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Geological Society, London, Special Publications
The onset of interaction between the hydrosphere and
oceanic crust, and the origin of the first continental
lithosphere
Maasrten J. De Wit and Andrew Hynes
Geological Society, London, Special Publications 1995, v.95;
p1-9.
doi: 10.1144/GSL.SP.1995.095.01.01
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The onset of interaction between the hydrosphere and oceanic crust,
and the origin of the first continental lithosphere
MAARTEN
J. D E W I T 1 & A N D R E W
HYNES 2
1Department of Geological Sciences, University of Cape Town,
Rondebosch 7700, South Africa
2Department of Earth and Planetary Sciences, McGill University,
3450, University Street, Montreal, Quebec H3A 2A7, Canada
Abstract: New continental crust forms above subduction zones through the recycling of
hydrated oceanic lithosphere. The most efficient process known for oceanic lithosphere
hydration takes place at the submerged mid-ocean ridges where the lithosphere is young and
warm, and cools through hydrothermal convection. Such mid-ocean ridge hydrothermal interactions were operative at least as far back as 3.5-3.8 Ga. The apparent absence of preserved
continental crust older than 4.0 Ga may reflect the absence of hydrothermal interaction before
that time. This model requires that prior to about 4.0 Ga mid-ocean ridges stood above sea level.
Our calculations show, however, that on a plate-tectonic early Earth with substantially less
continent, realistic higher heat flow, and a volume of sea water similar to that of today's ocean,
Archaean mid-ocean ridges would have remained below sea level. Only with a substantial
reduction of surface water would Earth have been able to recycle dry oceanic lithosphere, and
thus prevent the present day style of continental crust formation.
A 30% reduction of surface water is required to elevate early Earth's ridges above sea level.
This excess water may have been stored in nominally anhydrous minerals of the mantle. Early
Earth's mantle may have released a significant proportion of its initial water only gradually
through convective overturn of the oceanic floor. Given realistic ocean-floor creation rates,
it would have taken roughly 500 Ma to process the early Earth's mantle through a MORB
generation event if only the upper mantle was involved and considerably longer if whole mantle
convection was involved. The inefficiency of water extraction during this process is illustrated by
the amount of water apparently present in the source regions for present-day MORB. In this
scenario, the Hadean-Archaean transition may mark the time when Earth changed its style
of cooling from one dominated by heat exchange directly to the atmosphere to one dominated
by heat exchange with the hydrosphere, which still buffers Earth's heat loss today.
The oldest continental rocks discovered to date are
the Acasta gneisses from the Slave Province in
Canada (Bowring et al. 1989, 1990). Precise U/Pb
zircon dating has shown that these rocks are about
4.0 Ga old. Small areas of such old rocks (> 3.8 Ga)
have been found in only a few other places around
the world: over very limited areas of Antarctica,
Australia, China and Greenland (Bennett et al.
1993). One of the major conclusions of the work on
the Acasta gneisses is that there is nothing unusual
about these rocks. In fact they chemically so
resemble rocks that are forming above subduction
zones today, that Bowring and his colleagues feel
that plate tectonics might well have operated as
long ago as 4.0 Ga.
Continental rocks between 3.8 and 3.5 Ga old
have now been found on all continents, and two
sizable continental fragments, the Pilbara and
Kaapvaal cratons, have preserved continental areas
of > 0.5 • 106 km 2 which had stabilised by 3.0 Ga.
There is circumstantial evidence that these two
fragments were part of a much greater volume of
continental crust that must have appeared between
4.0 and 3.0 Ga (de Wit & Hart 1993; Myers this
volume).
A search in Archaean sediments has identified
single minerals older than 4.0 Ga in one place
(Frouche et al. 1983). A small percentage of zircon
grains from a 3.0 Ga sandstone in Western
Australia have yielded ages of c. 4.2 Ga. The
chemical compositions of these zircons are similar
to those found in continental rocks of all ages. It is
not clear yet if these tiny zircons indicate that there
were substantial continents in existence by 4.2 Ga.
If so, they have not yet been discovered or they
have been destroyed, perhaps by meteorite impacts
or by subduction. Hf isotope data from zircons in
Archaean sediments, however, provide no evidence
for an abundance of continental crust prior to
4.0 Ga (Stevenson & Patchett 1990; McCulloch &
From COWARD,M. E & PIES, A. C. (eds), 1995, Early PrecambrianProcesses,
Geological Society Special Publication No. 95, pp. 1-9.
