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At least 109 records · Page 6
All sourcesNASA NTRSDOE OSTI
NASA NTRS1985

Viscosity profile of the lower mantle

The viscosity of the earth's mantle is an important factor in studies of mantle convection and other problems in geodynamics. The present investigation is concerned with a determination of the variation of effective viscosity across the lower mantle from models of the Gibb's free energy of activation G(asterisk) and the adiabatic temperature profile. The variation of G(asterisk) with depth is calculated using both an elastic strain energy model, in which G(asterisk) is related to the seismic velocities, and a model which assumes G(asterisk) is proportional to the melting temperature.

Ellsworth, K.↗
NASA NTRS1992

Water inventories on Earth and Mars: Clues to atmosphere formation

Water is distributed differently on Earth and on Mars and the differences may have implications for the accretion of the two planets and the formation of their atmospheres. The Earth's mantle appears to contain at least several times the water content of the Martian mantle even accounting for differences in plate tectonics. One explanation is that the Earth's surface melted during accretion, as a result of development of a steam atmosphere, thereby allowing impact-devolitalized water at the surface to dissolve into the Earth's interior. In contrast, because of Mars' smaller size and greater distance from the Sun, the Martian surface may not have melted, so that the devolatilized water could not dissolve into the surface. A second possibility is suggested by the siderophile elements in the Earth's mantle, which indicates the Earth acquired a volatile-rich veneer after the core formed. Mars may have acquired a late volatile-rich veneer, but it did not get folded into the interior as with the Earth, but instead remained as a water rich veneer. This perception of Mars with a wet surface but dry interior is consistent with our knowledge of Mars' geologic history.

Carr, M. H.↗
NASA NTRS2001

Geodynamo Modeling of Core-Mantle Interactions

Angular momentum exchange between the Earth's mantle and core influences the Earth's rotation on time scales of decades and longer, in particular in the length of day (LOD) which have been measured with progressively increasing accuracy for the last two centuries. There are four possible coupling mechanisms for transferring the axial angular momentum across the core-mantle boundary (CMB): viscous, magnetic, topography, and gravitational torques. Here we use our scalable, modularized, fully dynamic geodynamo model for the core to assess the importance of these torques. This numerical model, as an extension of the Kuang-Bloxham model that has successfully simulated the generation of the Earth's magnetic field, is used to obtain numerical results in various physical conditions in terms of specific parameterization consistent with the dynamical processes in the fluid outer core. The results show that depending on the electrical conductivity of the lower mantle and the amplitude of the boundary topography at CMB, both magnetic and topographic couplings can contribute significantly to the angular momentum exchange. This implies that the core-mantle interactions are far more complex than has been assumed and that there is unlikely a single dominant coupling mechanism for the observed decadal LOD variation.

Kuang, Wei-Jia↗
NASA NTRS2011

Fortnightly Earth Rotation, Ocean Tides, and Mantle Anelasticity

Sustained accurate measurements of earth rotation are one of the prime goals of Global Geodetic Observing System (GGOS). We here concentrate on the fortnightly (Mf) tidal component of earth-rotation data to obtain new results concerning anelasticity of the mantle at this period. The study comprises three parts: (1) a new determination of the Mf component of polar motion and length-of-day from a multi-decade time series of space-geodetic data; (2) the use of the polar-motion determination as one constraint in the development of a hydrodynamic ocean model of the Mf tide; and (3) the use of these results to place new constraints on mantle anelasticity. Our model of the Mf ocean tide assimilates more than fourteen years of altimeter data from the Topex/Poseidon and Jason-1 satellites. The polar motion data, plus tide-gauge data and independent altimeter data, give useful additional information, with only the polar motion putting constraints on tidal current velocities. The resulting ocean-tide model, plus the dominant elastic body tide, leaves a small residual in observed length-of-day caused by mantle anelasticity. The inferred effective tidal 0 of the anelastic body tide is 90 and is in line with a omega-alpha frequency dependence with alpha in the range 0.2--0.3.

