Plate Tectonics: This Works As a Result of Hot Mantle Asthenosphere Ascending Beneath Mid- Ocean Ridges to Form the Plates

Prof. J. TARNEY

Wednesdays at 9.30 (Lecture) & Wednesdays at 11.30 (2 hr Practical or Lecture)

Program Second Semester 1998:

Wed Lecture 9.30 - 10.30 Wed Practical/Lecture 11.30 - 1.30 Week 6 Mantle Topology & Mineralogy (L) Mid-Ocean Ridge Processes (L) Week 7 Rifting and Wilson Cycle (L) Practical: Pacific Ocean Week 8 Extension & Sedimentary Basins (L) Subduction and Island Arcs (L) Practical: Mini practical Week 9 Active Margins & Accretion (L) Practical: Plate model exercises (1) Week 10 Plumes & plateaus (L) Practical: Plate model exercises (2) Collision; plate tectonics & Earth evolution

Useful General References: PRESS, F. & SIEVER, R. 1998. Understanding Earth (2nd Ed.) p.504–535 + CD-rom & Internet link: www.whfreeman.com/understandingearth

KEARY, P. & VINE, F.J. 1991. Global Tectonics. Blackwell Scientific Publ., 302pp.. (2nd Edition now out)

Plate Tectonics: This works as a result of hot mantle asthenosphere ascending beneath mid- ocean ridges to form the plates. These plates then subside into the mantle again at subduction zones, pulled by the excess density of the mafic ocean crust, which transform to eclogite. Plate Tectonics: GL209 Prof. John Tarney Lecture 1: Geological Aspects

mafic as a result of extraction of silicic granitic magmas from the MANTLE PETROLOGY IN RELATION TO PLATE deep crust? TECTONICS There a number of regions where we think tectonic activity has brought segments of the lower crust up for inspection. Notable examples are Kapuskasing and Pikwitonei in Canada (Precambrian Knowledge of mantle petrology and the constitution of the deeper crust), the Lewisian of NW Scotland (also Precambrian), Calabria in mantle is important in trying to understand several aspects of plate S. Italy and the Ivrea Zone in the Alps (both Phanerozoic). tectonics. For instance, is there whole-mantle convection or two- layer convection? What are mantle plumes? What are superplumes? Does the subducting slab penetrate into the lower mantle? What happens to the slab at depth? Is the sub-continental mantle different from the oceanic mantle? First, some basic facts.

Principal Internal Subdivisions of the Earth

Region Depth Mass Mass (km) (1025g) Fraction

Crust 0-Moho 2.4 0.004 Upper Mantle Moho-400 62 0.10 Transition Zone 400-1000 1000 0.17 Lower Mantle 1000-2900 245 0.41 Outer Core 2900-5154 177 0.30 Inner Core 5154-6371 12 0.02 Fig. 2. Diagram (based on field and geophysical studies) to show how deep crust can be thrust up to high crustal levels. Kapuskasing Note that the crust makes up quite a small proportion of the structure, Ontario, Canada). total Earth. The main problems that have occupied geologists over The Lewisian of NW Scotland can be interpreted similarly, the the years are: What is the nature of the crust-mantle boundary (the high-grade lower crustal granulites being thrust over amphibolites MOHO). What is the nature of the low velocity zone? Is the >2.5 Gyr ago. lithosphere diferent in composition from the asthenosphere? What happens in the transition zone? What is the nature of the deep mantle?

Fig. 3. Estimates as to the extent to which we have sections The Continental Crust thru two segments of Archaean crust and one segment of ‘recent’ Alpine crust. There is uncertainty as to whether actually we can directly sample the lowest crust. The Alpine Though we know quite a lot about the upper crust, there is sequence is in a series of thrust slices, but some rocks in still quite a lot of uncertainty about the lower crust. Is there a real region of the Ivrea zone have been down to greater depths Conrad discontinuity separating the lower from the upper crust? Is and have re-bounded again to the surface. the lower crust made up of dry granulite-facies rocks. Is it more Perhaps the more important question is what causes high-P mafic than the upper crust, perhaps as a result of intrusion of mafic rocks to exhume? magmas into the lower crust (called "underplating"). Or is it more

1 Plate Tectonics: GL209 Prof. John Tarney Lecture 1: Geological Aspects

The Moho and the Lower Crust

In the early 1960s there was considerable discussion between petrologists and geophysicists as to the nature of the MOHO. The P-wave velocity of most regions of the uppermost mantle beneath both continents and oceans lies in the range 8.2 ±0.2 km/sec. This in itself would restrict the composition of the mantle below the Moho to some combination of the following minerals (which have the appropriate properties):

Olivine Pyroxenes Garnet (minor spinel, hornblende, phlogopite)