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2
M. J. DE WIT • A. HYNES
Bennett 1993; Bennett et al. 1993). Moreover, the
general lack of inherited zircons does not provide
support for the presence of large volumes of very
ancient continental crust (> 4.0 Ga) preserved in
the old cratons such as in Greenland (Nutman et al.
1993).
In contrast to the continents, rocks of the present
ocean basins are always young (< 0.2 Ga). This is
because oceanic basalts are continuously recycled
into the asthenosphere at subduction zones. In
exceptional cases, old pieces of oceanic crust
(ophiolites) are preserved because they are
emplaced on the continents during mountain
building. These fragments offer the only clues to
ancient oceanic environments.
Continental rocks are 'second-hand' rocks (cf.
Taylor 1989) formed at subduction zones during
the recycling of the oceanic lithosphere. The
precise genesis of continental rocks above subduction zones is still a matter of debate, but almost
certainly devolatilization and dehydration of the
descending oceanic lithosphere plays a first-order
role in the production of these continental rocks
(Grove 8,: Kinzler 1986, Ellam & Hawkesworth
1988, Baker et al. 1994). Without the involvement
of fluids from the descending slab it is unlikely that
efficient formation of granitoid rocks, which predominate on the continents, would occur (Campbell
& Taylor 1983). Thus, a significant water content
in the oceanic lithosphere is a prerequisite for the
formation of continental rocks in today's plate
tectonic scheme. Where does the oceanic lithosphere obtain this water.*
Mid-oceanic ridge hydrothermai processes
The most efficient process known through which
oceanic material becomes hydrated takes place at
mid-ocean ridges during the formation of oceanic
lithosphere. Submarine volcanism at mid-ocean
ridges is the most important source of molten rock
of Earth's crust (Sclater et al. 1981). The heat from
igneous rocks, cooling within the oceanic crust,
drives the convection of cold seawater which
hydrothermally cools and chemically alters the
crust (Hart 1973; Spooner & Fyfe 1973; Lister
1977, de Wit & Stem 1976; Basaltic Volcanism
Study Project 1981; Seyfried 1987; Edmond
1992). Some 25% of Earth's total heat budget
is believed to be lost to the hydrosphere through
this hydrothermal heat exchange. Oxygen and
strontium isotopes used as tracers of hydrothermal activity indicate that ophiolite sections
up to 7 km thick have similarly interacted with seawater during their formation at spreading centres
(Gregory & Taylor 1981; Alt et al. 1986; Schiffman
& Smith 1988; Alexander et al. 1993).
Recently, similar processes have been recognized to have affected ophiolite-like sequences in
the oldest preserved greenstone belts. The most
detailed studies of this type have been carried out
on c. 3.5 Ga mafic-ultramafic rock sequences of
the Barberton greenstone belt in South Africa. The
comagmatic mafic-ultramafic rocks of the southern
Barberton terrain (referred to as the Jamestown
ophiolite complex) are interpreted to represent a
remnant of c. 3.5 Ga ocean-like lithosphere (de Wit
et al, 1987) which was obducted approximately
45 Ma after its formation (de Wit et al. 1992).
Evidence for submarine hydrothermal metamorphism is extensive (de Wit & Hart 1993;
de Ronde et al. 1994). The igneous and hydration
ages (3.48-3.49 Ga) of these mafic-ultramafic
rocks are synchronous; this provides the most
compelling evidence for mid-ocean ridge-like
hydrothermal interaction (de Wit & Hart 1993).
The extent of hydration and the chemical reactions
of meta-igneous rocks in the Jamestown ophiolite
complex are similar to those of hydrothermally
altered igneous rocks from the present-day oceaniccrust. Water contents of up to 16% have been
documented in the ultramafic rocks (de Wit & Hart
1993).
