Variations In Mantle Temperature Research Articles

Morphology and tectonics of the Mid‐Atlantic Ridge, 7°–12°S

We present swath bathymetric, gravity, and magnetic data from the Mid‐Atlantic Ridge between the Ascension and the Bode Verde fracture zones, where significant ridge–hot spot interaction has been inferred. The ridge axis in this region may be divided into four segments. The central two segments exhibit rifted axial highs, while the northernmost and southernmost segments have deep rift valleys typical of slow‐spreading mid‐ocean ridges. Bathymetric and magnetic data indicate that both central segments have experienced ridge jumps since ∼1 Ma. Mantle Bouguer anomalies (MBAs) derived from shipboard free air gravity and swath bathymetric data show deep subcircular lows centered on the new ridge axes, suggesting that mantle flow has been established beneath the new spreading centers for at least ∼1 Myr. Inversion of gravity data indicates that crustal thicknesses vary by ∼4 km along axis, with the thickest crust occurring beneath a large axial volcanic edifice. Once the effects of lithospheric aging have been removed, a model in which gravity variations are attributed entirely to crustal thickness variations is more consistent with data from an axis‐parallel seismic line than a model that includes additional along‐axis variations in mantle temperature. Both geophysical and geochemical data from the region may be explained by the melting of small (<200 km) mantle chemical heterogeneities rather than elevated temperatures. Therefore, there may be no Ascension/Circe plume.

Mantle compositional control on the extent of mantle melting, crust production, gravity anomaly, ridge morphology, and ridge segmentation: a case study at the Mid-Atlantic Ridge 33–35°N

Abstract Mantle temperature variation and plate spreading rate variation have been considered to be the two fundamental variables that determine the extent of mantle melting and ocean crust production. Along the length of a ∼200 km portion of the Mid-Atlantic Ridge (MAR) between the Oceanographer (35°N) and Hayes (33°N) transforms, the mantle potential temperature is the same, the plate spreading rate is the same, but the extent of mantle melting and crustal production vary drastically. In addition to the typical crustal thickness variation on ridge segment scales at the MAR, i.e. thicker at segment centers and thinner at segment ends, there exist between-segment differences. For example, the ∼90 km long segment OH-1 is magmatically robust with a central topographic high, thick crust, and a large negative gravity anomaly whereas the ∼45 km long segment OH-3 is magmatically starved with a deep rift valley, thin crust and a weak negative gravity anomaly. We demonstrate that the observed differences in the extent of mantle melting, melt production and crustal mass between segments OH-1 and OH-3 are ultimately controlled by their fertile mantle source compositional difference as reflected by the lava compositional differences between the two segments: >70% of OH-1 samples studied (N=57) are enriched MORB with [La/Sm]N>1, but >85% of OH-3 samples studied (N=42) are depleted MORB with [La/Sm]N

Slab temperature and thickness from seismic tomography: 2. Izu‐Bonin, Japan, and Kuril subduction zones

Delay times from teleseismic and local P wave arrivals are used to invert for a high‐resolution three‐dimensional velocity model beneath the northwest Pacific. The model shows high‐velocity slabs with average velocity anomalies of the order of 3–4%. Assuming the positive velocity deviations in the subducting lithosphere are to first order due to a temperature anomaly, the results of a theoretical slab temperature profile based on the diffusion equation are converted to a synthetic slab velocity model. Temperature variations between the ambient mantle and the interior of the slab are converted to P wave velocity perturbations using dVp/dT ≈ 4.8 × 10−4 km s−1 °C−1. A nonlinear optimization scheme compares the tomograms obtained via tomography to the theoretically predicted models in order to determine the optimal values for slab thickness and mantle potential temperature. Using 1180±100°C as the potential temperature, thickness estimates of 88±8 km, 85±8 km, and 84±8 km are obtained for the Izu‐Bonin, Japan, and Kuril slabs, respectively. A correlation exists between slab thickness and age, which is strong if mantle temperature variations along the slab strike can be ruled out. In the process of estimating slab thickness the predicted slab velocity model is used as a filter to enhance the initial minimum‐norm tomographic result. The initial tomogram is modified to closely resemble the synthetic slab tomogram by using only null‐space components. The use of the null‐space components guarantees that the enhanced solution will satisfy the original seismic delay times. The enhanced slab images show very continuous and narrow slabs compared to the initial tomographic results.

