Mantle Temperature Research Articles

Global trends in novel stable isotopes in basalts: Theory and observations

The geochemistry of global mantle melts suggests that both mid-ocean ridge basalts (MORB) and ocean island basalts (OIB) sample lithological and temperature heterogeneities originating in both the upper and lower mantle. Recently, non-traditional stable isotopes have been suggested as a new tool to complement existing tracers of mantle heterogeneity (e.g., major and trace elements, radiogenic isotopes), because mineral- and redox-specific equilibrium stable isotope fractionation effects can link the stable isotope ratios of melts to their source mineralogy and melting degree. Here, we investigate five stable isotope systems (Mg-Ca-Fe-V-Cr) that have shown promise in models or natural samples as tracers of mantle temperature and/or lithological heterogeneity. We use a quantitative model, combining thermodynamically self-consistent mantle melting and equilibrium isotope fractionation models, to explore the behaviour of the isotope ratios of these elements during melting of three mantle lithologies (peridotite, and silica-excess and silica-deficient pyroxenites), responding to changes in mantle mineralogy, oxygen fugacity, temperature and pressure. We find that, given current analytical precision, the stable isotope systems examined here are not predicted to be sensitive to mantle potential temperature variations through equilibrium isotope fractionation processes. By contrast, source lithological heterogeneity is predicted to be detectable in some cases in the stable isotope ratios of erupted basalts, although generally only at proportions of >10% MORB-like pyroxenite in the mantle source, given current analytical precision. Magnesium and Ca stable isotopes show most sensitivity to a garnet-bearing source lithology, and Fe and Cr stable isotopes are potentially sensitive to the presence of MORB-like pyroxenite in the mantle source, although the behaviour of Cr isotopes is comparatively under-constrained and requires further work to be applied with confidence to mantle melts. When comparing the magnitude and direction of predicted equilibrium isotopic fractionation of peridotite and pyroxenite melts to natural MORB and OIB data, we find that aspects of the natural data (including the mean Mg-Ca-Fe-V isotopic composition of MORB, the range of Mg-Ca isotopic compositions seen in MORB data, the mean Mg-Ca-Cr isotopic composition of OIB, and the range of Mg-V-Cr isotopic compositions in OIB data) can be matched by equilibrium isotope fractionation during partial melting of peridotite and pyroxenite sources – with pyroxenite required even for some MORB data. However, even when considering analytical uncertainty on natural sample measurements, the range in stable isotope compositions seen across the global MORB and OIB datasets suggests that kinetic isotope fractionation, or processes modifying the isotopic composition of recycled crustal material such that it is distinct from MORB, may be required to explain all the natural data. We conclude that the five stable isotope systems considered here have potential to be powerful complementary tracers to other geochemical tracers of the source lithology of erupted basalts. However, continued improvements in analytical precision in conjunction with experimental and theoretical predictions of isotopic fractionation between mantle minerals and melts are required before these novel stable isotopes can be unambiguously used to understand source heterogeneity in erupted basalts.

Mantle source heterogeneity in a Neoproterozoic back-arc basin: Geochemical and thermodynamic modeling of the volcanic section of Wadi Ghadir ophiolite, Egypt