Work to date indicates that mid-ocean ridge-like
hydrothermal interactions were operative at least
as far back as 3.5 Ga. More circumstantial evidence
(viz. the presence of pillow lavas and associated
BIF) from the oldest mafic-ultramafic rocks, such
as those in the deformed 3.8 Ga Isua supracrustal
sequence in Greenland, suggests that major interactions between the hydrosphere and ocean floor
were also active as far back as 3.8 Ga (Maruyama
et al. 1992; Nutman & Collerson 1991; Maruyama
pers. comm., 1992). The circumstances for generation of continental crust were therefore present
at least as early as 3.8 Ga, in conformity to the
presence of continental crust that old. The apparent
absence of preserved continental crust, older than
4.0 Ga, has led to the suggestion that the formation
of the first second-hand continental crust may have
been coincident with the onset of hydrothermal
alteration of oceanic crust (de Wit et al. 1992;
de Wit & Hart 1993). In this model, the first
production of substantial continental crust is
directly related to the onset of mid-ocean ridge-like
hydrothermal processes. The earliest hydrothermal
interaction must, therefore, have started by 4.0 Ga
and perhaps as early as 4.2 Ga.
If this model is correct, then prior to c. 4.0 Ga the
average mid-ocean ridge must have stood above
sea level. Under these circumstances the degree of
hydrothermal alteration of the newly formed crust
is highly dependent on the amount of precipitation
(i.e. rain etc.) the region experiences. There is, for
example more hydrothermal alteration in subaerial
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THE ORIGIN OF THE FIRST CONTINENTS
spreading centres on Iceland than in the Afar (both
of which are dominated by meteoric water (SIO
1977; Sveinbjornsdottir et al. 1986; Lonker et al.
1993), but even on Iceland alteration is less
pervasive by a factor of 3-10 and probably reaches
shallower depths than the alteration in ophiolite
complexes and ocean-floor rocks (Stefannson
1983). Furthermore, since most of the cooling of
oceanic crust occurs at very young ages, even if
such cooling is solely by conduction, the probability of substantial hydrothermal circulation after
submergence declines markedly with increase in
submergence age, and is probably neglible by an
age of 50 Ma. With emergent spreading ridges,
therefore, Earth's oceanic lithosphere would have
been recycled with much less contained water than
it has today, and there would have been no significant formation of continental crust. Such a model is
compatible with the geochemical observations that
the earliest preserved mafic-ultramafic rocks on
Earth were derived from mantle sources which
already had a long history of relative chemical
depletion and extraction of possibly earlier, but
recycled, oceanic crust (Hamilton et al. 1983;
DePaolo 1988; Hoffman 1988; Chase &
Patchett 1988; Carlson & Silver 1988; Bowring et
al. 1990; Galer & Goldstein 1991; McCulloch &
Bennett 1993). Assuming a closed-system for
argon and helium in the mantle, crust and
atmosphere, the model presented here is also
compatable with the high 4~
3He/4He
ratios of the present day mid-ocean ridge
mantle source, and with exhaustive Early Archaean
depletion of 36Ar relative to 4~ and 3He relative to
U-Th, from this mantle source (Schwartzman
1973, Hart et al. 1979, 1985); such depletion would
have accompanied the onset of mid-ocean
ridge hydrothermal activity and continental crust
formation.
Mid-ocean ridges of present Earth stand 22.5 km below present sea level. Only in a few
places do these spreading ridges become shallow
and subaerial (viz. Iceland and the Afar). Shallowwater spreading activity has been documented
during the formation of the Jamestown ophiolite
complex in Barberton (de Wit & Hart 1993;
de Ronde et al. 1994) but the setting of this
ophiolite might have corresponded to an unusual
area like the Afar. It is pertinent to ask, therefore,
what the average elevation of spreading ridges
might have been relative to sea level on an
Archaean Earth with little or no continental
material.
Below, we calculate the depths of mid-oceanic
ridges at 4.0 Ga, given a plate-tectonic Earth with
substantially less continent, a realistically higher
heat flow and a volume of seawater similar to that
of today's oceans.
3
Water depths of Archaean mid-oceanic
ridges
On present Earth, there is a direct relationship
between the elevation of oceanic lithosphere and
its age (Parsons & Sclater 1977) which is thought
to be a direct consequence of conductive cooling
following formation of the oceanic crust at ridges.
Following Parsons & Sclater (1977), an approximate expression for this relationship is:
e,,)
2(P0
Pw----------~
-
(~0 -" ~ )
(1)
where e(t) is the elevation of ocean floor of age
t, 190 is the density of the lithosphere, o~ is the
coefficient of thermal expansion for the ocean
floor, Pw is the density of water and ~: is the thermal
diffusivity. T 1 is the temperature at the base of the
plate using the 'plate' model of McKenzie (1967)
for the thermal structure of the oceans. In the plate
model, the plate is considered unable to convect
throughout its depth, and has a constant temperature at its base. Since the thermal boundary layer
Of Earth is probably able to convect at depths
considerably lower than those corresponding to the
plate base in the plate model (e.g. Parsons &
McKenzie 1978), the basal temperature used in the
model is not a true indicator of the temperature
at some specific depth; it is simply a convenient
device with which to parameterize oceanic thermal
evolution. It is, however, very similar in magnitude
to the temperature of the mantle beneath Earth's
thermal boundary layer (e.g. Parsons & Slater
1978, fig. 7) and may be treated as such for the
purposes of this paper.
