Variations In Mantle Temperature Research Articles

Response to Comment on "Discovery of davemaoite, CaSiO3-perovskite, as a mineral from the lower mantle".

Walter et al. issue a number of critical comments on our report about the discovery of davemaoite to the end that they believe to show that our results do not provide compelling evidence for the presence of davemaoite in the type specimen and that the hosting diamond had formed in the lithosphere. Their claim is based on a misinterpretation of the diffraction data contained in the paper, an insufficient analysis of the compositional data that disregards the three-dimensional distribution of inclusions, and the arbitrary assumption that Earth's mantle shows no lateral variations in temperature, inconsistent with state-of-the-art assessments of mantle temperature variations and with their own published results.

Electrical resistivity imaging of continental United States from three-dimensional inversion of EarthScope USArray magnetotelluric data

Long-period magnetotelluric (MT) sites, covering much of the continental United States (US) have been collected through the EarthScope USArray MT Transportable Array (TA) project. Previous regional studies using subsets of these data suggest large-scale variations in deep (sub-lithospheric) resistivity, often of significant amplitude. Here, we present a range of three-dimensional (3D) continental-scale electrical resistivity models from 3D inversion of the MT TA data, with a focus on testing robustness and resolution of this deep structure. The main features of our initial model, obtained with no constraints, are quite similar to those from previous studies, with significant (±1 order of magnitude) lateral variations in deep resistivity. We show that data fit, as measured by global normalized root-mean-square misfit, is not increased by replacing structures below 200 km depth with layer averages. A more careful examination of residuals leads to further refinements, with a slight improvement in data fit. These include a moderately conductive mantle transition zone (13Ω.m), and subtle (±1/4 order of magnitude) lateral variations in resistivity below 200 km. In contrast to the initial results, these variations can be explained by reasonable variations in mantle temperature and hydration. Overall, our results suggest that the asthenospheric mantle at 200–275 km is nearly dry on average and at greater depths contains 150–300 ppm water. Lateral variations may be explained by this range of water content, or by temperature variations of 80–120∘C. Patterns of resistivity variations below 200 km are consistent with known continental structure, and suggest deep roots (∼250 km) beneath cratons, and perhaps also beneath the mid-continent rift. Reduced resistivity, likely requiring greater hydration (or higher temperatures), occurs beneath the North-central part of the continental US, the oldest part of the continent, and beneath the Yellowstone hotspot.

Ice, Fire, or Fizzle: The Climate Footprint of Earth's Supercontinental Cycles

AbstractSupercontinent assembly and breakup can influence the rate and global extent to which insulated and relatively warm subcontinental mantle is mixed globally, potentially introducing lateral oceanic‐continental mantle temperature variations that regulate volcanic and weathering controls on Earth's long‐term carbon cycle for a few hundred million years. We propose that the relatively warm and unchanging climate of the Nuna supercontinental epoch (1.8–1.3 Ga) is characteristic of thorough mantle thermal mixing. By contrast, the extreme cooling‐warming climate variability of the Neoproterozoic Rodinia episode (1–0.63 Ga) and the more modest but similar climate change during the Mesozoic Pangea cycle (0.3–0.05 Ga) are characteristic features of the effects of subcontinental mantle thermal isolation with differing longevity. A tectonically modulated carbon cycle model coupled to a one‐dimensional energy balance climate model predicts the qualitative form of Mesozoic climate evolution expressed in tropical sea‐surface temperature and ice sheet proxy data. Applied to the Neoproterozoic, this supercontinental control can drive Earth into, as well as out of, a continuous or intermittently panglacial climate, consistent with aspects of proxy data for the Cryogenian‐Ediacaran period. The timing and magnitude of this cooling‐warming climate variability depends, however, on the detailed character of mantle thermal mixing, which is incompletely constrained. We show also that the predominant modes of chemical weathering and a tectonically paced abiotic methane production at mid‐ocean ridges can modulate the intensity of this climate change. For the Nuna epoch, the model predicts a relatively warm and ice‐free climate related to mantle dynamics potentially consistent with the intense anorogenic magmatism of this period.

