The mechanical and electrical properties of earth's asthenosphere

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Physics of the Earth and Planetary Interiors, 25 (198 I) 280— 296 Elsevier Scientific Publishing Company, Amsterdam — Printed in The Netherlands

The mechanical and electrical properties of Earth’s asthenosphere D.C. Tozer School ofPhysics, The University, Newcastle upon Tyne, NE! 7R4 (Great Britain) (Accepted for publication November 17, 1980)

Tozer, D.C., 1981. The mechanicaJ and electrical properties of Earth’s asthenosphere. Phys. Earth Planet. Inter., 25: 280—2%. An adequate theory of continental drift can be based on heat transfer theory, but it does demand the acceptance of a large downward revision of traditional estimates of average upper mantle temperatures and a consistent understanding of lithosphere and asthenosphere in terms of a difference in rheological behaviour under prolonged non-hydrostatic stress. The recognition that an extremely viscous average state of the upper mantle is self regulating both requires and permits an explanation of magma generation at a strictly limited rate (when averaged for the whole planet over a few years) in terms of unsteady and local deformationa] heating. The activity of water as a reducer of silicate creep resistance is used to develop the hypothesis that water produced by an amphibole dehydration has been effectively trapped in the Earth and is the underlying cause of a low seismic Q— 50 and an electrical conductivity l02_l0~ ohm~m~,at depths of —~l00km. At the predicted low horizontallyaveraged temperatures, the conductivity contrast of rock and aqueous solutions is very large, and mantle electrical conductivity studies now look best-suited to explore this trapping process, and the distinctly recognisable possibility that the uptake of ocean water in the subduction process exceeds the rate of loss that can be explained purely through magsnatic activity.

1. Inlroduction My particular aim in this paper will be an interpretation of minima in electrical resistivity and the seismic dissipation function Q at a depth of —~100 km in the Earth’s mantle that is consistent with basic arguments of heat transport theory, and observations of large-scale surface motions. At a time when we are still far from certain how geographically widespread such a resistivity minimum is, or whether there is a systematic difference in the depth to the two minima, it may look rather premature to select such poorly mapped features for special theoretical attention. However, others have already expressed surprise at conducohnf’ m1 in tivity values as high as 10~2~lO—1 any geographical location at a depth of 100 km

or less (Schmucker, 1972, Schock et al., 1977) and I shall try to explain that important new developments in our understanding of the thermal state and dynamics of Earth have made such fragmentary reports of high conductivity and a Q of 50—100 look even more interesting. Not only do such conductivities now make one very seriously doubt the way mantle electrical conductivities have been interpreted in the past, but the interpretation I shall offer suggests that a study of Earth’s conductivity structure at depths of 100 km may be a way in which geomagnetic induction studies can make a uniquely sensitive and important contribution to the elucidation of a process responsible for some distinctive geodynamic phenomena. My first task must be to explain the general developments that have forced this change of perspective.

0031-9201/81 /0000—0000/$02.50 © 1981 Elsevier Scientific Publishing Company

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2. The asthenosphere-lithosphere boundary and the mechanical properties of the upper mantle A depth of - 100 km has also often been suggested as the position of the boundary between lithosphere and asthenosphere so perhaps the first question one might ask is whether these resistivity and Q minima are a feature of the boundary region or whether they are intrinsic to one or other of these layers. However, if one asks how these layers are defined one will soon discover a very confused and controversial problem. Seismologists often appear to believe that an asthenosphere is an observational fact dictated by a decrease of S-wave velocity with depth in the upper mantle but this looks like a misconceived attempt to settle a long-standing issue rather than the result of any careful assessment of its nature as a slow deformation rheological problem. At all relevant times it has been widely known among experimentalists that short and long time-scale rheologies of a material have to be treated as independently varying properties and that any material can be induced to behave as everybody’s notion of what a “solid” or “liquid” is, if the applied stress time-scales are sufficiently short or long. Barrell, Daly and a host of followers, struggling to articulate their notion of a “lithosphere” and “asthenosphere” may not have specifically used the idea of a relaxing viscoelastic material, but the definitions of a lithosphere- asthenosphere boundary that makes it look like some sort of geological unconformity separating strata with intrinsically different properties, now seem unphysical and untenable. The rheological characterisation of the outer parts of Earth has not been helped by the use of the words “rigid plate” rather than “lithosphere” to distinguish superficial Earth material from what is underneath. In fact, the choice of “rigid plate” seems a singularly inappropriate term to apply to a material that shows such ubiquitous evidence of a past and often intense deformation, and which would actually preclude all the changes visualised by plate tectonics if taken at its face value. Plate tectonicians have tried to give physical plausibility to their concept of “rigidity” by also using the seismic low velocity layer to define the lower

boundaries of its occurrence. However, what here appears to have become confused is the purely kinematic idea of a rigid body motion as a rough approximation to the geologically recent movements of surface material in limited parts (plates) of Earth’s surface, and this long-standing but still quite unarticulated feeling that superficial material is somehow more difficult to deform on the timescale of continental drift movements than the material lying below it. It has been clear for some time that whatever plate tectonicians are trying to imply by “rigidity” cannot consistently have anything to do with a seismically determined rigidity modulus distribution because its lowest values are assigned to the “crust” (Tozer, 1973). Incidentally, the “crust” of the Earth also seems to have been a term earlier introduced in this perennial search to describe a rheological change with depth, and one that has now become irredeemably corrupted in meaning by similar well-meaning attempts to make it look precise by reference to the Earths rheology at seismic frequencies-in that case a conveniently located feature of the P wave velocity distribution. (When Mohorovicic made his choice of the 8 km s - ’ contour of the P-wave velocities to define a crustal base, he was probably strongly influenced by a then current and widespread prediction that “melting” would start at depths of the same order.) The reader might well see these criticisms of efforts to define a lithosphere and an asthenosphere as missing the point, because they do not alter a supposed geophysical “fact” lurking behind all these attempts to define a rheological transition with depth. From numerous calculations of the thermal state of the upper mantle, if not by “direct” geothermometric observations, he may well claim to know that Earth material forms a partially molten layer at depths of 100 km. I would challenge the “directness” of this inference of a molten layer, and suggest instead that it is merely the product of applying heat conduction theory to the Earth. Being a diffusion theory of heat transfer, it is inherently incapable of indicating how heat might repeatedly become spatially and/or temporally concentrated without postulating a correspondingly concentrated distribution of chemical or radiogenic heat sources. Since these postulates would raise several further questions that

