Subduction and the geochemical cycle

Tectonoph.vsics, 99

Convergence

271

( 1983) 21 l-211

Elsevier Science Publishers

B.V.. Amsterdam

- Printed

in The Netherlands

Tectonics and Sediment Subduction

SUBDUCTION

AND THE GEOCHEMICAL

CYCLE

W.S. FYFE Department of Geology. University of Western Ontario, London, N6A 587 (Canada) (Revised

September

7, 1981; received by Publisher

May 19, 1983)

ABSTRACT

Fyfe,

W.S..

1983. Subduction

and

the geochemical

cycle.

In: T.W.C.

Convergence

and Subduction.

Tectonophysics, 99: 21 l-217.

The present

balance

of crust

creation

to crust removal

processes

similar

been influenced surface

by surface

components

continents

hydrosphere,

are subducted,

and hydrosphere

at ocean

at subduction

ridges

atmosphere

and

subduction

S. Uyeda

(Editors).

zones appears

10% of the mass of the upper mantle

and crustal

then in a slowly cooling

can increase

and above

zones. Almost

Hilde

component

planet

contamination.

it is difficult

to be has

As long as

to see how the mass of

with time.

INTRODUCTION

One of the most fundamental problems of geological science is the understanding of the chemistry of the inaccessible portions of the earth. There are constraints imposed by the physical properties of the interior largely determined by seismic parameters but these are clearly ambiguous as shown by Anderson’s (1980) recent postulates that there is a great deal of eclogite in the lower mantle, a return to the older views of Goldschmidt (see Rankama and Sahama, 1950). The only definitive major constraint on bulk composition is that the material which rises near ocean ridges must be capable of producing a basaltic extract and a peridotite residue but the volume of mantle which is involved in the extraction process is still unknown (Turcotte, 1981) and depends on models of melt separation. Most model compositions of the petrologists involve a certain degree of circular argument or depend on analogies with a few tons of meteorite debris which may or may not be representative of our planet. Today we have a rather good idea of what materials come from the interior to form the new ocean floor crust with its ophiolite structure, the common model of such crust. We know the rate of crust production required by ocean-floor spreading and we know that this new crust is cooled partly by convective circulation of seawater deep into this crust. This process substantially modifies the primary 0040-1951/83/$03.00

0 1983 Elsevier Science Publishers

B.V.

272

mantle-derived chemistry but the exact chemistry of old ocean floor crust is not yet well quantified. We know that the ocean solute reservoir is fed by continental runoff. We know that ocean-floor crust created at ridges is almost quantitatively subducted; there is very little old ocean-floor crust in existence. We are beginning to think (Windley, 1979) that the modern type of convective structure of the planet commenced at the Archaean-Proterozoic boundary 2.5 Ga (billion years) ago, and has been going on, presumably with slowly diminishing intensity since that time. Given that new crust is formed at a rate of a little more than 10 km3 a- ‘, the total mass of crust that has circulated through the upper mantle is at a minimum of the order of 1O26g over 2.5 Ga. As the mass of the entire mantle is 4 * 102’ g this means that 2+% of the mass of the entire mantle has been through the crust cycle or about 10% of the upper mantle if convection is restricted to this region. The conclusion from such a simple consideration is that the mantle must contain a significant fraction of rock that has been modified and could hardly be expected to be a perfectly homogeneous body (Davies, 1981). Recent estimates of Karig and Kay (1981) indicate that andesite magma generation, the most obvious return flow process above subduction zones produces new crust at a rate of about 1.8. 10” g (0.5 km3) per year. These figures indicate that only a small percentage of subduction spilitic ophiolite returns; this remelting process may not be very efficient. But one must be cautious about such numbers until we really understand the root structure of continental regions like the Andes. An equally intriguing problem involves the possible subduction of sediments or direct tectonic erosion of continental edges in the subduction process. And related to this problem is the entire question of mechanisms of continental growth or decay. There is no doubt that in regions such as the Himalayas or Andes continental crust is thickened but this is a transient phenomena and is rapidly relaxed by erosion. Related to the process of ophiolite and sediment subduction, is the question of volatile recycling through the mantle and the resultant ambiguities in models of outgassing (or ingassing) of a planet which is vigorously mixing. ARE SEDIMENTS

SUBDUCTED?

