Search

SUMMARY This paper presents a strategy for extending scalar (P–V–T) equations of state to self-consistently model anisotropic materials over a wide range of pressures and temperatures under nearly hydrostatic conditions. The method involves defining a conventional scalar equation of state (V(P, T) or P(V, T)) and a fourth-rank tensor state variable $\boldsymbol {\Psi }(V,T)$ whose derivatives can be used to determine the anisotropic properties of materials of arbitrary symmetry. This paper proposes two functional forms for $\boldsymbol {\Psi }(V,T)$ and provides expressions describing the relationship between $\boldsymbol {\Psi }$ and physical properties including the deformation gradient tensor, the lattice parameters, the isothermal elastic compliance tensor and thermal expansivity tensor. The isothermal and isentropic stiffness tensors, the Grüneisen tensor and anisotropic seismic velocities can be derived from these properties. To illustrate the use of the formulations, anisotropic models are parametrized using numerical simulations of cubic periclase and experimental data on orthorhombic San Carlos olivine.

NASA's InSight lander successfully touched down on Mars in November 2018, and, for the first time, a seismometer was deployed on the surface of the planet. The seismic recordings reveal diurnal and seasonal changes of the broadband noise level that are consistent with variations of the local atmospheric conditions. The seismic data include a variety of spectral peaks, which are interpreted as wind‐excited, mechanical resonances of the lander, resonances of the subsurface, or artifacts produced in the measurement system. Understanding the origin of these signals is critical for the detection and characterization of marsquakes as well as for studies investigating the ambient noise. We identify the major spectral peaks up to 9 Hz, corresponding to the frequency range the most relevant to observed marsquakes. We track the variations in frequency, amplitude, and polarization of these peaks over the duration of the mission so far. The majority of these peaks can readily be classified as measurement artifacts or lander resonances (lander modes), of which the latter have a temperature‐dependent peak frequency and a wind‐sensitive amplitude. Of particular interest is a prominent resonance at 2.4 Hz, which is used to discriminate between seismic events and local noise and is possibly produced by a subsurface structure. In contrast to the lander modes, the 2.4 Hz resonance has distinctly different features: (1) a broad and stable spectral shape, slightly shifted on each component; (2) predominantly vertical energy; (3) temperature‐independent peak frequency; (4) comparatively weak amplification by local winds, though there is a slow change in the diurnal and seasonal amplitude; and (5) excitation during all seismic events that excite this frequency band. Based on these observations, we suggest that the 2.4 Hz resonance is the only mode below 9 Hz that could be related to a local ground structure., Bulletin of the Seismological Society of America, 111 (6), ISSN:0037-1106, ISSN:1943-3573