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would not be consistent with the fundamental assumption of a stationary heat transferring medium, the layer interpretation normally attached to the thermal/rheological state of erupted material has appeared to be the direct and only logical solution. However, now that the great majority of geoscientists accept the large-scale deformation of Earth material, is that still true? It is extremely difficult for me to decide how universally this model of a partially molten layer of silicates has ever been accepted by geologists, because there still seems to be a lot of talk about a magma generation problem, and there are some who specifically see the discovery of a mechanism for “focussing” heat (Bailey, 1977) as the missing key to an understanding of vulcanicity. My belief, developed below, is that a partially molten silicate layer in the upper mantle has to be rejected for basic theoretical reasons. One could point to movements on the scale and speed of continental drift as sufficient reason to abandon all temperature calculations for the mantIe based on heat conduction theory (Tozer, 1967) but a constructive approach that is at all likely to resolve the vulcanism puzzle and indeed, why continental drift occurs, requires an examination of the foundations on which a secure theory of planetary heat transfer can be built. In my experience, any feeling that this type of theory cannot give explanations of these problems can usually be traced either to making an insufficiently radical correction of Kelvin’s original erroneous assumption— that a demonstration of Earth’s tidal rigidity demands the use of a heat conduction theory— or to a mistaken view of what can and cannot be predicted by a planetary heat transport theory. Since Lord Rayleigh (1916) showed how the simplest convective heat transport theories scale with system size, it has been clear that the answer to a planetary heat transport problem can be totally changed ‘from that provided by heat conduction arguments, by the occurrence of finite (but by everyday standards, enormous) viscosities that give every appearance of “solidity” at tidal and seismic rates of deformation. Consequently, with a strongly temperature-dependent viscosity one has to bring the rheological ambivalence of “solids” and “liquids” into planetary heat transport theory ab -

initio. Hopefully, such a close integration of planetary deformation and thermal studies will remove the common tendency to see convection simply as a special hypothesis to explain continental drift. Rheological complexity and heterogeneity will always force us to give a purely descriptive account of many defonnational problems of intense geological interest. However, the failure to see that some key rheological and dynamical questions pertaming to large-scale deformation could be predicted closely enough to test a much more general heat transfer theory and form a basis for new development, came directly from the feeling that the thermal state of Earth’s interior was already “known”. Since the creep resistance of Earth material did not appear in heat conduction theory, there was no way to appreciate how it might determine and be determined by the heat transport process, and without a feeling for the importance of this coupling, insupportably narrow assumptions about the mantle composition and whether its creep strain rate varied as this or that power of the shear stress seemed to be essential before the problem would become in any useful way quantitatively predictive. When this coupling is taken into account with the efficiency of sub-solidus convection as a planetary-scale transporter of heat (Fig. I), the need for a major reappraisal of the interpretation of Earth structure at depths of 100 km becomes immediately evident. For example, traditional juggling with the radiogenic heat source and thermal conductivity distributions to prevent wholesale “melting” of the upper mantle, the basis of much thinking about the degree of upward differentiation of radioactivity, is seen to be quite unnecessary when plausible rheological behaviour is included in Earth’s heat transport problem. What one can summarise as a marked tendency of the actual temperature distribution to avoid widespread (see below) intersections with that of a solidus temperature can only be circumvented if the solidus temperature decreases quite significantly (>0.3 of its absolute value) with increasing depth. For example, a solidus temperature decreasing with depth by at least this fractional amount is the only circumstance to which one can plausibly

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spatially averaged effective viscosity, never found in a material that is near its solidus, and as we 2500 C.,

shall now see, a very small probability of finding “molten” silicate material along an Earth radius

2000 11500

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1000~20OOk~ 0

1 Heat

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Source Density (Unit~P,.sentChondrite Value.)

Fig. I. The steady state central temperature—heat source density relationship in homogeneous self gravitating objects. The 3 km, and those material objects radii are iO~,2 X l0~and 4 X 1O properties needed to define a heat transport problem have been chosen to approximate those of a stony meteorite containing —0.1% water. For small heat source densities there is convective stability and heat conduction indicates the steeply rising segment at the left. For greater heat source densities convective heat transfer affects an ever increasing fraction of the object’s interior. The increasing length scale of convective movements plus the increased buoyancy provided by a stronger gravitational acceleration in the outer parts of any self gravitating object are responsible for the sharp “knee” and “plateau” of this relationship, The position of this plateau on the temperature scale is fixed by the attainment of an effective viscosity of _l021 poise, the higher value for the 4000 km object being due to the effect of pressure which raises the creep resistance deep inside such a large object. The development of convection also shortens the thermal time constant to values that make it reasonable to use this steady state relationship in cases where the heat source density is not changing by a large factor in l0~y.

attribute the present survival of a planetary core sufficiently “liquid”-like to permit the generation of a magnetic field magnetohydrodynamically (Tozer, 1977), but a solidus that fractionally decreases so much with depth is not normally thought to be the situation in Earth’s upper mantle. However, if one adopts a consistent and more general meaning of melting than that in which it has come to be used by igneous petrologists, it turns out to be a very likely and self sustaining situation for the upper mantle over many thousands of millions of years. Before explaining this, some comments are needed to calm the storm of criticism that normally erupts when one suggests that upper mantle temperatures are generally several hundred °Cless than those calculated with conduction theory. To be more precise, one has predicted a very large

chosen from ical distinction a failure at random. tobetween keep Most inhorizontal of mind a local theacriticism necessary temperature has theoretcome and the temperature after averaging. The latter is the more relevant quantity to consider when discussing possible causes of an apparent horizontal layering of Earth structure and is the quantity nowinestimated be several hundred degrees less the upperto mantle. However, an interesting possibility to emerge from the new solutions to the heat transfer problem is that one can now give reasons for believing positive ternperature anomalies, each involving just a few km3 of Earth material up to several hundred °Cabove the horizontally averaged value for the correspondmg depth, could have been generated and dispersed millions of times throughout the planet’s history. (Uncertainties in the theory allow a tradeoff between the volume of material involved and its temperature rise. I have chosen these particular values merely to indicate the volume of magma that could form in one anomaly, though the peak temperature reached could be much higher than the solidus in a correspondingly smaller volume. Such anomalies could recur in roughly the same place at intervals of the order of a century.) The cause of this peculiar behaviour is the fact that any convection process is to some extent a generator as well as a transporter of heat, since irrecoverable deformation is involved, but I can only sketch here the reasons why it takes this highly localised and intermittent character in the Earth. In total and time-averaged amount, it is readily shown that deformational heating becomes a larger fraction of the primary heat sources as the length scale of the convection they may induce increases. This fraction, the “thermodynamic efficiency” of the convection seen as a heat engine that has to dissipate it’s own output, can be estimated to be a few percent for convection in Earth’s mantle (Tozer, 1967, 1978). This is still a small quantity in comparison with our uncertainty about the primary (radiogemc) heat sources in the Earth, and furthermore, since deformational heating can only alter

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the distribution but not the total amount of heat crossing the planet’s surface, one might easily conclude that it is of little global importance or observational consequence. However, since the point at issue is the relationship of the temperatures reached by extruded material to the much lower horizontally averaged temperatures predicted for the upper mantle, it is more relevant to notice that this estimated rate of energy dissipation is of the same order as the observationally computed rate of energy release in active vulcanism. The fact that nothing remotely resembling such a concentrated ‘dissipation has been (or ever will be) seen in a laboratory experiment is easily attributable to the extremely small thermodynamic efficiency on that scale, and indeed, a partial answer to the lower level of volcanic activity on the smaller planets can be attributed to this efficiency growing faster than the square of the radius of self-gravitating spheres of similar material. (More precisely, the external radius minus about 1000 km, since smaller objects than this are not expected to convect their radiogemc heat (Tozer, 1972).] Planetary size also plays a key role in determining why the deformational heating is so concentrated in space and time, in that it is simply on account of this size that the heat transfer process regulates an extremely large horizontally averaged viscosity. This regulation ensures that an average of —‘ 102 J m3 of shear strain energy is stored throughout the region of convection, despite the extremely small average irrecoverable strain rate (10 14_ 10 15 s I) the heat transport process imposes on the planetary material. This represents a store of energy, equal in amount to the computed average dissipation rate for several decades, that could in principle be totally liberated as heat mostly along a small number of separate internal surfaces in little more than the few minutes it takes seismic energy to travel the length scale of the convective flow. However, theoretical as well as experimental studies of the comparable “stick-slip” sliding process indicate no possibility of this happening under planetary conditions unless there are already surfaces of weakness within the convecting material (equivalent to the prior existence of the rubbing surfaces in the “stick-slip” experiments) to nucleate a localised release of this diffusely stored energy. In a planet —