Ray, Richard D.↗
NASA NTRS1993

Three-dimensional spherical models of layered and whole mantle convection

We present numerical calculations of three-dimensional spherical shell thermal convection for constant viscosity and stratified viscosity models of whole-layer and two-layer mantle convection. These four examples are intended to provide theoretical guidance for determining the style of convection that is occurring in Earth's mantle. An impermeable interface between the upper and lower convecting shells in the two-layer solutions is placed at a depth of 670 km to coincide with the mantle seismic discontinuity that divides the upper and lower mantle. The interface results in an internal thermal boundary layer that raises the mean temperature in the lower shell by about 1400 K compared to the whole-layer solutions. The patterns of convection in the upper part of the whole-layer solutions are dominated by narrow arcuate sheetlike downflows in a background of weak upflow. In contrast, the upper shells of the two-layer solutions have complicated networks of convective rolls with the upflows and downflows having very similar structure. The structure of convection in the lower shells is similar to that in the lower part of the whole-layer solutions. Based on the horizontal structure of subduction zones on Earth's surface and on tomographic images of temperature variations in Earth's mantle, we conclude that the style of convection in Earth's mantle is more like that of the whole-mantle models.

Glatzmaier, Gary A.↗
NASA NTRS2008

How Deep and Hot was Earth's Magma Ocean? Combined Experimental Datasets for the Metal-silicate Partitioning of 11 Siderophile Elements - Ni, Co, Mo, W, P, Mn, V, Cr, Ga, Cu and Pd

Since approximately 1990 high pressure and temperature (PT) experiments on metal-silicate systems have showed that partition coefficients (D) for siderophile (iron-loving) elements are much different than those measured at low PT conditions. The high PT data have been used to argue for a magma ocean during growth of the early Earth. Initial conclusions were based on experiments and calculations for a small number of elements such as Ni and Co. However, for many elements only a limited number of experimental data were available then, and they only hinted at values of metal-silicate D's at high PT conditions. In the ensuing decades there have been hundreds of new experiments carried out and published on a wide range of siderophile elements. At the same time several different models have been advanced to explain the siderophile elements in the earth's mantle: a) intermediate depth magma ocean; 25-30 GPa, b) deep magma ocean; up to 50 GPa, and c) early reduced and later oxidized magma ocean. Some studies have drawn conclusions based on a small subset of siderophile elements, or a set of elements that provides little leverage on the big picture (like slightly siderophile elements), and no single study has attempted to quantitatively explain more than 5 elements at a time. The purpose of this abstract is to update the predictive expressions outlined by Righter et al. (1997) with new experimental data from the last decade, test the predictive ability of these expressions against independent datasets (there are more data now to do this properly), and to apply the resulting expressions to the siderophile element patterns in Earth's upper mantle. The predictive expressions have the form: lnD = alnfO2 + b/T + cP/T + d(1Xs) + e(1Xc) + SigmafiXi + g These expressions are guided by the thermodynamics of simple metal-oxide equilibria that control each element, include terms that mimic the activity coefficients of each element in the metal and silicate, and quantify the effect of variable oxygen fugacity. Preliminary results confirm that D(Ni) and D(Co) converge at pressures near 25-30 GPa and approximately 2200 K, and show that D(Pd) and D(Cu) become too low at the PT conditions of the deepest models. Furthermore, models which force fit V and Cr mantle concentrations by metal-silicate equilibrium overlook the fact that at early Earth mantle fO2, these elements will be more compatible in Mg-perovskite and (Fe,Mg)O than in metal. Thus an intermediate depth magma ocean, at 25-30 GPa, 2200 K, and at IW-2, can explain more mantle siderophile element concentrations than other models.

Righter, Kevin↗
NASA NTRS2002

Constraining the Material that Formed the Moon: The Origin of Lunar V, CR, and MN Depletions

The mantles of the Earth and Moon are similarly depleted in V, Cr, and Mn relative to chondritic values. Core formation deep within the Earth was suggested by as the origin of the depletions. Following Earth's core formation, the Moon was proposed to have inherited its mantle from the depleted mantle of the Earth by a giant impact event. This theory implied the Moon was primarily composed of material from the Earth's mantle. Recent systematic metal-silicate experiments of V, Cr, and Mn evaluated the behavior of these elements during different core formation scenarios. The study found that the V, Cr, and Mn depletions in the Earth could indeed be explained by core formation. The conditions of core formation necessary to deplete V, Cr, and Mn in the Earth's mantle were consistent with the deep magma ocean proposed to account for the Earth's mantle abundances of Ni and Co. Using the parameterizations of for the metal-silicate partition coefficients (D) of V, Cr, and Mn, we investigate here the conditions needed to match the depletions in the silicate Moon and determine if such conditions could have been present on the giant impactor.