The two principal rock types carrying these minerals are: PERIDOTITE (olivine + pyroxenes) and ECLOGITE (pyroxene + garnet), which are of ultramafic and mafic composition respectively. The MOHO and the gabbro-eclogite transformation: Basalt The nature of the lower crust is less certain. Exhumed transforms to eclogite at high pressures according to the equation: Precambrian high-grade granulite-facies rocks (as in Figs. 2 & 3), which have equilibrated at depths of 25-30 km, have an intermediate Olivine + pyroxene + plagioclase > jadeitic pyroxene + garnet (dioritic) bulk composition. But deep crustal xenoliths brought up in volcanic breccia pipes tend to be more mafic (gabbroic) in For this to be capable of explaining the MOHO it must be a composition. So has the continental crust in part been underplated relatively sharp transition, of no more than a few km. Green and subsequently by mafic magma? Also, recent deep seismic Ringwood (1967) studied this experimentally to 30 kilobars (= ca investigations have revealed strong horizontal reflections in the deep 100 km) using a quartz tholeiite and an alkali basalt (Fig 5). (mainly post-Archaean) crust - what is the cause of these reflections? Do they represent mafic intercalations, differences in fluid content, crustal viscosity, or the bottoming out of shear zones (cf. Kusznir & Matthews, 1988; Meissner & Kusznir, 1987; Warner, 1990; Reston, 1990a). Comparison of crustal reflection profiles across the various tectonic zones of Europe (Wever et al., 1987; Sadowiak et al., 1991) has identified several different types of deep crustal structure: (a) abundant lamellae above the Moho as in BIRPS SWAT 4, (b) bands of reflectors as in BIRPS WINCH 3, (c) hyperbola-like diffractions as in BIRPS SWAT 6-9, (d) "crocodile" diverging reflectors observed in old collision zones, but not so far in the UK, (e) "ramp and flat" stuctures as in BIRPS SWAT 4 2/3, and (f) "fishbone" pattern observed across the Brabant massif. These features (cf. Meissner, 1989) are thought to represent different types of crustal structure. However it is felt that lower crustal viscosity (Meissner & Kusznir, 1987) is a more important factor controlling development of reflectors than is composition, and the current view is that good lower crust reflectors might characterise mature crust, but that this pattern could be destroyed by either compression/collision ("crocodiles", etc.), igneous intrusions or significant extension. Nonetheless it is commonly assumed that the lower crust is gabbroic in composition, either igneous (gabbro), or metamorphic amphibolite (wet) or granulite (dry).

The MOHO as a phase transition

Fig. 4 shows various suggestions (made at one time or other) for how the MOHO beneath oceans and continents could be a phase transition (change of mineralogy, but not a major change in composition). Serpentine is the hydrated variety of peridotite (with ca 12% water, thus lower density). Eclogite is the high- pressure form of basalt or gabbro. But are these models realistic? The serpentine-peridotite model is now discounted. Fig. 5. Experimental studies of Green & Ringwood (1967) on quartz tholeiite basalt showed that transformation to eclogite occurred over a considerable depth interval. Note that eclogite has a lot more quartz than the equivalent basalt. But mantle eclogites have no quartz. Where does the silica go? 2 Plate Tectonics: GL209 Prof. John Tarney Lecture 1: Geological Aspects

With both compositions the transformation was found to be gradual. (4) Modelling "backwards" from erupted basalt compositions. The disappearance of the low density phase (plagioclase) and its replacement by the high density phases (garnet and jadeite) occurred (1) Olivine rich nodules are quite common in erupted alkali basalts over a pressure range of ca 10 kb (= ca 25 km). Thus the MOHO worldwide, and are almost all of spinel lherzolite composition cannot be a phase transformation and must be a compositional (olivine, orthopyroxene, Cr-diopside, spinel). In kimberlite transition. (diamond) pipes there is a greater diversity in that both garnet peridotite and eclogite xenoliths occur (the former dominant). Some The MOHO as a chemical transition nodules contain diopside-rich, phlogopite-rich or amphibole-rich veins. It is thought that these nodules are representative of the sub- Possible scenarios for the oceans and continents are shown in Fig. 6. continental mantle. This material seems to be rather refractory (could not yield much basalt on melting), but at the same time can be quite enriched in incompatible trace elements such as Sr, Ba, K, Rb and the light rare- earths. (2) The peridotitic material in ophiolite complexes (obducted ocean floor) is mainly HARZBURGITE (ol+opyx) or DUNITE (ol), often cut by pyroxenite (opyx) veins and sometimes having chromite segregations (podiform chromites). There is a consensus that this mantle is the refractory residue left after basalt extraction at mid- ocean ridges. But are the pyroxenite veins the result of silica-rich fluids coming off the subduction zone? (see in Fig. 5a all the free quartz present in eclogite in subduction zones) - note that many ophiolites are thought to be fragments of "back-arc" spreading centres.