Wadi Ghadir ophiolite in the Egyptian Eastern Desert, which forms the northern part of the Nubian Shield, is considered as one of the best-preserved segments of the Neoproterozoic oceanic lithosphere on Earth. Primary melt calculations and thermodynamic modeling of new geochemical data for the Wadi Ghadir pillow lavas and dike complexes, integrated with comprehensive petrographic investigations, are here employed to unravel the geodynamic evolution of this ophiolitic section.Whole-rock geochemical and clinopyroxene data indicate that the pillow lavas and sheeted dike complexes preserve geochemical signatures of N-MORB, E-MORB and OIB-like basalts. Less abundant, discrete dikes have a distinctly LILE-enriched island arc tholeiite (IAT) composition. The pillow lavas and sheeted dike complexes replicate mixed liquid lines of descent, whereas the discrete dikes exhibit a calc-alkaline differentiation trend. The pillow lavas were produced by limited (5–7%) mantle partial melting as a result of isothermal decompression over a wide pressure range (5 – 19 kbar) and restricted mantle potential temperatures (Tp ≈ 1260–1300 °C), suggesting ponding of rising melts from depths of ∼60 to 15 km in dry mantle. Primary melts of the sheeted dike complexes were produced by 9–11% partial melting at P ≈ 9.7 ± 2.5 kbar, Tp ≈ 1290 °C, and P ≈ 14.7 ± 1.4 kbar, Tp ≈ 1325 °C, suggesting a role of varying hydrostatic pressure. The magma source for the discrete dikes occurred at 10.7 ± 0.6 kbar and Tp of 1230–1300 °C by 11–13% partial melting of a metasomatized mantle region.The calculated Tp ranges for the different melts are consistently lower than temperatures of sub-ridge ambient mantle. Together with the variable concentrations of K2O and other incompatible elements, the estimated low temperatures of melt generation emphasize mixed sources and wet peridotite melting. The modeled primary melts designate crustal growth during back‐arc basin opening and closure. During the basin opening, decompression melting of mildly enriched mantle produced high-Ti tholeiitic (pillow lavas) and transitional (sheeted dike complexes) melts, whereas subduction initiation during basin closure promoted melt-peridotite interaction. Lowering of the solidus by subduction-related components triggered melting of a spinel lherzolitic mantle and produced low-Ti calc-alkaline melts that sourced the discrete dikes. The mixed MORB and SSZ geochemical characteristics of the studied volcanic rocks, coupled with the various modeled melts and lack of significant thermal anomalies in the mantle conditions, are here interpreted as manifestations of mantle source heterogeneity in a marginal oceanic basin during the accretionary stages of the Arabian-Nubian Shield.

High Electrical Conductivity of Olivine at Oxidizing Conditions of the Shallow Mantle and Geophysical Implications

AbstractHigh electrical conductivity in the shallow mantle has long been recognized by electromagnetic depth soundings, but the origin remains debated. Various candidates, of which the two most popular are water enrichment in olivine (the dominant mineral in the upper mantle) and partial melt, have been proposed to explain the electrical anomalies. These models nearly exclusively assume a very low conductivity for dry or water‐poor olivine. Here, we show by experimental work that, under well‐controlled conditions of pressure, temperature, oxygen fugacity (fO2), and water inventory, the conductivity of olivine is in fact insensitive to water content at mantle temperature (e.g., >1,000°C), although the enhancement is substantial at relatively low temperature. In particular, the conductivity of olivine at temperature beyond ∼1,000°C is strikingly high, about 0.01–0.1 S/m, at the oxidizing conditions of the shallow mantle. The high conductivity is comparable to the geophysically measured high conductivity in many conductive regions of the shallow mantle. The results provide a new framework for understanding the electrical anomalies and electrical structure of the shallow mantle.

Dynamic slab segmentation due to brittle-ductile damage in the outer rise.

Subduction is the major plate driving force, and the strength of the subducting plate controls many aspects of the thermochemical evolution of Earth. Each subducting plate experiences intense normal faulting1-9 during bending that accommodates the transition from horizontal to downwards motion at the outer rise at trenches. Here we investigate the consequences of this bending-induced plate damage using numerical subduction models in which both brittle and ductile deformation, including grain damage, are tracked and coupled self-consistently. Pervasive slab weakening and pronounced segmentation can occur at the outer-rise region owing to the strong feedback between brittle and ductile damage localization. This slab-damage phenomenon explains the subduction dichotomy of strong plates and weak slabs10, the development of large-offset normal faults6,7 near trenches, the occurrence of segmented seismic velocity anomalies11 and distinct interfaces imaged within subducted slabs12,13, and the appearance of deep, localized intraplate areas of reduced effective viscosity14 observed at trenches. Furthermore, brittle-viscously damaged slabs show a tendency for detachment at elevated mantle temperatures. Given Earth's planetary cooling history15, this implies that intermittent subduction with frequent slab break-off episodes16 may have been characteristic for Earth until more recent times than previously suggested17.