Morphology of the transition from an axial high to a rift valley at the Southeast Indian Ridge and the relation to variations in mantle temperature

The Southeast Indian Ridge exhibits a transition in axial morphology from an East Pacific Rise‐like axial high near 100°E to a Mid‐Atlantic Ridge‐like rift valley near 116°E but spreads at a nearly constant rate of 74–76 mm/yr. Assuming that the source of this transition lies in variations in mantle temperature, we use shipboard gravity‐derived crustal thickness and ridge flank depth to estimate the variations in temperature associated with the changes in morphological style. Within the transitional region, SeaBeam 2000 bathymetry shows scattered instances of highs, valleys, and split volcanic ridges at the axis. A comparison of axial morphology to abyssal hill shapes and symmetry properties suggests that this unorganized distribution is due to the ridge axis episodically alternating between an axial valley and a volcanic ridge. Axial morphology can then be divided into three classes, with distinct geographic borders: axial highs and rifted highs are observed west of a transform fault at 102°45′E; rift valleys are observed east of a transform fault at 114°E; and an intermediate‐style morphology which alternates between a volcanic ridge and a shallow axial valley is observed between the two. One segment, between 107° and 108°30′E, forms an exception to the geographical boundaries. Gravity‐derived crustal thickness and flank depth generally vary monotonically over the region, with the exception of the segment between 107°E and 108°30′E. The long‐wavelength variations in these properties correlate with the above morphological classification. Gravity‐derived crustal thickness varies on average ∼2 km between the axial high and rift valley regions. The application of previous models relating crustal thickness and mantle temperature places the corresponding temperature variation at 25°C–50°C, depending on the model used. The average depth of ridge flanks varies by ∼550 m over the study area. For a variation of 25°–50°C, thermal models of the mantle predict depth variations of 75–150 m. These values are consistent with observations when the combined contributions of crustal thickness and mantle density to ridge flank depth are considered, assuming Airy isostasy. Crustal thickness variations differ at the two transitions described above: A difference of 750 m in crustal thickness is observed at the rift valley/intermediate‐style transition, suggesting small variations in crustal thickness and mantle temperature drive this transition. At the axial high‐rifted high/intermediate‐style transition, crustal thickness variations are not resolvable, suggesting that this transition is controlled by threshold values of crustal thickness and mantle temperature, and is perhaps related to the presence of a steady state magma chamber.

The Southeast Indian Ridge between 88°E and 118°E: Variations in crustal accretion at constant spreading rate

The temperature of the mantle and the rate of melt production are parameters which play important roles in controlling the style of crustal accretion along mid‐ocean ridges. To investigate the variability in crustal accretion that develops in response to variations in mantle temperature, we have conducted a geophysical investigation of the Southeast Indian Ridge (SEIR) between the Amsterdam hotspot and the Australian‐Antarctic Discordance (88°E–118°E). The spreading center deepens by 2100 m from west to east within the study area. Despite a uniform, intermediate spreading rate (69–75 mm yr−1), the SEIR exhibits the range in axial morphology displayed by the East Pacific Rise and the Mid‐Atlantic Ridge (MAR) and usually associated with variations in spreading rate. The spreading center is characterized by an axial high west of 102°45′E, whereas an axial valley is prevalent east of this longitude. Both the deepening of the ridge axis and the general evolution of axial morphology from an axial high to a rift valley are not uniform. A region of intermediate morphology separates axial highs and MAR‐like rift valleys. Local transitions in axial morphology occur in three areas along the ridge axis. The increase in axial depth toward the Australian‐Antarctic Discordance may be explained by the thinning of the oceanic crust by ∼4 km and the change in axial topography. The long‐wavelength changes observed along the SEIR can be attributed to a gradient in mantle temperature between regions influenced by the Amsterdam and Kerguelen hot spots and the Australian‐Antarctic Discordance. However, local processes, perhaps associated with an heterogeneous mantle or along‐axis asthenospheric flow, may give rise to local transitions in axial topography and depth anomalies.

