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Downloaded from http://sp.lyellcollection.org/ at Pennsylvania State University on April 24, 2014 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 Email alerting service click here to receive free e-mail alerts when new articles cite this article Permission request click here to seek permission to re-use all or part of this article Subscribe click here to subscribe to Geological Society, London, Special Publications or the Lyell Collection Notes © The Geological Society of London 2014 Downloaded from http://sp.lyellcollection.org/ at Pennsylvania State University on April 24, 2014 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. Downloaded from http://sp.lyellcollection.org/ at Pennsylvania State University on April 24, 2014 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 Downloaded from http://sp.lyellcollection.org/ at Pennsylvania State University on April 24, 2014 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 Downloaded from http://sp.lyellcollection.org/ at Pennsylvania State University on April 24, 2014 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 Downloaded from http://sp.lyellcollection.org/ at Pennsylvania State University on April 24, 2014 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. Downloaded from http://sp.lyellcollection.org/ at Pennsylvania State University on April 24, 2014 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. Downloaded from http://sp.lyellcollection.org/ at Pennsylvania State University on April 24, 2014 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. 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