Depletion of the upper mantle by convergent tectonics in the Early Earth

Partial melting of mantle peridotites at spreading ridges is a continuous global process that forms the oceanic crust and refractory, positively buoyant residues (melt-depleted mantle peridotites). In the modern Earth, these rocks enter subduction zones as part of the oceanic lithosphere. However, in the early Earth, the melt-depleted peridotites were 2–3 times more voluminous and their role in controlling subduction regimes and the composition of the upper mantle remains poorly constrained. Here, we investigate styles of lithospheric tectonics, and related dynamics of the depleted mantle, using 2-D geodynamic models of converging oceanic plates over the range of mantle potential temperatures (Tp = 1300–1550 °C, ∆T = T − Tmodern = 0–250 °C) from the Archean to the present. Numerical modeling using prescribed plate convergence rates reveals that oceanic subduction can operate over this whole range of temperatures but changes from a two-sided regime at ∆T = 250 °C to one-sided at lower mantle temperatures. Two-sided subduction creates V-shaped accretionary terrains up to 180 km thick, composed mainly of highly hydrated metabasic rocks of the subducted oceanic crust, decoupled from the mantle. Partial melting of the metabasic rocks and related formation of sodic granitoids (Tonalite–Trondhjemite–Granodiorite suites, TTGs) does not occur until subduction ceases. In contrast, one sided-subduction leads to volcanic arcs with or without back-arc basins. Both subduction regimes produce over-thickened depleted upper mantle that cannot subduct and thus delaminates from the slab and accumulates under the oceanic lithosphere. The higher the mantle temperature, the larger the volume of depleted peridotites stored in the upper mantle. Extrapolation of the modeling results reveals that oceanic plate convergence at ∆T = 200–250 °C might create depleted peridotites (melt extraction of > 20%) constituting more than half of the upper mantle over relatively short geological times (~ 100–200 million years). This contrasts with the modeling results at modern mantle temperatures, where the amount of depleted peridotites in the upper mantle does not increase significantly with time. We therefore suggest that the bulk chemical composition of upper mantle in the Archean was much more depleted than the present mantle, which is consistent with the composition of the most ancient lithospheric mantle preserved in cratonic keels.

The Effects of Planetary and Stellar Parameters on Brittle Lithospheric Thickness

AbstractThe thickness of the brittle lithosphere—the outer portion of a planetary body that fails via fracturing—plays a key role in the geological processes of that body. The properties of both a planet and its host star can influence that thickness, and the potential range of those properties exceeds what we see in the Solar System. To understand how planetary and stellar parameters influence brittle lithospheric thickness generally, we modeled a comprehensive suite of combinations of planetary mass, surface and mantle temperature, heat flux, and strain rate. Surface temperature is the dominant factor governing the thickness of the brittle layer: smaller and older planets generally have thick brittle lithospheres, akin to those of Mercury and Mars, whereas larger, younger planets have thinner brittle lithospheres that may be comparable to the Venus lowlands. But certain combinations of these parameters yield worlds with exceedingly thin brittle layers. We predict that such bodies have little elevated topography and limited volatile cycling and weathering, which can be tested by future telescopic observations of known extrasolar planets.

The Role of Temperature in the Along‐Margin Distribution of Volcanism and Seismicity in Subduction Zones: Insights From 3‐D Thermomechanical Modeling of the Central Andean Margin

AbstractThe distribution of volcanic and seismogenic zones is segmented along the trench‐parallel direction in the Central Andes, and factors controlling their clustering are not fully understood. Here we present a 3‐D thermomechanical model of the subduction zone at 18°–26°S to examine the role that temperature and mantle flow play in the distribution of active volcanoes and seismicity. We applied a steady state approach in which solid‐state flow is driven by a kinematically prescribed slab with realistic geometry (including changes along the Bolivian Orocline) and using a 3‐D model of the continental crust thickness. The obtained temperature distribution is consistent with proxies for isotherms derived from independent geophysical data, except below the Eastern Cordillera at 21°–23°S. The computed mantle flow pattern reveals the presence of along‐margin dynamic pressure gradients. This 3‐D preferential flow results in mantle temperatures of 1200–1400°C at 80–100 km depth below the arc, with comparatively higher temperatures at ∼22°–25°S. The obtained along‐margin variations in temperature and in estimated melt velocity suggest that the subarc mantle south of 22°S exhibits more favorable conditions for generation and upward migration of partial melts. This segment coincides with the higher concentration of active arc volcanoes and the presence of the Altiplano‐Puna Volcanic Complex in the backarc. Intermediate‐depth seismicity concentrates roughly below where the slab top is at 400–800°C, suggesting that temperature exerts some control on the first‐order distribution of intraslab seismicity. However, most intraslab seismicity occur at pressure‐temperature conditions which are outside of the stability field expected for key dehydration reactions in slabs.