Chabot, N. L.↗
NASA NTRS2012

Core-Mantle Partitioning of Volatile Siderophile Elements and the Origin of Volatile Elements in the Earth

There are currently several hypotheses on the origin of volatile siderophile elements in the Earth. One hypothesis is that they were added during Earth s accretion and core formation and mobilized into the metallic core [1], others claim multiple stage origin [2], while some hypothesize that volatiles were added after the core already formed [3]. Several volatile siderophile elements are depleted in Earth s mantle relative to the chondrites, something which continues to puzzle many scientists. This depletion is likely due to a combination of volatility and core formation. The Earth s core is composed of Fe and some lighter constituents, although the abundances of these lighter elements are unknown [4]. Si is one of these potential light elements [5] although few studies have analyzed the effect of Si on metal-silicate partitioning, in particular the volatile elements. As, In, Ge, and Sb are trace volatile siderophile elements which are depleted in the mantle but have yet to be extensively studied. The metal-silicate partition coefficients of these elements will be measured to determine the effect of Si. Partition coefficients depend on temperature, pressure, oxygen fugacity, and metal and silicate composition and can constrain the concentrations of volatile, siderophile elements found in the mantle. Reported here are the results from 13 experiments examining the partitioning of As, In, Ge, and Sb between metallic and silicate liquid. These experiments will examine the effect of temperature, and metal-composition (i.e., Si content) on these elements in or-der to gain a greater understanding of the core-mantle separation which occurred during the Earth s early stages. The data can then be applied to the origin of volatile elements in the Earth.

Nickodem, K.↗
NASA NTRS2013

Water Contents of the Mantle Beneath the Rio Grande Rift: FTIR Analysis of Kilbourne Hole Peridotite Xenoliths

Although nominally anhydrous mantle minerals contain only trace amounts of water, they are the main reservoir of water in the mantle. Added up at the scale of the Earth's mantle, these trace amounts of water represent oceans worth in mass]. Mantle xenoliths from Kilbourne Hole in southern New Mexico are ideal to study mantle water distribution in a rift tectonic setting as they come from a recently‐erupted maar in the middle of the Rio Grande Rift. Eleven lherzolites, one harzburgite, and one dunite are being analyzed for water contents by FTIR. The xenoliths will also be analyzed for major and trace element composition, Fe3+/Summation (Fe) ratios, and characterized petrologically. Olivines exhibit variable water contents with less water at the rims compared to the cores. This is probably due to H loss during decompression and xenolith transport by the host magma. Mantle water contents appear to have been primarily preserved in the core of the olivines, based on diffusion modeling of the typically plateau-shaped water content profiles across these grains. Water concentrations are in equilibrium between clino- and orthopyroxene, but olivine concentrations are typically not in equilibrium with those of either pyroxene. Lherzolites analyzed so far have water contents of 2‐12 ppm H2O in olivines, 125‐165 ppm H2O in orthopyroxenes, and 328‐447 ppm H2O in clinopyroxenes. These water contents are similar to, but with a narrower range, than those for the respective minerals in other continental peridotite xenoliths. The lherzolites have bulk-rock (BR) Al2O3 contents that range between 3.17 and 3.78 wt%, indicating similar degrees of partial melting, which could explain the narrow range of their pyroxene water contents. Primitive mantle normalized rare earth element (REE) profiles of the bulk lherzolites vary from light REE depleted to flat, with no significant differences between, nor relation to, their mineral water contents. Consequently, the metasomatic agents that enriched LREEs in these lherzolites were most likely water-poor. The harzburgite and the dunite have lower weight percent Al2O3 compared to the lherzolites (2.11% and 0.34% respectively) indicating higher degrees of melting. Their olivine water contents, however, are similar to those of the lherzolites. Moreover, no correlations are observed between pyroxene water contents and indices of melting or metasomatism between the lherzolite group, the harzburgite, and the dunite, although the latter has the lowest pyroxene water contents. More samples will be analyzed to determine if the water contents are controlled by melting, metasomatism, or a combination of the two in the Kilbourne Hole mantle.