of terrestrial size such nucleation is made possible by the radial temperature gradient near the external surface, which becomes so steep that a layer of cold, cracked and unannealing material forming an outer shell of the planet gets incorporated in the convective flow. Another important factor is that increasing planetary size also raises surface gravity, permitting volatiles like water to be retamed on the surface and entrained into this cold subducted material as the lubricant for a future potential shear zone. As soon as even such a tiny fraction of the Earth’s interior as a few kd are liquified in a slanting, thin sheet extending over many kilometres in depth, very large forces tending to cxtrude it to the external surface are brought into being by its density contrast with the surrounding material 0.1 g cm3). As a result, it is local upward movement rather than thermal dissipation by conduction that is the dominant way by which these anomalies have been removed from the interior. This brief account should indicate that planetary mantles are in a quite unfamiliar regime of viscoelastic convective heat transfer and it is obviously rather naïve to think that the state of material appearing at Earth’s surface from the depths is representative of the horizontally averaged conditions at the depths at which it originates. It will be argued that even if one has now correctly identified the regime in which individual solutions to Earth’s heat transport problem are likely to lie, this cannot begin to answer most questions a geologist is going to ask about that particular configuration of Earth material we call Earth. Any theoretician will accept the criticism for small-scale phenomena, but I would admit that even on such a question as the geologically recent pattern of continental drift, theorists must not pretend to predict most of what has been inferred from observation. This is not a defect of the theory but an admission that we have no knowledge of an earlier flow pattern that would make any prediction of the present surface velocity field the basis of a meaningful test of the theory. (Of course, the literature contains many detailed predictions of a mantle convection pattern that make no mention of this dependence on initial conditions. The illusion of determinacy in this problem can be traced (—j

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to logical errors: the use of hydrodynamic stability arguments to predict velocity patterns rather than to reject them; the groundless assumption that the convection equations have a unique solution to given spatial boundary conditions; and/or to constraints applied to the problem to facilitate its machine storage and computation.) In reality, anyone seeking a detailed explanation of visible rock deformation with a heat/mass transport theory is in a similar position to a meteorologist trying to predict today’s weather without a knowledge of yesterday’s— it cannot be done. He has to be content to give a “climatic” account of geodynamic phenomena, i.e. to find quantities or qualitative behaviour that have much the same numerical value or character for the vast ensemble of particular solutions to the heat transport problem that are all possible if one has no knowledge of its “initial” conditions (Tozer, 1970, 1972). He has to rely on the Earth settling down into some kind of quasi-steady state with it’s own heat sources. This can be demonstrated to have taken about 500 My, a small fraction of Earth’s present age and small enough in comparison with the decay time of the principal heat-producing isotopes to make the application of quasi-steady state arguments to the interpretation of recent geological movements look plausible. Horizontally averaged temperatures in the upper mantle are uncertain mainly due to ignorance about the temperature dependence of in situ mantle material creep properties, but a survey of the heat transport process in cases where the creep behaviour has been chosen to be different functions of temperature has shown the heat transport process to be a self regulating mechanism for the average creep resistance of Earth material at depths greater than some tens of kilometres. The exact depth at which self regulation controls an effective viscosity varies slightly to offset the different estimates of the creep dependence on temperature. An effective viscosity is a simplification of the creep behaviour to be expected under arbitrary conditions of pressure and temperature (see below), but this survey clearly shows that the thermal evolution of Earth’s deep interior has been determined by whether the effective viscosity of the deep interior was initially generally greater or

less than a value of 102) poise, and whether the function that specifies the temperature, pressure and shear stresses at which such large viscosities are attained has any long term tendency to change due to a gradual differentiation of the starting material. If “initial” viscosity throughout the intericr of a homogeneous planet were, for any reason, much different from 1021 poise, the regulation provided,by the heat transport process would have led to a comparable value within a few hundred million years. Given the regulation of such a large viscosity, it can be seen that any subsequent differentiation of the planetary material, if it were well mixed at a scale of hundreds of metres or less, depends entirely on whether deformational heating produces the type of high temperature—low viscosity anomalies mentioned above. Since the light components in the presumed mantle starting material tend to be the more fusible, differentiation to a crust— mantle situation has led to a secular rise in the horizontally averaged temperatures of the Earth’s deep interior, and so far this has probably more than compensated for any dedine due to the decay of radiogemc heat sources. The self-regulation of an effective viscosity of — 1021 poise below depths of a few tens of kilometres can readily be shown to be associated with large scale mass transfer at speeds of a few cm y and shear stresses a 10 bar (Tozer, 1972). If one could therefore demonstrate that such speeds are communicated to the much cooler superficial material, one will probably have gone about as far as one can to explain the continental drift phenomenon in the absence of initial flow data. The temperatures at which Earth material at various depths in the upper mantle attains an effective viscosity of 1021 poise are probably in the range 500—1000°C(Tozer, 1972), but even this indirect way of estimating lower horizontally averaged temperatures in the upper mantle is still indicating values more than twice the absolute value of the external surface temperature. If the typical cxponential temperature dependence of a rock’s effective viscosity were the only relevant factor, one might easily infer values of 1050 poise or more in the surface material. Though at first sight large enough to justify some new notion of plate “rigidity”, one can say that none of the deformation —‘

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ubiquitously registered in the visible rocks would ever have occurred as a result of the heat transfer process if such an effective viscosity were remotely descriptive of the rheological response to the stresses it imposes on these rocks. It is important to see here how this puzzle can be resolved, because it shows both the asthenosphere—lithosphere boundary and its relationship to the Q and electrical resistivity minima in a new and more plausible light. So far I have been only concerned to explain the planetary significance of the general observation that the irrecoverable deformability of materials can often be expressed by an effective viscosity function of the temperature, pressure and cornposition. It is now appropriate to evaluate the significance of this being an approximation only valid within certain conditions. These are that the shear stresses are small compared with the ambient pressure and that the temperature is not low enough to prevent a complete annealing of the material in the time a small irrecoverable strain is imposed upon it. The justification of the approach to the problem of the Earth’s heat transfer using a viscosity is that its resultant solutions satisfy these conditions throughout much the larger part of the planet’s interior, but it is clear that neither condition is met within a few kilometres of the Earth’s surface. Since imperfect annealing would imply that the deformability of the surface rocks at any one time depends on how they have been treated in the past, one could easily imagine a situation where the answer to the question of whether or not the surface rocks are deformed by the heat transport process depends on whether it had deformed them earlier. Before one gets too concerned about the intractable analytical difficulties this reflexive rheological behaviour might give rise to, one should again remind oneself that it is illusory to seek more than a “climatic” understanding of the deformation process after 4 X l0~y of planetary evolution. At this more modest level of enquiry, a possible way of avoiding these difficulties is to use the general observation that the result of a large deformation of cold rock from an annealed state tends, if anything, to make it more deformable, through the introduction of cracks, faults, etc. If one can show