(3) In the Ivrea Zone and the Lanzo-Seisia Zone of the Italian Alps, the peridotite slices are overlain by layered gabbros. They are mainly LHERZOLITE (ol-opyx-cpyx -spinel, or -hornblende or - phlogopite). Most carry veins composed of orthopyroxene, orthopyroxene-spinel, diopside-orthopyroxene, phlogopite-diopside, or hornblende. Different segments different veins, suggesting a complex make-up of the sub-continental mantle. Some of the rock types are similar to those brought up in volcanic breccia pipes.

(4) Considerable progress has been made in understanding the compositions of basaltic rocks in recent years, and interpreting them in terms of melting models. So it is possible to "model back" to the primary mantle from which the basalt was derived, and estimate its composition. We now know that there are several distinct types of mantle, that have been kept separated for many hundreds of millions of years. They have distinct trace element and isotopic characteristics. (You may come across references to them as DMM, HIMU, EM1, EM2 and PREMA), but there is uncertainty as to In oceanic regions the form of the MOHO is known from dredging at where they are located. fracture zones where peridotites (often serpentinised) have been recovered by dredging and drilling along with cumulate gabbros and pillow basalts. Actual sections of ocean floor are preserved in ophiolite complexes. Upper Mantle Mineralogy The variation in upper mantle mineral assemblages with The nature of the sub-continental MOHO is less certain, partly due to temperature and pressure can be determined experimentally. But lack of knowledge of the lower crust. there is uncertainty about composition. Because a lot of observed mantle material is not primary, but has had a liquid (basalt) fraction removed by partial melting, Ringwood coined the term 'PYROLITE' Composition of the Upper Mantle for primitive fertile mantle - in effect dunite with basalt put back in!

Our petrological knowledge of the upper mantle composition comes PYROLITE = 3 parts DUNITE + 1 part BASALT from several sources: (1) Nodules brought up in volcanic pipes; Mantle mineralogy varies mainly on account of the nature of the (2) Large sections of mantle found in obducted ophiolites; aluminous phase, which is P-T dependent, i.e. (3) Slices of mantle thrust up in mountain belts such as the Alps (Ivrea-Verbano Zone) and the Caledonides (especially Norway); and

3 Plate Tectonics: GL209 Prof. John Tarney Lecture 1: Geological Aspects

Olivine (MgFe)2SiO4 + Orthopyroxene (MgFe)2Si2O6 as major which are already known (isomorphs). For instance we can compare phases, plus: silicates with germanates because germanates form a series of crystal structures closely parallel to those of silicates, but the Plagioclase CaAl2Si2O8 or transformations occur at lower pressures. Spinel (MgFe)Al2O4 or Thermodynamics requires that the high pressure Clinopyroxene (NaCa)2(AlSi)2O6 or polymorph be denser, which limits possible structures. Once Garnet Mg3Al2Si3O12 or structure is known the bond lengths between cations and anions Hornblende or enable densities to be calculated. Phlogopite The Radius Ratio (Rcation/Ranion) determines type of crystal structure. At high pressures effective radii contract Phase diagram (Fig. 7), though rather complicated, shows that differentially, thus altering the radius ratio. Thus a new high pressure plagioclase peridotite can exist only at very shallow depths where the phase may appear when radius ratios exceed certain critical values. geothermal gradient is high; spinel- and pyroxene peridotites have a Large ions (e.g. Oxygen 1.40Å) contract more under pressure than larger stability field in the upper mantle; but garnet peridotite will small ions. Oxygen is more polarizable than smaller cations: occur at deeper levels (hence common as nodules in kimberlite pipes). Element Polarizability Radius (Å) O-- 3.1 × 10-24cm3 1.4 Si4+ 0.04 × 10-24cm3 0.26 Mg2+ 0.12 × 10-24cm3 0.72

Pressure thus increases covalent component of chemical bond.

Phase transitions in the deeper mantle Refinement of seismic wave data has shown number of discontinuities (Fig. 8):

Fig. 7. Summary of phase relations in pyrolite (after Green & Ringwood) appropriate to upper mantle conditions. Wet solidus for small amount of hornblende breakdown. Note that only oceanic geotherm intersects this wet solidus.

Methods for investigating deep mantle mineralogy Direct sampling of the deeper mantle is obviously impossible. Observed seismic velocity-depth functions however constrain the densities of likely mantle phases. Moreover, possible phase These zones are: transformations in the transition zone of the mantle are very difficult to verify experimentally. For instance, in the 400-900 km depth (1) LOW VELOCITY ZONE: from below lithosphere to about region pressures are in the range 130-340 kilobars and temperatures 200-250 km. Not always present. Asthenosphere has S-wave 1500-3000°C ... beyond the range of most experimental equipment attenuation . . small amount of liquid, perhaps ca 1% melting? until recently. Now with diamond anvil apparatus, laser heating and (2) MINOR DISCONTINUITY around 350 km. on-line X-ray determinations it is possible to reach into this range, at (3) MAJOR DISCONTINUITY at 400 km. least momentarily. (4) MAJOR DISCONTINUITY at 650 km. Indirect methods have also proved reasonably successful. Fortunately high pressure phases tend to crystallise in structures