I think that one of the most important aspects of the Texas conference was the evidence presented that sediment subduction is common if not ubiquitous (T. Wilde, 1983, this volume). It appears that accretionary sedimentary prisms of offscraped sediments are common only where the sediment load carried to the subduction site is very massive. Further, the mechanisms for locking initially light materials into the descending slab appear to be rdated to the roughness of the surface of the descending slab-a roughness first created by the ubiquitous block fat& patterns created at spreading centres and secondly by massive cr&tig of the bending slab. Thus, ideas concerning sediment subduction so elegantly expressed by Gifluly (197 1) and supported by workers such as Garrels and Lerman (1977) and Uyeda

(1978, p. 187) appear who proposed

the “indestructibility

tively low density”

inescapable. explain

criticism

the “irreversible

is clearly not valid. Thus the possibility

shown to be possible The mantle

becomes

by Molnar sediments

kyanite-bearing

(1978)

of its rela-

differentiation

of subduction

and Gray (1978) appears

even more muddy!

for these should produce

by Moorebath

sialic crust because

with this concept

the great lack of meta-pelagic

California

Typical

of continental

and associated

of the upper mantle” light material

to be substantiated.

of some now to be

These new observations

in regions

also

like the Franciscan

schists (Fyfe,

of

1980) and kyanite

blue schists are rare. THE GEOCHEMICAL

CYCLE

The major geochemical

cycle of the elements

involves:

(a) new crust production at ridges; (b) modification of new ocean floor crust by hydrosphere-atmosphere (c) (d) (e) (f) Of

subduction of modified ocean floor crust: return flow above subduction zones; erosion off the continents; return tectonic flow of sediments to the mantle. all these processes (a), (c) and (e) are reasonably

well known

exchange;

but there are

great gaps in our knowledge of (b), (d), and (f). Until these gaps are quantified any final arguments about detailed upper mantle chemistry and the past and future mass history

of continents

will be unsatisfactory.

NEW CRUST ALTERATION

It is now generally agreed that almost half of the energy focussed at ocean ridge systems is removed by deep convective circulation of sea water through the cracks and pores of the cooling crust. Anderson et al (1979) suggest that this process may continue in crust that is over 50 Ma old and that more than one third of the modern ocean floor crust is involved in such circulation at the present time. It is now clear that massive

exchange

processes

occur during

this process.

As recently

summarized

by Fyfe and Lonsdale (198 1) O,, CO,, S, H,O are volatiles added in significant quantities while Na, K, Mg, Rb, U are added and displace Ca and transition metals which may be removed Bischoff (1979) propose

and contribute to metal-enriched sediments. Bloch and that as much as half the K delivered to the oceans may

become fixed in spilites while Aumento (1979) has shown that uranium is substantially enriched. For species such as strontium, it has been clearly demonstrated that the 87Sr/86Sr ratio increases towards sea-water values as alteration proceeds (Spooner, 1976). Present sea-water strontium isotopes appear to show a balance between typical runoff and hydrothermal discharge values. But there are few elements for which the data are sufficient to allow realistic balance to be established.

214

it is unlikely “ typical”

RETURN

that this situation

ocean-floor

will improve

until there is sufficient

deep drilling

o!

crust.

FLOW

It is clear that for some species

there must be moderately

efficient

return

flow

processes. I shall here assume that most pelagic sediments are subducted. The present pelagic sedimentation rate is in the range 0.3- 1.O km3 a- ’ (Sibley and Vogel, 1976). First, consider water as an example. Water is fixed in hydrated upper altered layers of sea-floor basal& and in serpentized ultramafic

phases of the rocks, There is

clear evidence that such alteration proceeds to depths of several kilometers (Fyfe and Lonsdale, 1981) or even to 8- 10 km (Lewis and Snydsman, 1977). Assuming that average ocean-floor crust contains 5% H,O and the pelagic sediments 10% bound water in clays and chlorites, the present bound-water subduction rate is of the order of 1.5 . lOi g a-‘. At this rate the entire ocean mass (1.4 - 1O24 g) will be subducted in a billion years. Clearly there must be a return flow. First, as materials are compressed

and

metamorphosed

during

descent

and glaucophane

schist-eclogite

facies rocks are formed, there must be massive outflow of water. There is clear evidence for such flow in the metasomatic changes observed in some glaucophane schist terrains (Fyfe and Zardini, 1967) and there is even evidence for very high fluid pressures. Thus Essene and Fyfe (1967) have described glaucophane schist fluidbreccias with major gas cavities and even minerals such as omphacite, the characteristic eclogite pyroxene, growing into apparent free space. There is no doubt that explosive expulsion of water could be related to seismic activity (see Ruff and Kanamori, 1983, this volume). Water locked in amphiboles and in particular in phlogopite McBirney,