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.

Effect of the cold Nazca Slab on the depth of the 660 km discontinuity in South America

The Nazca plate subducting beneath South America makes the 660 km discontinuity deeper and the mantle transition zone (MTZ) thicker under the continent. MTZ variations are often associated with mantle temperature and, therefore, can help confirm the slab position at greater depths. Recent P- and S-wave tomography results show the Nazca plate, near 20° S, being held below the MTZ for longitudes between 70° W and 55° W. We used 63,809 P-wave receiver functions from 1216 stations (using the LQT components of the incident ray system) to image the MTZ in South America. The receiver functions were corrected for move-out, stacked in cells of 3° x 3° degrees every 1° x 1°. We obtained 54,389 RF traces imaging the 410 km and the 660 km discontinuities. The discontinuity times were corrected using the SL2013 global tomography model to obtain depths. A thickened MTZ follows the trend of the Nazca plate beneath the sub-Andes. To the north of 18oS the thickened MTZ is only about 250 km wide; to the south, the thickened zone reaches up to a 1100 km width. This observation clearly indicates that the Nazca slab flattens close to the 660 km discontinuity lowering the mantle temperature and thickening the MTZ. The 660 km discontinuity is more affected than the 410 km, which is consistent with the Nazca slab being held just below the MTZ and not inside the MTZ in most of the region.

Can sub-slab low-velocity anomalies be an artifact caused by anisotropy? A case study from the Alboran slab area in the western Mediterranean

Although it is well understood that mantle flow in a subduction setting can develop seismic anisotropy, many tomographic studies still assume an isotropic mantle. In this study, we explore how unaccounted-for seismic anisotropy contributes to P-wave anomalies, focusing on apparent sub-slab low velocities. We use data from the westernmost Mediterranean, where the Alboran slab has subducted beneath the Gibraltar Arc. Our isotropic P-wave velocity model shows a low-velocity anomaly at depths of 75–225 km to the west of the high-velocity-anomaly that represents the Alboran slab. We explore whether this low-velocity anomaly could be an anisotropy-induced artifact as a significant shear-wave-splitting (SWS) along the arc is observed. We consider five different hypothetical anisotropy models representing different mantle configurations. The anisotropy models are based on 1) the SWS observations, 2) the depth distribution of the low-velocity anomaly and 3) geodynamic modeling of the development of entrained and toroidal mantle flow in generic subduction zones. Given the underdetermined nature of an anisotropic inversion, we include the anisotropy models as a priori constraints on the tomography. The tomographic results including anisotropy models have smaller travel time residuals and isotropic anomalies in the region of interest while fitting the SWS observation better than the isotropic model. We propose that unaccounted-for seismic anisotropy can produce artifacts in isotropic tomography of real datasets, and this is at least partially the case for the low-velocity anomaly below the Alboran slab. Further, we show that not taking this into account can lead to erroneous conclusions about mantle temperature and the presence of melt. From the insights that our anisotropy models provide, we also suggest that toroidal mantle flow is currently dominant below and around the Alboran slab.

Modelling of thermal stratification at the top of a planetary core: Application to the cores of Earth and Mercury and the thermal coupling with their mantles