High H2O Content in Pyroxenes of Residual Mantle Peridotites at a Mid Atlantic Ridge Segment

Global correlations of mid-ocean-ridges basalt chemistry, axial depth and crustal thickness have been ascribed to mantle temperature variations affecting degree of melting. However, mantle H2O content and elemental composition may also play a role. How H2O is distributed in the oceanic upper mantle remains poorly constrained. We tackled this problem by determining the H2O content of orthopyroxenes (opx) and clinopyroxenes (cpx) of peridotites from a continuous lithospheric section created during 26 Ma at a 11°N Mid-Atlantic Ridge segment, and exposed along the Vema Transform. The H2O content of opx ranges from 119 ppm to 383 ppm; that of cpx from 407 ppm to 1072 ppm. We found anomalous H2O-enriched peridotites with their H2O content not correlating inversely with their degree of melting, although H2O is assumed to be incompatible during melting. Inverse correlation of H2O with Ce, another highly incompatible component, suggests post-melting H2O enrichment. We attribute a major role to post-melting temperature-dependent diffusion of hydrogen occurring above the melting region, where water-rich melt flows faster than residual peridotites through dunitic conduits cross-cutting the uprising mantle. Accordingly, estimates of the H2O content of the MORB mantle source based on H2O in abyssal peridotites can be affected by strong uncertainties.

The Meaning of Global Ocean Ridge Basalt Major Element Compositions

Mid-ocean ridge basalts (MORB) are arguably the most abundant and also the simplest igneous rocks on the Earth. A correct understanding of their petrogenesis thus sets the cornerstone of igneous petrogenesis in general and also forms the foundation for studying mantle dynamics. Because major element compositions determine the mineralogy, phase equilibria and physical properties of rocks and magmas, understanding global MORB major element systematics is of prime importance. The correlated large MORB major element compositional variations are well understood as the result of cooling-dominated crustal-level processes (e.g. fractional crystallization, magma mixing, melt–rock assimilation or reaction, and other aspects of complex open-magma chamber processes), but it remains under debate what messages MORB major elements may carry about mantle sources and processes. To reveal mantle messages, it is logical to correct MORB melts for the effects of crustal-level processes to Mg# ≥ 0·72 to be in equilibrium with mantle olivine of ≥Fo90. Such corrected MORB major element (e.g. Si72, Ti72, Al72, Fe72, Mg72, Ca72 and Na72) compositional variations thus reflect fertile mantle compositional variation, composition-controlled mantle physical property variation (e.g. density and solidus), variation in the extent and pressure of melting, and uncertainties associated with the correction. The correction-related uncertainties can be removed through justified heavy averaging. Because ridge axial depth variation (∼0 to ∼6000 m below sea level) and plate spreading rate variation ( 150 mm a–1) are the two largest known physical variables along the global ocean ridge system, possible correlations of MORB major element compositions at Mg# ≥ 0·72 with these two physical variables are expected to reveal intrinsic controls on global MORB petrogenesis and ocean ridge dynamics. Indeed, global MORB major element data averaged with respect to both ridge axial depth intervals and ridge spreading rate intervals show significant first-order correlations. These correlations lead to the conclusion that the ridge axial depth variation and MORB chemistry variation are two different effects of a common cause, induced by fertile mantle compositional variation. The latter determines (1) variation in both composition and mode of mantle mineralogy, (2) variation of mantle density, (3) variation of ridge axial depth, (4) source-inherited MORB compositional variation, (5) density-controlled variation in the maximum extent of mantle upwelling, (6) apparent variation in the extent of melting, and (7) the correlated variation of MORB chemistry with ridge axial depth. These correlations also confirm the recognition that the extent of mantle melting increases with, and is caused by, increasing plate spreading rate. Mantle temperature variation could play a part, but its overstated role in the literature results from a basic error (1) in treating ridge axial depth variation as solely caused by mantle temperature variation by ignoring the intrinsic control of mantle composition, (2) in treating mantle plume-influenced ridges (e.g. Iceland) as normal ridges of plate spreading origin, and (3) in treating seismic low velocity at great depths (>300 km) beneath these mantle plume-influenced ridges as evidence for hot ridge mantle. There is no evidence for large mantle temperature variation beneath ridges away from mantle plumes. The suggested conclusions of this study may continue to be debated, but they are most objective, and are most consistent with petrological, geochemical, geological and geophysical principles and observations.

