Protostars and Planets VI

When continents rift to form new ocean basins, the rifting is sometimes accompanied by massive igneous activity. We show that the production of magmatically active rifted margins and the effusion of flood basalts onto the adjacent continents can be explained by a simple model of rifting above a thermal anomaly in the underlying mantle. The igneous rocks are generated by decompression melting of hot asthenospheric mantle as it rises passively beneath the stretched and thinned lithosphere. Mantle plumes generate regions beneath the lithosphere typically 2000 km in diameter with temperatures raised 100–200°C above normal. These relatively small mantle temperature increases are sufficient to cause the generation of huge quantities of melt by decompression: an increase of 100°C above normal doubles the amount of melt whilst a 200°C increase can quadruple it. In the first part of this paper we develop our model to predict the effects of melt generation for varying amounts of stretching with a range of mantle temperatures. The melt generated by decompression migrates rapidly upward, until it is either extruded as basalt flows or intruded into or beneath the crust. Addition of large quantities of new igneous rock to the crust considerably modifies the subsidence in rifted regions. Stretching by a factor of 5 above normal temperature mantle produces immediate subsidence of more than 2 km in order to maintain isostatic equilibrium. If the mantle is 150°C or more hotter than normal, the same amount of stretching results in uplift above sea level. Melt generated from abnormally hot mantle is more magnesian rich than that produced from normal temperature mantle. This causes an increase in seismic velocity of the igneous rocks emplaced in the crust, from typically 6.8 km/s for normal mantle temperatures to 7.2 km/s or higher. There is a concomitant density increase. In the second part of the paper we review volcanic continental margins and flood basalt provinces globally and show that they are always related to the thermal anomaly created by a nearby mantle plume. Our model of melt generation in passively upwelling mantle beneath rifting continental lithosphere can explain all the major rift-related igneous provinces. These include the Tertiary igneous provinces of Britain and Greenland and the associated volcanic continental margins caused by opening of the North Atlantic in the presence of the Iceland plume; the Parana and parts of the Karoo flood basalts together with volcanic continental margins generated when the South Atlantic opened; the Deccan flood basalts of India and the Seychelles-Saya da Malha volcanic province created when the Seychelles split off India above the Reunion hot spot; the Ethiopian and Yemen Traps created by rifting of the Red Sea and Gulf of Aden region above the Afar hot spot; and the oldest and probably originally the largest flood basalt province of the Karoo produced when Gondwana split apart. New continental splits do not always occur above thermal anomalies in the mantle caused by plumes, but when they do, huge quantities of igneous material are added to the continental crust. This is an important method of increasing the volume of the continental crust through geologic time.

Magmatism at rift zones: The generation of volcanic continental margins and flood basalts

Assembly, configuration, and break-up history of Rodinia: A synthesis

Basaltic volcanism 'samples' the Earth's mantle to great depths, because solid-state convection transports deep material into the (shallow) melting region. The isotopic and trace-element chemistry of these basalts shows that the mantle contains several isotopically and chemically distinct components, which reflect its global evolution. This evolution is characterized by upper-mantle depletion of many trace elements, possible replenishment from the deeper, less depleted mantle, and the recycling of oceanic crust and lithosphere, but of only small amounts of continental material.

Mantle geochemistry: the message from oceanic volcanism

Obesity induces an insulin-resistant state in adipose tissue, liver, and muscle and is a strong risk factor for the development of type 2 diabetes mellitus. Insulin resistance in the setting of obesity results from a combination of altered functions of insulin target cells and the accumulation of macrophages that secrete proinflammatory mediators. At the molecular level, insulin resistance is promoted by a transition in macrophage polarization from an alternative M2 activation state maintained by STAT6 and PPARs to a classical M1 activation state driven by NF-kappaB, AP1, and other signal-dependent transcription factors that play crucial roles in innate immunity. Strategies focused on inhibiting the inflammation/insulin resistance axis that otherwise preserve essential innate immune functions may hold promise for therapeutic intervention.

Macrophages, Inflammation, and Insulin Resistance

Syndrome X, typified by obesity, insulin resistance (IR), dyslipidemia, and other metabolic abnormalities, is responsive to antidiabetic thiazolidinediones (TZDs). Peroxisome proliferator-activated receptor (PPAR) γ, a target of TZDs, is expressed abundantly in adipocytes, suggesting an important role for this tissue in the etiology and treatment of IR. Targeted deletion of PPARγ in adipose tissue resulted in marked adipocyte hypocellularity and hypertrophy, elevated levels of plasma free fatty acids and triglyceride, and decreased levels of plasma leptin and ACRP30. In addition, increased hepatic glucogenesis and IR were observed. Despite these defects, blood glucose, glucose and insulin tolerance, and insulin-stimulated muscle glucose uptake were all comparable to those of control mice. However, targeted mice were significantly more susceptible to high-fat diet-induced steatosis, hyperinsulinemia, and IR. Surprisingly, TZD treatment effectively reversed liver IR, whereas it failed to lower plasma free fatty acids. These results suggest that syndrome X may be comprised of separable PPARγ-dependent components whose origins and therapeutic sites may reside in distinct tissues.

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Adipose-specific peroxisome proliferator-activated receptor gamma knockout causes insulin resistance in fat and liver but not in muscle.

Mantle Convection in the Earth and Planets is a comprehensive synthesis of all aspects of mantle convection within the Earth, the terrestrial planets, the Moon, and the Galilean satellites of Jupiter. The book includes up-to-date discussions of the latest research developments that have revolutionized our understanding of the Earth and the planets. It is suitable as a text for graduate courses in geophysics and planetary physics, and as a supplementary reference for use at the undergraduate level. It is also an invaluable review for researchers in the broad fields of the Earth and planetary sciences including seismologists, tectonophysicists, geodesists, mineral physicists, volcanologists, geochemists, geologists, mineralogists, petrologists, paleomagnetists, planetary geologists, and meteoriticists. The book features a comprehensive index, an extensive reference list, numerous illustrations (many in color) and major questions that focus the discussion and suggest avenues of future research.