that irrecoverable deformation of Earth’s superficial material would occur even if it were in a well-annealed state initially, then the case for connecting surface tectonics with the heat transfer process has been adequately made. Then, it is only those who try to estimate how long the connection would continue in the future as radiogenic heat sources decline that need concern themselves with a deformation-induced deformability. The simplest manifestation of any material’s memory of past deformation is its regime of elasticity, since any applicability of a rheological relation involving strain necessarily requires a memory of the configuration of no strain that remains valid for at least the period of observation. For present purposes, what I have described as a smaller deformability of well annealed material can be usefully expressed by saying that the range of shear stresses in which elasticity is a good approximation to the rheological behaviour increases with the degree of annealing. Then, the proposed approach to the study of the connection between surface tectonics and heat transfer in an arbitrary planet can be broken down into two problems. Firstly one has to decide whether a transition from quasi-viscous to quasi-elastic behaviour occurs under the longest time-scale stresses applied by the heat transfer process as the external surface is approached. Assuming that such a shell of potentially elastic response has been identified, it remains to decide whether the magnitude of the non-hydrostatic stresses being applied to it exceed those expected for its elastic limit when considered as a well-annealed material. From our general observation of convection in systems at Rayleigh numbers comparable to those that can be estimated for the Earth’s deep interior from the self regulation argument (l06_ l0~see Tozer, 1972), we expect the convection velocity field to be unsteady, the spatial pattern of motions changing radically in times of the same order as the turnover time of 108 y. A transition to quasi-elasticity will therefore occur if there is a region in which the characteristic relaxation time of the planetary material r ( ~ where ~ is the effective viscosity and ~uthe shear modulus for short-period deformation), exceeds this turnover time. Taking the seismically determined shear -

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modulus for CL, one estimates that the lower boundary of any potentially elastic region in the Earth on this time scale is fixed by the position of the - lo*’ poise viscosity contour (surface). One could debate whether the best definition of a lower boundary to the region might be given by other viscosity surfaces labelled by values differing by as much as a factor of ten, but it should be noticed that one is inevitably talking about the boundary of a region of high thermal gradient lying wholly within the upper thermal boundary layer of the heat transport process. The radial gradient of an effective viscosity is here so large that such debate can make virtually no difference to the estimate of an average thickness for the shell of potentially elastic behaviour. I estimate this thickness will be lo-20 km below the sea floor, and perhaps a factor of at least two deeper under the continental shields. Although one can also predict and use effects resulting from a systematic variation of this thickness within oceanic basins (see below), it will now be shown to be of crucial importance to the surface tectonics problem that it everywhere lies at much shallower depths than the low velocity layer. It is readily shown that the condition for an elastostatic equilibrium of a superficial shell of thickness t, subjected to shear stresses (I on its undersurface by convection with horizontal length scale L, is that the elastic limits in tension and compression must exceed (L/t)a. Rutting L = 2000 km, t = 15 km and u = 10 bar, one sees that the elastic limits of the superficial Earth material have to be at least 1 kbar to avoid fracture and large subsequent displacements. Rocks that contain no obvious structural defects introduced by past deformation are much weaker than 1 kbar in tension (typical breaking stress < 100 bar) and with the normal static fatigue effects induced by contact with water (Anderson and Grew, 1977), there is no doubt that cracks would propagate downwards for several kilometres. This raises the stress concentration factor L/t to an even higher value, though the effect of this on downward crack propagation is gradually reduced by the rising lithostatic pressure. One can understand how a crack could reach the base of a quasi-elastic layer 10 km thick but it is entirely out of the question that it should propagate to the low velocity layer where the pressure is

- 25 kbar. In fact, the question of whether it would go all the way through a lo-20 km layer looks so delicately balanced that the factor of two difference in thickness between oceanic and continental regions may well be the reason why it is sea floor rather than continents that contain the lines of positive divergence (spreading) in the surface velocity field. Rocks are very significantly stronger in axial compression than tension, and since the shell of quasi-elastic behaviour is much too thick to exhibit any flexural elastic instability, failure in compression must necessarily accompany failure in tension if there is to be a large horizontal displacement of the surface material. Due to the general downward trend in the isothermal surfaces as one goes from places of ascending to descending material in the underlying convection, one may also predict that any critical effective viscosity surface similarly slopes downwards, further emphasising that failure in compression is the more critical condition to satisfy. However, its maximum depth may still be little more than 20 km, and one cannot discount the possibility that this thickening of the quasielastic layer may be more than compensated by an increasing thermal blanketing effect provided by up to several kilometres of water-filled sediments having virtually no compressive strength. This triggering effect on the failure of the underlying basement rocks would neatly explain why deep sedimentary basins seem to be so exclusively selected as the sites of future mountain building. (The reader may well have detected that I have not entirely avoided the reflexive nature of the surface tectonics process by this appeal to thick sediments - that indirectly assumes the prior existence of surface deformation in the form of mountains. This may well be an important factor in sustaining surface tectonics at the present stage of Earth history. The whole process may only have started because radiogenic heating was - 8 times higher when the planet was formed or because of a large input of accretional energy. With the quasi-elastic layer thinner by a similar factor, compressive failure would then have easily occurred without any sedimentation.) to a transition from quasi-viscous to quasi-elastic behaviour in Earth material on a time scale of

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108 y, one may well ask whether its position may be marked by some observable structure that would clearly distinguish it from the seismic low velocity layer. I have already mentioned a downward drift of the isothermal surfaces as the distance from a region of positive divergence, in the surface velocity field increases, and one will notice that this also implies that a greater thickness of the near horizontally moving material will have made the transition from quasi-viscous to quasi-elastic behaviour the greater this distance. Such a rheological transition is just where one would expect to see a strain-induced anisotropy of properties, and of course, the sloping isotherms are exactly suited for such a strain-induced anisotropy to become “frozen” into the quasi-elastic region. A seismic velocity anisotropy with fast and slow axes definitely correlated with magnetically-inferred surface horizontal velocity and at depths estimated to be 10—20 km has now been observed at a number of oceanic sites (e.g. Raitt et al., 1969; Forsyth, 1975). 3. Q at depths of

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A major aim of the above discussion has been to demonstrate that once one has got rid of obviously inconsistent and out of date arguments about the thermal state of the upper mantle, one is left with a clear-cut case for seeing the relatively low shear velocity, low Q and low electrical resistivity at depths of 80—100 km as a set of distinctive and associated structural features embedded well within the region expected to exhibit a quasi-viscous response at the mean strain rate induced by the planetary heat transfer process—the region I would choose to call the “asthenosphere”. My aim is now to develop the reasons for thinldng that this distinctive structure around a depth of 100 km is due to the presence of water as a separate phase. This idea has grown from well-established knowledge of water as a potent modifier of silicate creep resistance and its less well-known but spectacular effect on anelastic dampjng. By its effect in lowering the creep resistance at a particular ternperature and the action of the heat transport process in regulating the creep resistance, we have the quite novel possibility of a volumetrically very —~