There is, furthermore, a simple relationship
between the area of ocean floor younger than a
specified age and that specified age (Parsons 1982):
A ( t ) = C 0t
1-
(2)
where A(t) is the area of ocean floor of age less than
or equal to age t, C Ois the rate of generation of new
oceanic crust, and tm is the age of the oldest
oceanic crust preserved in situ. Parsons (1982)
showed that a relationship of this form would be a
direct consequence of the consumption of oceanic
crust (by subduction) at uniform rates regardless of
its age, and that this uniform consumption rate is
approximately true on present Earth.
From equation 2:
2 A(tm)
tm = ~
Co
(3)
where tm is the age of the oldest extant oceanic
lithosphere and A(tm) is the total area of ocean floor
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4
M.J. DE WIT & A. HYNES
on the planet. The cumulative conductive heat loss
from ocean floor of age greater than or equal to
t, Q(t), is approximately given by (Parsons 1982):
1 ~_t)l~1
-
-3\tm]
J
(4)
where k is the thermal conductivity. Equations 3
and 4 with t = tm may be solved for tm and C o if
Q(tm), the total heat flow from the oceans, T 1 and
A(tm), the total area of ocean floor, are known.
After this, Equations 1 and 2 may be used to
calculate the total volume of the oceans beneath
the ridge crests and, if the total volume of oceanic
water is known, the height of the ridge crest with
respect to sea level.
These relationships depend only on the
assumptions of a "plate' cooling model for the
ocean floor and a subduction rate that is independent of the age of the ocean floor being subducted.
They are not affected by the thickness of the
oceanic crust, which may have been thicker or
thinner in the Archaean than at present (cf. Sleep
& Windley 1982, or Nisbet & Fowler 1983,
respectively). This is because any continental crust
present would have adjusted itself to stable thicknesses giving rise to minimal freeboard as it does
today, through a combination of orogenic and
erosional processes. It is unlikely that any
significant proportion of the continental crust
would have been deeply submerged for long time
intervals.
These relationships have been used to calculate
the elevation of the mid-ocean ridges for a range of
assumed values of total heat loss from the oceans,
plate basal temperature and total oceanic area.
Other parameters in Equations 1 to 4 were assigned
the values given in Table 1.
On modem Earth the temperature at the base of
the plates is about 1350~ (Parsons & Sclater
1977). Spreading-ridge heights are calculated
Table 1. Values of constants used
Constant
Po
ot
Pw
k
a
Value
3330 kg m-3
3.28 • 10-5 ~
7 • 10-7 rn2 s-1
1000 kg m-3
3.14 W m-1 ~
1.25 • 105 m
Values of constants used in equations 1 to 4,
after Parsons & Sclater (1977) & Parsons
(1982).
relative to sea level, assuming this value for the
basal temperature T 1, but allowing total heat flow
Q(tm) to increase incrementally to up to four times
its present value, and with oceanic surfaces (A(tm))
ranging up to 100% of the total planetary surface
(Fig. la). It is apparent that ocean ridges would
have stood below sea level for most combinations
of heat flow and oceanic area. Only those for which
the total heat flow out of the oceans is less than
roughly one and a half times present give ridges
that emerge, and these do so only for oceanic areas
considerably larger than at present, so that the total
heat loss from Earth would be less than that
indicated by past/present total heat-flow ratio.
These models are clearly unrealistic. Total Earth's
heat flow in the Archaean must have been at least
2-3 times that of present heat loss (Burke et al.
1976; Basalt Volcanism Study Project 1981).
If the total heat flow through the oceans was
twice that today, ridges could have emerged
provided there was very little continental crust, and
the basal temperature was considerably higher than
at present. For example, with a basal temperature
of 1550~
ridges would have been emergent
if oceanic crust covered 95% of Earth's surface
(Fig. lb). For this value of total heat flow, however, the ridges would never have been emergent
with basal temperatures of only 1450~ (Fig. lb).