Crustal thickness anomalies in the Indian Ocean inferred from gravity analysis

AbstractOceanic crustal thickness is a critical parameter for understanding the magmatic, tectonic and hydrothermal processes at mid‐ocean ridges. Gravity anomalies can be mapped across the oceans through satellite altimetry, and these data hold valuable information about the nature and spatial variation of oceanic crust. Residual mantle Bouguer anomaly (RMBA) reflects variations in crustal thickness and crustal and mantle densities. It was simply treated as an upper mantle gravity component and a crustal gravity component in our model of a lithospheric depth. The upper mantle gravity component results from variations in mantle temperature, mantle porosity and mantle depletion due to partial melting initiating at depths of 80–100 km. The crustal gravity component results from variations in crustal thickness. RMBA contributed by mantle temperature variation of 200 K, mantle depletion of 5% and mantle porosity of 5% are 46, 16 and 21 mGal, respectively. Upper mantle gravity variations do not exceed 46 mGal, and variations in oceanic crustal thickness or deeper mantle effects have to be considered for areas with RMBA variations greater than 46 mGal. Assuming a standard RMBA of a reference crust is 0 mGal, we partitioned the RMBA in the Indian Ocean into three categories: ‘negative’ (less than −46 mGal), ‘normal’ (between −46 and 46 mGal) and ‘positive’ (>46 mGal), corresponding to thick, normal and thin oceanic crust, respectively. Thick oceanic crustal anomalies are associated with hotspots or plumes and are distributed along hotspot tracks. Prominent thin oceanic crustal anomalies are observed along fracture zones, around Rodriguez Triple Junction (RTJ) and Australian‐Antarctic Discordance (AAD). Besides the possible effects of fracture zones, oceanic core complex or some other enigmatic deep mantle factors must be responsible for the positive gravity anomalies and crustal thinning. Copyright © 2016 John Wiley & Sons, Ltd.

Effect of GIA models with 3D composite mantle viscosity on GRACE mass balance estimates for Antarctica

Seismic data indicate that there are large viscosity variations in the mantle beneath Antarctica. Consideration of such variations would affect predictions of models of Glacial Isostatic Adjustment (GIA), which are used to correct satellite measurements of ice mass change. However, most GIA models used for that purpose have assumed the mantle to be uniformly stratified in terms of viscosity. The goal of this study is to estimate the effect of lateral variations in viscosity on Antarctic mass balance estimates derived from the Gravity Recovery and Climate Experiment (GRACE) data. To this end, recently-developed global GIA models based on lateral variations in mantle temperature are tuned to fit constraints in the northern hemisphere and then compared to GPS-derived uplift rates in Antarctica.We find that these models can provide a better fit to GPS uplift rates in Antarctica than existing GIA models with a radially-varying (1D) rheology. When 3D viscosity models in combination with specific ice loading histories are used to correct GRACE measurements, mass loss in Antarctica is smaller than previously found for the same ice loading histories and their preferred 1D viscosity profiles. The variation in mass balance estimates arising from using different plausible realizations of 3D viscosity amounts to 20 Gt/yr for the ICE-5G ice model and 16 Gt/yr for the W12a ice model; these values are larger than the GRACE measurement error, but smaller than the variation arising from unknown ice history. While there exist 1D Earth models that can reproduce the total mass balance estimates derived using 3D Earth models, the spatial pattern of gravity rates can be significantly affected by 3D viscosity in a way that cannot be reproduced by GIA models with 1D viscosity. As an example, models with 1D viscosity always predict maximum gravity rates in the Ross Sea for the ICE-5G ice model, however, for one of the three preferred 3D models the maximum (for the same ice model) is found near the Weddell Sea. This demonstrates that 3D variations in viscosity affect the sensitivity of present-day uplift and gravity rates to changes in the timing of the ice history. In particular, low viscosities (<1019 Pas) found in West Antarctica make the mantle very sensitive to recent changes in ice thickness.