minor component being able to change the horizontally averaged temperature of the Earth’s interior below a depth of a few tens of kilometres very significantly. We shall see that a particularly convincing and attractive aspect of the water hypothesis is that an initial presence of water in the planetary material itself leads to the maintenance of physical conditions that favour its very prolonged retention as an intergranular fluid at depths of 100 km in the case of the Earth. While any geologist or geochemist would accept the existence of some water in the Earth’s deep interior as a direct inference from volcanic activity, the various questions of origin, distribution and the physical state of this water are greatly confused and complicated by the possibility that we are now principally seeing the effects of a geologically recent subduction of ocean water. Consequently, my approach has been to use the existence of terrestrial oceans merely as an indicator that water was present in the material from which the Earth accumulated, and then ask whether and how this original water could be retained in the deep interior in detectable amounts after 4.5 X l0~y of Earth history. Of course, any amount of water initially present in the Earth material in excess of that which could be incorporated into hydrates and which formed a connected network of channels, would have been expelled from the interior by the rising differential pressure gradient between the water and rock at a very early stage of the accumulation process. Our attention focusses on the dehydration process for various phases thought to be present in Earth material and whether an intergranular aqueous phase produced in this way could still be present in the deep interior. This production of a fluid phase by dehydration is the cause of the semantic difficulties surrounding the idea of a solidus temperature which I have referred to already. Experimental petrologists do not recognise the liberation of water from hydrated minerals as partial or incongruent melting, and invariably draw what they call the solidus only at values of temperature and pressure at which they sense the generation of “liquid” silicate material. This convention reflects their preoccupation with magmatism, but as far as planetary

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dynamics is concerned and particularly when the words “partially molten” are being applied simply as a label for a low Q, low velocity region in Earth’s interior, it becomes quite arbitrary and highly confusing. Consistency demands that they be brought under the same heading. With such a general definition of a “solidus” one has to decide what reaction or process is now likely to mark the appearance of a liquid phase in the upper mantle as a whole, i.e. away from any zones that could be contaminated with ocean water or hydrates formed by contact with it. Immediately after the expulsion of a directly accreted water phase, any one of many low temperature dehydration reactions up to and including the deserpentinisation of olivines, could have marked the solidus position. However, all these reactions are untenable candidates for a definition of the solidus at the present time because there is no prospect of a self-regulating state of convective heat transfer ensuring the survival of such phases in the mantle over the intervening period of Earth history— the high creep resistance and low superadiabatic temperature gradients would probably preclude convection altogether. Under the influence of accretional and radiogenic heating, this fraction of the hydration water would have been chemically released and also squeezed from the interior at a very early stage of Earth material history. (Studies of the amount of energy dissipated in the accretion process suggest that this loss of water occurred while Earth material still formed several proto-objects, and while the water could still escape from the surface to interplanetary space. Interesting indirect evidence that water could have been squeezed from earth material at such an early stage has come from Jovian satellites Europa and Callisto. Despite the smallness of their mass (<1% of Earth mass), a surface temperature <170 K has prevented its escape, and an icy shell now surrounds a rocky core.) However, once deep temperatures in the growing and less numerous proto-Earths had risen to a mean value somewhere in the range 500—1000°C, the upward trend of the average temperature would have been quite abruptly halted. This occurs not only because the exponential rate of decrease of

the effective viscosity with temperature would be rapidly increasing the Rayleigh number of the heat transport process to highly supercritical values, but also because of the creep weakening effect of free water released by the higher temperature hydrates in contact with the silicate rock phases. The particular case of this weakening happening as a result of the dehydration of amphiboles has been studied by Riecker and Rooney (1969), but it seems to be a very widespread phenomenon among silicates (Murrell and Ismail, 1976). (The effective viscosity function of the mantle is often equated with that of olivine, on the grounds of its presumed dominant abundance. There are good grounds for using olivine data to set an upper bound to the mantle’s creep resistance, but an abundance of even 60—70% is still a quite insubstantial argument for thinking it controls the creep resistance appearing in planetary heat transfer theory. This is because this process imposes stress on an enormously different length scale from the expected size of the olivine crystals, making it quite possible for the mantle material to deform without involving the olivine crystals at all: compare the deformability of beach sand to the foot and that of the abundant quartz crystals that comprise it.) The dehydration of amphiboles is not only made noteworthy by its occurrence under conditions in which the effects of temperature and water weakening will have reduced the creep resistance to the point of promoting a very significant convective heat transfer. Its peculiarity in occurring at lower temperatures as pressure increases (see, e.g., Wyllie, 1971; Green, 1972) also acts to make further evolution of the horizontally averaged upper mantle temperatures to values much higher than —~ 800°C an extremely slow, if not impossible process to accomplish within the time-scale of significant radiogenic heat production ~ l0”~y). To see this, suppose for simplicity that water from the lower temperature dehydration processes has been released and expelled from the interior so that the remaining water content is everywhere equal to that which could be or has been already released by an amphibole dehydration. Let us also assume for the moment that there are no significant temperature anomalies so that local and horizontally averaged temperatures at the corre-

290

sponding depth coincide. Then, because the amphibole dehydration temperature (the “solidus”) increases with decreasing depth to values (1000— 1100°C)much greater than the values one predicts for the horizontally averaged temperature at those shallow depths where cracks generated by the stresses of the heat transport process could provide channels for free water to the surface, no mechanism exists for loss of this hydration water to the outside. This simplified model is quite adequate for objects of less than about lunar size, but the retention of the amphibole dehydration water becomes less perfect in bigger objects where there is a much higher rate of viscous dissipation producing positive temperature anomalies of hundreds of deg. Celsius. The localised upward movement of this exceptionally hot material at temperatures ~ 1100°Cwould constitute a “leak” in the barrier to water loss presented by the amphibole dehydration curve. However, it should be noticed in the first place that this “leak” is small in the sense that at the predicted rate of dissipation only a few cubic km y —‘ of Earth material are involved and it would take much longer than the present age of the Earth for all upper mantle material to have a high probability of being involved in such hot “zones”. Secondly, for those objects big enough for dissipation to make such molten zones, surface gravity is also strong enough for water to be retained on their surfaces, and the surface material is likely to be involved in the movements of the heat transport process. l’his makes it possible for water to be returned to the interior. Furthermore, since this resorption involves the interaction of cold rock with superficial water probably retained from earlier as well as the current dehydration of amphiboles, the subduction of water may well occur at a faster rate than the loss through high temperature magmatism. Before discussing the consequences of such an imbalance in the Earth, the generality of the above argument for the thermal evolution of terrestrial planets becoming arrested at the staae of a partial amphibole dehydration (Fig. 2) if they initially contain ~ 0.1% water makes it worth enquiring whether any other object in the Solar System shows evidence of now being in this persistent state. The Moon would seem to be an almost ideal

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60

70

80

90

100

110

120

130

(kms)

Fig. 2. Horizontally averaged temperature vs. depth in —~6000 km radius objects, having wet (WP) and dry (DP) peridotite mantles. In all cases the mean heat source density has been equated with that of chondrites, and the difference in the WP and DP curves is only due to the effect of water (0.1%) on the creep resistance of such rock. Only the rough position of the amphibole dehydration and some important decarbonation actions are shown—their position changes somewhat with H 20 and CO2 concentrations (Wylie, 1977). Amphibole dehydration defines the generalised solidus (see text) only at depths greater than that marked by S; at shallower depths it is marked by the appearance of “liquid” silicate phases. Hence, a partially molten layer will only form in Earth from dehydrationat ~‘ 100 km depth. The two curves for a dry peridotite mantle show how a self regulating viscosity leads to horizontally averaged temperatures below a few tens of kilometres being virtually independent of surface temperature. Since the high surface temperatureof the curve marked Venus is itself to how the expulsion comparison with the curve WP due shows the extent of of C02, nearsurface decarbonation critically depends on water content.