Twice the present total heat flow through the
oceans is still an improbably low figure for the
Archaean if total oceanic area was larger than at
present, and it can be seen from Fig. la that
raising the heat flow further would only result in
submergence of the ridges, even for basal temperatures as high as 1550~ This modelling indicates,
therefore, that mid-ocean ridges are unlikely to
have been emergent in the Archaean if there was as
much oceanic water on the surface as there is today.
In Fig. 2 and Table 2 we illustrate, however, using
what we believe to be a realistic thermal model for
the Archaean planet, a total oceanic heat-flow three
times the present one and basal temperatures of
1550~ that a reduction of the volume of the
oceans by 30% would have caused the ridges to
remain above sea level.
In conclusion, it appears from our calculations
that with the present volume of seawater, the midocean-ridges would have remained below sea level
throughout the Archaean. Although increase in
basal temperature tends to favour the emergence of
ridges, its effect is far outweighed by that of the
accompanying increase in total heat loss from the
oceans. With constant oceanic volume, interaction
between oceanic lithosphere and the hydrosphere
would thus have been operative before the time
of the first preserved continental fragments. A
relationship between the onset of hydrothermal
processes and continental crust formation therefore
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THE ORIGIN OF THE FIRST CONTINENTS
.~
(b)
Ca)
i
,,,,
om
, , i , ~ , , i , , , , i , , , , i~,
4
i Var'a
'+
5
, ,
Heat flow 2 times present
4
Variable basal T
-!
0
g
-4
I-,
50
60
70
80
90
100
oceanic surface (% total)
50
,
.
,
,
I
60
. . . .
I
70
,
~ L ~ I
80
~ ~ =,,
I
,
,
,
90
,
100
oceanic surface (% total)
Fig. 1. (a) Height of the mid-ocean ridge above sea level as a function of the percentage of the Earth's surface
covered by oceanic crust, assuming a basal temperature (approximately equivalent to the mantle temperature below
the thermal boundary-layer; see text) of 1350~ The curves are for different assumed values for the total heat flow
out of the ocean floor, expressed as multiples of the present total heat flow out of the oceans (1, 1.5, 2, 2.5, 3, 3.5 and
4 times; every second curve is labelled), which is taken to be 29.0 x 1012 W, after Parsons (1982). The total volume
of oceanic water used to calculate the height of the ridges is the present volume of the oceans (1.349 x 1018 m 3 ;
Emiliani 1987).(b) as for (a), but assuming a total heat flow out of the oceans of twice the present value. The curves
are for different assumed values of the basal temperature (1350, 1400, 1450, 1500, 1550, 1600 and 1650~ the
lowest- and highest-temperature curves are labelled).
seems unlikely unless the volume of the seawater in
the oceans was substantially smaller than today.
Below, we explore the likelihood of such a reduced
volume of ocean water on Archaean Earth. We do
not, however, consider the potential addition of
exogenous water via cometary impact about that
time (cf. Cogley & Henderson-Sellers 1984; Chyba
& Sagan 1992 ).
Water content of the mantle
Recent work has indicated that the Earth's mantle
may contain more water than has previously been
suspected (Smyth 1987; Smyth et al. 1991; Bell &
Rossman 1992a, b; Thompson 1992). Nominally
anhydrous minerals believed to be present in the
mantle are potential reservoirs for substantial
amounts of water (see Bell & Rossman 1992a, and
Thompson 1992 for comprehensive lists of these
minerals).
Measured concentrations of water in MORB
from the Juan de Fuca ridge and the Pacific-Nazca
ridge have led to estimates of water concentrations
in their mantle source regions of 250-300 ppm and
100-180ppm, respectively (Dixon et al. 1988;
Michael 1988) and incompatible-enriched basalts
from Pacific-Nazca region require H20 contents in
the source region of 250-450 ppm (Michael 1988).
Even higher values have been reported by Jambon
& Zimmerman (1990). The infrared spectroscopic
data of Bell & Rossman (1992a) indicate that
olivine, orthopyroxene, clinopyroxene and garnet
could accommodate most or all of this water.
The potential importance of the mantle as a
reservoir for water is illustrated by the fact that
100 ppm of H20 distributed uniformly through the
upper mantle (shallower than 670 km) would be
equivalent to 7.5% of the present oceanic volume
(for an oceanic volume of 1 . 3 4 9 • 1018m3;
Emiliani 1987) and this figure rises to 26.5% if the
entire mantle is considered to have H20 concentrations of this magnitude. If undepleted or
enriched mantle is considered these figures become
correspondingly higher again.