The Global Systematics of Ocean Ridge Basalts and their Origin

Tests of models of melt generation and mantle source variations beneath mid-ocean ridges require a definitive set of mid-ocean ridge basalt (MORB) compositions corrected for shallow-level processes. Here we provide such a dataset, with both single sample and segment means for 241 segments from every ocean basin, which span the entire range of spreading rate, axial depth, and MORB chemical composition. Particular attention is paid to methods of fractionation correction. Values corrected to 8 wt % MgO are robust as they are within the range of the data. Extrapolation to equilibrium with mantle olivine is a non-unique procedure that is critically dependent on the MgO content where plagioclase first appears. MORB data, trace element ratios and calculated liquid lines of descent provide consistent evidence that plagioclase fractionation primarily occurs between 8 and 9 wt % MgO, with the exception of hydrous magmas mainly from back-arc segments.Varying the MgO content of plagioclase appearance over large ranges does not produce the observed systematics at 8 wt % MgO, but may contribute to the spread of the data. Data were evaluated individually for each segment to ensure reliable fractionation correction, and segment means are reported normalized both to MgO of 8 wt % and also to a constant Mg/(MgþFe) in equilibrium with Fo90 olivine. Both sets of corrected compositions show large variations in Na2O and FeO, good correlations with segment depth, and systematic relationships among the major elements. A particularly good correlation exists between Al90 and Fe90.These new data are not in agreement with the presentation of Niu & O’Hara (Journal of Petrology 49, 633^664, 2008), whose results relied on an inaccurate fractionation correction procedure, which led them to large errors for highand low-FeO magmas. The entire dataset is provided in both raw and normalized form so as to have a uniform basis for future evaluations. The new data compilation permits tests of competing models for the primary causes of variations in MORB parental magmas: variations in mantle composition, mantle temperature, reactive crystallization or lithospheric thickness. The principal component of chemical variation among segment mean compositions is remarkably consistent with variations in mantle temperature of some 2008C beneath global ocean ridges. Comparisons with experimental data, pMELTS and other calculations show that variations in mantle fertility at constant mantle potential temperature produce trends that are largely orthogonal to the observations. At the same time, there is clear evidence for mantle major element heterogeneity beneath and around some hotspots and beneath back-arc basins. Super slowspreading ridges display a characteristic chemical signature of elevated Na90 and Al90 and lowered Si90 relative to faster-spreading ridges. If this signature were produced by reactive crystallization, Si90 should be higher rather than lower in these environments owing to the thicker lithosphere and lower temperatures of mantle^melt reaction. Instead, the data are consistent with lower extents of mantle melting beneath a thicker lithosphere. Hence, variations in extent of melting appear to be the dominant control on the major element compositions of MORB parental magmas. Trace elements, in contrast, require a large component of mantle heterogeneity, apparent in the factor of 50 variation in K90. Such variations do not correlate with the other major elements, showing that major element and trace

Helium isotopic textures in Earth's upper mantle

AbstractWe report 3He/4He for 150 mid‐ocean ridge basalt (MORB) glasses from the Southeast Indian Ridge (SEIR). Between 81°E and 101°E 3He/4He varies from 7.5 to 10.2 RA, encompassing more than half the MORB range away from ocean island hot spots. Abrupt transitions are present and in one case the full range occurs over ∼10 km. Melting of lithologically heterogeneous mantle containing a few percent garnet pyroxenite or eclogite leads to lower 3He/4He, while 3He/4He above ∼9 RA likely indicates melting of pyroxenite‐free or eclogite‐free mantle. Patterns in the length scales of variability represent a description of helium isotopic texture. We utilize four complementary methods of spectral analysis to evaluate this texture, including periodogram, redfit, multitaper method, and continuous wavelet transform. Long‐wavelength lobes with prominent power at 1000 and 500 km are present in all treatments, similar to hot spot‐type spectra in Atlantic periodograms. The densely sampled region of the SEIR considered separately shows significant power at ∼100 and ∼30–40 km, the latter scale resembling heterogeneity in the bimodal distribution of Hf and Pb isotopes in the same sample suite. Wavelet transform coherence reveals that 3He/4He varies in‐phase with axial depth along the SEIR at ∼1000 km length scale, suggesting a coupling between melt production, 3He/4He and regional variations in mantle temperature. Collectively, our results show that the length scales of MORB 3He/4He variability are dominantly controlled by folding and stretching of heterogeneities during regional (∼1000 km) and mesoscale (∼100 km) mantle flow, and by sampling during the partial melting process (∼30 km).