Introduction To understand solar system formation it is critical to know how Uranus and Neptune formed. This requires knowledge of internal composition. Uranus and Neptune are generally referred to as ”ice-giants” in recent literature, as it has been inferred that their interiors are ice-dominated. Physical measurements from the Voyager 2 flybys include mass, radius, oblateness, low order gravity coefficients, moments of inertia, and magnetic field snapshots. One fundamental issue is that high temperature and pressure ice mixtures have similar densities to silicates mixed with hydrogen and helium. Therefore, existing physical constraints cannot by themselves distinguish between ice or rock-dominated interiors and almost any interior model fits the observations [5,10]. Measurements of atmospheric composition and temperature provide a possible complementary window into these planets’ interiors and may provide a way to break the degeneracy [12]. Here we consider the case for ice and rock-dominated interiors and attempt to propose a consistent explanation. The case for Ice Giants Internally-generated magnetic fields are observed at Uranus and Neptune [11]. Fields are highly non-dipolar, suggesting a shallow origin. Magnetic field generation requires conducting fluid and the conventional explanation is super-ionic water at high temperature and pressure, implying ice-dominated interiors. Spectroscopic atmospheric CO observations provide further evidence favouring the ice giant model. CO has higher abundance in Uranus’ and Neptune’s stratospheres than in their tropospheres, indicating an external source [7]. Uranus has ~8 ppb stratospheric CO and More relevant to the interior is that Neptune appears to have ~100 ppb tropospheric CO. Conventionally, this is explained by quenching CO dredged up from the deep interior by Neptune’s vigorous tropospheric mixing. Thermochemical models predict ~400 x O/H enrichment over solar abundance is required to reproduce this CO amount [2,8]. This extreme enrichment requires ~90% water ice in Neptune’s interior, again implying an ice-dominated interior. It is usually extrapolated that Uranus is also an ice giant with a similarly extreme oxygen and ice abundance, where the lack of CO in Uranus’ troposphere is conveniently explained by more sluggish tropospheric mixing. Issues with the Ice Giant model Although ice-dominated interiors can explain many observational aspects of Uranus and Neptune, there are also some worrying discrepancies. 1) Most icy bodies in the outer solar system have rock fractions of ~70%. If Uranus and Neptune formed from similar objects, then we require some explanation of where the missing rock fraction has gone or why the planetesimals that formed Uranus and Neptune are different to anything we observe today. 2) Measurements of atmospheric methane on Uranus and Neptune suggest deep abundances of a few percent [6]. This implies a C/H enrichment of ~50–100 x solar [1], which is much lower than that inferred for O/H from tropospheric CO. 3) D/H is ~4x10-5 on both Uranus and Neptune [3]. This is much lower than D/H observed in modern solar system icy objects such as comets, which typically have D/H ∼15–60x10-5. If interiors of Uranus and Neptune are well mixed and equilibrated, this implies only ~15% of the interiors can be ice, suggesting ~50–100 x solar enrichment [12]. Again, much lower than inferred from CO. A way around this is for interiors to only be partially mixed and equilibrated, with more D hiding in the unobservable deep atmosphere. Alternatively, some form of extinct exotic ices with lower D/H could be the source material. In summary, exotic ices, incomplete interior mixing, and unusually ice-rich planetesimals have all been invoked to make atmospheric observation consistent with the ice giant model. Not impossible, but also not entirely convincing as an explanation. Rock Giant interiors as a potential solution The alternative is that Uranus and Neptune’s interiors are rock-dominated. In this case we need to explain magnetic field generation and Neptune’s tropospheric CO. Recent work shows mixtures of silicates, hydrogen, and helium may be conductive at relevant pressures and temperatures, so super-ionic water is not necessarily required to generate magnetic fields [4]. Alternatively, there is no-doubt some ice in Uranus and Neptune’s interiors, which may form thin shell dynamos and explain non-dipolar field structures. Recent work also shows tropospheric CO may not actually be present throughout the troposphere and may be limited to the upper troposphere [12,13]. In this case, CO could be entirely sourced externally from comets. Profiles with CO limited to pressures Conclusion Recent advances in our understanding of CO profiles on Neptune and high-pressure conductivity of silicate/hydrogen/helium mixtures suggests that rock-dominated interiors for Uranus and Neptune are becoming more plausible than conventional ice giant scenarios. Such a rock giant could be formed from planetesimals with similar rock:ice ratios and D/H ratios to modern-day outer solar system comets, Kuiper belt objects, and icy moons. Interiors could also be well mixed and equilibrated. This opens the possibility of simpler formation mechanisms for Uranus and Neptune, with both planets forming in similar ways, and avoiding any requirements for dubious ice compositions. References [1] Atreya+ 2020. https://ui.adsabs.harvard.edu/abs/2020SSRv..216...18A/abstract [2] Cavalié+ 2017. https://ui.adsabs.harvard.edu/abs/2017Icar..291....1C/abstract [3] Feuchtgruber+ 2013. https://ui.adsabs.harvard.edu/abs/2013A%26A...551A.126F/abstract [4] Gao+ 2022. https://ui.adsabs.harvard.edu/abs/2022PhRvL.128c5702G/abstract [5] Helled+ 2020. https://ui.adsabs.harvard.edu/abs/2020RSPTA.37890474H/abstract [6] Irwin+ 2019. https://ui.adsabs.harvard.edu/abs/2019Icar..331...69I/abstract [7] Lellouch+ 2005. https://ui.adsabs.harvard.edu/abs/2005A%26A...430L..37L/abstract [8] Luszcz-Cook+de Pater 2013. https://ui.adsabs.harvard.edu/abs/2013Icar..222..379L/abstract [9] Moreno+ 2017. https://ui.adsabs.harvard.edu/abs/2017A%26A...608L...5M/abstract [10] Neuenschwander+Helled 2022. https://ui.adsabs.harvard.edu/abs/2022MNRAS.512.3124N/abstract [11] Soderlund+Stanley 2020. https://ui.adsabs.harvard.edu/abs/2020RSPTA.37890479S/abstract [12] Teanby+ 2020. https://ui.adsabs.harvard.edu/abs/2020RSPTA.37890489T/abstract [13] Teanby+ 2019. https://ui.adsabs.harvard.edu/abs/2019Icar..319...86T/abstract

SUMMARY Seismic observations indicate that the lowermost mantle above the core–mantle boundary (CMB) is strongly heterogeneous. Body waves reveal a variety of ultra-low velocity zones (ULVZs), which extend not more than 100 km above the CMB and have shear velocity reductions of up to 30 per cent. While the nature and origin of these ULVZs remain uncertain, some have suggested they are evidence of partial melting at the base of mantle plumes. Here we use coupled geodynamic/thermodynamic modelling to explore the hypothesis that present-day deep mantle melting creates ULVZs and introduces compositional heterogeneity in the mantle. Our models explore the generation and migration of melt in a deforming and compacting host rock at the base of a plume in the lowermost mantle. We test whether the balance of gravitational and viscous forces can generate partially molten zones that are consistent with the seismic observations. We find that for a wide range of plausible melt densities, permeabilities and viscosities, lower mantle melt is too dense to be stirred into convective flow and instead sinks down to form a completely molten layer, which is inconsistent with observations of ULVZs. Only if melt is less dense or at most ca. 1 per cent more dense than the solid, or if melt pockets are trapped within the solid, can melt remain suspended in the partial melt zone. In these cases, seismic velocities would be reduced in a cone at the base of the plume. Generally, we find partial melt alone does not explain the observed ULVZ morphologies and solid-state compositional variation is required to explain the anomalies. Our findings provide a framework for testing whether seismically observed ULVZ shapes are consistent with a partial melt origin, which is an important step towards constraining the nature of the heterogeneities in the lowermost mantle and their influence on the thermal, compositional and dynamic evolution of the Earth.