Spreading-rate dependence of the extent of mantle melting beneath ocean ridges

Abyssal peridotites and mid-ocean-ridge basalts (MORBs) are complementary products of the mantle melting and melt-extraction processes that create the ocean crust. Studies of abyssal peridotites1–4 and MORBs2 have showed that the extent of mantle melting is high beneath hotspot-influenced shallow ridges, and is low beneath deep ridges away from hotspots. These results have led to the recognition of a global correlation of MORB composition with ridge depth5, and to the notion that mantle temperature variation exerts the primary control on the extent of melting beneath ocean ridges2,5–8. This conclusion is, however, based largely on data from slow-spreading ridges in the Atlantic and Indian oceans. At the fast-spreading East Pacific Rise (EPR), there is little correlation between MORB chemistry and ridge depth9,10, an observation that has proved puzzling8–12. Here we show that abyssal peridotites from the EPR13–21 are extremely depleted in basaltic major-element components—significantly more so than peridotites from ridges away from hotspots in the Atlantic and Indian oceanss—indicating that the EPR peridotites are residues of the highest extents of melting. These abyssal peridotite data and existing MORB major-element data12 both suggest that the extent of mantle melting beneath normal ocean ridges increases with increasing spreading rate.

Transitions in axial morphology along the Southeast Indian Ridge

Shipboard bathymetric and magnetic profiles across the Southeast Indian Ridge (SEIR) were analyzed in order to examine the nature of along‐axis variations in axial morphology at this intermediate spreading rate ridge. Three types of axial morphology are observed along the SEIR: an axial high, a shallow (200–700 m deep) axial valley and a deep (>1000 m deep) axial valley. An axial high is found to the east of the Australian‐Antarctic Discordance (AAD) (east of 128°E) and between 82°E and 104°E. A shallow rift valley is found from 104°E to 114°E and from 82°E westward past the Amerstdam/St. Paul hotspot (ASP) to about 30°S, 75°E. Deep rift valleys are found from 114°E to 128°E in the vicinity of the AAD and from the Indian Ocean Triple Junction (IOTJ) at 25°S, 70°E to about 30°S, 75°E. The transition near 30°S occurs in an area of constant zero‐age depth and does not appear to result from an increase in mantle temperature. It could be the result of the rapid increase in spreading rate along that portion of the SEIR. The most likely cause of the other transitions in axial morphology is variations in mantle temperature. The transitions between the different types of axial morphology are well defined and occur over a limited distance. Transitions in axial morphology are accompanied by significant changes in ridge flank topographic roughness. The transitions from axial valleys to axial highs are also accompanied by changes in the amplitude of the seafloor magnetic anomalies. Our observations suggest that there are distinct modes rather than a continuum of axial morphology on the SEIR and that there appears to be a “threshold” mechanism for a rapid change between different states of axial morphology. The ASP has only a limited influence on the SEIR. The ridge axis is marked by an axial valley for the entire distance from the IOTJ up to and past the ASP. The ridge axis becomes shallower as the ASP is approached from the northwest but only by about 300 m over a distance of 800 km. In addition, the ridge continues to become shallower away from Amsterdam Island toward the transition to an axial high at 82°E, 350 km to the east of the ASP. The Kerguelen hotspot appears to exert a major influence on the morphology of the SEIR by feeding asthenospheric material to the ridge axis. A long, narrow finger‐like gravity high extends ENE away from the Kerguelen Plateau for a distance of 500 km. Shipboard data show that the gravity high results from a large volcanic ridge. The ridge appears analogous to the Rodriguez Ridge extending from the Reunion hotspot toward the Central Indian Ridge. A series of lower and broader lineated gravity highs extend from the volcanic ridge toward the SEIR in the ridge segment between the 81°E and 85°E transforms, which is the westernmost segment with an axial high. The only region of significant off‐ridge seismicity on the Antarctic flank of the SEIR is a diffuse band of epicenters extending from Kerguelen to the SEIR within the segment between the 81°E and 85°E fracture zones. The along‐axis gradient in depth from 86°E to the AAD and the transitions in axial morphology at 104°E and 114°E most likely reflect along‐axis variations in mantle temperature and melt production rate due to distance from the Kerguelen hotspot and the influence of the AAD.