Effects of continental configuration on mantle heat loss

AbstractTwo‐dimensional (2D) and three‐dimensional (3D) simulations are used to explore the effects of continental distribution on mantle convection. In both 2D and 3D, at total surface areas &lt; 50%, internal temperature is weakly sensitive to continental configuration. Mantle heat flux values show mild variations with changing configuration. In 3D, at total continental area &gt; 50%, the dependence of mantle heat loss on continental configuration becomes stronger. Mantle temperature continues to increase with total continental area but now varies by 5%–7% with changing configurations. When distributed, continents can cause flow patterns to become locked. This leads to significant variations in mantle temperature below continental and oceanic regions. This differs from the expectation that supercontinents would preferentially lead to large lateral variations in mantle temperatures and suggests that insulation‐induced thermal anomalies could exist below continents today if they have remained fixed relative to mantle flow (e.g., Africa).

Insights into mantle composition and mantle melting beneath mid‐ocean ridges from postspreading volcanism on the fossil Galapagos Rise

New major and trace element and Sr, Nd, and Pb isotope data, together with 39Ar‐40Ar ages for lavas from the extinct Galapagos Rise spreading center in the eastern Pacific reveal the evolution in magma compositions erupted during slowdown and after the end of active spreading at a mid‐ocean ridge. Lavas erupted at 9.2 Ma, immediately prior to the end of spreading are incompatible element depleted mid‐ocean ridge tholeiitic basalts, whereas progressively younger (7.5 to 5.7 Ma) postspreading lavas are increasingly alkalic, have higher concentrations of incompatible elements, higher La/Yb, K/Ti, 87Sr/86Sr, and lower 143Nd/144Nd ratios and were produced by smaller degrees of mantle melting. The large, correlated variations in trace element and isotope compositions can only be explained by melting of heterogenous mantle, in which incompatible trace element enriched lithologies preferentially contribute to smaller degree mantle melts. The effects of variable degrees of melting of heterogeneous mantle on lava compositions must be taken into account when using mid‐ocean ridge basalt (MORB) to infer the conditions of melting beneath active spreading ridges. For example, the stronger “garnet signature” inferred from Sm/Nd and 143Nd/144Nd ratios for postspreading lavas from the Galapagos Rise results from a larger contribution from enriched lithologies with high La/Yb and Sm/Yb, rather than from a greater proportion of melting in the stability field of garnet peridotite. Correlations between ridge depth and Sm/Yb and fractionation‐corrected Na concentrations in MORB worldwide could result from variations in mantle fertility and/or variations in the average degree of melting, rather than from large variations in mantle temperature. If more fertile mantle lithologies are preferentially melted beneath active spreading ridges, then the upper mantle may be significantly more “depleted” than is generally inferred from the compositions of MORB.

Constraints on the deep thermal structure of the Dharwar craton, India, from heat flow, shear wave velocities, and mantle xenoliths

[1] We have used constraints from seismic shear wave vertical velocity profiles, geothermobarometry estimates on mantle xenoliths, and surface heat flux and heat production measurements to analyze the thermal regime of the deep lithosphere beneath India. In the Dharwar craton of southern India, the shear wave velocity gradient in the mantle, as well as xenolith geothermobarometry data, suggests a low mantle heat flux, 14–20 mW m−2, consistent with surface heat flux measurements. However, for standard cratonic mantle composition, seismic velocities require Moho and mantle temperatures to be about 300 K higher than inferred from heat flux and xenolith data. This discrepancy can be only resolved by changing the mantle composition, specifically by increasing the Fe number. The shear wave velocities are highest beneath north central India, where calculated S wave travel times are 2 s shorter than in the Dharwar craton. These differences in traveltime and the very steep gradient in the shear wave velocity profiles in north central India cannot be explained by variations in mantle temperature but require differences in mantle composition.

Small-scale sublithospheric convection reconciles geochemistry and geochronology of ‘Superplume’ volcanism in the western and south Pacific

