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Mid-Ocean Ridges: Mantle Convection and Formation of the Lithosphere

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Mid-Ocean Ridges: Mantle Convection and Formation of the Lithosphere MID-OCEAN RIDGES: MANTLE CONVECTION AND FORMATION OF THE LITHOSPHERE G. Ito and R. A. Dunn, University of Hawai’i at Manoa, mantle matrix and buoyantly rises toward the sur- Honolulu, HI, USA face, where it forms new, basaltic, oceanic crust. The & 2009 Elsevier Ltd. All rights reserved. crust and mantle cool at the surface by thermal conduction and hydrothermal circulation. This cooling generates a thermal boundary layer, which is rigid to convection and is the newly created edge of the tectonic plate. As the lithosphere moves away Introduction from the ridge, it thickens via additional cooling, Plate tectonics describes the motion of the outer becomes denser, and sinks deeper into the underlying lithospheric shell of the Earth. It is the surface ex- ductile asthenosphere. This aging process of the pression of mantle convection, which is fueled by plates causes the oceans to double in depth toward Earth’s radiogenic and primordial heat. Mid-ocean continental margins and subduction zones (Figure 2), ridges mark the boundaries where oceanic plates where the oldest parts of plates are eventually thrust separate from one another and thus lie above the downward and returned to the hot underlying upwelling limbs of mantle circulation (Figure 1). The mantle from which they came. upwelling mantle undergoes pressure-release partial Mid-ocean ridges represent one of the most im- melting because the temperature of the mantle sol- portant geological processes shaping the Earth; they idus decreases with decreasing pressure. Newly produce over two-thirds of the global crust, they are formed melt, being less viscous and less dense than the primary means of geochemical differentiation in the surrounding solid, segregates from the residual the Earth, and they feed vast hydrothermal systems 180 Ϫ150 Ϫ120 Ϫ90 Ϫ60 Ϫ30 0 30 60 90 120 150 90 90 75 75 IcelandIceland 60 60 e 45 g 45 idged RRidge AzoresAzores Ri ic t n 30 la TransformTransform 30 t A offsetoffset id RidgeRidge 15 MidMMid-Atlantic Atlantic 15 segmentsegment e s 0 i GalapagosGalapagos 0 R c i SpreadingSpreading CenterCenter f i c Ϫ15 a Ϫ15 P t s a Ϫ30 EastE Pacific Rise EasterEaster I.I. Ϫ30 an Indi est thw Ϫ45 SouthwestSou Indian Ϫ45 ge RidgeRid Ϫ60 Ϫ60 Ϫ75 Ϫ75 180 Ϫ150 Ϫ120 Ϫ90 Ϫ60 Ϫ30 0 30 60 90 120 150 Ϫ8000 Ϫ7000 Ϫ6000 Ϫ5000 Ϫ4000 Ϫ3000 Ϫ2000 Ϫ1000 0 1000 2000 3000 4000 5000 6000 7000 8000 Topography (km) Figure 1 Map of seafloor and continental topography. Black lines mark the mid-ocean ridge systems, which are broken into individual spreading segments separated by large-offset transform faults and smaller nontransform offsets. Mid-ocean ridges encircle the planet with a total length exceeding 50 000 km. Large arrows schematically show the direction of spreading of three ridges discussed in the text. 867 868 MID-OCEAN RIDGES: MANTLE CONVECTION AND FORMATION OF THE LITHOSPHERE (a) 3 4 5 Depth (km) 6 Plate HSC 7 0 50 100 150 Seafloor age (My) (b) 1 0.5 Height 0 0 2 4 6 8 10 12 Width Figure 2 (a) Global average (dots) seafloor depth (after correcting for sedimentation) and standard deviations (light curve) increase with seafloor age. Dashed curve is predicted by assuming that the lithosphere cools and thickens indefinitely, as if it overlies an infinite half space. Solid curve assumes that the lithosphere can cool only to a maximum amount, at which point the lithosphere temperature and thickness remain constant. (b) Temperature contours in a cross section through a 2-D model of mantle convection showing the predicted thickening of the cold thermal boundary layer (i.e., lithosphere, which beneath the oceans reaches a maximum thickness of B100 km) with distance from a mid-ocean ridge (left side). Midway across the model, small-scale convection occurs which limits the thickening of the plate, a possible cause for the steady depth of the seafloor beyond B80 Ma. (a) Modified from Stein CA and Stein S (1992) A model for the global variation in oceanic depth and heat flow with lithospheric age. Nature 359: 123–129, with permission from Nature Publishing Group. (b) Adapted from Huang J, Zhong S, and van Hunen J (2003) Controls on sublithospheric small-scale convection. Journal of Geophysical Research 108 (doi:10.1029/2003JB002456) with permission from American Geophysical Union. that influence ocean water chemistry and support Mantle Flow beneath Mid-ocean enormous ecosystems. Around the global ridge sys- Ridges tem, new lithosphere is formed at rates that differ by more than a factor of 10. Such variability causes large While the mantle beneath mid-ocean ridges is mostly differences in the nature of magmatic, tectonic, and solid rock, it does deform in a ductile sense, very hydrothermal processes. For example, slowly spread- slowly on human timescales but rapidly over geo- ing ridges, such