object to test the water-trapping hypothesis, on the grounds that it was probably formed at a distance from the Sun where one would expect some water in its mitial composition and a basaltic fraction that could form amphiboles is known to be abundant in its composition. Lastly, it is too small for the generation of positive temperature anomalies responsible for “leaks” through the amphibole dehydration barrier to be significant. Of course, the Moon is frequently described as totally devoid of water but this may be an overstatement for the Moon as a whole. The absence of water from the Apollo samples is not very remarkable when one recalls they were erupted as lavas into an ultra high vacuum environment from which water could quickly escape to interplanetary space. Historically, there are many observations by reliable observers of sporadic phenomena that are interpreted as an emission of volatiles from the lunar interior

291

(Middlehurst, 1967), though of course, their composition is unknown. The extreme dryness of lunar surface rocks has pointed the way to a new understanding of the factors that can effect a seismic Q value, and since Q values are often cited as evidence that both Earth’s upper mantle and the deep lunar interior contain molten silicate material, this is a convenient way to approach our problem. The very distinctive seismic reverberation for several hours that follows a sharp impact on the lunar surface was quickly attributed by the seismologists to the combined effect of an intense wave scattering and a Q of several thousand, but it was only through the challenge of making this interpretation appear physically plausible that we have come to realise how sensitive Q values can be to an intergranular water content. Experiments removing the occluded water from rocks by evacuating them, first at backing pump pressures (Pandit and Tozer, 1970) and later using ultra high vacua (Tittman et al., 1974, 1977), have shown that Q can be reversibly changed from values of <20 to several thousand • by manipulating a tiny amount of water content (—~0.1%). Such experiments reveal the weakness of the argument that the low Q of Earth’s upper mantle (Anderson and Hart, 1978) require “partial melting” in the igneous petrologist’s sense of the word. Goetze (1977) made the same point, without even referring to the potent effect of occluded water on Q, but it could be argued that all Q observations refer to rocks at low pressure and, in particular, those on the effect of water are irrelevant to water produced by dehydration at pressures approaching 30 kbar. The experimental measurement of Q at pressures of this order is extremely difficult, although I believe they are at present being attempted. In the meantime, the simplest models of ~ in dehydrating material suggest it is the so called “effective” pressure (lithostatic pressure minus a pore pressure) rather than the lithostatic pressure itself, that is the significant parameter. The key question would then be whether the water released by amphibole dehydration is sufficient in amount to make the effective pressure very small. On this point the seismic study of the Moon has proved extremely interesting and in my opinion,

beautifully illustrates how comparative planetology can illuminate a terrestrial problem. Not only was a region of low Q found to begin at a lunar depth of —~ 700 km, the depth at which I judge the pressure and horizontally averaged temperature to be not significantly different from that at a depth of 100 km in Earth. It also contained several sources of moonquakes having their recurrent activity correlated with tidally-induced stresses arising from the varying Earth—Moon distance. Since these varying tidal stresses have an amplitude < I bar (Goulty, 1979), there has naturally been speculation as to whether they could ever induce seismic activity on their own or whether they merely augment a much larger pre-existmg tectonic stress field to a value sufficient to cause sudden slippage or fracture. Both the tiny size and recurrence of moonquakes from individual sources weigh against this second hypothesis, but it has in any case been neatly disposed of by the observation of inverted polarity events from one of the most active and important moonquake sources. This can only be understood as a capacity of tidal stresses acting alone to induce sudden source displacements in opposite directions. While Toksoz et al. (1977) noted this significance of the inverted polarity events, their efforts to visualise moonquakes as a “fault” motion do not look consistent with their estimates (using heat conduction theory) of such high temperatures that molten silicate phases were their interpretation of the low Q coexisting at the same depths. Are faults, let alone seismically-active ones, possible in those circumstances? To me, a much more acceptable picture is that of an inviscid fluid permeating rocks whose creep resistance is high enough to imply little or no tendency for adhesion of those neighbouring rock surfaces that happen to touch. With an “effective” pressure of essentially zero, one can visualize the moonquakes as a slight adjustment of a few of the billions of contacts existing between neighbouring rock masses in this heterogeneous medium that happen to be critically poised to respond to such small shear stresses. Similar adjustments in the rocks at a depth of 100 km on Earth would be quite undetectable in the much more seismically noisy terrestrial enViroflmeflt.

292

4. The electrical conductivity at several hundred km depthinEartli The above lunar seismic arguments cannot chemically identify the phases constituting the heterogeneous material in which moonquakes occur, but the inferenceof a very low “effective” pressure —it cannot be more than —~l0~ of the lithostatic pressure if tidal stresses are to stimulate lunar seismicity— is itself indicative of a very high degree of connectivity in the liquid phase permeating the relatively very creep resistant rock. (The necessary lack of adhesion over periods of a month

couldmeans be the taken to imply a Maxwell-type This time of an rocks effective in contact viscosity of at >5least X relaxation 1018 as poise long. under stresses of at most a few bar, a condition that is guaranteed by the self regulation of viscosity by the heat transport process.) Given the strength of the heat transfer argument that widespread “melting” is excluded as a feature of the quasi-steady state unless liquid is produced at lower temperatures the higher the pressure, and the coincidence of low values in the Q distributions of both Earth and Moon with the position in which one would predict amphibole to dehydrate, I find an identification of this liquid phase with water very compelling, Even without a chemical identification of the liquid phase one could appeal to the correlation that exists between an ionic electrical conductivity and viscosity (Tozer, 1979) as grounds for believing the relatively very inviscid liquid phase controls the bulk electrical conductivity of the material even if it formed only a very small volume fraction of the total. Of course, the same point can be made more quantitatively with the water hypothesis. Any free water would probably be more accurately described as a solution saturated with metallic ions, and at the horizontally averaged temperatures of 500—750°Cone now predicts for the region of dehydration its specific conductivity would be > l0~times greater than that of any major rock-forming phase (Fig. 3). In other words, one may easily have a situation at these depths similar to that in the uppermost parts of the Earth’s crust, where the conductivity sensed by geomagnetic disturbances seems to be controlled

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Fig. 3. Electrical conductivity of important minerals, rock types and saturated aqueous solutions. The scatter of conductivity data from individual specimens of nominally the same material make it meaningful to refer only to strips of the log a— l/T plane within which one may expect such data to lie. A strip bounded by an arbitrarypair of lines P and Q is denoted below by (PQ). (AB): “Wet” granites and basalts. (CE): “Dry” granites and basalts. (DF): Olivines and pyroxenes, the single crystal and more magnesium rich specimens tending to be nearest line F. An aqueous solution curve, with an uncertainty of 3 in conductivity, has been obtained by averaging and extrapolating solubility data, and using Walden’s rule with the measured effects of temperature and pressure on the viscosity of water. Even if one ignores the exceptionally high conductivity of saturated solutions, thegeological enormous materials range of conductivity values seen among different should correct any tendency to believe that the interpretation of mantle conductivities can only be improved by more careful measurements on a narrow class of well characterised specimens.

by meteoric water contained in cracks and pores (Bayard-Ducleux, 1933; Brace et al., 1965). The suggestion of a conductivity maximum in Earth’s upper mantle due to an aqueous solution presupposes that an extensive network of relatively light and inviscid fluid can be maintained in the gravitational field. Although I have already referred to an initial expulsion of free water not involved in hydration reactions by the difference in pressure gradients in water and rock, it is not complete if the planet is in a state of convection.