Schaffer, Lillian A.↗
NASA NTRS2005

The Effect of Metal Composition on Fe-Ni Partition Behavior between Olivine and FeNi-Metal, FeNi-Carbide, FeNi-Sulfide at Elevated Pressure

Metal-olivine Fe-Ni exchange distribution coefficients were determined at 1500 C over the pressure range of 1 to 9 GPa for solid and liquid alloy compositions. The metal alloy composition was varied with respect to the Fe/Ni ratio and the amount of dissolved carbon and sulfur. The Fe/Ni ratio of the metal phase exercises an important control on the abundance of Ni in the olivine. The Ni abundance in the olivine decreases as the Fe/Ni ratio of the coexisting metal increases. The presence of carbon (up to approx. 3.5 wt.%) and sulfur (up to approx. 7.5 wt.%) in solution in the liquid Fe-Ni-metal phase has a minor effect on the partitioning of Fe and Ni between metal and olivine phases. No pressure dependence of the Fe-Ni-metal-olivine exchange behavior in carbon- and sulfur-free and carbon- and sulfur-containing systems was found within the investigated pressure range. To match the Ni abundance in terrestrial mantle olivine, assuming an equilibrium metal-olivine distribution, a sub-chondritic Fe/Ni-metal ratio that is a factor of 17 to 27 lower than the Fe/Ni ratios in estimated Earth core compositions would be required, implying higher Fe concentrations in the core forming metal phase. A simple metal-olivine equilibrium distribution does not seem to be feasible to explain the Ni abundances in the Earth's mantle. An equilibrium between metal and olivine does not exercise a control on the problem of Ni overabundance in the Earth's mantle. The experimental results do not contradict the presence of a magma ocean at the time of terrestrial core formation, if olivine was present in only minor amounts at the time of metal segregation.

Holzheid, Astrid↗
NASA NTRS1993

Effective elastic thickness of the Venusian lithosphere with lateral viscosity variations in the mantle

Both the Earth and Venus have a convecting mantle at the top of which is a relatively strong, mechanical boundary layer. The surface topography and gravity signals which result from the convection within the viscous mantle are modified by the elastic properties of this lithospheric boundary layer. In particular the ability of the lithosphere to support loads and transmit stresses from below is a function of the wavelength of the load--the lithosphere is strong to loading at shorter wavelengths. As a consequence it is usual to expect that long wavelength topography cannot be supported by the mechanical strength of the lithosphere and must be compensated--isostatically or dynamically--within the uppermost mantle or the crust. The flexural rigidity of the lithosphere can therefore be determined by estimating the greatest wavelength at which uncompensated surface topography can be supported, usually by measuring the admittance as a function of wavelength. In fact this procedure for determining the elastic thickness relies upon being able to distinguish topography with underlying support from that supported by the brittle lithosphere on the basis of their each having a characteristic value of the admittance. However, in the presence of lateral viscosity variations in the mantle, it is possible for topography to be generated which is NOT compensated by density anomalies in the underlying mantle at the same wavelength. Although this effect is not likely to be important for the Earth, on Venus, where the high surface temperatures would be expected to give a weaker lithosphere, lateral viscosity variations in the mantle can give a misleadingly large apparent elastic thickness for the lithosphere.

Moresi, Louis↗
NASA NTRS1983

Shock temperatures of SiO2 and their geophysical implications

High pressure shock state temperatures of SiO2 were measured in specimens of single crystal alpha-quartz and fused quartz. Projectile impact and pyrometry techniques were employed to impart pressures in the 60-140 GPa range. Shock-induced phase transformations were observed near 70 and 50 GPa along the alpha- and fused-quartz Hugoniots. It is suggested that the transformation consists of melting of the shock-synthesized stishovite, with melting delayed by the superheating of the crystalline phase. Details of the stishovite-liquid phase boundaries have been determined, including stishovite melting at 4500 K under 70 GPa, with the melting accompanied by a relative volume change of about 2.7% and a latent heat of fusion of about 2.4 MJ/kg. At 100 GPa, an approximate limit of 3500 K for the melting of SiO2 is indicated for solid mantle mineral assemblages, such as the earth's mantle

Lyzenga, G. A.↗
NASA NTRS1986

Komatiite genesis in the Archaean mantle, with implications for the tectonics of Archaean greenstone belts

The presence of ultramafic lavas (komatiites) associated with Archean greenstone belts has been suggested to indicate very high increments (50-80%) of partial melting of the Archean mantle. Such extensive melting of the Earth's mantle during the Archean might have profound effects on the early tectonic and chemical evolution of the planet, although problems associated with keeping the komatiite liquid in equilibrium with the residual mantle at such high increments of melting has cast doubt upon aspects of extensive melting. Two important aspects of the origin of komatiites are discussed below.