We present a new numerical scheme for solving one-dimensional conduction problems and in the radial direction of a spherically symmetric body or shell with variable spatial domain. This numerical scheme adopts a solution of the conduction equation in each interval of the chosen discretization which is valid if the fluxes at the interval boundaries are constant in time. This ‘piece-wise steady flux’ (PWSF) numerical scheme is continuous and differentiable in the space domain. These smoothness properties are convenient for implementing the numerical scheme in an energy-conserved thermal evolution approach for a terrestrial core in which a conductive thermally stratified layer is considered that develops below the core-mantle boundary when the heat flux drops below the adiabatic heat flux. The influence of a time-variable thermally stratified region on the general evolution of the planetary body is examined, in comparison to imposing an adiabatic temperature profile for the entire core. Also, the dependence of the model's accuracy to the applied grid size of the numerical scheme is studied. We showed that a very low numerical resolution of this approach suffices for obtaining a thermal evolution of a partially conductive planetary core with high accuracy.By considering thermal stratification in a planetary core where the heat flux is lower than the adiabatic heat flux, radial variations in the cooling rate are accounted for whereas otherwise the distribution of energy in the core is fixed by the imposed adiabat. During the growth of the thermally stratified region, the deep part of the core cools more rapidly than the outer part of the core. Therefore, the inner core grows to a larger size and the temperature and heat flux at the core-mantle boundary are higher and larger, respectively, if a thermally stratified region is considered. For the Earth, the implications are likely very minor and can be neglected in thermal evolution studies that are not specifically interested in the thermally stratified region itself. For Mercury, however, these implications are larger. For example, the age of Mercury's inner core can be underestimated by about a billion of years if thermal stratification is neglected. The consideration of thermal stratification in Mercury's core is also important for the evolution of Mercury's mantle. It increases the mantle temperature, leads to a higher Rayleigh number and therefore a larger heat flux into the lithosphere and prolongation of mantle convection.

Melting conditions and mantle source composition from probabilistic joint inversion of major and rare earth element concentrations

The chemical composition of erupted basalts provides a record of the thermo-chemical state of their source region and the melting conditions that lead to their formation. Here we present the first probabilistic inversion framework capable of inverting both trace and major element data of mafic volcanic rocks to constrain mantle potential temperature, depth of melting, and major and trace element source composition. The inversion strategy is based on the combination of (i) a two-phase multi-component reactive transport model, (ii) a thermodynamic solver for the evolution of major elements and mineral/liquid phases, (iii) a disequilibrium model of trace element partitioning and (iv) an adaptive Markov chain Monte Carlo algorithm. The mechanical and chemical evolution of melt and solid residue are therefore modelled in an internally- and thermodynamically-consistent manner.We illustrate the inversion approach and its sensitivity to relevant model parameters with a series of numerical experiments with increasing level of complexity. We show the benefits and limitations of using major and trace element compositions separately before demonstrating the advantages of a joint inversion. We show that such joint inversion has great sensitivity to mantle temperature, pressure range of melting and composition of the source, even when realistic uncertainties are assigned to both data and predictions. We further test the reliability of the approach on a real dataset from a well-characterised region: the Rio Grande Rift in western North America. We obtain estimates of mantle potential temperature (∼1340 °C), lithospheric thickness (∼60 km) and source composition that are in excellent agreement with numerous independent geochemical and geophysical estimates. In particular, this study suggests that the basalts in this region originated from a moderately hot upwelling and include the contribution from a slightly depleted source that experienced a small degree of melt or fluid metasomatism. This component is likely associated with partial melting of the lower portions of the lithosphere. The flexibility of both the melting model and inversion scheme developed here makes the approach widely applicable to assessing the thermo-chemical structure and evolution of the lithosphere-asthenosphere system and paves the way for truly joint geochemical-geophysical inversions.