4 Plate Tectonics: GL209 Prof. John Tarney Lecture 1: Geological Aspects

(5) Between 900 and 2700 km no major discontinuities, but some Fig. 9. Density changes with depth in the mantle, and smaller ones. In general increase in seismic velocities and density the changes in mineral structure that have been explained by self-compression of homogenous material. proposed to explain them The following explanations have been proposed to explain these discontinuities (Fig. 9): A point of interest is whether this sharp increase in density at 650- 700 km acts as a barrier to mantle convection. If the slab cannot 350 km. Pyroxene forms a complex solid solution with pre-existing penetrate this boundary, does it pile up above 700km? garnet in which one-quarter of silicon atoms are octahedrally co- Are there two convecting zones in the mantle: one above, one below ordinated, leading to 10% increase in density of pyroxene the 700km discontinuity? Does this also coicide with a chemical component: boundary? Is there any chemical interchange across the boundary Mg3(MgSi)Si3O12 & Ca3(CaSi)Si3O12 layer? Fate of the subducted slab: Ringwood 1991 Model 400 km. Olivine transforms to beta-Mg2SiO4 which has SPINEL One of the problems of plate tectonics is the fate of the structure and is 8% denser than olivine. subducting slab. This can be traced, from seismic evidence, to descend to about 650 km; but the evidence is somewhat conflicting 500-550 km. Calcium silicate CaSiO3 component of garnet regarding the extent to which it penetrates the dense 650 km transforms to extremely dense PEROVSKITE structure. Also beta- discontinuity. (See references by Jordan and Hilst). Because the Mg2SiO4 transforms to gamma-Mg2SiO4 with 2% increase in phase changes with depth are now known in some detail, both for density. ultramafic mantle material and for subducted basaltic ocean crust, it is possible to calculate their modal compositions with depth. For 650 km. The spinel structure disproportionates to MgSiO3 with instance, the modal composition of pyrolite with depth is shown in PEROVSKITE structure and MgO with a ROCK SALT structure, Fig. 10: i.e. Mg2SiO4 > MgSiO3 + MgO. Additionally the MgSiO3.Al2O3 component transforms to an ILMENITE structure and any sodium present will transform to a high pressure form of NaAlSiO4 having CALCIUM FERRITE structure.

Lower Mantle. Below 700 km no more major transformations are possible - the minerals are as close-packed as they can get. There is thus then a slow progressive increase in density to the mantle-core boundary.

Fig. 10 Modal composition of pyrolite with depth.

5 Plate Tectonics: GL209 Prof. John Tarney Lecture 1: Geological Aspects

Fig. 13. Density differences between basalt - pyrolite and harzburgite - pyrolite as the subducted ocean crust sinks. The basaltic slab becomes less dense than mantle pyrolite in the depth range 650 - 750 km. Fig. 11 shows the same calculations for basaltic ocean crust. The important point is that the subducted plate sinks Note that the plate which is subducting is not uniform mantle because the basaltic component of the slab (now eclogite) is ca. 0.2 - pyrolite but, because of melting at the ridge axis, it has segregated 0.1 g/cc more dense than the enclosing host pyrolite to depths of 650 into a basaltic ocean crust (ca 5 km thick), residual harzburgite (from km, and exerts a strong 'slab-pull' force at subduction zones. The which the basalts were extracted) underlain by ordinary pyrolite. harzburgite part of the plate may also be slightly more dense initially Knowing the mineral proportions and the densities of the minerals in because it is cold, but is inherently less dense once it has thermally each of the main rock types, undepleted pyrolite, depleted equilibrated with the surrounding mantle pyrolite. However, because harzburgite, and basaltic ocean crust, it is then possible to calculate of the phase changes in pyrolite at the 670 km discontinuity, the the density changes in each of these rock types with depth. The basaltic crust suddenly becomes 0.2 g/cc less dense than the pyrolite thermally equilibrated densities for these three rock types with depth in the depth range 650-750 km, whereas the harzburgite component are shown in Fig. 12. of the slab becomes very slightly more dense. These effects are very clearly shown in Fig. 13. Ringwood (1991) argues that these changes then have the effect of trapping subducted basaltic ocean crust at the 670 km discontinuity, as shown in Fig. 14.

Fig. 12. Densities (g/cc) of thermally equilibrated basaltic ocean crust, subducted harzburgite lithosphere compared with undepleted pyrolite mantle to depths of 800 km. Note that the ocean crust is mostly more dense and the harzburgite is less dense than pyrolite down to 650km depth, but then their positions are reversed. Fig. 14. The effect of density differences is that basaltic ocean crust becomes trapped at the 670km discontinuity.