293

There is a stable fractional (volume) concentration ~m such that the mean upward speed of the fluid through the host rock due to the differential pressure gradient is of the same order as the typical mixing speed V, provided by the planetary heat transfer process. Using the idealised model of a permeable medium shown in Fig. 4, one can read ily show that 2~V 022XI0 m

II

II II

where 1 is the spacing of the water channels, ~j the solution viscosity and v ~, the difference in pressure gradients in rock and water. Measurements of the effects of pressure and temperature on a water viscosity (Bridgeman, 1951) and the effect of dissolved salts on it, lead to an estimate of s~—~2 X l0~ poise under the conditions of an amphibole dehydration in the upper mantle. With V being regulated at —~iO~ cm s~ and ~ 2 < I ~3 dyn cm3 we obtain Om —~ l0~~// (cm)

a~O/3 where a~is the electrical conductivity of the network material. From a survey of electrolyte conductivities and solubility data, and using the above argument for 0 —, °m at depths much greater than the depth of dehydration, I estimate that water =

II

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IIII

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H

I H H. U

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-_________________________________

VP

1 is not a known parameter for an in situ mantle material, but it certainly cannot be smaller than the crystal size of its important minerals—a quantity much discussed by heat transfer theorists who try to predict in advance a precise creep resistance function for the mantle material. While their estimates of crystal size range anywhere from 1 ~ to kilometres, any value of 1 in this range would indicate a °mvalue that is smaller than the volume fraction of water likely to form from amphibole dehydration. It is therefore reasonable to conclude that only a small part of the water released by amphibole dehydration would be carried to much greater depths by the convective motions as a network of water-filled channels. Using the same model (Fig. 4), one can show that the bulk conductivity due to the network material alone is given by

II I

II

II

I

I

I

II

II

Fig. 4. Idealised model of a permeable medium.

could not raise the bulk conductivity much above that of the principal rock minerals (10 ~— 10—2 ohm~m’) except in the vicinity of the dehydration. ‘This indication that water might collect at the

•

depth ‘at which it is released by dehydration, as well as the possibility of ocean water being carried to the Earth’s deep interior faster than it can be removed by the strictly limited rate of magma generation and migration to the external surface, raises the interesting question of a value to which the water concentration might rise before some other water loss mechanism became rapid enough to stabilise it. One can broadly classify these mechanisms as percolative, i.e. involving only an outward movement of the aqueous solution, or diapiric, a much larger-scale flow involving both the water and its host rock. The diapiric mode of loss will inevitably be the dominant process if percolation cannot balance the mantle water budget at a low enough water concentration to prevent a significant density inversion. Notice that high lithostatic pressures make it quite discuss law; mantle water lationinappropriate in terms of toDarcy’s any rockpercowill immediately collapse if thereis insufficient fluid to fill its interstices. One can also be confident that the pressure destroys all possibility of reintroducing water into a previously impermeable material even over geological periods of time. The evidence

294

for this is the existence of effective cap rocks for oil and gas reservoirs, in some cases 108 y old, under the relatively trivial pressures of the, upper crust. The occlusion of channels can also. be inferred from the way pore pressures approach lithostatic pressures at depths of only a few kilometres in sedimentary basins (Bredehoeft and Handshaw, 1968). One can therefore see that water released within a body of rock by dehydration is not only uniquely sited to create the channels necessary for its percolation, but these channels would be the only way subducted ocean water could become widely distributed throughout the mantle. The fraction of the subducted ocean water that avoids eruption in andesitic-type vulcanism above Benioff zones would only be able to travel great distances horizontally through the host rock alter it had reached depths of —~80 km (where amphibole dehydration would have already created a network of channels for horizontal diffusion. With the rocks between 80 km deep and the surface acting as a cap rock, diapiric movements are the only way of balancing the water budget if subduction exceeds the magmatic losses of water. In estimating the water concentration at depths 80—100 km at which diapirism would commence, a natural standard of comparison is the thermally induced density differences responsible for the water imbalance in the first place. From heat transfer theory, as well as using the observed dcpartures from hydrostatic equilibrium in the external gravity field, one infers a fractional density difference of —~ l0~on level surfaces at a length scale of several hundred kilometres. From this I conclude that total water concentration in a layer such as that visualised for amphibole dehydration could not be increased by much more than l0~ before diapirism would balance any possible rate of gain from this tendency of the heat transfer motions to remix the oceans and upper mantle Despite these generally very tiny concentrations, electrical conductivity studies are a uniquely powerful tool with which to explore these diverse water problems. At the much. lowdr horizontally averaged temperatures (750°C) now predicted for the whole depth range mcluding and above an amphibole dehydration, the conductivity of any saturated aqueous solution is so much greater (<

l0~times) than that of any abundant rock phase that a bulk electrical conductivity determination is bound to put interesting limits on the amount of water contained in connected channels. The upper limit of —‘ 10~placed on the total volume concentration of water by the initiation of diapirism puts an upper bound of 10-’ ohm’ m’ on the bulk conductivity of any extensive layer of the Earth’s upper mantle. This conductivity is high enough to indicate that the distribution of a very geologically significant amount of mantle water in connected channels could be usefully defined by its effect on geomagnetic variations with periods from hours to days. The first priority must be the difficult observational task of deciding how widespread are the present indications of a conductivity maximum at depths of 100 km. Now that definite theoretical reasons have been given for the existence of such a maximum, there may be less reluctance to explore whether models that do have such a conductivity maximum provide the best interpretation of one’s observations. Given the much lower temperatures in the upper mantle, nothing would more quickly refute current models of mantle conductivity as a thermally-induced conductivity of its major but exceptionally resistive silicate phases than the demonstration of a conductivity decline with depth. 5. Condusions My aim has been to indicate some of the important predictions that accompany the explanation of the Earth’s large-scale deformation in terms of heat transfer theory, and hence the general direction in which I believe the interpretation of mantle electrical conductivity now has to evolve if it is to make a contribution to the elucidation of this most important of planetary problems. So often in the past electrical conductivity interpreta*

My previous estimate of averaged temperatures “somewhere in the range 500-1000°C”was made without any attempt to fix a water concentration. If one has been convinced by the argument that by water creates conditions necessaryone forcan its own retention acting as a the creep strength reducer, put the average temperature somewhere in the lower half of this range.