may not be released till very great 1975). Mixing of ultramafic materials

depths are attained (Fyfe and and potassic clays or feldspars

must generate phlogopite, e.g., KAISi,Os + 3Mg,SiO, + H,O + KMg,(AlSi,O,,)(OH), + 3MgSi0, (or talc). Fluids in such phases can persist to depths of 100 km or so (Wyllie, 1971). At the same time carbonates can be stable to even greater depths. Fluids

released from amphiboles

and micas carried

to great depth may either lead to

partial melting of the subducted material itself or extremely high pressure fluids which must be very rich in soluble components (K,O, Na,O, SiO,, etc.) may trigger melting in the overlying hotter mantle and cause strong enrichment in typical crustal elements. Uranium nicely illustrates the balance problem. Uranium moves to the crust from ocean ridge magmatism and subduction zone magmatism. It is quite difficult to decide on the uranium content of mantle magmas as they may be contaminated by crustal sources during ascent (Fyfe, 1979). Data from Aumento et al. (1976) might indicate a primary uranium content in ridge magmas of the order of 0.1-0.2 ppm.

275

This gives an annual and

Kay’s

subduction (0.2-0.4

figure

addition

of 1.8 * lOI

of 1S-3.0.

IO9 g of uranium

g a- ’ for the rate of andesite

zones and data from Carmichael ppm) in oceanic

per year. If we use Karig

regions then about

melt production

et al (1974) on U contents 1 . lo9 g of U returns

at

of andesites

above subduction

zones annually. Present uranium

water flux off the continents at about

is 3.6. lOI g a-’

the 0.1 ppb level. Thus

oceans

annually.

contain exactly

2-3 ppm U. Thus such sediments balancing the river input. Aumento

enriched sediment

Pelagic

in uranium, subduction

sediments

forming

about

and surface

waters carry

3.6. lo9 g is transported

at the rate of l-3.

lOI

to the

g per year

fix 2-6. lo9 g of U annually, almost (1979) has shown that spilites are also

even up to 4 ppm. If we use such figures and allow for pelagic we have:

U to surface = 2-4.

lo9 g a- ’

U subducted

lo9 g a-’

= 3-9.

All one can conclude is that the system is about that uranium is being purged from the mantle--if

balanced! There is little indication sediments are subducted!

CONCLUSION

At recent conferences I have listened to a number of workers indicate that convective motions may link ridges and subduction zones as shown in Fig. 1. Such models are attractive in explaining the almost steady state chemistry of the present earth. The mantle

is recharged

as fast as it is depleted

and models of heterogeneous

mantle are logical (Davies, 1981). Such models might also suggest that hot spot magmas simply represent small amounts of leakage off the deep conveyor belt and

Fig. 1. A possible taking

scenario

some sediments

for present-day

with it. Convective

trench-subduction.

Volcanic-plutonic

the deeper material

leads to hot-spot

convection.

New lithosphere

created

at ridges is subducted

flow goes from ridge to ridge at depths

mountain volcanism,

belts are built above

the subduction

and rain moves continental

materials

of 600-700

km via

zones, leakage to the oceans.

off

216

even features like high CO, magma types could fit with subducted materials. But a cooling planet cannot be in a perfect steady state. The present mass balance of crust and mantle is not stable in a cooling and mixing planet. Typical surface components such as K,O-SiO,-H,O-CO, would be more stable in phases such as phlogopite, pyroxene, carbonate in a cooler mantle and lithosphere: they are returned to the surface by energetic igneous processes. We need to pay attention to evidence for reduction in the volume of the hydrosphere and crust through time and keep very open minds on the old dogmas of continuous irreversible degassing and continental growth. Will we end up looking like Mars? REFERENCES

Anderson,

D.L., 1980. Early evolution

Anderson,

R.H., Hobart,

and sediments Aumento,

in the Indian

Ocean.

F., 1979. Distribution

Sot. London, Aumento,

1980 (3): 3-7.

1979. Geothermal

convection

through

oceanic

crust

Science, 204: 828-832.

and evolution

W.S. and Fratta,

of time and depth.

Bloch, S. and Bischoff, of potassium. Carmichael,

Episodes,

M.G.,

of uranium

in the oceanic

lithosphere.

Philos. Trans.

R.

Ser. A, 291: 423-431.

F., Mitchell,

a function

of the mantle.