6 Plate Tectonics: GL209 Prof. John Tarney Lecture 1: Geological Aspects

Ringwood has suggested that the slab piles up at the base of the upper mantle, as shown in Fig. 14. By the end of the Archaean (2500 my ago) he envisages that the mantle structure around the 650km discontinuity would be as shown in Fig. 15. This layer is source for diamond-bearing kimberlite magmas according to Ringwood et al. (1992).

Fig. 16. Models of mantle differentiation involving storage of ocean crust at the 650 km discontinuity.

This has interesting consequences as a mechanism to generate mantle plumes and 'hotspots'. Most plumes need to be generated at a discontinuity, either the 650 km one or at the core-mantle boundary. The mantle model (Fig. 16) shows that these plumes rise and penetrate the lithosphere to become the source of hotspot ocean islands. If these rising diapirs cannot penetrate the lithosphere, they Fig. 15. Likely mantle structure at the end of the Archaean may just add to the base of the lithsophere, and melts may penetrate as a result of subducted mafic ocean crust piling up at the it and metasomatise and chemically alter it. The ocean lithosphere 650 km discontinuity (after Ringwood). is young (almost all less than 200 my) whereas the sub-continental lithosphere is older, cooler, thicker and more complex, as shown in both Figs. 16 & 17.

Because of the different thermal regimes, and the influences of plumes, it is likely that there have been differences in the make-up of the lithosphere during Earth history. Fig. 18 (below) shows the likely structure of the modern mature Phanerozoic ocean lithosphere (left), which is regarded as being less depleted with increasing depth. Ocean plateaus (centre) have a very thick ocean crust, with (implicitly) a much more depleted harzburgitic mantle underlying it. In the Archaean (right) one suggestion is that the high mantle temperatures would have led to very high degrees of melting, to produce high-Mg komatiitic lavas, and leaving an extremely depleted pure-olivine dunitic residue. This oceanic structure is much more like that of modern oceanic plateaus, so was there plate tectonics in the Archaean or plateau tectonics (see later PlateLect-F)?

Fig. 17. (above) Comparison between oceanic and continental lithosphere.

Assuming constant spreading rates (present day) it can be calculated that, throughout Earth history, the amount of ocean crust which may have accumulated at the 650 km discontinuity would be at least 100 km thick. However because the harzburgite is inherently less dense and potentially more buoyant than the surrounding mantle, then when it heats up it may begin to ascend as blobs or diapirs, as shown in Fig. 16 (below).

7 Plate Tectonics: GL209 Prof. John Tarney Lecture 1: Geological Aspects

Mantle Convection Models of mantle differentiation The diagrams below show some conceptual models of how This widely accepted model implies that mantle (or at least the mantle may be convecting, and possible relationships between the upper mantle) is continually differentiating to form continental the upper and lower mantle (after Allègre et al.). There is still a very crust by a two-stage process. The crust formed is permanent and is intense debate on whether the lower mantle is involved in not recycled back into the mantle. manconvection. (1) Primitive pyrolite mantle rises at mid-ocean ridges, melts to form basaltic ocean crust overlying refractory harzburgite plate. (2) Plate sinks back into the mantle at subduction zones. Hydrated altered ocean crust dehydrates and causes melting of the basaltic ocean crust and of the overlying mantle wedge to yield andesitic magmas. (3) Andesitic magmas fractionate en route to the surface to produce more siliceous magmas. Hence sialic crust accretes laterally at continental margins, is of low density and is indestructible.

Fig. 20. Box models for crust-mantle evolution. On the left continental growth occurs through igneous contributions from both the upper and lower mantle. On the right the continental crust has mainly been extracted from the upper mantle, which is therefore "depleted" relative to lower mantle.

Fig. 19. Simple "box model" of mantle evolution, showing how melting at spreading ridges produces ocean crust, which Slab Penetration into Lower Mantle? is then altered by hydrothermal activity and then subducted. Part of this subducted crust is then melted to form continental crust, and the residues then subducted to become part of the reservoir of the depleted (DMM) mantle. Small degree melts migrate upwards to enrich the sub-continental mantle and provide the source for alkali basalts. Sediment subduction may modify the sub-continental lithosphere. (after Tarney et al. 1980)

A consequence of course is that if the continental crust has been extracted from the convecting mantle, the convecting mantle must have become progressively depleted in lithophile elements. This is now known as the 'DM' mantle reservoir. This is the reservoir that supplies depleted mid-ocean ridge basalt ("MORB"). The real story is a little more complicated, as may be deduced form Fig. 19. Sediments may be subducted and contaminate the lithosphere under continental margins as well as the material stored at the 650 km discontinuity. Small degree melts permeate upwards and vein both the sub-continental and sub-oceanic lithosphere, but because the former is older, we generally observe more complex effects under the continents. Fig. 22. Cartoon showing how subducting slabs may either lay themselves out along the 650 km discontinuity (a), or penetrate the discontinuity to enter the lower mantle as in (b). The latter gives active back-arc spreading (see later lecture).