Elthon, D.↗
NASA NTRS1979

Chemical stratification of the mantle

A possible scenario for the chemical stratification of the earth's mantle is presented. Differentiation of the mantle by either the production of basaltic magmas or partial melting by the upper mantle is proposed to lead to a thick basalt layer, the lower part of which is converted to eclogite as the earth cools. Density estimates indicate that the eclogite formed would not be able to sink to below 670 km. The eclogite layer is thus demonstrated to be trapped as a result of whole-mantle convection and possible irreversible differentiation of the mantle into eclogite and overlying residual peridotite layers.

Anderson, D. L.↗
NASA NTRS1984

Time-dependent models of single- and double-layer mantle convection

Time-dependent numerical models of two-dimensional convection which simulate mantle convection are presented. Starting from a fluid initially at rest with a purely conductive temperature profile, it is found that for a Rayleigh number of ten million, the model experiences a transient period of double-layer convection which lasts on the order of hundreds of millions of years when scaled to the earth's mantle. It is suggested that transient periods of double-layer convection in the earth's mantle may have existed in the past, whether or not such a period exists today.

Boss, A. P.↗
NASA NTRS1992

Volcanism by melt-driven Rayleigh-Taylor instabilities and possible consequences of melting for admittance ratios on Venus

A large number of volcanic features exist on Venus, ranging from tens of thousands of small domes to large shields and coronae. It is difficult to reconcile all these with an explanation involving deep mantle plumes, since a number of separate arguments lead to the conclusion that deep mantle plumes reaching the base of the lithosphere must exceed a certain size. In addition, the fraction of basal heating in Venus' mantle may be significantly lower than in Earth's mantle reducing the number of strong plumes from the core-mantle boundary. In three-dimensional convection simulations with mainly internal heating, weak, distributed upwellings are usually observed. We present an alternative mechanism for such volcanism, originally proposed for the Earth and for Venus, involving Rayleigh-Taylor instabilities driven by melt buoyancy, occurring spontaneously in partially or incipiently molten regions.

Tackley, P. J.↗
NASA NTRSDate not supplied

Carbonatite metasomatism and water systematics of peridotite xenoliths from Lanzarote, Canaries

Water in Earth’s mantle, present as structural hydrogen in minerals, influences large scale processes like mantle deformation, plate tectonics, and melting. To better understand the processes controlling water concentrations in the Earth’s upper mantle, we analyzed hydrogen (calculated as ppm wt. H2O), major, and trace element concentrations in minerals from harzburgite xenoliths of Lanzarote, Canaries. Previous studies have shown evidence for carbonatite metasomatism in Lanzarote peridotites and carbonatite magmas have erupted in the Canaries. Thus, the Canaries offer a rare opportunity to examine the role of metasomatism and carbonatitic magmas in rehydrating the oceanic lithosphere during plume-lithosphere interaction. Major element data (obtained by electron microprobe) show that these peridotites are highly depleted compared to the primitive mantle, with a high average olivine Mg# (91.1 ± 0.5), low bulk rock Al2O3 (0.4 to 1.4 wt.%), high spinel Cr# (40 to 72) from which high degrees of melting were estimated (15 to 21%). There is no evidence of hydrous phases (e.g., amphibole and phlogopite), thus water in these peridotites is thought to be stored only in nominally anhydrous minerals and melt inclusions. Water concentrations (determined by polarized FTIR spectroscopy), in the peridotite silicate minerals range from 2.7 to 13 ppm in olivine, 42 to 265 ppm in orthopyroxene (Opx), and 76 to 470 ppm in clinopyroxene (Cpx). Reconstructed bulk rock water concentrations range from 5 to 115 ppm, generally lower than estimates for the depleted mantle (DM, 50-200 ppm). Trace element data in Opx, Cpx, and bulk rock frequently show high La/Yb ratios (up to 12.2, 71.5, 45.2; respectively) and strong negative Zr, Hf, and Ti anomalies relative to REE, consistent with carbonatite metasomatism. With decreasing Ti/Eucpx ratios, a proxy for carbonatite metasomatism, the bulk water increases. The calculated trace element compositions of melts in equilibrium with Cpx strongly resemble those of carbonatites. However, the bulk rock water concentrations are two magnitudes lower than what is expected by equilibrium with a carbonatite melt. Evidently, carbonatitic metasomatism did rehydrate of the Lanzarote mantle lithosphere but less so than expected, implying lower partitioning of H in the presence of CO2than a silicate melt.

S. Patterson↗