Scaling laws for stagnant-lid convection with a buoyant crust

SUMMARY Stagnant-lid convection, where subduction and surface plate motion is absent, is common among the rocky planets and moons in our solar system, and likely among rocky exoplanets as well. How stagnant-lid planets thermally evolve is an important issue, dictating not just their interior evolution but also the evolution of their atmospheres via volcanic degassing. On stagnant-lid planets, the crust is not recycled by subduction and can potentially grow thick enough to significantly impact convection beneath the stagnant lid. We perform numerical models of stagnant-lid convection to determine new scaling laws for convective heat flux that specifically account for the presence of a buoyant crustal layer. We systematically vary the crustal layer thickness, crustal layer density, Rayleigh number and Frank–Kamenetskii parameter for viscosity to map out system behaviour and determine the new scaling laws. We find two end-member regimes of behaviour: a ‘thin crust limit’, where convection is largely unaffected by the presence of the crust, and the thickness of the lithosphere is approximately the same as it would be if the crust were absent; and a ‘thick crust limit’, where the crustal thickness itself determines the lithospheric thickness and heat flux. Scaling laws for both limits are developed and fit the numerical model results well. Applying these scaling laws to rocky stagnant-lid planets, we find that the crustal thickness needed for convection to enter the thick crust limit decreases with increasing mantle temperature and decreasing mantle reference viscosity. Moreover, if crustal thickness is limited by the formation of dense eclogite, and foundering of this dense lower crust, then smaller planets are more likely to enter the thick crust limit because their crusts can grow thicker before reaching the pressure where eclogite forms. When convection is in the thick crust limit, mantle heat flux is suppressed. As a result, mantle temperatures can be elevated by 100 s of degrees K for up to a few Gyr in comparison to a planet with a thin crust. Whether convection enters the thick crust limit during a planet’s thermal evolution also depends on the initial mantle temperature, so a thick, buoyant crust additionally acts to preserve the influence of initial conditions on stagnant-lid planets for far longer than previous thermal evolution models, which ignore the effects of a thick crust, have found.

The Seismicity of Indonesia and Tectonic Implications

AbstractIndonesian seismicity provides important insights into the tectonics and hazards of a region that is, characterized by a remarkable diversity in faulting, including subduction, extension, thrusting, and strike‐slip faulting. We present a synthesis of Indonesian seismotectonics by documenting the distributions of hypocenters (≥M 4.6) and focal mechanisms (≥M 5.0) over ∼20 years, quantifying seismicity rates, and comparing observed seismicity trends with recent tectonic and tomography models. Of the 20,622 events ≥M 4.6 observed in the study region, ∼77% are shallow (≤70 km depth) and of magnitudes <M 5.0 (68%). Sixty one events ≥M 7.0 occurred, five of which exceeded M 8.0, including the 2004 Mw 9.1 Sumatra‐Andaman earthquake. Regionally, about ∼320 ≥ M 5.0 earthquakes occur per year, and rates decrease exponentially with increasing temperatures and pressures between 50 and 300 km. Seismicity rates increase in the Mantle Transition Zone (MTZ) likely due to viscous resistance at the 410, 520, and 660 km discontinuities. Intermediate and deep events (≥70 km depth) trace the Wadati‐Benioff zones of several subducting slabs exhibiting geometries consistent with recent tomography models. Seismicity extends to a maximum depth of 678 km. Oblique convergence, lithospheric age, ambient mantle temperatures, and plate velocity likely contribute to the non‐uniform depth distribution of intermediate and deep earthquakes. Shallow seismicity provides insight into how complex oblique convergence is accommodated near the surface, with primary sources including megathrusting, crustal faulting, and shallow intraslab faulting. All sources of shallow seismicity constitute significant seismic hazards.

Thermoelasticity of Water in Silicate Melts: Implications for Melt Buoyancy in Earth's Mantle

AbstractA comprehensive analysis of experimental data and theoretical simulations on the partial molar volume of water in silicate melt indicates that finite strain theory successfully describes the compression of the H2O component dissolved in silicate melt at high pressures and temperatures. However, because of the high compressibility of the water component, a fourth order equation of state fit is required to accurately simulate experimental results on water's volume in silicate melts at a deep upper mantle, transition zone, and lower mantle pressures. Data from previous shock compression experiments on hydrous minerals in which melting occurs along the Hugoniot are used to provide an experimental constraint on the partial molar volume of water in silicate melt at deep mantle temperatures and pressures. The equation of state of the water component indicates that, depending on elastic averaging technique, the amount of water that could be present in neutrally or negatively buoyant mafic/ultramafic melts above the 410 km seismic discontinuity is upper‐bounded at 5.6 wt%: smaller than previously inferred, and consistent with melt being confined to a narrow depth range above the 410 km discontinuity. If melt is predominantly distributed along grain boundaries in low aspect ratio films, extents of melting as low as 2% could produce observed seismic velocity reductions. The ability of the lowermost mantle to contain negatively buoyant hydrous liquids hinges on the trade‐off between iron content and hydration: at these depths, substantially higher degrees of hydration could be present within partial melts.