Earth science has been heavily data-driven due to the abundance in data. Yet, when there are relatively a small number of hypotheses to verify, the inverse problem becomes a classification problem. It is then worth directly examining observed seismic data against predicted data. Concretely, we chain forward modelling from geodynamics to seismology. We call this process ‘waveform Seismic Low Filtering of Earth’s models’ (SeLFiE). We take seismic signals of the snapshots of forwardly generated Earth models with that of the actual Earth, as if we took a photo of ourselves. Although there have several studies on how the seismological tomographic technique can perceive the geodynamical models, there are few studies on the seismic waveform sensitivity to geodynamical or petrological parameters. A pilot test of our SeLFiE methodology was encouraging, since we used only one seismic station to constrain the melt transportation manner beneath the Réunion island (Franken et al. 2020). Here in this contribution we present our strategy and developed tools towards the waveform filtering that have been developed during and after the CLEEDI week in August, 2020.

The National Aeronautics and Space Administration InSight mission has deployed the seismic experiment, SEIS, on the surface of Mars, and has recorded a variety of signals including marsquakes and dust devils. This work presents results on the tilt and local noise sources, which provide context to aid interpretation of the observed signals and allow an examination of the near-surface properties. Our analysis uses data recorded by the short-period sensors on the deck, throughout deployment and in the final configuration. We use thermal decorrelation to provide an estimate of the sol-to-sol tilt. This tilt is examined across deployment and over a Martian year. After each modification to the site, the tilt is seen to stabilize over 3–20 sols depending on the action, and the total change in tilt is

This paper presents a strategy for consistently extending isotropic equations of state to model anisotropic materials over a wide range of pressures and temperatures under nearly hydrostatic conditions. The method can be applied to materials of arbitrary symmetry. The paper provides expressions for the deformation gradient tensor, the lattice parameters, the isothermal elastic compliance tensor and thermal expansivity tensor. Scalar properties including the Gibbs energy, volume and heat capacities are inherited from the isotropic equation of state. Other physical properties including the isothermal and isentropic stiffness tensors, the Grueneisen tensor and anisotropic seismic velocities can be derived from these properties.The equation of state is demonstrated using periclase (cubic) and San Carlos olivine (orthorhombic) as examples.

SUMMARY Mantle convection and long-term lithosphere dynamics in the Earth and other planets can be treated as the slow deformation of a highly viscous fluid, and as such can be described using the compressible Navier–Stokes equations. Since on Earth-sized planets the influence of compressibility is not a dominant effect, density deviations from a reference profile are at most on the order of a few percent and using the full governing equations poses numerical challenges, most modelling studies have simplified the governing equations. Common approximations assume a temporally constant, but depth-dependent reference profile for the density (the anelastic liquid approximation), or drop compressibility altogether and use a constant reference density (the Boussinesq approximation). In most previous studies of mantle convection and crustal dynamics, one can assume that the error introduced by these approximations was small compared to the errors that resulted from poorly constrained material behaviour and limited numerical accuracy. However, as model parametrizations have become more realistic, and model resolution has improved, this may no longer be the case and the error due to using simplified conservation equations might no longer be negligible: while such approximations may be reasonable for models of mantle plumes or slabs traversing the whole mantle, they may be unsatisfactory for layered materials experiencing phase transitions or materials undergoing significant heating or cooling. For example, at boundary layers or close to dynamically changing density gradients, the error arising from the use of the aforementioned compressibility approximations can be the dominant error source, and common approximations may fail to capture the physical behaviour of interest. In this paper, we discuss new formulations of the continuity equation that include dynamic density variations due to temperature, pressure and composition without using a reference profile for the density. We quantify the improvement in accuracy relative to existing formulations in a number of benchmark models and evaluate for which practical applications these effects are important. Finally, we consider numerical aspects of the new formulations. We implement and test these formulations in the freely available community software aspect, and use this code for our numerical experiments.

We present new time series partial melting experiments performed on a natural enstatite chondrite (EL6), aimed at investigating the textural and geochemical changes induced by silicate-metal equilibration during early planetary differentiation. The starting material of our experiments consisted of small fragments (ca. 50 mg) obtained from the interior of the enstatite chondrite MCY 14005 (MacKay Glacier, Antarctica), collected during the XXX° Italian Expedition in Antarctica (PNRA). Experiments were performed in graphite capsules at a pressure of 1 GPa, at temperature ranging from 1100 to 1300 °C, with run durations from 1 to 24 h. The initial phase assemblage of the enstatite chondrite, mostly composed by granular enstatite and Fe-Ni metal (up to 400 µm in size) with minor amounts of sulphides and plagioclase, undergoes significant changes with increasing temperature and run duration. At 1100 °C, no silicate melt is produced and subsolidus reactions occur at the contact between the metal and silicate phases. At 1200 °C, small amounts of silicate melt are produced at the grain boundaries and enstatite grains in contact with the melt grow Fe-enriched rims. The metal portions are characterized by two immiscible liquid phases that exhibit rounded shapes when in contact with the silicate melt, whereas smaller (micrometric) liquid metal spheres occur isolated within the silicate melt throughout the experimental charges. These features are already observed in the 1 h experiment but become increasingly evident with increasing run duration, and at higher temperatures. In the experiments performed at 1300 °C, the amount of silicate melt increases and new silicate minerals form (olivine and low-Ca-pyroxene).Enstatite chondrites are characterized by an oxygen isotope composition similar to that of the bulk Earth and Moon, and are considered to have initially formed in the terrestrial planetary zone of the solar nebula. For this reason, they represent a suitable material to investigate the early planetary differentiation processes that occurred in the proto-Earth system. Preliminary results from our experiments indicate that, at the investigated oxygen fugacity (1-2 log units below the IW buffer), the Fe-Si exchange between the metal and silicate phases allows the formation of silicate melt and silicate phases such as olivine and low-Ca-pyroxene. At the same time, the change in shape of the metal grains (increasingly circular/spherical with increasing temperature) and the overall reduction of their number density with increasing experimental time point to rapid aggregation of the metal phase and, possibly, to fast silicate-metal differentiation in small planetesimals.

