Chapter 5 The eclogite engine The World's great age begins anew, being recognized that large blocks of eclogite in The golden years return, the mantle may be an important fertility source. Delaminated continental crust differs in many The Earth doth like a snake renew important respects from recycled MORB. It does Her winter weeds outgrown not go through subduction zone and seafloor pro­ Shelley cessing, it starts out hotter than MORB, it may occur in bigger blobs, and it is not accompanied by the same amount, if any, of buoyant infer­ The water cycle drives geological processes at tile harzburgite. Figure 5.1 illustrates the lower the surface. TI1e fact that water coexists as fluid, crust delamination cycle. The crust thickens by vapor and solid is crucial in shaping the Earth's tectonic or igneous processes, eventually forming surface. The fact that conditions in the upper dense eclogite that detaches and sinks into the mantle can readily convert eclogite to magma to mantle. It reaches a level of neutral buoyancy and basalt, and back, with enormous density changes, starts to warm up. Eventually it rises and forms a is crucial in global magmatism and tectonics. warm fertile patch in the mantle. If the overlying Phase changes in the mafic components of the continents have moved off, a midplate magmatic upper mantle are larger than thermal expansion province is the result. effects and they drive the eclogite engine. Thermal expansion is the main source of There are several ways to generate massive buoyancy in thermal convection of simple flu­ melting in the mantle; one is to bring hot mate­ ids. Phase changes can be more important in rial adiabatically up from depth until it melts; the mantle. When basalt converts to eclogite, the other is to insert low-melting point fertile or when it melts, there are changes in density material - delaminated lower arc-crust, for exam­ that far exceed those associated with thermal ple - into the mantle from above and allow the expansion. Removal of the dense lower continen­ mantle to heat it up. Both mechanisms may tal crust is an important element of plate tecton­ be involved in the formation of large igneous ics that complements normal subduction zone provinces - LIPs . The timescale for heating and and ridge processes. It causes uplift and magma­ recycling of lower-crust material is much less tism and introduces distinctive materials into the than for subducted oceanic crust because the for­ mantle that are dense, fertile and have low melt­ mer starts out much hotter and does not sink as ing points. After delamination, eclogitic lower deep. crust sinks into the mantle where it has rela­ TI1e standard petrological models for magma tively low seismic velocities and melting point genesis involve a homogenous pyrolite mantle, (Figures 5.2 and 5.4) compared with normal man­ augmented at times by small-scale pyroxenite tle peridotite. These fertile mafic blobs sink to veins or recycled oceanic crust. It is increasingly various depths where they warm up, melt and MANTLE STRATIGRAPHY 59 ROOT FORMATION higher than their starting temperatures, but lower than normal mantle temperatures. They heat up by conduction from the surrounding mantle (Figure 5.2); they are fertile blobs in the mantle, in spite of being cold. Trenches and continents move about on the ridge DELAMINATION Earth's surface, refertilizing the underlying man­ tle. Ridges also migrate across the surface, sam­ pling and entraining whatever is in the mantle beneath them. Sometimes a migrating ridge will override a fertile spot in the asthenosphere; a SPREADING melting anomaly ensues, an interval of increased magmatic output. Ridges and trenches also die and reform elsewhere. What is described is a form of convection, but it is quite different from heating the kind of convection that is usually treated by geodynamicists. It is more akin to fertilizing and UPWELLING mowing a lawn. The mantle is not just convect­ ing from a trench to a ridge; the trenches and 4 ridges are visiting the various regions of the man­ tle. The fertile dense blobs sink, more-or-less ver­ tically into the mantle and come to rest where SPREADING they are neutrally buoyant, where they warm up and eventually melt (Figures 5.3 and 5.4). They 5 will represent more-or-less fixed points relative to the overlying plates and plate boundaries. They are chemically distinct from 'normal' mantle. @ji The delamination cycle. Eclogites are not a uniform rock type; they come in a large variety of flavors and densities and end return to the surface as melting anomalies, often up at