Travel‐time residuals of teleseismic P‐waves at the Rodriguez Triple Junction in the Indian Ocean using ocean‐bottom seismometers

We conducted three weeks of seismic observations at the Rodriguez Triple Junction (RTJ) in the Indian Ocean using 18 ocean‐bottom seismometers over an area of 90km × 90km. We identified six teleseismic events and obtained significant anomalies in the relative travel‐time residuals of the teleseismic P‐waves. The residuals are positive at the RTJ, the northern part of the Southeast Indian Ridge (SEIR), and the eastern side of the Central Indian Ridge (CIR), and are negative at and around the Southwest Indian Ridge (SWIR). It is suggested that there is a relatively hotter mantle under the triple junction and the northern part of the SEIR segment, along‐axis variations in mantle temperature along the SEIR segment, and cooler mantle under the SWIR segment. The CIR segment has an asymmetrical distribution of relative residuals.

An inversion of gravity and topography for mantle and crustal structure on Mars

Analysis of the gravity and topography of Mars presently provides our primary quantitative constraints on the internal structure of Mars. We present an inversion of the long‐wavelength (harmonic degree ≤ 10) gravity and topography of Mars for lateral variations of mantle temperature and crustal thickness. Our formulation incorporates both viscous mantle flow (which most prior studies have neglected) and isostatically compensated density anomalies in the crust and lithosphere. Our nominal model has a 150‐km‐thick high‐viscosity surface layer over an isoviscous mantle, with a core radius of 1840 km. It predicts lateral temperature variations of up to a few hundred degrees Kelvin relative to the mean mantle temperature, with high temperature under Tharsis and to a lesser extent under Elysium and cool temperatures elsewhere. Surprisingly, the model predicts crustal thinning beneath Tharsis. If correct, this implies that thinning of the crust by mantle shear stresses dominates over thickening of the crust by volcanism. The major impact basins (Hellas, Argyre, Isidis, Chryse, and Utopia) are regions of crustal thinning, as expected. Utopia is also predicted to be a region of hot mantle, which is hard to reconcile with the surface geology. An alternative model for Utopia treats it as a mascon basin. The Utopia gravity anomaly is consistent with the presence of a 1.2 to 1.6 km thick layer of uncompensated basalt, in good agreement with geologic arguments about the amount of volcanic fill in this area. The mantle thermal structure is the dominant contributor to the observed geoid in our inversion. The mantle also dominates the topography at the longest wavelengths, but shorter wavelengths (harmonic degrees ≥4) are dominated by the crustal structure. Because of the uncertainty about the appropriate numerical values for some of the model's input parameters, we have examined the sensitivity of the model results to the planetary structural model (core radius and core and mantle densities), the mantle's viscosity stratification, and the mean crustal thickness. The model results are insensitive to the specific thickness or viscosity contrast of the high‐viscosity surface layer and to the mean crustal thickness in the range 25 to 100 km. Models with a large core radius or with an upper mantle low‐viscosity zone require implausibly large lateral variations in mantle temperature.