The Earth's mantle hosts a variety of reduced and oxidized phases, including iron-bearing alloys, diamond, and sulfide and carbonate melts. In the upper mantle, increasing pressure favors the stabilization of reduced iron-bearing phases via disproportionation of ferrous iron into ferric and metallic iron. Pressure-driven disproportionation is thought to continue into the transition zone, based on the extrapolation of experiments conducted at lower pressures. To test this hypothesis, we performed high-temperature and high-pressure experiments on basaltic and peridotitic compositions at pressures of 10 to 20 GPa, buffered at different oxygen fugacities. Under these conditions, majoritic garnet is the dominant ferric-iron bearing phase. We analyze our experimental run products for their ferric iron concentrations with EELS and Mossbauer spectroscopy. Contrary to expectations, results show that at iron saturation, ferric iron content of majorite peaks in the upper transition zone and then decreases between 500 and 650 km depth, destabilizing and resorbing reduced phases. This peak can be explained by decreases in the effective volume of ferrous minerals in transition zone assemblages. We also show that natural diamond-hosted majorite inclusions that equilibrated in the sublithospheric mantle grew from variably reduced fluids. These results are consistent with the idea that these diamonds formed during progressive reduction of an originally carbonatitic melt.

Eutectic melting curves in the system MgO–SiO2 have been experimentally determined at lower mantle pressures using laser-heated diamond anvil cell (LH-DAC) techniques. We investigated eutectic melting of bridgmanite plus periclase in the MgO–MgSiO3 binary, and melting of bridgmanite plus stishovite in the MgSiO3–SiO2 binary, as analogues for natural peridotite and basalt, respectively. The melting curve of model basalt occurs at lower temperatures, has a shallower d T / d P slope and slightly less curvature than the model peridotitic melting curve. Overall, melting temperatures detected in this study are in good agreement with previous experiments and ab initio simulations at ∼25 GPa ( Liebske and Frost, 2012 ; de Koker et al., 2013 ). However, at higher pressures the measured eutectic melting curves are systematically lower in temperature than curves extrapolated on the basis of thermodynamic modelling of low-pressure experimental data, and those calculated from atomistic simulations. We find that our data are inconsistent with previously computed melting temperatures and melt thermodynamic properties of the SiO2 endmember, and indicate a maximum in short-range ordering in MgO–SiO2 melts close to Mg2SiO4 composition. The curvature of the model peridotite eutectic relative to an MgSiO3 melt adiabat indicates that crystallization in a global magma ocean would begin at ∼100 GPa rather than at the bottom of the mantle, allowing for an early basal melt layer. The model peridotite melting curve lies ∼ 500 K above the mantle geotherm at the core–mantle boundary, indicating that it will not be molten unless the addition of other components reduces the solidus sufficiently. The model basalt melting curve intersects the geotherm at the base of the mantle, and partial melting of subducted oceanic crust is expected.

International audience; A reflector 70-130 km below the base of the transition zone beneath Tibet is observed in receiver functions and underside seismic reflections, at depths consistent with the transition of garnet to bridgmanite. Contrast in water storage capacity between the minerals of the Earth's transition zone and lower mantle suggests the possibility for dehydration melting at the top of the lower mantle. First-principles calculations combined with laboratory synthesis experiments constrain the mantle water capacity across the base of the transition zone and into the top of the lower mantle. We interpret the observed seismic signal as consistent with 3-4 vol % hydrous melt resulting from dehydration melting in the garnet to bridgmanite transition. Should seismic signals evident in downwelling region result from water contents representative of upper mantle water globally, this constrains the water stored in nominally anhydrous minerals in the mantle to

A large class of solid solution models are built on the premise that exchange of chemical species takes place on a finite number of unique sites, and that the thermodynamic properties of the solution are a function of the proportions of species occupying each of the sites. The site-occupancy spaces spanned by such models are geometrically equivalent to convex polytopes, n-dimensional generalisations of polygons and polyhedra. The endmembers of these solid solutions correspond to the vertices of the polytopes.We present a set of mathematical tools based on this geometrical equivalence which aid the creation and manipulation of solution models. Vertex enumeration methods can be used to compute the total set of endmembers in a solution from the valences of the species occupying each site and the total charge of the species not involved in site exchange. The number of independent endmembers constructed in this way is equal to n site-species − n sites + c, where n site-species is the total number of potential site-species occupancies in the solution and n sites is the number of distinct sites. If charge-balance constraints are active, c = 0, otherwise c = 1. We also present the linear algebra required to transform solution model parameterisations between different independent endmember bases. The same algorithms can also be used to derive macroscopic endmember interactions from microscopic site interactions. This algebra is useful both in the initial design of solution models, and when making thermodynamic calculations in restricted chemical subsystems.