as the Mid-Atlantic Ridge (MAR) and logic time. ‘Flow’ of the solid mantle is therefore Southwest Indian Ridge (SWIR), exhibit heavily often described using fluid mechanics. The equation faulted axial valleys and large variations in volcanic of motion for mantle convection comes from mo- output with time and space, whereas faster-spreading mentum equilibrium of a fluid with shear viscosity Z ridges, such as the East Pacific Rise (EPR), exhibit and zero Reynolds number (i.e., zero acceleration): smooth topographic rises with more uniform mag- matism. Such observations and many others can be d Z V VT P z 2=3Z dV r r þr Àr þr½ðÀ Þr largely understood in context of two basic processes: ÂÃÀÁ1 f Drg 0 1 asthenospheric dynamics, which modulates deep þ ðÞÀ ¼ ½ temperatures and melt production rates; and the bal- The first term describes the net forces associated with ance between heat delivered to the lithosphere, largely matrix shear, where V is the velocity vector, V is the by magma migration, versus that lost to the surface by velocity gradient tensor, and VT is its transpose;r the conduction and hydrothermal circulation. Unraveling second term P is the non-hydrostaticr pressure the nature of these interrelated processes requires the gradient; the thirdr term describes matrix divergence integrated use of geologic mapping, geochemical and dV (with effective bulk viscosity z) associated with petrologic analyses, geophysical sensing, and geody- meltr transport; and the last term is the body force, namic modeling. with f being the volume fraction occupied by melt MID-OCEAN RIDGES: MANTLE CONVECTION AND FORMATION OF THE LITHOSPHERE 869 Imposed surface motion to simulate seafloor spreading 0 1500 Rigid, cool lithosphere 1300 20 Hot ductile 18 as thenosphere 1000 40 14 10 6 60 2%/My Depth (km) 500 80 100 0 Ϫ150 Ϫ100 Ϫ50 0 50 100 150 Temp. (ЊC) Across-axis distance from ridge (km) Figure 3 Cross section of a 2-D numerical model that predicts mantle flow (shown by arrows whose lengths are proportional to flow rate), temperatures (shading), and melt production rate beneath a mid-ocean ridge (contoured at intervals of 2 mass %/My). In this particular calculation, asthenospheric flow is driven kinematically by the spreading plates and is not influenced by density variations 1 (i.e., mantle buoyancy is unimportant). Model spreading rate is 6 mm yrÀ . and r being the density contrast between the solid seismic anisotropy is thought to be caused by lattice- andr melt. To first order, the divergence term is neg- preferred orientation of olivine crystals due to ligible, in which case eqn [1] describes a balance mantle flow. Global studies show that at depths of between viscous shear stresses, pressure gradients, 200–300 km beneath mid-ocean ridges, surface and buoyancy. Locally beneath mid-ocean ridges, the waves involving only horizontal motion (Love spreading lithospheric plates act as a kinematic waves) tend to propagate slower than surface waves boundary condition to eqn [1] such that seafloor involving vertical motion (Rayleigh waves). This spreading itself drives ‘passive’ mantle upwelling, suggests a preferred orientation of olivine consistent which causes decompression melting and ultimately with vertical mantle flow at these depths but not the formation of crust (Figure 3). Independent of much deeper. plate motion, lateral density variations can drive ‘active’ or ‘buoyant’ mantle upwelling and further contribute to decompression melting, as we discuss Mantle Melting beneath Mid-ocean below. Ridges Several lines of evidence indicate that the up- welling is restricted to the upper mantle. The pres- Within the upwelling zone beneath a ridge, the sures at which key mineralogical transitions occur in mantle cools adiabatically due to the release of the deep upper mantle (i.e., at depths 410 and pressure. However, since the temperature at which 660 km) are sensitive to mantle temperature, but the mantle begins to melt drops faster with de- global and detailed local seismic studies do not reveal creasing pressure than the actual temperature, the consistent variations in the associated seismic struc- mantle undergoes pressure-release partial melting ture in the vicinity of mid-ocean ridges. This finding (Figure 3). In a dry (no dissolved H2O) mantle, indicates that any thermal anomaly and buoyant melting is expected to begin at approximately 60-km flow beneath the ridge is confined to the upper depth. On the other hand, a small amount of water mantle above the discontinuity at the depth of in the mantle (B102 ppm) will strongly reduce the 410 km. Regional body wave and surface wave mantle solidus such that melting can occur at depths studies indicate that it could even be restricted to the 4100 km. Detailed seismic studies observe low-vel- upper B200 km of the mantle. Although some global ocity zones extending to depth of 100–200 km, tomographic images, based on body wave travel which is consistent with the wet melting scenario.
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