Some theoretical predictions on the relationships among spreading rate, mantle temperature, and crustal thickness

A series of numerical experiments on mantle flow and melting predict a positive relation between mantle temperature and crustal thickness. The models also demonstrate that crust formed at slow spreading rates is more sensitive to variations in mantle temperature than crust formed at fast rates so that the range of calculated thicknesses is much greater for crust formed at slower rates. An instantaneous mantle temperature increase results in a transient pulse of melt production that is also more pronounced at slower spreading rates. The predicted behavior is caused by the interplay between mantle flow driven by plate separation and that driven by thermal, compositional, and melt‐related buoyancy. A temperature increase results in a decrease in mantle viscosity and an increase in the depth at which melting begins. A lower viscosity leads to stronger buoyancy‐driven flow that carries more mantle to shallow depths below the ridge. Thermal buoyancy effects, which may result in cooling and mixing of depleted and undepleted material under the ridge, appear to be of greater importance at slower spreading rates. The steady state results are broadly consistent with global compilations of oceanic crustal thickness that show larger variations in crustal thickness at slower spreading rates than at faster rates. Thicknesses estimated from seismic refraction data from crust formed within a single segment of the Mid‐Atlantic Ridge but at different spreading rates (1.0 to 1.9 cm/yr) are consistent with (but do not prove) the model results. The transient pulse of melt production associated with a rapid increase in mantle temperature might occur when a ridge becomes proximal to a hot spot.

Melting and mantle flow beneath a mid-ocean spreading center

A series of numerical experiments on mantle flow, melt generation, and melt migration beneath mid-ocean ridges is reported. In these experiments, flow in the mantle is driven by the motion of the surface plates and melting-induced mantle density variations. Mantle densities are controlled by the proportions of minerals in the residual mantle and their Fe/Mg ratio. Flow due to thermal variations in density are ignored. Melting results from the decompression of the upwelling mantle beneath the ridge, and melt production rates are determined from variations in mantle temperature above a solidus that depends explicitly upon pressure, mantle mineralogy, and mantle oxide composition. Melt migration is driven by mantle pressure gradients and melt buoyancy forces. The mantle temperature is influenced by the latent heat of melting and the advection of heat due to flow of the melt and mantle. Experiments with solely plate driven flows illuminate the results of including these latter thermal effects. The latent heat of melting lowers mantle temperatures to the solidus wherever melting occurs and also has an influence on the distribution of melting. Advection of heat due to melt migration increases melting rates up the melting column. For low mantle viscosities (10 18 Pa s) the latter effect is minor as melt rises essentially vertically. The thermal effect of the melt, however, will become more important as melting rates increase and if melt is focused into a narrow region. At slow spreading rates (1 cm yr −1), depletion-driven mantle flow markedly increases crustal thicknesses as melting rates increase to match the enhanced advection of mantle heat. This additional flow also causes the melting region to become narrower but is an insufficient mechanism for focusing melt to the ridge axis. The rapid extraction of the melt phase from the mantle results in an enhanced upwelling beneath the ridge which in turn leads to higher melt production rates and greater crustal thicknesses. Solidification of the melt phase at its liquidus results in melt freezing rates of order ten times greater than average melt production rates. High freezing rates are the result of high vertical temperature gradients in the cool oceanic lithosphere where the freezing occurs. The release of latent heat upon solidification has a negligible effect upon melting rates within the melting regime. Buoyancy forces resulting from the presence of an interstitial melt phase can result in a significant narrowing of the mantle upwelling beneath the ridge and also the width of the melt regime. The importance of melt buoyancy forces in shaping mantle flow depends on their magnitude relative to buoyancy forces resulting from density changes in the solid mantle.

Thermal core‐mantle interactions and time‐averaged paleomagnetic field

Recent studies of historical geomagnetic secular variation and lower mantle seismic velocity suggest the Earth's magnetic field is controlled by lateral variations in mantle temperature. In this paper the average expected magnetic field pattern is compared with the time‐averaged field obtained from paleomagnetic pole positions from the last 5 m.y. The historical record suggests that the time‐averaged inclination anomalies are negative (“far‐sided”) in the Atlantic region north of 30°S and the west Pacific and positive in the South Atlantic, extreme North and South Pacific, and east Pacific. The pole positions give positive inclination anomalies in the South Atlantic and extreme North Pacific, and negative anomalies elsewhere, in agreement with the theory except for the east Pacific. This suggests that the historical record is not long enough to give an accurate picture of long term secular change in the eastern Pacific but that elsewhere the time‐averaged field of paleomagnetism is predicted satisfactorily. The preponderance of “far sidedness” in the paleomagnetic record is a consequence of the present distribution of temperature at the base of the mantle rather than a fundamental property of the geodynamo, and it may therefore change on the long time scale of mantle convection. In the historical record declinations average to negative values in the Atlantic hemisphere and positive values in the Pacific which, if typical of longer‐term behavior, could explain the preponderance of positive declinations for reversed polarity, which are mainly in the Atlantic hemisphere.