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6
M. J. DE WIT & A. HYNES
.
.
i , , ,, i i , , ,it,
, , i , ,., ,1
Heat flow 3 times present
Basal T 1 550~
Variable w a t e r v o l u m e
.
.
0.5
With the m a x i m u m measured OH contents in
nominally anhydrous minerals, an a m o u n t of water
equivalent to c. 85% of the current ocean mass can
be accommodated in the whole mantle. Alternatively, if the oceans were derived from the mantle
(which is not established) and originated from that
portion modelled to have degassed to yield Earth's
atmosphere (c. 46%), then the m a x i m u m observed
OH concentrations suggest that up to 40% of
present oceans could originally have been stored in
this form (Bell & R o s s m a n 1992a).
It is thus possible that during Earth's early
history a greater volume of water was stored in the
mantle. Given such a scenario, we investigate first
if Earth could have retained a realistic volume of
water in the mantle for a long enough period to
have allowed any m i d - o c e a n ridge to have
remained above sea levels for a realistic period of
time; and secondly, h o w long it would have taken
to dehydrate the entire mantle.
the
fo
g
-4- , , , I , , , , I , , , , 1 , , , , I , , , ,
50
60
70
i
80
oceanic surface
90
100
(% total)
Fig. 2. As for Fig. la, but assuminga total heat flow
out of the oceans of three times the present value and
a basal temperature of 1550~ The curves are for
different assumed total volumes of oceanic water,
expressed as multiples of the present oceanic volume
(0.5, 0.6, 0.7, 0.8, 0.9 and 1.0 times present volume;
the lowest- and highest-volume curves are labelled).
Mantledehydration
This problem is approached using the results of our
earlier calculations, since these calculations yield
a creation rate for the ocean floor, expressed as
area per unit time (e.g. Table 2). Using these
creation rates, together with estimates of the crustal
thickness created and the a m o u n t of m a n t l e
involved in (depleted by) the creation of the crust,
it is then possible to determine h o w long it would
Table 2. Ridge heights for Archaean Earth model and variable ocean volume
Area of
ocean
floor
(% total
planet area)
60
70
80
90
100
60
70
80
90
100
60
70
80
90
100
Oceanbasin
volume
(multiple
of
present)
Ocean
floor
creation
rate
(kin2 a-1)
Oldest
extant
ocean
floor
(Ma)
Ocean
volume
above
ridge
crests
(109 km 3)
Ocean
volume
below
ridge
crests
(109 km 3)
Ridge
crest
height
(~,n)
1.0
1.0
1.0
1.0
1.0
0.7
0.7
0.7
0.7
0.7
0.5
0.5
0.5
0.5
0.5
23.35
20.02
17.51
15.57
14.01
23.35
20.02
17.51
15.57
14.01
23.35
20.02
17.51
15.57
14.01
26.21
35.67
46.59
58.97
72.80
26.21
35.67
46.59
58.97
72.80
26.21
35.67
46.59
58.97
72.80
0.346
0.471
0.615
0.779
0.962
0.346
0.471
0.615
0.779
0.962
0.346
0.471
0.615
0.779
0.962
1.003
0.878
0.734
0.570
0.387
0.598
0.473
0.329
0.165
0
0.328
0.203
0.059
0
0
-3.277
-2.459
-1.798
-1.242
-0.760
-1.955
-1.325
-0.806
-0.360
0.699
-1.073
-0.570
-0.145
1.193
1.849
Age of
ocean
floor at
sea level
(Ma)
2.844
8.292
19.906
Results of representative calculations of mid-ocean-ridge height, for a basal temperature of 1550~ and a total oceanic
heat flow of three times present, for variously reduced volumes of oceanic water. These results are some of those
plotted on Fig. 2.
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THE ORIGIN OF THE FIRST CONTINENTS
take to deplete the mantle through the creation of
the ocean floor. Using an ocean-floor creation-rate
of 14.01 x 106 m 2 a-1 (the last entry from Column
3, Table 2) as an example, for a crustal thickness of
15 kin, extracted from a mantle column initially
three times as thick (33% melting by volume),
6.30 • 1011 m 3 of mantle is depleted each year.