295

tions have seemed more concerned to show their consistency with heat conduction calculations of upper mantle temperature, than with any new insight they might offer into the state of the interior. The central result motivating many changes of perspective about the interior is the recent understanding of how the convective heat transfer process and the effective viscosity mutually interact to regulate the horizontally averaged value of the

tamed throughout the continental drift process. The greater creep resistance and a slightly lower density of the sub-continental upper mantle at a particular temperature and pressure would prevent remixing and give support to Wegener’s original conception of the continents as “rafts”. Perhaps an even more profound consequence of water weakening in our planet has been the way it could haye facilitated the retention of other volatiles, notably CO2 in the form of carbonates. Venus,

latter at —~ 1021 poise. Not only does this interaction make convection at a few cm y —‘ in the mantle look inevitable, but it also precludes the melting of silicates as a layer of the upper mantle. A careful assessment of why Earth’s surface rocks are also involved in the convective movements has resulted in new definitions of lithosphere and asthenosphere, the net result of which is to make the various structural features at a depth —‘ 100 km look quite separate from any notion of a transition between the two regions. Reasons are given for believing the Earth and perhaps some other terrestrial planets are trapped in a state of partial dehydration of amphiboles, and that the collection of a free water phase is the cause of the distinctive structure at —~ 100 km depth under the

that most Earth-like of the other planets in its gross characteristics, but where there is virtually no water in the atmosphere to indicate that water was ever incorporated in the planetary material in rheologically significant amounts, could be vividly illustrating this effect. With no water weakening of its silicates to prevent the horizontally averaged temperature from reaching —~ 1000°C at quite shallow depths and with no surface water to facilitate carbonate sedimentation, a massive CO2 atmosphere would have been formed in less than l0~y. In turn, the “greenhouse” effect of this atmosphere would have promoted a further decarbonisation of the interior by raising the surface temperature to the present value of nearly 500°C (see Fig. 2). It is probably only the absence of water that makes it rheologically possible for the present Venusian surface to maintain topographic differences exceeding 5 km over vast areas, despite this high surface temperature. On Earth there are a variety of geological phenomena: kimberlite intrusions, crypto-volcanic cxplosions, voluminous pyroclastic deposits, etc., that point to sub-solidus, volatile-rich eruptions, which may collectively represent attempts by the Earth to balance a tendency to absorb more water from the oceans than it can possibly get rid of by magmatic activity. Though classed with the magma eruptions as volcanic activity, the recent activity of Mount St. Helens may have its origin in this water budget problem rather than the deformational heating responsible for the other types of vulcanism.

ocean basins. In the changed thermal conditions

visualised for the mantle, electrical conductivity studies now look the most sensitive tool with which to elucidate the distribution of water, a minor mantle component that has probably had major dynamical and evolutionary consequences for the Earth through its weakening effect on the creep resistance of the mantle silicates. As an example of such evolutionary trends, if the water hypothesis is accepted as the explanation of the low velocity layer, the observed systematic differences in this layer under oceans and continents can only be attributed to the systematic removal of water from the underlying mantle as the visible expression of a continent grew laterally above it. The absence of a water weakening effect in the sub-continental mantle could also be an important factor in explaining how such sys-

.

tematic differences in the low velocity layer, as

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Anderson, O.L. and Grew, P.C., 1977. Stress corrosion theory of crack propagation application to geophysics. Rev. Geophys. Space Phys.,with 15: 77—104.

296 Anderson, D.L. and Hart, R.S., 1978. Q of the Earth. I. Geophys. Res., 83: 5869—5882. Bailey, D.K., 1977. Igneous rocks and the degassing of the Earth. 9th Tomkieff Memorial Lecture. Dep. Geol., Univ. Newcastle upon Tyne. Bayard-Ducleux, F., 1933. C.R. Acad. Sci., 197: 854—856. Brace, W.F., Orange, A.S. and Madden, T.R., 1965. The effect of pressure on the electrical resistivity of water saturated rocks. J. Geophys. Res., 70: 5669—5678. Bredehoeft, i.D. and Handshaw, B.B., 1968. On the maintenance of anomalous fluid pressures: I. Thick sedimentary sequences. Geol. Soc. Am. Bull., 79: 1097—1106. Bndgeman, P.W., 1951. The Physics of High Pressure. 2nd edn., Bell, London. Forsyth, D.W., 1975. The early structural evolution and anisotropy of the oceanic upper mantle. Geophys. J.R. Astron. Soc., 43: 103—162. Goetze, C., 1977. The effect of volatiles and partial melt on the physics of the upper mantle. In: M.H. Manghani and S. Akimoto (Editors), High-Pressure Research: Applications to Geophysics. Academic Press, New York, NY, pp. 3—23. Goulty, N.R., 1979. Tidal triggering of deep moonquakes. Phys. Earth Planet. Inter., 19: 52—58. Green, D.H., 1972. Magmatic activity as the major process in the chemical evolution of the Earth’s crust and mantle. Tectonophysics, 13: 47—71. Middlehurst, B.M., 1967. An analysis of lunar events. Rev. Geophys., 5: 173—189. Murrell, S.A.F. and Ismail, I.A.H., 1976. The effect of decomposition of hydrous minerals on the mechanical properties of rocks at high pressures and temperatures. Tectonophysics, 31: 207—258. Pandit, B.!. and Tozer, D.C., 1970. Anomalous propagationof elastic energy in Moon. Nature (London), 226: 335—336. Raitt, R.W., Shor, G.G., Francis, T.J.G. and Morris, G.B., 1969. Anisotropy of the Pacific upper mantle. J. Geophys. Res., 74: 3095—3109. Rayleigh, Lord, 1916. On convection currents in a horizontal layer of fluid, when the higher temperature is on the underside. Philos. Mag., 32: 529— 546. Riecker, R.E. and Rooney, T.P., 1969. Water induced weakening of hornblende and amphibolite. Nature (London), 224: 1299.

Schmucker, U. and Jankowski, J., 1972. Geomagnetic studies and the electrical state of the upper mantle. Tectonophysics, 13: 233—256. Schock, R.N., Duba, A.G., Heard, H.C. and Stromberg, H.D., 1977. The electrical conductivity of polycrystalline olivine and pyroxene under pressure. In: M.H. Manghani and S. Akimoto (Editors), High Pressure Research: Applications to Geophysics. Academic Press, New York, NY, pp. 39—51. Tittman, B.R., Housley, R.M., Alers, G.A. and Cirlin, E.H., 1974. Internal friction in rocks and its relationship to volatiles on the Moon. Proc. Lunar Sci. Conf., 5th, pp. 2913—2918. Tittman, B.R., 1977. Lunar rock Q in 3000—5000 range achieved in laboratory. Philos. Trans. R. Soc. London, Ser. A, 285: 475—479. Toksoz, M.N., Goins, N.R. and Cheung, C.H., 1977. Moonquakes: mechanisms and relation to tidal stresses. Science, 196: 979—981. Tozer, D.C., 1967. Towards a theory of thermal convection in the mantle. In: T.F. Gaskell (Editor), The Earth’s Mantle. Academic Press, London, pp. 325—353. Tozer, D.C., 1970. Temperature, conductivity, compositions and heat flow. J. Geomagn. Geoelectr., 22: 35—43. Tozer, D.C., 1972. The present thermal state of the terrestrial planets. Phys. Earth Planet. Inter., 6: 182—197. Tozer, D.C., 1973. The concept of a lithosphere. Geofis. mt., 13: 363—388. Tozer, D.C., 1977. The thermal state and evolution of the Earth and terrestrial planets. Sci. Progr., 64: 1—28. Tozer, D.C., 1978. Terrestrial planet evolution and the observational consequences of their formation. In: S.F. Dermott (Editor), The Origin of the Solar System. Wiley, New York, NY, pp. 433—462. Tozer, D.C., 1979. The interpretation of upper mantle electrical conductivities, 56: 147—163. Wyllie, P.J., 1971. The Dynamic Earth. Wiley, New York, NY, pp. 167—176. Wylie, P.J., 1977. CO 2 and H2O in the uppermantle. In: M.H. Manghani and S. Aicimoto (Editors), High-Pressure Research: Applications to Geophysics. Academic Press, New York, NY, pp. 77—106.