Analysis of the cooling of a variable-viscosity fluid with applications to the Earth

Analysis shows that the convective cooling of a fluid with a temperature-dependent viscosity should exhibit simple behaviour which is consistent with geophysical data on the cooling of the oceanic lithosphere and asthenosphere. The cooling rate of convecting fluid is predicted to vary approximately linearly with (time)−1/2. This relationship has also been demonstrated for numerical solutions of the full governing equations of convection. It is found that a similarity solution can describe the evolution of temperature within the stagnant high-viscosity lid which forms over an actively convecting region. The rate of thickening of the lid (or lithosphere) is directly proportional to a single parameter, λ; and the overall cooling rate is also a function of λ. Expressions are derived which relate changes in the parameter (λ) to the initial average viscosity (or temperature of the fluid) and to the temperature and pressure dependence of viscosity. With assumptions about how the cooling of variable-viscosity fluids can be used to describe the cooling of the oceanic lithosphere and asthenosphere, values of sea-floor depth and the isostatic geoid height can be predicted as a function of time. Sea-floor depth is predicted to increase linearly with (time)1/2 and geoid height to vary nearly linearly with time in agreement with observations. The rate of change of the depth or geoid height is a strong function of the initial temperature of the convecting region. The mantle is taken to have physical properties appropriate for olivine. A change in the mantle temperature under the ridge of 150K causes only about a 20 per cent change in the rate of subsidence but causes nearly a 100 per cent change in the rate of change of the geoid height with time. The observed variations in these rates over different sections of oceanic ridges are of about this magnitude. Simple conductive cooling would require more than 1000 K variation in mantle temperature to account for the geoid height observations. This analysis can also be used to estimate the cooling rate of magma sills.

Structure and seismotectonics of the Vema Fracture Zone, Atlantic Ocean

The structure and seismo-tectonics of the Vema Fracture Zone and the adjacent regions of its transverse ridge to the south and the Mid-Atlantic Ridge spreading centre to the north is discussed, based on an analysis of seismic refraction profiles, micro-earthquake activity and gravity anomalies. Crustal structures were determined by analysis of wide angle seismic data from explosive and airgun sources recorded on arrays of ocean bottom seismometers. Micro-earthquakes were recorded by six instruments for a period of 14 days in an array to the north of the ridge-transform intersection. Immature crust, similar to that reported by Fowler (1976) from the FAMOUS area, lies beneath the spreading centre. The lower crust thins and normal mantle appears as the fracture zone is approached. The thinnest crust lies beneath the northern end of the topographic deep at the fracture zone intersection. This is also the locus of current microearthquake activity at sub-bottom depths of 3–4 km, which a composite fault-plane solution suggests may originate on normal faults that parallel the trend of the local topography on the eastern wall of the median valley. Within the central region of the fracture zone valley, the crust thickens but still retains an anomalous velocity structure. Seismic layer 3 is absent and crustal velocities are low. This is probably the consequence of attenuated crustal accretion, of faulting and cracking, and of hydrothermal alteration. The pattern of residual gravity anomalies, after correction for topography, for sediment thickness and for mantle temperature variations, is consistent with the crustal structure defined by the seismic experiments. The gravity measurements indicate that there is a broad thinning of the crust over 20–40 km in the vicinity of the fracture zone, on which are superimposed localized regions of low and high densities. To the south of the fracture zone, the crust thickens and assumes a normal velocity structure including a mature seismic layer 3. The seismic Moho is warped upwards under the transverse ridge, suggesting a tectonic origin from buckling rather than uplift by serpentinization.