Thermal Structure of Eastern Australia's Upper Mantle and Its Relationship to Cenozoic Volcanic Activity and Dynamic Topography

AbstractSpatio‐temporal changes of upper mantle structure play a significant role in generating and maintaining surface topography. Although geophysical models of upper mantle structure have become increasingly refined, there is a paucity of geologic constraints with respect to its present‐day state and temporal evolution. Cenozoic intraplate volcanic rocks that crop out across eastern Australia provide a significant opportunity to quantify mantle conditions at the time of emplacement and to independently validate geophysical estimates. This volcanic activity is divided into two categories: age‐progressive provinces that are generated by the sub‐plate passage of mantle plumes and age‐independent provinces that could be generated by convective upwelling at lithospheric steps. In this study, we acquired and analyzed 78 samples from both types of provinces across Queensland. These samples were incorporated into a comprehensive database of Australian Cenozoic volcanism assembled from legacy analyses. We use geochemical modeling techniques to estimate mantle temperature and lithospheric thickness beneath each province. Our results suggest that melting occurred at depths ≤80 km across eastern Australia. Prior to, or coincident with, onset of volcanism, lithospheric thinning as well as dynamic support from shallow convective processes could have triggered uplift of the Eastern Highlands. Mantle temperatures are inferred to be ∼50–100°C hotter beneath age‐progressive provinces that demarcate passage of the Cosgrove mantle plume than beneath age‐independent provinces. Even though this plume initiated as one of the hottest recorded during Cenozoic times, it appears to have thermally waned with time. These results are consistent with xenolith thermobarometric and geophysical studies.

X‐Discontinuity Beneath the Indian Shield—Evidence for Remnant Tethyan Oceanic Lithosphere in the Mantle

AbstractBeneath the continents and island arcs, a seismic discontinuity is often detected around 300 km depth, referred to as the X‐discontinuity. Various mineralogical and petrological mechanisms have been put forth to explain its occurrence. Because of the large depth variability, it is challenging to explain its origin invoking a single mineralogical phase transition. In order to investigate this discontinuity beneath India, we analyzed 10,216 P wave receiver functions at seismological stations deployed on the Indian shield and the Himalayas. We detect the X‐discontinuity as a sporadic and thin feature, in the depth range of 246–335 km, with a sharp shear velocity jump of 2.5%–3.6%. It neither bears a clear tectonic affinity nor has any correlation with the transition zone discontinuities. Interpreting its origin due to a single mineralogical change warrants a large spatial variation in the mantle temperature. Therefore, we suggest that the observed widespread X‐discontinuity beneath the Indian shield owes its origin to two mechanisms, that is, Orthoenstatite to high pressure Clinoenstatite transformation which shifts to lower pressures (∼2 GPa) due to the presence of water (0.13 wt% H2O) in MgSiO3 and coesite‐stishovite transition occurring at 8–11 GPa due to excess silica in an eclogitic component derived from the Tethys oceanic lithosphere subducted during lower Eocene. The identification of such a discontinuity could allow tracking of subducted material within the upper mantle providing a measure of mantle geochemical heterogeneity.

ERRATA: Synthesizing EarthScope data to constrain the thermal evolution of the continental U.S. lithosphere

The panels of Figure 8 are reversed relative to the caption, which should read “Maps of upper mantle temperatures at grain sizes of (A) 103, (B) 102, and (C) 10 μm.” In the Grain Size section on p. 1730, the third sentence should read “The model with the smallest grain size (10 μm) exhibits the lowest temperatures, while the largest grain size (1 mm) exhibits the highest.”

Mantle temperature and density anomalies: The influence of thermodynamic formulation, melt, and anelasticity

Deriving a quantitative geodynamical interpretation from tomography models is a difficult task. It is also a timely task, particularly given the many high-resolution tomography models published in recent years. It is important to assess mantle structure in terms of density and temperature anomalies. Here, we provide a new estimation of the conversion factors between seismic velocity anomalies and density anomalies, Rρ/v, and between seismic velocity anomalies and temperature anomalies, RT/v. These conversion factors are found by considering the constraints provided by thermodynamics, the influence of melt, and anelasticity. To determine the elastic component, we performed thermodynamic simulations by considering several compositions representative of normal or plume-enriched mantle. We also explore the effects of the presence of melt on seismic velocity and consider a radial attenuation profile to account for anelasticity. We provide an envelope of acceptable models, by combining the range of possible solutions combining the elastic, anelastic, and melt relationships used in deriving the conversion factors. Several of the frequently used velocity-density scaling relationships are encompassed in this envelope. We assess the validity of our result by calculating temperature anomalies from the tomographic models using our scaling relationships—these anomalies lie within the range constrained by melting temperatures registered in volcanic rocks (±300 K). Further, we show that the geoid model computed through our velocity-density scaling from the S40RTS tomography provides a good fit to the observed geoid.

