60 & 10

km respectively here), and

c = 11b-1/2 (km1/2).

Material needs for the 1st Myr of lithosphere

as a function of spreading rate

0 10 20 30 40 50 60 70 800

400

800

1200

1600

2000

Half spreading rate R1/2 (mm/yr)

= 14.667 R1/2Forming lithospheric mantle

by cooling

= 24.667 R1/2Lithospheric mantle + 5 km crust

To

tal m

ass f

lux (

km

3)

per

km

rid

ge le

ngth

in

the

first

1 M

yr

LVZ

Log (Viscosity) (Pa-s)

Asthenosphere

D" BoundaryLayer

Lithosphere

Dep

th

Mesosphere Continental

18 20 22 24

The exponential increase in mantle viscosity with depth

requires that lateral asthenospheric flow via the low-viscosity

LVZ be the primary material supply to ridges

Phipps Morgan et al. (J. Geophys. Res., 1995)

Lambeck & Johnston (in The Earth’s Mantle, 1998)

NOTE: The required material supply to ridges, i.e., RIDGE

SUCTION, flows in direction against the overlying plate motion.

(1) The spreading lithospheric plate is necessarily decoupled from the LVZ

atop the asthenosphere; (2) The degrees of decoupling increase with

increasing spreading rate.

The decoupling is implicit: (1) Phipps Morgan & Morgan, EPSL, 1999; (2) Niu et al., JGR, 1999

~ 2

00 k

m

Ridge Suction

La/Sm, Rb/Sr & 87Sr/86Sr

Sm/Nd and 143Nd/144Nd

Varying extents of melting or varying proportions of

of the two componnets in a two-component mantle

De

crea

sin

g t

he

en

rich

ed

co

mp

on

ent

in

the

me

lt a

s a r

esu

lt o

f d

ilu

tio

n

Niu & Batiza [EPSL, 1997]; Niu et al.

[JGR, 1996, 1999; EPSL, 2001, 2002]

1

10

0.3

4

40 Nb

Ta

y = -0.587 + 18.201xr = 0.998

0.1 1.00.03 0.4

Rb/Sr

87Rb/86Sr

0.7022

0.7024

0.7026

0.7028

0.7030

0.7032

0.00 0.02 0.04 0.06 0.08

87Sr/86Sr

Rb/Sr

MORB & Seamount lavas

Nature of mantle plume materials:

Two component mantle

Enriched component of variable size

(high La/Sm, Rb/Sr & 87Sr/86Sr; low Sm/Nd and143Nd/144Nd)

Depleted peridotite matrix

(low La/Sm, Rb/Sr & 87Sr/86Sr; high Sm/Nd and143Nd/144Nd)

Low Tsolidus

easily-melted

High Tsolidus

Refractory

E

D

E

D

OIB

SOURCE

MORB

SOURCE

E

D >

EXAMPLES:

Ridge-suction-induced

(1) ridge-ward flow

(2) decompression melting

of plume materials.

0°

10°S

20°S

30°S

40°S

180° 160°W 140°W 120°W 100°W

Example 1: Easter Hotline in the South Pacific

Easter

Microplate

Juan

Fernandez

Microplate

S. EPR

PAR

After Smith, W.H.F. & Sandwell, D.T. [Global seafloor topography from

satellite altimetry and ship depth soundings, Science, 277, 1956-1962, 1997].

East Rift Salas-y-Gomez

Easter Hot Line Towards EPR

20

19

18

1

3

2

4

2

4

6

8

10

114 112 110 108 106 104

206P

b/2

04P

b[L

a/S

m] N

MgO

(w

t. %

)

Longitude (°W)

Salas y Gomez

Islands

Easter

Is land

Easter Microplate

East Rift

Data Source:

Pan & Batiza [J. Geophys. Res., 103,

5287-5304, 1998]

Hekinian et al. [ J. Vol. Geotherm. Res.,

72, 259-289, 1996]

Kingsley et al. [J. Geophys. Res., 103,

24159-24177, 1998]

Easter Hot Line Towards EPR

Enriched dikes/veins

Flow of plume materials

Decompression melting

Depleted peridotitic matrix

Lithosphere

Distance to East Rift (km)

100 200 300 400 500 600 700 800 9000

Ridge suction

Lava compositional depletion (e.g., K/Ti, La/Sm, 206Pb/204Pb) due to

depletion of enriched dikes/veins in the two component mantle as a

result of progressive decompressiuon melting duirng ridge-ward

flow.

Salas y Gomez

IslandEaster Island

Easter Microplate

East Rift

The base of normal oceanic

lithosphere without hot plume influence

Ridgeward Flow

Decompression:Partial melting &

Seaamount volcanism

Ridge-suction-induced (1) ridge-ward flow & (2) decompression melting

of Plume Materials

Easter

Microplate

Juan

Fernandez

Microplate

S. EPR

PAR

Easter Hotline

0°

10°S

20°S

30°S

40°S

180° 160°W 140°W 120°W 100°W

After Smith, W.H.F. & Sandwell, D.T. [Global seafloor topography from

satellite altimetry and ship depth soundings, Science, 277, 1956-1962, 1997].