Residual geoid anomalies in Atlantic Ocean Basin: Relationship to mantle plumes

The relations of geoid data from radar altimeter measurements (GEOS 3) to water depth and geochemical variations along the Mid‐Atlantic Ridge are investigated. Eight geophysical lines across the ridge in the Atlantic Ocean were constructed approximately normal to the local ridge trend. Slopes of geoid per kilometer of topographic relief show an asymmetry between the two sides of the ridge that generally is not matched by topography, which has better symmetry between the two flanks. Subtracting a regional field, based on previous work, produces residual geoid anomalies whose source must be in the earth's outer 600 km. The residual geoid anomalies of the North Atlantic show striking correlation with the axis of the Mid‐Atlantic Ridge, but those in the South Atlantic do not. Instead, major positive residual geoid anomalies appear over the Rio Grande and Walvis ridges. Determination of slope S, residual geoid per million years crustal age, reveals that S increases with greater residual geoid anomaly and with higher (shallower) crestal ridge topography. The slope of 22 cm/m.y. west of the Azores high (near 38°N) suggests that the anomalously high crestal residual geoid and depth anomalies may not be entirely of thermal origin, which may normally produce slopes only of about 8 cm/m.y. as found near 28°N away from shallow ridge topography. To account for this difference in slopes simply by a variation in mantle temperature would require a difference of about 940°C between the two sites, which appears excessive. Also, the decrease in topographic height with crustal age away from the Azores high is much greater than predicted by theoretical cooling curves. Correlation of residual geoid anomalies with depth, geochemical variations along the length of the Mid‐Atlantic Ridge, and the chemistry of the Rio Grande Rise and Walvis Ridge suggest that many regions of anomalous residual geoid and crestal depth values may be in large part a function of crustal thickness and density variations induced by plume intrusion and volcanism in addition to their transient thermal effects.

Minimal upper mantle temperature variations consistent with observed heat flow and plate velocities

The momentum equations applied to 5° block means are integrated from the observed surface plate velocities downward to a depth of 280 km, assuming no lateral heterogeneities in density or viscosity. It is assumed that 85% of the global heat production QG = 0.85 × 4.0 ×1013 W comes from below 280 km and that at this level the transfer is fully convective. A temperature field 7 is inferred at depth 280 km by minimizing the quantity∫|T− T0|n dS + nλ{QG−pC∫(T−T0)υrdS}, where the integrals are over the sphere, ρ is density, C is heat capacity, υr is radial velocity, T0 is a prescribed mean, and λ is a Lagrangian multiplier. Norms n, ranging from 1.5 to 2.5, are tried. The intervening temperature fields are then inferred, integrating the energy equation downward by using the previously calculated velocity field. This integration is subject to the limitations that the derivative of the temperature with respect to depth is everywhere sufficient to attain the fully convecting temperature, but never less than adiabatic. A surface heat flow based on observations plus age and tectonic setting is used. The principal inferences are: (1) the greatest lateral variations in temperature, ∼1000°C, occur with the top 20 km; (2) the greatest advection, ∼200°C/m.y., occurs within the top 20 km; (3) below 50 km, the greatest departures of temperature from the mean are negative “tongues,” reaching an extreme of about −800°C at depth 100 km; (4) below 50 km, heat transfer becomes more convective than conductive; (5) at the fully convecting level, 280 km, temperature variations are at least ±180°C about the mean. The principal defect in the entire calculation is unrealistically low temperatures arising from unrepresentatively low surface heat flows. The principal defect of the model probably arises from the assumption that all heat transfer at a depth of 280 km is representable by 5° means in velocity and temperature.