The Earth’s mantle is characterized by a sharp seismic discontinuity at a depth of 660 km that can provide insights into deep mantle processes. The discontinuity occurs over only 2 km—or a pressure difference of 0.1 GPa—and is thought to result from the post-spinel transition, that is, the decomposition of the mineral ringwoodite to bridgmanite plus ferropericlase. Existing high-pressure, high-temperature experiments have lacked the pressure control required to test whether such sharpness is the result of isochemical phase relations or chemically distinct upper and lower mantle domains. Here, we obtain the isothermal pressure interval of the Mg–Fe binary post-spinel transition by applying advanced multi-anvil techniques with in situ X-ray diffraction with the help of Mg–Fe partition experiments. It is demonstrated that the interval at mantle compositions and temperatures is only 0.01 GPa, corresponding to 250 m. This interval is indistinguishable from zero at seismic frequencies. These results can explain the discontinuity sharpness and provide new support for whole-mantle convection in a chemically homogeneous mantle. The present work suggests that distribution of adiabatic vertical flows between the upper and lower mantles can be mapped on the basis of discontinuity sharpness. The post-spinel transition in mantle composition, which occurs at 660-km depth in Earth’s mantle, takes place over a pressure range equivalent to 250 m in depth, according to multi-anvil experiments for realistic mantle compositions and temperatures.

Recent studies have revealed that Earth's deep mantle may have a wider range of oxygen fugacities than previously thought. Such a large heterogeneity might be caused by material subducted into the deep mantle. However, high-pressure phase relations are poorly known in systems including Fe3+ at the top of the lower mantle, where the subducted slab may be stagnant. We therefore conducted high-pressure and high-temperature experiments using a multi-anvil apparatus to study the phase relations in a Fe3+-bearing system at 26 GPa and 1573–2073 K, at conditions prevailing at the top of the lower mantle. At temperatures below 1923 K, MgSiO3-rich bridgmanite, an Fe3+-rich oxide phase, and SiO2 coexist in the recovered sample. Quenched partial melt was observed above 1973 K, which is significantly lower than the solidus temperature of an equivalent Fe3+-free bulk composition. The partial melt obtained from the Fe3+-rich bulk composition has a higher iron content than coexisting bridgmanite, similar to the Fe2+-dominant system. The results suggest that strong mantle oxygen fugacity anomalies might alter the subsolidus and melting phase relations under lower mantle conditions. We conclude that (1) a small amount of melt may be generated from an Al-depleted region of a stagnant slab, such as subducted former banded-iron-formation, and (2) Fe3+ is not transported into the deep part of the lower mantle because of its incompatibility during melting.

International audience; Abstract The Interior exploration using Seismic Investigations, Geodesy, and Heat Trans-port (InSight) Mission will focus on Mars’ interior structure and evolution. The basic structure of crust, mantle, and core form soon after accretion. Understanding the early differentiation process on Mars and how it relates to bulk composition is key to improving our understanding of this process on rocky bodies in our solar system, as well as in other solar systems. Current knowledge of differentiation derives largely from the layers observed via seismology on the Moon. However, the Moon’s much smaller diameter make it a poor analog with respect to interior pressure and phase changes. In this paper we review the current knowledge of the thickness of the crust, the diameter and state of the core, seismic attenuation, heat flow, and interior composition. InSight will conduct the first seismic and heat flow measurements of Mars, as well as more precise geodesy. These data reduce uncertainty in crustal thickness, core size and state, heat flow, seismic activity and meteorite impact rates by a factor of 3–10× relative to previous estimates. Based on modeling of seismic wave propagation, we can further constrain interior temperature, composition, and the location of phase changes. By combining heat flow and a well constrained value of crustal thickness, we can estimate the distribution of heat producing elements between the crust and mantle. All of these quantities are key inputs to models of interior convection and thermal evolution that predict the processes that control subsurface temperature, rates of volcanism, plume distribution and stability, and convective state. Collectively these factors offer strong controls on the overall evolution of the geology and habitability of Mars.

We present an updated model for estimating the lander mechanical noise on the InSight seismometer SEIS, taking into account the flexible modes of the InSight lander. This new flexible mode model uses the Satellite Dynamics Toolbox to compute the direct and the inverse dynamic model of a satellite composed of a main body fitted with one or several dynamic appendages. Through a detailed study of the sensitivity of our results to key environment parameters we find that the frequencies of the six dominant lander resonant modes increase logarithmically with increasing ground stiffness. On the other hand, the wind strength and the incoming wind angle modify only the signal amplitude but not the frequencies of the resonances. For the baseline parameters chosen for this study, the lander mechanical noise on the SEIS instrument is not expected to exceed the instrument total noise requirements. However, in the case that the lander mechanical noise is observable in the seismic data acquired by SEIS, this may provide a complementary method for studying the ground and wind properties on Mars.