Warm and Slightly Reduced Mantle Under the Off-Rift Snæfellsnes Volcanic Zone, Iceland

AbstractOlivine (Fo75-91) with spinel inclusions (Cr# 10–61) in basaltic lavas/tephras from the off-rift Snæfellsnes Volcanic Zone in Iceland record the chemistry, temperature and oxygen fugacity of fractionating magmas. After a detailed assessment of equilibrium conditions, crystallization temperatures and oxygen fugacity can be calculated from the composition of homogeneous Cr-spinel and Al-chromite inclusions in olivine phenocrysts. Geologically meaningful results can occasionally be obtained when homogenous spinel is enclosed in mildly zoned olivine and KDMg-Fe [(Mg/Fe)olivine/(Mg/Fe2+)spinel] is within the range for homogenous spinel in homogeneous olivine (3.5–4.3 for our samples). Spinel in normal zoned Fo84.7–90.9 olivine records the primitive stages of magma fractionation and has crystallized from clinopyroxene-free primitive melts, probably at Moho depth and/or below. Discrepancies between Tol-liq (Mg-Fe2+ diffusion sensitive) and TAL (diffusion insensitive) suggest that some primitive olivines experienced magma mixing, completely overprinting their Fo content. Consequentially, Tol-liq in primitive olivines occasionally records residence rather than crystallization conditions. Temperature (1187–1317°C) gradually decreases across normal zoned Fo84.7–90.9 olivine and controls fO2 (Δlog fO2 (QFM) −0.6 ± 0.2). Recharge-related primitive Fo83.8–86.8 mantles of reverse zoned olivine contain the most primitive Cr-spinel linked to crustal magma storage zones. These spinels are mostly antecrysts with high Cr# (41.1–47.9) similar to spinel in normal zoned olivines that were captured by olivine and equilibrated in terms of Mg-Fe2+. A rare olivine macrocryst crystallized alongside clinopyroxene (wehrlite) and includes abundant homogeneous Al-rich Cr-spinels. These are unique because they appear to record closed-system fractional crystallization rather than magma mixing and because they show that Cr-poor, Al-rich spinel crystallized alongside clinopyroxene. The macrocryst olivine–spinel pairs record lower crustal crystal mush conditions with fO2 around the QFM buffer and Tol-liq of ∼1200°C, similar to recharge-related mantles of reverse zoned olivine. More evolved compositions occur in the cores of reverse zoned olivine (Fo75-85) that contain Cr-spinel, Fe-spinel and Al-magnetite. Contrary to spinel in more primitive olivine, these compositions are diverse and follow increasing 100Fe3+/(Cr+Al+Fe3+) of 12.3 to 54.8 and TiO2 (3.3 to 14.7 wt %) at decreasing Mg# (57.4 to 24.1) and Cr# (30.4 to 9.9) and rapidly increasing oxygen fugacities (Δlog fO2 (QFM) +0.2 to +2.0) over only a limited temperature decrease (Tol-liq: 1190 to 1145°C). These compositions span the ‘spinel gap’ and are extremely rare globally. Their preservation is probably related to high-temperature crystallization followed by rapid cooling. These compositions occur at two of the four investigated volcanic centres (Búðahraun and Berserkjahraun) and indicate a strong influence of crustal magmatic processes on crystal composition and fO2, which is absent in the other two locations (Ólafsvíkurenni and Nykurhraun). Spinel and olivine compositions support the tectonically controlled decompression melting of a fertile peridotitic source at elevated mantle temperatures relative to MORB and more reducing conditions than other off-rift magmatism in Iceland.