Cretaceous volcanism in the West Pacific Seamount Province (WPSP), and Tertiary volcanism along the Cook-Australs in the South Pacific are associated with the same broad thermochemical anomaly in the asthenosphere perhaps related to the Pacific ‘Superplume.’ Abundant volcanism has usually been attributed to secondary plumelets rising from the roof of the Superplume. The Cook-Australs display distinct geochemical trends that appear to geographically project, backward in time, to corresponding trends in the WPSP. However, the implied close proximity of source regions (i.e., ∼ 1000 km) with very different geochemical fingerprints and their longevity over geological time (> 100 Myrs) appear to be at odds with the secondary plumelet hypothesis, a mechanism with a typical timescale of ∼ 30 Myrs. Moreover, ages sampled along the individual volcano chains of the Cook-Australs, and of the WPSP violate the predictions of the plumelet hypothesis in terms of linear age–distance relationships. Our numerical models indicate that small-scale sublithospheric convection (SSC) as likely triggered by the thermochemical anomaly of the ‘Superplume’ instead reconciles complex age–distance relationships, because related volcanism occurs above elongated melting anomalies parallel to plate motion (‘hot lines’). Furthermore, SSC-melting of a mantle source that consists of pyroxenite veins and enriched peridotite blobs in a matrix of depleted peridotite creates systematic geochemical trends over seafloor age during volcanism. These trends arise from variations in the amount of pyroxenite-derived lavas relative to peridotite-derived lavas along a ‘hot line,’ therefore stretching between the geochemical end-members HIMU and EMI. These predicted trends are consistent with observed trends in radiogenic isotopic composition from the Wakes, Marshalls, Gilberts (i.e., the individual volcano groups of the WPSP) and the Cook-Australs. For increasing mantle temperatures, volcanism is further predicted to occur at greater seafloor ages and with a more EMI-like signature, a result that can explain many of the observed systematics. Thus, SSC explains many of the geochemical observations with long-term temporal variations in mantle temperature, instead of persistent intermediate-scale (∼ 1000 km) compositional heterogeneity.

The Seismic Structure and Dynamics of the Mantle Wedge

Seismic imaging provides an opportunity to constrain mantle wedge processes associated with subduction, slab dehydration, arc volcanism, and backarc spreading. The mantle wedge is characterized by a low attenuation forearc, an inclined zone of low velocity and high attenuation underlying the volcanic front, and a broad region of low velocity and high attenuation beneath the backarc spreading center when present. Seismic velocities, bathymetry, and basalt chemistry suggest mantle temperature variations of ∼100°C between different backarc regions. Rock physics experiments and geodynamic modeling are essential for interpreting seismic observations. Seismic anisotropy indicates a complex pattern of mantle flow that can be modeled with along-strike flow in a low viscosity channel beneath the arc and backarc. Comparison of geodynamic models with seismic tomographic results using experimentally derived relations between velocity, attenuation, and temperature suggests the existence of small melt fractions in the mantle at depths of 30–150 km.

Thermal regime of the Southeast Indian Ridge between 88°E and 140°E: Remarks on the subsidence of the ridge flanks

The flanks of the Southeast Indian Ridge are characterized by anomalously low subsidence rates for the 0–25 Ma period: less than 300 m Ma−1/2 between 101°E and 120°E and less than 260 m Ma−1/2 within the Australian‐Antarctic Discordance (AAD), between 120°E and 128°E. The expected along‐axis variation in mantle temperature (∼50°C) is too small to explain this observation, even when the temperature dependence of the mantle physical properties is accounted for. We successively analyze the effect on subsidence of different factors, such as variations in crustal thickness; the dynamic contribution of an old, detached slab supposedly present within the mantle below the AAD; and depletion in ϕm, a parameter here defined as the “ubiquitously distributed melt fraction” within the asthenosphere. These effects may all contribute to the observed, anomalously low subsidence rate of the ridge flanks, with the most significant contribution being probably related to the depletion in ϕm. However, these effects have a deep‐seated origin that cannot explain the abruptness of the transition across the fracture zones that delineate the boundaries of the AAD, near 120°E and near 128°E, respectively.

Variation in styles of rifting in the Gulf of California

Constraints on the structure of rifted continental margins and the magmatism resulting from such rifting can help refine our understanding of the strength of the lithosphere, the state of the underlying mantle and the transition from rifting to seafloor spreading. An important structural classification of rifts is by width, with narrow rifts thought to form as necking instabilities (where extension rates outpace thermal diffusion) and wide rifts thought to require a mechanism to inhibit localization, such as lower-crustal flow in high heat-flow settings. Observations of the magmatism that results from rifting range from volcanic margins with two to three times the magmatism predicted from melting models to non-volcanic margins with almost no rift or post-rift magmatism. Such variations in magmatic activity are commonly attributed to variations in mantle temperature. Here we describe results from the PESCADOR seismic experiment in the southern Gulf of California and present crustal-scale images across three rift segments. Over short lateral distances, we observe large differences in rifting style and magmatism--from wide rifting with minor synchronous magmatism to narrow rifting in magmatically robust segments. But many of the factors believed to control structural evolution and magmatism during rifting (extension rate, mantle potential temperature and heat flow) tend to vary over larger length scales. We conclude instead that mantle depletion, rather than low mantle temperature, accounts for the observed wide, magma-poor margins, and that mantle fertility and possibly sedimentary insulation, rather than high mantle temperature, account for the observed robust rift and post-rift magmatism.