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Mantle Convection in the Earth and Planets

Numerical calculations of fluid dynamos powered by thermal convection in a rotating, electrically conducting spherical shell are analyzed. We find two regimes of nonreversing, strong field dynamos at Ekman number 10 -4 and Rayleigh numbers up to 11 times critical. In the strongly columnar regime, convection occurs only in the fluid exterior to the inner core tangent cylinder, in the form of narrow columnar vortices elongated parallel to the spin axis. Columnar convection contains large amounts of negative helicity in the northern hemisphere and positive helicity in the southern hemisphere and results in dynamo action above a certain Rayleigh number, through a macroscopic α 2 mechanism. These dynamos equilibrate by generating concentrated magnetic flux bundles that limit the kinetic energy of the convection columns. The dipole-dominated external field is formed by superposition of several flux bundles at middle and high latitudes. At low latitudes a pattern of reversed flux patches propagates in the retrograde direction, resulting in an apparent westward drift of the field in the equatorial region. At higher Rayleigh number we find a fully developed regime with convection inside the tangent cylinder consisting of polar upwelling and azimuthal thermal wind flows. These motions modify the dynamo by expelling poloidal flux from the poles and generating intense toroidal fields in the polar regions near the inner core. Convective dynamos in the fully developed regime exhibit characteristics that can be compared with the geomagnetic field, including concentrated flux bundles on the core-mantle boundary, polar minima in field intensity, and episodes of westward drift.

Numerical modeling of the geodynamo: Mechanisms of field generation and equilibration

Summary

We analyse ~ 50 3-D numerical calculations of hydrodynamic dynamos driven by convection in a spherical shell. We examine rigid and stress-free boundaries, with Prandtl number 1, magnetic Prandtl numbers in the range 0.5–5, Ekman numbers E=10− 3–10− 4 and Rayleigh numbers to 15 times critical. No parametrizations such as hyperviscosities are used. Successful dynamos are compared with non-magnetic convection solutions. Results for various spectral truncations suggest that the calculations are well resolved when the kinetic and magnetic energy drops by more than two orders of magnitude from the spectral peak to the cut-off, although the basic features are still captured at lower resolution. With few exceptions we obtain dipole-dominated magnetic fields. The dynamos operate in the strong-field regime where Lorentz and Coriolis forces are of similar order. The critical magnetic Reynolds number for self-sustained dynamos is of order Rm=50. However, we also find that the field can die away when Rm is too large. The minimum magnetic Prandtl number at which we find dynamo action depends on the Ekman number to the 3/4 power. Dynamos at E=10− 3 are subcritical whereas those at E=10− 4 are generally supercritical. The presence of the magnetic field tends to break the equatorial symmetry of the flow and favours convection inside the inner core tangent cylinder. With stress-free boundaries, dynamo action suppresses the axisymmetric azimuthal wind that dominates in non-magnetic convection. The field morphology is broadly similar for both kinds of boundary conditions. When low-pass filtered, several models exhibit field structures that resemble the geomagnetic field at the core–mantle boundary to a surprising degree.

Numerical modelling of the geodynamo: a systematic parameter study

Results of fluid dynamical experiments on the behavior of subducted slabs are presented that address two important characteristics of subduction: slab penetration through the transition zone and horizontal slab migration. Cold, negatively buoyant molded slabs of concentrated sucrose solution, with viscosities of 3–5 × 106 P were introduced into a more dilute, two-layered sucrose solution representing the upper and lower mantle. The transition zone was modeled by a step increase in both density and viscosity. Ratios of slab thickness to upper layer depth, the relative viscosities in each layer, and the effective Rayleigh number were close to mantle values. The initial configuration consisted of two plates separated by a trench gap, with one plate attached to a dipping slab, simulating a developing subduction zone with an overriding plate. Two different boundary conditions were used at the trailing (ridge) end of the subducting plate: (1) fixed end, corresponding to zero ridge motion, and (2) free end, corresponding to zero ridge resistance. Based on results from 15 experiments with various combinations of upper layer, lower layer, and slab densities (ρU, ρL, ρS) and viscosities, we find that slab penetration depth is sensitive to both the normalized slab density anomaly R = (ρS - ρL)/(ρS - ρU) and the dip angle. Three penetration regimes are recognized: R ≳ 0.5, in which the slab sinks into the lower layer without distortion; −0.2 ≲ R ≲ 0.5, which permits limited penetration, the amount dependent on dip angle (in this regime the slab develops a thick root just beneath the transition zone); and R < −0.2, in which the slab is deflected by the transition, and little or no penetration occurs. Retrograde slab motion and trench migration occurred in nearly every case, with the overriding plate moving parallel to but faster than the trench, causing the gap between subducting and overriding plates to close with time. Prograde trench motion was never observed, and the trench remained stationary only in the extreme case (free ridge condition with equal upper and lower layer densities). These results suggest that the transient extension found in many back arc basins may be a direct consequence of the tendency for retrograde subduction and the amount of extension may be governed in part by the degree of slab penetration through the transition zone.

Peter Olson

Papers

Adipose-specific peroxisome proliferator-activated receptor gamma knockout causes insulin resistance in fat and liver but not in muscle.

Mantle Convection in the Earth and Planets

Numerical modeling of the geodynamo: Mechanisms of field generation and equilibration

Numerical modelling of the geodynamo: a systematic parameter study

An experimental study of subduction and slab migration