8 Plate Tectonics: GL209 Prof. John Tarney Lecture 1: Geological Aspects

Fig. 23 (after Kerr et al. 1995) shows 2-layer convection of The most recent analysis of the fate of the oceanic crust as it the mantle, as subducting plates lodge at the 670km discontinuity, or subducts into the mantle beneath the West Pacific island arcs (van get convected back into the upper mantle; but with periodic der Hilst & Seno, 1993), suggests that whereas that subducting interchange between the two as cold plates avalanche down into the beneath the Izo-Bonin arc and Shikoku Basin, south of Japan may be lower mantle, and deep mantle plumes are displaced and rise to form deflected and "laid-out" along the 650 km discontinuity in the major oceanic plateaus and continental large igneous provinces transition zone (Fig. 22(a)), that further south beneath the Mariana ("LIPs"). There may be composition differences between upper Arc may penetrate into the lower mantle (Fig. 22(b)): mantle and lower mantle as a result of such processes through Earth history. UPDATES (1994/6) Larson and Kincaid (1996) then go on to argue that the breakup of Ringwood's Megalith Model: major continents, as occurred with the Gondwana supercontinent in the Mesozoic (ca. 130Ma), leads to more rapid subduction of old The essence of the Ringwood megalith model is that while ocean cold ocean crust. These cold slabs then avalanche down and lithosphere subduction is initially thermally driven because the penetrate the 670km thermal boundary layer into the lower mantle. downgoing slab is cold [the basaltic (eclogite) layer is 5% more One effect is to raise the 670km TBL; another is to displace material dense than the surrounding mantle, and compensates for the depleted from the deep lower mantle (D") which appears as major mantle harzburgite which is 2% lighter – see Fig. 13], the compositional plumes during the mid-Cretaceous magnetic superchron (120 Ma - buoyancy difference between the two becomes significant at 700km 80 Ma). See later notes on mantle plumes. depth, resulting in the mechanical separation of basaltic from harzburgite/dunite components (see Fig. 15). However, recent modelling by Gaherty & Hager (1994), using a range of viscosity contrasts for eclogite vs harzburgite, shows that the two are unlikely to separate. The slab buckles and folds as it reaches the 700 km discontinuity, but there is no obvious separation of eclogitic and harzburgitic components. The compositional buoyancy differences are subordinate to the overall thermal buoyancy.

Nature of the Lower Mantle:

While it is generally known that the convecting Upper Mantle (above the 670km discontinuity) is chemically depleted in lithophile elements because of the progressive growth and extraction of the continental crust from it, it has been commonly thought that the Lower Mantle is largely undepleted. However, Kerr et al. (1995) have proposed that the Lower Mantle is also depeleted, in part because of the return of subducting slab material right through the 670km discontinuity into the lower mantle: see also van der Hilst & Seno (1993). This implies that there is much more interchange between Upper and Lower mantle than was first thought. The Lower Mantle feeds into the upper mantle in the form of large hot plumes (see later lecture). Figure 23 below shows how cool subducted material may go right into lower mantle, or get stuck termporarily at the 670km discontinuity and then 'drip' into the lower mantle:

9 Plate Tectonics: GL209 Prof. John Tarney Lecture 1: Geological Aspects

RINGWOOD, A.E. 1985. Mantle dynamics and basalt petrogenesis. REFERENCES: Mantle Mineralogy Tectonophysics 112, 17-34. RINGWOOD, A.E. 1986. Dynamics of subducted lithosphere and implications for basalt petrogenesis. Terra Cognita 6, 67-77. (These references are probably more than you require at this stage, RINGWOOD, A.E. 1991. Phase transitions and their bearing on the but as some aspects are followed up in more detail later in your constitution and dynamics of the mantle. Geochimica et courses, they may be useful to you.) Cosmochimica Acta 55, 2083-2110. RINGWOOD, A.E. & IRIFUNE, T. 1988. Nature of the 650-km ALLÈGRE, C.J. 1982. Chemical geodynamics. Tectonophysics 81, seismic discontinuity: implications for mantle dynamics and 109-132. differentiation. Nature 331, 131- 136. ALLÈGRE, C.J. & TURCOTTE, D.L. 1986. Implications of a two- RINGWOOD, A.E., KESSON, S.E., HIBBERSON, W. & WARE, component marble-cake mantle. Nature 323, 123-127. N. 1992. Origin of kimberlite and related magmas. Earth and BLUNDELL, D.J. 1990. Seismic images of continental lithosphere. Planetary Science Letters 113, 521-538. Journal of the Geological Society, London 147, 895-913. SADOWIAK, P., WEVER, T. and MEISSNER, R. 1991. Deep GAHERTY, J.B. & HAGER, B.H. 1994. Compositional vs. thermal seismic reflectivity patterns in specific tectonic units of Western buoyancy and the evolution of subducted lithosphere. and Central Europe. Geophysics Journal International 105, 45- Geophysics Research Letters 21, 141-144. 54. IRIFUNE, T. & RINGWOOD, A.E. 1987. Phase transformations in a TARNEY, J., WOOD, D.A., SAUNDERS, A.D., CANN, J.R. & harzburgite composition to 26 GPa: implications for dynamical VARET, J. 1980. Nature of mantle heterogeneity in the North behaviour of the subducting slab. Earth and Planetary Science Atlantic: evidence from deep sea drilling. Phil. Trans. Roy. Soc. Letters 86, 365-376. London A297, 179-202. IRIFUNE, T. & RINGWOOD, A.E. 1993. Phase transformations in van der HILST, R. & SENO, T. 1993. Effects of relative plate subducted ocean crust and buoyancy relationships at depths of motion on the deep structure and penetration depth of slabs 600-800 km in the mantle. Earth and Planetary Science Letters below the Izu-Bonin and Mariana island arcs. Earth and 117, 101-110. Planetary Science Letters 120, 395-407. JORDAN, T.H. 1975. The continental tectosphere. Review of WEVER, T., TRAPPE, H. and MEISSNER, R. 1987. Possible Geophysics and Space Physics 13, 1-12. relations between crustal reflectivity, crustal age, heat flow and JORDAN, T.H. 1978. Composition and development of the viscosity of the continents. Annales Geophysicae 5B, 255-266. continental tectosphere. Nature 274, 544-548. WARNER, M.R. 1990. Basalts, water or shear zones in the lower JORDAN, T.H. 1981. Continents as a chemical boundary layer. continental crust? Tectonophysics 173, 163-173. Philosophical Transactions of the Royal Society, Lond. A301, WYLLIE, P.J. 1971. The Dynamic Earth. Wiley, London 359-373. KEARY, P. & VINE, F.J. 1991. Global Tectonics. Blackwell Scientific Publ., 302pp. KERR, A.C., SAUNDERS, A.D., TARNEY, J., BERRY, N.H & HARDS, V.L. 1995. Depleted mantle-plume geochemical signatures: no paradox for plume theories. Geology 23, 843-846. KUSZNIR, N. and MATTHEWS, D.H. 1988. Deep seismic APPENDIX: reflections and the deformational mechanics of the continental Germanates as high pressure models of silicates lithosphere. Journal of Petrology Special Lithosphere Issue, pp. 63-87. Trying to elucidate the petrological nature of the deep mantle is not LARSON, R.L. & KINCAID, C. 1996. Onset of mid-Cretaceous easy because it is difficult to re-create such high-pressure - high- volcanism by elevation of the 670 km thermal boundary layer. temperature conditions in the laboratory. At least for any length of Geology 24, 551-554. time: Laser heating and momentary shock treatment can do it for a MEISSNER, R. 1989. Rupture, creep, lamellae and crocodiles: short time, but as silicate reactions usually take a long time to reach happenings in the continental crust. Terra Nova 1, 17-28. equilibrium condition, this leads to huge uncertainties in P-T MEISSNER, R. and KUSZNIR, N. 1987. Crustal viscosity and the parameters. However, in the early years, mineral chemistry reflectivity of the lower crust. Annales Geophysicae 5B, 365- principles were used to predict high pressure behaviour. 373. MEISSNER, R., MATTHEWS, D.H. and WEVER, T. 1986. The Use of germanates to model high pressure silicates was first Moho in and around Britain. Annales Geophysicae 4B, 659-666. suggested by Goldschmidt in 1931. Si and Ge are tetravalent and in MENZIES, M.A. 1990. Archaean, Proterozoic, and Phanerozoic same group in the Periodic Table. lithospheres. In: M.A. Menzies (Editor) Continental Mantle, Clarendon Press, Oxford, pp.67-86. RADII: Si4+ 0.26A Ge4+ 0.40A RINGWOOD, A.E. 1974. The petrological evolution of island arc systems. Journal of the Geological Society, London 130, 183- Silicates and germanates usually isostructural and there is a wide 204. range of germanate structures. So, if it is possible to synthesize a RINGWOOD, A.E. 1975. Composition and Petrology of the Earth's germanate structure at moderate pressures it is likely that an Mantle. McGraw-Hill, New York. equivalent silicate structure will exist at higher pressures. If a RINGWOOD. A.E. 1982. Phase transformations and differentiation germanate displays a phase transformation at a given pressure, the in subducted lithosphere: implications for mantle dynamics, corresponding silicate often displays the same transformation but at a basalt petrogenesis, and crustal evolution. Journal of Geology 90, much higher pressure. This is because the critical radius ratio 611-643.