Foundering of Continents and its Relation to Isostasy, Crustal Thinning and Mantle Density Changes: ABSTRACT

Current theories on geosynclinal subsidence and on the disappearance of ancient land masses postulate crustal thinning by subcrustal processes. This idea was a necessary implication of the two premises popular a decade ago, namely: (1) surface elevation is related to crustal thickness according to Airy's model of isostatic compensation; (2) either the crust as defined by the Mohorovicic discontinuity is comparable with the crust envisioned by Airy, who separated a solid crust from a fluid substratum, or there is no significant regional difference in mantle density, and there has been none during the past. Recent geophysical investigations have indicated that isostatic adjustment could result not only from changes in crustal thickness, but also from variation in upper mant e density. This new discovery permits the postulate that thinning of continental crust can be related to removal of surficial sialic material by supracrustal processes, such as erosion, gravity sliding, and overthrusting from elevated regions which owed their surface elevation to an abnormally low mantle density. Subsequent increase in mantle density because of variations in mantle temperature leads to isostatic subsidence of regions of thin crust. Such a combination of crustal thinning by supracrustal processes and isostatic subsidence related to mantle density changes could explain adequately (1) the disappearance of ancient land, (2) the formation of a new geosyncline at the site of an ancient land, and (3) the estimated chemical composition of the earth's crust. The efficacy of supra rustal processes to remove large quantities of sialic materials has been discussed. It is not necessary to postulate subcrustal processes which either must assume lateral transfer of vast quantities of crustal material or that the Mohorovicic discontinuity is a phase change boundary. An isostatic model relating surface elevation, crustal thickness, and mantle density variation during an orogenic cycle is presented. End_of_Article - Last_Page 345------------

Bull. Geol. Soc. Amer. : Shipek C.G., 1962. Photographic survey of sea floor on southwest slope of Eniwetok Atoll. 73 (7): 805–812

Basalts and basaltic rocks are the most abundant igneous rocks on the earth and their petrologic and geochemical studies have formed our knowledge base on the thermal structure and composition of the mantle with which we have developed workable models on the chemical differentiation of the earth. All this would not have been possible without innovative and painstaking experimental petrology on mantle peridotite melting, basaltic magma generation and evolution largely done in the period of 1960s -1980s. However, the ~30 year lively debate on the nature of “primary magma” among experimental petrologists and the petrology community during this time had inadvertently shelved the development of consensus models on mantle melting in the context of plate tectonics. Continued experimental petrology in parallel with worldwide sampling and study of mid-ocean ridge basalts (MORB) brought about new insights, culminating with a model in 1980s that mantle potential temperature (TMP) variation controls the extent and pressure of mantle melting and basalt compositions. The tenet of this model is that hotter rising mantle begins to melt deeper and thus has greater decompression depth interval to melt more with the melt having the petrological signature of higher extent and pressure of melting than cooler mantle. This model has gained wide acceptance in MORB studies and has also been invoked in the study of intra-plate basalts in ocean basins and in continental settings. Basalt generation above subduction zones, on the other hand, has been generally accepted as resulting from slab-dehydration induced mantle wedge melting since early 1980s, but recent studies also advocate mantle temperature variation as the primary control on the extent of mantle wedge melting. All these views with laudable merits have formed a paradigm on mantle melting and basaltic magmatism. In this paper, I review the historical developments towards this paradigm and demonstrate in simple clarity that it is the lithosphere thickness, not TMP, that controls the extent of mantle melting, depth of melt extraction and basalt compositions, i.e., the lid effect. The lithospheric lid caps the rising melting mantle, thus limiting the extent of decompression melting and equilibrium pressure/depth of melt extraction, which is well registered in the compositions of MORB, intra-plate ocean island basalts (OIB), volcanic arc basalts above subduction zones (VAB) and basalts in continental interiors (CIB). Hence, lithosphere thickness is the governing variable that controls mantle melt compositions in all tectonic settings on earth. Major element compositions (e.g., Si-Mg-Fe) of erupted basalts have no memory of initial depth of melting because of effective and efficient melt-solid (e.g., olivine [Mg,Fe]2SiO4) equilibration in the rising melting mantle. Therefore, basalt-olivine based thermobarometry, albeit useful, supplies no information on TMP. It is also the lithosphere thickness that controls whether “mantle plumes” can surface or not and the large igneous provinces (LIPs) serve as effective manifestations for thin or thinned lithosphere at the time of emplacement. This new understanding based on global observations, well-understood experimental petrology and rigorous analysis is fundamental and requires a major change to the current paradigm.