various depths in the mantle. at ridges. Large melting anomalies that form on or near ridges and triple junctions may be due to the resurfacing of fertile blobs, including delam­ Mantle stratigraphy inated continental crust. This is expected to be especially prevalent around the 'passive' margins The densities and shear-velocities of crustal and of former supercontinents: Bouvet, Kerguelen, mantle minerals and rocks are arranged in order Broken Ridge, Crozet, Mozambique ridge, Marion, of increasing density (Figure 5.2). Given enough Bermuda, Jan Mayen, Rio Grande rise and Walvis time, this is the stratification toward which the ridge may be examples; the isotopic signatures mantle will evolve - the neutral-density profile of of these plateaus are expected to reflect lower the mantle. Such density stratification is already crustal components. evident in the Earth as a whole, and in the crust and continental mantle. This stratification, at least of the upper mantle, is temporary, however, The eclogite cycle even if it is achieved. Cold eclogite is below the melting point but eclogite melts at much lower Cold oceanic crust and warm lower continen­ temperatures than the surrounding peridotite. As tal crust are continuously introduced into the eclogite warms up, by conduction of heat from mantle by plate tectonic and delamination pro­ the surrounding mantle, it will melt and become cesses. These materials melt at temperatures buoyant, creating a form of yo-yo tectonics. 60 THE ECLOGITE ENGINE depth Rock type SHEAR VELOCITY (P = 0) SLAB EQUILIBRIUM reflectors density Vs 3 4 5 6 (km) (glee) km/s Eclogite Melts granodiorite 2.68 3.68 13 gneiss 2.79 3.57 anorthosite 2.80 3.73 serpentinite 2.81 3.83 20 metabasalt 2.87 3.28 gabbro 2.95 3.64 amphibolite 3.07 4.30 granulite-mafic 3.10 4.05 amphibole 3.20 3.80 pyroxenite 3.23 4.43 60 eclogite 3.24 4.28 continental Avg.ultramafic rock 3.29 4.68 moho cpx 3.30 4.60 80 PHN1569 3.31 4.87 sp.perid. 3.35 4.52 90 G1.Lhz. 3.35 4.83 UPPER .0 eclogite 3.37 4.90 Ui 130 PHN1611 3.42 4.76 MANTLE "' eclogite 3.43 4.58 LVZ eclogite 3.46 4.77 200 Hawaii Lhz. 3.47 4.72 220 ~-spinei(O FeO) 3.47 5.73 eclogite 3.49 4.95 280 majorite 3.53 5.43 330 y-spinel 3.55 5.79 garnet 3.57 5.08 0 20 40 60 80 100 400 P · spine~.1 FeO) 3.59 5.54 Time since Subduction (Myr) 410 gr.garnet 3.60 5.45 TZ eclogite 3.60 4.86 eclogite 3.61 4.69 LVZ Heating rates of subducted slabs due to mj 3.61 5.65 conduction of heat from ambient mantle. Delaminated eclogite(cold) 3.67 4.90 3 4 5 6 continental crust starts hot and will melt quickly. The total 500 y· spine~ . 1 FeO) 3.68 5.59 py.garnet 3.71 5.01 recycle time, including reheating, may take 30-75 Myr. If 'ilmenite ~. 1 FeO) 3.92 5.71 delamination occurs at the edge of a continent, say along a mj 4.00 5.63 suture belt, and the continent moves off at 3.3 em/year, the 650 mw( Mg.8) 4.07 5.08 710 perovskite(pV) 4.10 6.11-..._ average opening velocity of the Atlantic ocean, it will have 1000 LOWER MANTLE moved I 000 km to 2500 km away from a vertically sinking Density and shear-velocity of crustal and mantle root. One predicts paired igneous events, one on land and minerals and rocks at STP, from standard compilations. The one offshore (see Figure 5.5). ordering approximates the situation in an ideally chemically stratified mantle. The materials are arranged in order of increasing density. The STP densities of peridotites vary from 3.3 to 3.47 g/cm3; eclogite densities range from 3.45 to 3.75 g/cm3. The lower densicy eclogites (high-MgO. low-Si02) have densities less than the mantle below 41 0 km and will 8 therefore be trapped at that boundary. even when cold, Q) ::; creating a LVZ. Eclogites come in a large variecy of Iii Q) compositions, densities and seismic velocities. They have a. much lower melting points than peridotites and will E ~ eventually heat up and rise, or be entrained. If the mantle is close to its normal (peridotitic) solidus, then eclogitic blobs will eventually heat up and melt. Cold oceanic crust can contain perovskite phases and can be denser than shown here. Regions of over-thickened crust can transform to eclogite and become denser than the underlying mantle. The Pressure (GPa) upper mantle may contain peridotites, lherzolites and some Melting relations in dry lherzolite and eclogite of the less garnet-rich, high-MgO, eclogites. based on laboratory experiments. The dashed line is the I 300-degee mantle adiabat. showing that eclogite will melt as it sinks into normal temperature mantle, and upwellings from the shallow mantle will extensively melt gabbro and eclogite.