NASA’s InSight Mission will deploy two three-component seismometers on Mars in 2018. These short period and very broadband seismometers will be mounted on a three-legged levelling system, which will sit directly on the sandy regolith some 2–3 meters from the lander. Although the deployment will be covered by a wind and thermal shield, atmospheric noise is still expected to couple to the seismometers through the regolith. Seismic activity on Mars is expected to be significantly lower than on Earth, so a characterisation of the extent of coupling to noise and seismic signals is an important step towards maximising scientific return. In this study, we conduct field testing on a simplified model of the seismometer assembly. We constrain the transfer function between the wind and thermal shield and tripod-mounted seismometers over a range of frequencies (1–40 Hz) relevant to the deployment on Mars. At 1–20 Hz the displacement amplitude ratio is approximately constant, with a value that depends on the site (0.03–0.06). The value of the ratio in this range is 25–50% of the value expected from the deformation of a homogeneous isotropic elastic halfspace. At 20–40 Hz, the ratio increases as a result of resonance between the tripod mass and regolith. We predict that mounting the InSight instruments on a tripod will not adversely affect the recorded amplitudes of vertical seismic energy, although particle motions will be more complex than observed in recordings generated by more conventional buried deployments. Higher frequency signals will be amplified by tripod-regolith resonance, probably reaching peak-amplification at $\sim 50$ Hz. The tripod deployment will lose sensitivity at frequencies $>50$ Hz as a result of the tripod mass and compliant regolith. We also investigate the attenuation of seismic energy within the shallow regolith covering the range of seismometer deployment distances. The amplitude of surface displacement decays as $r^{-n}$ , where $1.5 < n < 2$ . This exceeds the value expected for a homogeneous isotropic elastic halfspace ( $n \sim 1$ ), and reflects an increase in Young’s modulus with depth. We present an updated model of lander noise which takes this enhanced attenuation into account.

Oceanic fracture zone traces are widely used in studies of seafloor morphology and plate kinematics. Satellite altimetry missions have resulted in high-resolution gravity maps in which all major fracture zones and other tectonic fabric can be identified, and numerous scientists have digitized such lineaments. We have initiated a community effort to maintain low-cost infrastructure that allows seafloor fabric lineaments to be stored, accessed, and updated. A key improvement over past efforts is our processing software (released as a GMT5 supplement) that allows for semiautomatic corrections to previously digitized fracture zone traces given improved gridded data sets. Here we report on our seafloor fabric processing tools, which complement our database of seafloor fabric lineations, magnetic anomaly identifications, and plate kinematic models.

There were regrettably a few typos that appeared in the published version of Myhill et al. (2016).

Grain size plays a key role in controlling the mechanical properties of the Earth's mantle, affecting both long-time-scale flow patterns and anelasticity on the time scales of seismic wave propagation. However, dynamic models of Earth's convecting mantle usually implement flow laws with constant grain size, stress-independent viscosity, and a limited treatment of changes in mineral assemblage. We study grain size evolution, its interplay with stress and strain rate in the convecting mantle, and its influence on seismic velocities and attenuation. Our geodynamic models include the simultaneous and competing effects of dynamic recrystallization resulting from dislocation creep, grain growth in multiphase assemblages, and recrystallization at phase transitions. They show that grain size evolution drastically affects the dynamics of mantle convection and the rheology of the mantle, leading to lateral viscosity variations of 6 orders of magnitude due to grain size alone, and controlling the shape of upwellings and downwellings. Using laboratory-derived scaling relationships, we convert model output to seismologically observable parameters (velocity and attenuation) facilitating comparison to Earth structure. Reproducing the fundamental features of the Earth's attenuation profile requires reduced activation volume and relaxed shear moduli in the lower mantle compared to the upper mantle, in agreement with geodynamic constraints. Faster lower mantle grain growth yields best fit to seismic observations, consistent with our reexamination of high-pressure grain growth parameters. We also show that ignoring grain size in interpretations of seismic anomalies may underestimate the Earth's true temperature variations.

High-pressure and high-temperature melting experiments were conducted in the systems Mg2SiO4–H2O and MgSiO3–H2O at 6 and 13 GPa and between 1150 and 1900 °C in order to investigate the effect of H2O on melting relations of forsterite and enstatite. The liquidus curves in both binary systems were constrained and the experimental results were interpreted using a thermodynamic model based on the homogeneous melt speciation equilibrium, H2O + O2− = 2OH−, where water in the melt is present as both molecular H2O and OH− groups bonded to silicate polyhedra. The liquidus depression as a function of melt H2O concentration is predicted using a cryoscopic equation with the experimental data being reproduced by adjusting the water speciation equilibrium constant. Application of this model reveals that in hydrous MgSiO3 melts at 6 and 13 GPa and in hydrous Mg2SiO4 melts at 6 GPa, water mainly dissociates into OH− groups in the melt structure. A temperature dependent equilibrium constant is necessary to reproduce the data, however, implying that molecular H2O becomes more important in the melt with decreasing temperature. The data for hydrous forsterite melting at 13 GPa are inconclusive due to uncertainties in the anhydrous melting temperature at these conditions. When applied to results on natural peridotite melt systems at similar conditions, the same model infers the presence mainly of molecular H2O, implying a significant difference in physicochemical behaviour between simple and complex hydrous melt systems. As pressures increase along a typical adiabat towards the base of the upper mantle, both simple and complex melting results imply that a hydrous melt fraction would decrease, given a fixed mantle H2O content. Consequently, the effect of pressure on the depression of melting due to H2O could not cause an increase in the proportion, and hence seismic visibility, of melts towards the base of the upper mantle.