M.A. and Langseth,

M., 1976. Interaction

I. Field evidence.

between

Can. Mineral.,

J.L., 1979. The effect of low-temperature

Geology,

sea water and oceanic

layer two as

14: 269-290. alteration

of basalt on the ocean budget

7: 193- 196.

I.S.E., Turner,

F.J. and Verhoogen,

J., 1974. Igneous

Petrology.

McGraw-Hill,

New York,

N.Y. Davies,

G.F.,

1981. Earth’s

neodymium

budget

and structure

and evolution

of the mantle.

Nature,

290:

208-213. Essene,

E.J. and

Petrol.,

Fyfe,

W.S.,

1967. Omphacite

in Californian

metamorphic

rocks.

Contrib.

Mineral.

15: l-23.

Fyfe, W.S., 1976. Hydrosphere

and continental

crust:

growing

or shrinking?

Geosci.

Fyfe, W.S., 1979. The geochemical

cycle of uranium.

Philos. Trans. R. Sot. London,

Fyfe, W.S.. 1980. Crust formation

and destruction.

In: D.E. Strangway

and Its Mineral

Deposits.

Fyfe, W.S. and McBimey,

(Editor),

Can., 3: 82-83. Ser. A, 291: 433-445.

The Continental

Crust

Geol. Assoc. Can., Spec. Pap., 20: 77-88.

A.R.,

1975. Subduction

and the structure

of andesitic

volcanic

belts. Am. J.

Sci., 275-A: 285-297. Fyfe, W.S. and Lonsdale,

P., 1981. Ocean floor hydrothermal

activity.

In: C. Emiliani

(Editor),

The Sea.

Wiley, New York, N.Y., 7: 589-638. Fyfe,

W.S. and Zardini,

California. Garrels,

R.M.

Alterations Gilluly,

and

Lerman,

A.,

by Man. Abakon

in the Franciscan

and magmatic

1983. Sediment

Uyeda (Editors),

subduction

Convergence

D.E. and Kay,

Philos. Trans.

R.W.,

1977. In: W. Stumm Verlagsgesellschaft,

J., 1971. Plate tectonics

Hilde, T.W.C., Karig,

R., 1967. Metaconglomerate

formation

near Pacheco

Pass,

Am. J. Sci., 265: 819-830.

evolution.

Lewis, B.R.T. and Snydsman,

around

Tectonophysics,

1981. Fate of sediments

R. Sot. London,

Global

Chemical

Cycles

and

their

Bull. Geol. Sot. Am., 82: 2387-2396.

versus accretion

and Subduction.

(Editor),

Berlin, pp. 23-32. the Pacific.

In: T.W.C.

Hilde and S.

99: 381-397.

on the descending

plate at convergent

margins.

A 301: 233-250.

W.E., 1977. Evidence

for a low velocity

layer at the base of the oceanic

crust. Science, 266: 340-344. Molnar,

P. and Gray,

Geology,

7: 58-63.

D., 1979. Subduction

of continental

lithosphere:

some constraints

and uncertainties.

211

Moorebath,

S., 1978. Age and isotope

Sot. London, Rankama,

evidence

K. and Sahama,

Ruff, L. and Kanamori, and S. Uyeda

Th.G.,

1950. Geochemistry.

H., 1983. Seismic coupling

(Editors),

Convergence

and the role of pelagic E.T.C.,

interaction. Turcotte.

D.L.,

sediments.

Science,

1976. The strontium

Earth Planet.

University

and uncoupling

and Subduction.

Sibley, D.F. and Vogel, T.A., 1976. Chemical Spooner,

for the evolution

of continental

crust.

Philos. Trans.

isotopic

of Chicago

Tectonophysics,

mass balance

Press, Chicago,

at subduction

Ill.

zones. In: T.W.C. Hilde

99: 99-l 17.

of the earth’s crust:

the calcium

dilemma(?)

192: 551-553. composition

of sea water

and sea water-oceanic

crust

Sci. Lett., 31: 167-174.

1981. Mechanisms

of magma

migration

S.. 1978. The New View of the Earth.

Freeman,

beneath

ocean

ridges. A.G.U.

Chapman

Conf.:

Airlie. Va.. (abstr.). Uyeda, Windley.

R.

Ser. A, 288: 401-413.

B.F., 1979. Tectonic

Wyllie, P.J., 1971. The Dynamic

evolution

of continents

San Francisco, in the Precambrian.

Earth. Wiley, New York, N.Y.

Calif. Episodes,

1979(4):

12-16.