Evaluating hot spot–ridge interaction in the Atlantic from regional‐scale seismic observations

We probe variations in mantle temperature, composition, and fabric along hot spot–influenced sections of the Mid‐Atlantic Ridge (MAR), using surface waves from nearby ridge earthquakes recorded on broadband island‐based seismic stations. We invert frequency‐dependent phase delays from these events to estimate one‐dimensional mean shear velocity and radial shear anisotropy profiles in the upper 200 km of the mantle within two seafloor age intervals: 5–10 Ma and 15–20 Ma. Mean shear velocity profiles correlate with apparent hot spot flux: lithosphere formed near the low‐flux Ascension hot spot is characterized by high mantle velocities, while the MAR near the higher‐flux Azores hot spot has lower velocities. The impact of the high‐flux Iceland hot spot on mantle velocities along the nearby MAR is strongly asymmetric: the lithospheric velocities near the Kolbeinsey ridge are moderately slow, while velocities near the Reykjanes ridge estimated in previous studies are much slower. Within each region the increase in shear velocity with age is consistent with a half‐space cooling model, and the velocity variations observed between Ascension, the Azores, and Kolbeinsey are consistent with approximately ±75° potential‐temperature variation among these sites. In comparison, the Reykjanes lithosphere is too slow to result purely from half‐space cooling of a high‐temperature mantle source. We speculate that the anomalously low shear velocities within the lithosphere produced at the Reykjanes ridge result from high asthenospheric temperatures of +50–75 K combined with ∼12% (by volume) gabbro retained in the mantle due to the imbalance between high hot spot–influenced melt production and relatively inefficient melt extraction along the slow spreading Reykjanes. Radial shear anisotropy in the upper 150 km also indicates an apparent hot spot influence: mantle fabric near Ascension is quite weak, consistent with previous models of anisotropy produced by corner flow during slow seafloor spreading. The fabric near the Azores and the Kolbeinsey ridge is stronger, suggesting that the hot spot increases mantle deformation beyond that produced by slow seafloor spreading in these regions.

Mantle temperature variations beneath back-arc spreading centers inferred from seismology, petrology, and bathymetry

Variations in seismological structure, major element composition, and axial depth between different back-arc spreading centers provide constraints on physical conditions associated with back-arc melt production. We invert vertical and transverse seismograms from several representative paths traversing the Mariana, Lau, North Fiji, and East Scotia back-arc basins. Seismic velocity varies substantially at depths of 40–100 km, with differences of up to 7% between the slowest (Lau) and the fastest (Mariana) structures. These mantle seismological structures correlate with major element systematics and the elevations of the ridge axes, consistent with differences in average upper mantle temperatures. In contrast to the temperature correlation, mantle seismic structure shows no apparent correlation with petrologically inferred water content. Petrological indicators suggest a ∼ 100 °C range in mantle potential temperature, consistent with the seismic velocity variations, assuming experimentally determined temperature derivatives. The temperature variations, however, must extend throughout the upper ∼ 200 km of the mantle wedge to produce the observed ridge elevation differences. Fast slab rollback and the influx of hot Samoan mantle may contribute to high temperatures in the Lau Basin.

Temporal variation of oceanic spreading and crustal production rates during the last 180 My