At this rate, the mantle above the 670 km discontinuity would be overturned only in 483 Ma,
and the entire mantle, if it is involved, only in
1703 Ma.
Although there is uncertainty in these figures,
they serve to illustrate that the time necessary to
overturn the upper mantle is of at least the same
order as the time between Earth formation and the
age of the oldest preserved continent fragments
(4.5 Ga minus 4.0 Ga). It is therefore quite possible
that oceanic volume rose to its present value only
gradually in the first 0.5 Ga of its history.
Discussion and conclusions
The calculations presented here show that with the
present ocean volume, ocean ridges would have
remained below sea level given realistic Archaean
heat content/flux. Only with a substantial reduction
of surface water would Earth have been able to
recycle dry oceanic lithosphere. It has also been
shown that, given our present understanding of
the water content of Earth's mantle, a realistic
reduction of early Earth surface water could have
been attained for a period long enough to allow a
gradual rise in sea levels of Hadean Earth. It is
possible, therefore, that the higher mantle temperatures of Hadean Earth, and lower ocean volumes,
may have kept mid-oceanic ridges above sea level.
The gradual dehydration of Earth's mantle may
then have drowned the mid-oceanic ridges by the
onset of the Archaean, resulting in hydrothermal
alteration of the ocean floor and, in turn, the
formation of continental crust above subduction
zones. For the first time in the history of Earth there
was henceforth efficient and continuous interaction
of our planet's hydrosphere and mantle. Although
this may have been a gradual process, it would
nevertheless have initiated a major change in the
efficiency of Earth's chemical differentiation, and
would have resulted in a change from planetary
cooling dominated by heat exchange directly to the
atmosphere (perhaps with a Venus-style mantle
convection, cf. Solomon 1993) to one dominated by
heat exchange with the hydrosphere, which still
buffers Earth's heat loss today.
The initial rate at which continental crust formation took place is not known, but this must have
been related to prevailing rates of oceanic lithosphere formation, which is believed to have been at
least 3 to 10 times greater in the early Archaean
7
than today (Burke et al. 1976; Basaltic Volcanism
Study 1981; Abbott & Hoffman; 1984, de Wit &
Hart 1993; Table 2). Furthermore, there are
compelling arguments for the continuous recycling
of continental crust into the mantle (Armstrong
1990). The geological record indicates, however,
that by 3.5 Ga the net rate of continental crust
formation was in the order of 0.05 k m 3 a -1 (de Wit
& Hart 1993), more than sufficient to produce
the granite-greenstone nuclei of the presently
preserved old cratons, which, together with their
depleted dunitic-harzburgitic keels, provided
continental kernels for further (plate) tectonic
accretion and collisions (de Wit et al. 1992; de Wit
& Hart 1993).
A critical feature of the evolution model proposed in this paper is that Earth's mantle released
a significant proportion of its initial water only
gradually, through convective overturn of the ocean
floor, over the first 500 Ma if the upper mantle only
was involved, and considerably longer, if whole
mantle convection was involved. This feature may
appear inconsistent with inferences that Earth
underwent essentially complete melting after core
formation (e.g. Nisbet & Walker 1982). If this
inference is correct, our model requires that this
whole-Earth melting did not lead to complete
extraction of the water from the mantle. While it
is beyond the scope of this paper to discuss the
reasons for this, it is consistent with the compelling
evidence for significant amounts of water in the
source regions for MORB. Although it is possible
that some of this water has been recycled from
the hydrosphere through subduction processes
(e.g. Jambon & Zimmerman 1990) the depleted
character and low 87Sr/86Sr of N-MORB make it
unlikely that all the water is derived from this
source. The Sr ratios would be particularly difficult
to explain, since any water transported into the
mantle would probably carry sufficient Sr to
dominate the mantle signature.
This study serves to illustrate the importance of
a greater understanding of the involvement of water
in mantle melting-processes. We need to know
more about the influence of water on this melting
and on the efficienty of water extraction during
melting. An implication of the model is that
melting in the early Archaean may have occurred
in the presence of larger amounts of water than at
present. There is the potential to test this through
the detailed examination of water-beating phases
such as pyroxene and amphibole in Archaean
mafic-ultramafic rocks.
M. J. de W. acknowledges Foundation for Research and
Development (South Africa) support, and A.H. was
supported by Natural Sciences and Engineering Research
Council (Canada). Two reviewers provided helpful
suggestions to sharpen our arguments.
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8
M. J, DE WIT & A. HYNES
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