10 Plate Tectonics: GL209 Prof. John Tarney Lecture 1: Geological Aspects

RGe/ROxygen for transformation to a new phase is attained at much Fig. 12. Densities (g/cc) of thermally equilibrated basaltic ocean lower pressures with Ge. Some germanates (e.g. GeO2) can crust, subducted harzburgite lithosphere compared with undepleted crystallise at zero pressure while the equivalent silicate needs 100 kb pyrolite mantle to depths of 800 km. Note that the ocean crust is pressure. mostly more dense and the harzburgite is less dense than pyrolite down to 650km depth, but then their positions are reversed. Many transformations in germanates involve change from 4- to 6- Fig. 13. Density differences between basalt - pyrolite and harzburgite fold co-ordination with oxygen. Compare: - pyrolite as the subducted ocean crust sinks. The basaltic slab becomes less dense than mantle pyrolite in the depth range 650 - 750 NaAlSi2O8 >NaAlSi2O6 + SiO2 28kb (silicate) km. Albite > Jadeiite + Rutile str NaAlGe2O8 > NaAlGe2O6 + GeO2 5kb (germanate) Fig. 14. The effect of density differences is that basaltic ocean crust 2CoSiO3 >2Co2SiO4 + SiO2 100kb (silicate) becomes trapped at the 670km discontinuity. Pyroxene > Spinel + Rutile str 2CoGeO3 >2Co2GeO4 + GeO2 10kb (germanate) Fig. 15. Likely mantle structure at the end of the Archaean as a result of subducted mafic ocean crust piling up at the 650 km discontinuity (after Ringwood). These lines of reasoning allowed predictions to be made as to what types of structure might exist at depth in the Earth. Fig. 16 (below). Models of mantle differentiation involving storage Figure Captions of ocean crust at the 650 km discontinuity.

Fig. 1. The Earth in proportion (and the crust thickness has still been Fig. 17. (above) Comparison between oceanic and continental exaggerated!). Upper convecting mantle is quite a thin layer. Do lithosphere. superplumes strat at core-mantle boundary? FIg. 19. Simple "box model" of mantle evolution, showing how Fig. 2. Diagram (based on field and geophysical studies) to show melting at spreading ridges produces ocean crust, which is then how deep crust can be thrust up to high crustal levels. Kapuskasing altered by hydrothermal activity and then subducted. Part of this structure, Ontario, Canada) subducted crust is then melted to form continental crust, and the residues then subducted to become part of the reservoir of the Fig. 3. Estimates as to the extent to which we have sections thru two depleted (DMM) mantle. Small degree melts migrate upwards to segments of Archaan crust and one segment of recent Alpine crust. enrich the sub-continental mantle and provide the source for alkali basalts. Sediment subduction may modify the sub-continental Fig. 4. Various early models interpreting the Moho as a phase change lithosphere. (after Tarney et al. 1980) (no real change in composition). Fig. 20 (above). Box models for crust-mantle evolution. On the left Fig. 5a. Experimental studies of Green & Ringwood (1967) on quartz continental growth occurs through igneous contributions from both tholeiite basalt showed that transformation to eclogite occurred over the upper and lower mantle. On the right the continental crust has a considerable depth interval. Note that eclogite has a lot more mainly been extracted from the upper mantle, which is therefore quartz than the equivalent basalt. But mantle eclogites have no "depleted" relative to lower mantle. quartz. Where does the silica go? Fig. 21 (below) implies that the size of the convective cell depends Fig. 5b. Transformation of alkali basalt into eclogite also occurs over on the size of the ocean: the large Pacific ocean with whole-mantle considerable depth range. Not sharp enough to explain Moho. Note convection, the smaller Indian ocean with only upper mantle no quartz. convection. But evidence is slight!

Fig. 6. Moho as compositional change: various models. FIg. 22 (above). Cartoon showing how subducting slabs may either lay themselves out along the 650 km discontinuity (a), or penetrate Fig. 7. Summary of phase relations in pyrolite (after Green & the discontinuity to enter the lower mantle as in (b). The latter gives Ringwood) appropriate to upper mantle conditions. Wet solidus for active back-arc spreading (see later lecture). small amount of hornblende breakdown. Note that only oceanic geotherm intersects this wet solidus. Fig. 23 (after Kerr et al. 1995) shows 2-layer convection of the mantle, as subducting plates lodge at the 670km discontinuity, or get Fig. 9. Density changes with depth in the mantle, and the changes in convected back into the upper mantle; but with periodic interchange mineral structure that have been proposed to explain them. between the two as cold plates avalanche down into the lower mantle, and deep mantle plumes are displaced and rise to form major Fig. 11. Modal composition of subducted basaltic ocean crust with oceanic plateaus and continental large igneous provinces ("LIPs"). depth. Compare with pyrolite in Fig. 10. There may be composition differences between upper mantle and lower mantle as a result of such processes through Earth history.