Hydrous melting at high pressures affects the physical properties, dynamics and chemical differentiation of the Earth. However, probing the compositions of hydrous melts at the conditions of the deeper mantle such as the transition zone has traditionally been challenging. In this study, we conducted high pressure multianvil experiments at 13 GPa between 1200 and 1900 °C to investigate the liquidus in the system MgO-SiO2-H2O. Water-rich starting compositions were created using platinic acid (H2Pt(OH)6) as a novel water source. As MgO:SiO2 ratios decrease, the T-XH2OXH2O liquidus curve develops an increasingly pronounced concave-up topology. The melting point reduction of enstatite and stishovite at low water contents exceeds that predicted by simple ideal models of hydrogen speciation. We discuss the implications of these results with respect to the behaviour of melts in the deep upper mantle and transition zone, and present new models describing the partitioning of water between the olivine polymorphs and associated hydrous melts.

Plumes are thought to transport water-rich material from the deep mantle to Earth’s surface. High-pressure experiments identify a hydrous mineral phase that is stable under lower-mantle conditions and could provide a source for this water.

NASA’s InSight lander will deploy a tripod-mounted seismometer package onto the surface of Mars in late 2018. Mars is expected to have lower seismic activity than the Earth, so minimisation of environmental seismic noise will be critical for maximising observations of seismicity and scientific return from the mission. Therefore, the seismometers will be protected by a Wind and Thermal Shield (WTS), also mounted on a tripod. Nevertheless, wind impinging on the WTS will cause vibration noise, which will be transmitted to the seismometers through the regolith (soil). Here we use a 1:1-scale model of the seismometer and WTS, combined with field testing at two analogue sites in Iceland, to determine the transfer coefficient between the two tripods and quantify the proportion of WTS vibration noise transmitted through the regolith to the seismometers. The analogue sites had median grain sizes in the range 0.3–1.0 mm, surface densities of $1.3\mbox{--}1.8~\mbox{g}\,\mbox{cm}^{-3}$ , and an effective regolith Young’s modulus of $2.5^{+1.9}_{-1.4}~\mbox{MPa}$ . At a seismic frequency of 5 Hz the measured transfer coefficients had values of 0.02–0.04 for the vertical component and 0.01–0.02 for the horizontal component. These values are 3–6 times lower than predicted by elastic theory and imply that at short periods the regolith displays significant anelastic behaviour. This will result in reduced short-period wind noise and increased signal-to-noise. We predict the noise induced by turbulent aerodynamic lift on the WTS at 5 Hz to be $\sim2\times10^{-10}~\mbox{ms}^{-2}\,\mbox{Hz}^{-1/2}$ with a factor of 10 uncertainty. This is at least an order of magnitude lower than the InSight short-period seismometer noise floor of $10^{-8}~\mbox{ms}^{-2}\,\mbox{Hz}^{-1/2}$ .

The high-pressure phases Fe4O5 and Fe5O6 have recently been added to the list of known iron oxides. As mixed-valence phases, it has been suggested that they could form in the Earth’s mantle once the dominant minerals become saturated in ferric iron. The possibility that Fe4O5 could exist in the mantle is also supported by the fact that it forms extensive solid solutions with both Mg2+ and Cr3+. In this study, we present the results of high-pressure and high-temperature multi-anvil experiments performed between 5 and 24 GPa at 1000–1400 °C aimed at constraining the stability field of the Fe4O5 phase. We combine these results with published phase equilibria, equation of state and Fe–Mg partitioning data to estimate the thermodynamic properties of Fe4O5, Fe5O6 and the (Mg,Fe)2Fe2O5 solid solution. Using our thermodynamic model, the oxygen fugacity at which the high-pressure iron oxides become stable is calculated and the redox stability of (Mg,Fe)2Fe2O5 in an assemblage of olivine and pyroxene is calculated as a function of the bulk Fe/(Fe + Mg) ratio. Fe4O5 and (Mg,Fe)2Fe2O5 are stable at oxygen fugacities higher than the diamond stability field and are, therefore, unlikely to be found as inclusions in diamonds. The stability field of Fe5O6, on the other hand, extends to oxygen fugacities compatible with diamond formation. Using the Mg–Fe solid solution model, we show that Fe4O5-structured phases would be restricted to aluminium-poor environments in the mantle such as dunites or silica–iron oxide-rich sediments transported into the mantle via subduction.

Earth’s lower mantle, extending from 660 to 2891 km depth, contains ~70% of the mass of the mantle and is ~50% of the mass of the entire planet [Dziewonski and Anderson, 1981]. Due to its size, the lower mantle has a large potential to bias the composition of the bulk silicate Earth (BSE), which in major element terms is generally assumed to be the same as the upper mantle [Allegre et al., 1995; McDonough and Sun, 1995; O’Neill and Palme, 1998]. Any chemical differences between the upper and lower mantle would have major implications for our understanding of the scale of mantle convection, the origin of the terrestrial heat flux, and aspects such as the volatile content of the interior. Furthermore, many constraints on the processes involved in the accretion and differentiation of Earth would be lost if element concentrations in the upper mantle could not be reliably assumed to reflect the mantle as a whole. Ultimately the best prospect for determining whether the lower mantle has the same major element composition as the upper mantle is to compare seismic reference models for shear and longitudinal wave velocities in the lower mantle with mineral physical estimates for what these velocities should be if the mantle is isochemical [Birch, 1952; Anderson, 1968; Jackson, 1983; Stixrude and Jeanloz, 2007; Murakami et al., 2012; Cottaar et al., 2014; Wang et al., 2015]. Such comparisons require not only high‐pressure and high‐temperature equation‐of‐state data for mantle minerals but also knowledge of the proportion and composition of minerals in the lower mantle for a given bulk composition. Significant uncertainties exist for both types of data. It must also be recognized that any fit to observed Chemistry of the Lower Mantle