Example 2: Foundation Hotline in the South Pacific

Foundation Hot Line Towards PAR

Dada from: Hekinian et al. [J. Geophys. Res., 102, 12265-12286, 1997] Hekinian et al. [Mar. Geol., 160, 199-223,

1999] Devey et al. [Mar. Geol., 137, 191-200, 1997] Maia et al. [Geochem. Geophys. Geosyst., 2001]

Foundation Hot Line Towards PAR

Distance to Pacific-Antarctic Ridge Axis (km)

Lithosphere

800 700 600 500 400 300 200 100 0900100011001200

Pacific-Antarctic

Ridge

Lava compositional depletion (e.g., Sr, Ce/Yb, Dy/Yb, Zr/Y etc.) due to

depletion of enriched dikes/veins in the two component mantle as a

result of progressive decompressiuon melting duirng ridge-ward flow.

Hotspot seamount chain

Foundation hotspot

seamounts

Ridge suctionThe base of normal oceanic lithosphere

without hot plume influence

Foundation hotline

volcanic ridges

Ridgeward Flow

Decompression:

Partial melting &

Seaamount volcanism

EXAMPLES:

Ridge suction [of plume materials] increases with

increasing spreading rate!

Iceland

Reykjanes Ridge

South

Kolbeinsey

Ridge

Example 3: S. Kolbeinsey

vs. Reykjanes

Dada from: Sun et al. [Science, 190, 143-147, 1977] Schilling et al. [Am. J. Sci., 283, 510-586, 1983] Schilling et al. [J. Geophys. Res., 104,

10543-10569, 1999] Hanan et al. [Geochem. Geophys. Geosyst., 2000]

NOAA Geophysical Data Center

1

10

[La/Sm]N

0.7035

0.7030

87Sr/86Sr

16

12

[3He/4He]N

18

19

206Pb/204Pb

0

1

2

35355575961636567

Depth (km)

Lantitude (°N)

Iceland Reykjanes Ridge

South Kolbeinsey Ridge

Sp

ar

FZ

Gib

bs

FZSpreading raste

R1/2 < 10 mm/yr R1/2 > 10 mm/yr

[La/Sm]N

87Sr/86Sr

206Pb/204Pb

[3He/4He]N

Depth (km)

Iceland

C.G

. F

Z

Sp

ar

FZ

Reykjanes R.

S. K. R.

Latitudes (°N)

Mantle plumes/hotspots are abundant near the MAR (e.g., Iceland, Azores, Ascension,

Tristan, Gough, Shona and Bouvet), but rare along the entire EPR (notably, the Easter

hotspot at ~ 27°S on the Nazca plate).

From: Jian Lin [Hitting the hotspots, Oceanus, 41, 34-37, 1998]

Such apparent unequal hotspot distribution would allow a prediction of more enriched

MORB at the MAR than at the EPR.

This prediction is, however, wrong!

Example 4: Slow MAR vs. Fast EPR

0.1

1

0.01 0.1 1

Nd143Nd/144Nd

206Pb/204Pb

207Pb/204Pb

208Pb/204Pb

EP

RR

SD

%

MARRSD%

87Sr/86Sr

EPR/MAR = 0.992

R = 1.000

(~ 100 %)

R = 0.990

(EPR/MAR)RSD%

= 0.353

Sr

6Pb

7Pb

8Pb

1

10

30

100

3

1 10 30 1003

MAR-Mean (N = 169 to 265)

EP

R-M

ean

(N

= 1

51

to 2

01

)

Sr, N

d, a

nd

Pb

Isoto

pic

rato

ios

SrZr

Nb

Ba

U

Th

Pb

Rb

Ta

ErGd

Dy La

Nd

Ce

Y

Yb

HfPr

Ho

EuTb

Tm

Lu

Sm

EPR/MAR = 1.0229

R = 0.947

(> 99.9 %)

0.1

1

10

100

0.1 1 10 100

MAR-Mean (N = 21 to 256)

EP

R-M

ean

(N

= 4

3 t

o 2

24) In

co

mp

atib

le T

race E

lemen

ts

Data from the LDEO Global MORB Data Base

[Lehnert et al., 2000]

The mean composition of MORB mantle

beneath slow-MAR and fast-EPR is the

same.

Rarity of near-EPR hotspots results from fast spreading. The fast spreading creates

large ridge suction forces that do not allow the development of surface expressions of

mantle plumes as such, but draw plume materials to a broad zone of sub-ridge

upwelling, giving rise to random distribution of abundant enriched MORB and

elevated and smooth axial topography along the EPR [Niu et al., JGR, 1996, 1999;

EPSL, 2001, 2002].

Imaging the Deep Seismic Structure Beneath a Mid-Ocean Ridge: The MELT Experiment

[The MELT Seismic Team, Science, 280, 1215-1238, 1998]

Ridge-suction induced melting anomalies would become “plumes/hotspots”

if suction is weak at slow-spreading ridges

Conclusion: 1. The subridge asthenosphere represents regions of lowest pressures in the entire

asthenosphere - which acts actively as a driving force that sucks (i.e., ridge suction)

asthenospheric materials to flow towards ridges.

2. This very driving force determines the so-called “plume-ridge interactions”.

3. The asthenospheric flow is necessarily decoupled from the motion of the overlying

lithospheric plates.

4. The degree of this decoupling increases with increasing spreading rate because of

the greater material demand to form the crust beneath ridges and the lithospheric

mantle in the vicinity of the ridges.

Ridge Suction Drives Plume-Ridge Interactions

By Yaoling Niu & Roger Hékinian ~

20

0 k

m

今天就到这儿，下次见！

谢谢！