We present a re-evaluation of seafloor spreading and generation rates, mainly based on a direct measurement of the remaining surfaces of oceanic crust and isochron lengths defined in the most recent isochron maps [J.Y. Royer, R.D. Müller, L.M. Gahagan, L.A. Lawyer, C.L. Mayes, D. Nürnberg, J.G. Sclater, A global isochron chart, Tech. Rep. 117, Austin, Univ. of Tex. Inst. for Geophys., 1992; R.D. Müller, W.R. Roest, J.Y. Royer, L.M. Gahagan, J.G. Sclater, Digital isochrons of the world's ocean floor, J. Geophys. Res., 102 (1997), 3211–3214]. Our evaluation of the amount of oceanic crust per unit age {d A/d t} as a function of age, which can be expressed as d A/d t= C o(1− t/ t m), is in fairly good agreement with previous determinations [J.G. Sclater, B. Parsons, C. Jaupart, Oceans and continents: similarities and differences in the mechanisms of heat loss, J. Geophys. Res., 86 (1981) 11,535–11,552; D.B. Rowley, Rate of plate creation and destruction: 180 Ma to present, Geol. Soc. Amer. Bull., 114 (2002) 927–933], with C o=2.850±0.119 km 2 year −1 and t m=180.2±9.7 Ma. Dividing these d A/d t by the ridge lengths L, defined as the isochron length at each epoch allowed us to compute the evolution of global half-spreading rates. These have been roughly constant at 25.9±3.3 mm year −1 for at least the last 150 Ma. We propose that the global seafloor surface generation rate is roughly constant as well, with a mean half-value of 1.298±0.284 km 2 year −1 and varying ±20% with time. This study corroborates the recent conclusion of Rowley [D.B. Rowley, Rate of plate creation and destruction: 180 Ma to present, Geol. Soc. Amer. Bull., 114 (2002) 927–933], of a constant generation rate since 180 Ma, and completely contradicts the commonly accepted idea of high seafloor spreading and surface generation rates during a large part of the Cretaceous. Combining the oceanic surface generation rates derived here with crustal thicknesses deduced from the chemical composition of old oceanic crusts and seismic measurements [E. Humler, C.H. Langmuir, V. Daux, Depth versus age: new perspectives from the chemical compositions of ancient crust, Earth Planet Sci. Lett., 173 (1999) 7–23], the magmatic flux at young (0–80 Ma) oceanic ridges appears to be about 18.1±3.4 km 3 year −1 and was possibly 15% to 30% higher during the Mesozoic. We propose that mantle temperature variation provides an alternative mechanism to spreading rate for the Cretaceous highstand in sea-level and atmospheric CO 2 generation.

A dynamic model for the Iceland Plume and the North Atlantic based on tomography and gravity data

SUMMARY The North Atlantic around Iceland is characterized by a large geoid high and an anomalous shallow ocean floor; both observations are presumably due to upwelling of hot material in the mantle. We extracted this region from a global tomography data set to study the structure of the mantle in more detail. The tomography data reveal a confined low-velocity structure from the core–mantle boundary (CMB) to the upper mantle, stretching from a location at the CMB below southwest Greenland towards the upper mantle in a strongly eastward inclination. In the present study we compare the observed gravity potential field in the North Atlantic with a modelled field based on mantle temperature variations estimated from tomography. Seismic traveltime residuals are converted to temperature variations assuming a linear relation between seismic velocity and density and a pressure-dependent thermal expansivity. We found a maximum excess temperature in the plume conduit at the CMB of ∼250 °C, weakly decreasing towards the phase transition zone (PTZ). In the PTZ a temperature rise of ∼50–70 °C is found, which is in agreement with the latent heat release by the olivine phase transitions. The 3-D temperature field is then used as the driving force for viscous flow in a Cartesian convection model. The model dimensions are chosen four times as large as the tomographic section to allow resolution for long wavelengths and convective return flow. For a number of constant viscosity and temperature- and depth-dependent viscosity cases the dynamic topography, gravity anomaly and geoid undulations are calculated and compared with the EGM96 potential field coefficients in the wavelength range of 400 to 4000 km. The observation data were also corrected for ocean lithosphere cooling and isostatic compensation of continental crust. The best agreement between observation and modelled data (78 per cent fit for geoid and 47 per cent for gravity) is obtained for a temperature-dependent viscosity of about one order of magnitude for 500 °C temperature variation and an increase of viscosity with depth by no more than a factor of 50 from the upper to the lower mantle. The generally good spatial agreement supports the tomographic model, at least for the upper mantle, and indicates that the East Greenland margin as well as the outer Faroer Ridge are dynamically supported. Low-density material west of the Kolbeinsey Ridge might be linked to low-density anomalies below the Greenland Shield. The presence of lower mantle anomalies causes a large-scale geoid high of ∼3 to 5 m in agreement with observations, but our approach cannot further constrain the spatial distribution of anomalies in the lower mantle.