Review of scientific instruments 87(2), 024501(2016). doi:10.1063/1.4941716, We have generated over 40 GPa pressures, namely, 43 and 44 GPa, at ambient temperature and 2000 K, respectively, using Kawai-type multi-anvil presses (KMAP) with tungsten carbide anvils for the first time. These high-pressure generations were achieved by combining the following pressure-generation techniques: (1) precisely aligned guide block systems, (2) high hardness of tungsten carbide, (3) tapering of second-stage anvil faces, (4) materials with high bulk modulus in a high-pressure cell, and (5) high heating efficiency., Published by American Institute of Physics, [S.l.]

Earthquakes beneath the southwest Pacific occur from the surface down to 700 km depth. Teleseismic waveforms created by some of these earthquakes are almost identical. We investigate Tonga–Kermadec and Vanuatu subduction zone earthquake P-coda waveforms using a cross-correlation technique and hierarchical clustering algorithm in order to determine the origin of waveform similarity and the distribution of earthquakes producing similar waveforms. We show that scatterers forming the majority of power in the P-wave coda are localised around the receiver. As a result, waveform similarity provides a much weaker constraint on source separation than in local studies. Waveform similarity can provide stronger constraints on focal mechanism. Most earthquake multiplets within the Tonga–Fiji–Kermadec Wadati–Benioff zone are found at depths between 0–60 km and 520–620 km. A significant proportion of all deep-focus events in south Pacific subduction zones have waveforms similar to those of at least one other event. Relative relocation of events within the largest identified multiplet reveals a planar zone of seismicity sub-parallel to the nodal plane of a related centroid moment tensor solution. Groups of earthquakes with similar waveforms remain active on at least the 14-year recording timescale. We equate this to repeated rupture on single or closely related shear systems within the subducting slabs.

[1] We make use of the effects of rupture directivity on waveform shape to analyze the rupture processes of 58 large, deep earthquakes in the Izu-Bonin-Marianas subduction zone. For more than half of the analyzed earthquakes, we determine a best-fitting unilateral rupture direction using teleseismic data. For 20 of these earthquakes, the constraints on the rupture direction allow the fault plane to be identified. Where the subducting slab dips at a moderate angle, near-horizontal fault planes dominate at all studied depths (50–600 km). Within more steeply dipping slabs, fault planes tend to dip toward the south and west. Rotated into the plane of the slab, the poles of the definitively identified faults form a single tight cluster pointing up and toward the surface of the slab. Identified ruptures have a tendency to propagate away from the top surface of the slab between 100 and 300 km depth, but appear to be randomly oriented at greater depths. The occurrence of predominantly near-horizontal faults at intermediate depths agrees with previous observations in several other subduction zones. However, at depths ≥300 km, the results from the Izu-Bonin-Marianas system differ from those previously obtained for Tonga-Kermadec, where both horizontal and vertical faults were identified. We consider various physical mechanisms to explain our observations and conclude that, while pre-existing slab structures may be reactivated if they are favorably oriented, the observed asymmetry in deep fault orientations may instead result from external forces acting on the slab, and resulting changes in slab morphology.

Narrow, discontinuous bands of high-grade subophiolitic metamorphic rocks, comprising predominantly amphibolite facies metabasites with rare metasediments, are observed at the contact between the complexes and subjacent melanges of the Mesohellenic Ophiolite exposed in northwestern Greece. Both conventional and pseudosection thermobarometry have been used to yield estimated peak pressure-temperature (P-T) conditions of these tectonic sheets. Toward the leading edge of the ophiolite, subophiolitic rocks of the Vourinos Complex record peak metamorphic temperatures of 770 ± 100 °C. Pressures of 4 ± 1 kbar beneath the Vourinos are estimated on the basis of hornblende composition and are similar to the expected pressures from ophiolitic overburden. Beneath the exposed Dramala Complex, at the trailing edge of the ophiolitic body southwest of the Vourinos, estimated temperatures reached 800 ± 40 °C and 12.00 ± 1.27 kbar at the top of an apparent inverted metamorphic gradient imposed by discrete phases of accretion. High pressure assemblages beneath ophiolitic bodies imply exhumation relative to the overlying ophiolite. Estimated homologous temperatures in the upper plate are similar to those inferred for channeled exhumation during continental collision. Mineral assemblages lower in the Dramala sole indicate reduced temperatures and peak pressures. Similar pressures obtained within lower temperature sole rocks beneath Vourinos and Pindos suggest that a shallowly dipping thrust may have been responsible for obduction. Peak temperatures and pressures are in agreement with those estimated for secondary thrust propagation beneath a proto-arc after subduction in an intra-oceanic setting.