Clapeyron Slope Research Articles

The Origin of the Lehmann Discontinuity Beneath the Ancient Craton: Insight From the High Pressure‐Temperature Elasticity Measurements of Topaz

AbstractIn this study, we concentrate on the seismic signature of subducted sediments and suggest the formation of the L‐discontinuity beneath the ancient craton related to migrated sediment dehydration. We first determined the single‐crystal elasticity of topaz, the product of sediment dehydration, at high pressures and temperatures by Brillouin scattering. Using the derived elastic parameters, we establish the velocity and density profiles of subducted sediments in the upper mantle. According to our modeling results, 8.5–17.5 vol.% sediments intruding into the upper mantle will induce a 2%–4% low‐VS anomaly at 210–260 km. Meanwhile, continuous heating will lead to the dehydration of phengite in sediments. The dehydration of this amount sediments can generate a 3%–6% ISS with negative Clapeyron slopes, satisfying the observed L‐discontinuity in northern Finland and northern America without the anisotropy changes but accompanied by low‐velocity anomalies. Our study thus provides new insights into the origin of the L‐discontinuity.

(De)hydration Front Propagation Into Zero‐Permeability Rock

AbstractHydration and dehydration reactions play pivotal roles in plate tectonics and the deep water cycle, yet many facets of (de)hydration reactions remain unclear. Here, we study (de)hydration reactions where associated solid density changes are predominantly balanced by porosity changes, with solid rock deformation playing a minor role. We propose a hypothesis for three scenarios of (de)hydration front propagation and test it using one‐dimensional hydro‐mechanical‐chemical models. Our models couple porous fluid flow, solid rock volumetric deformation, and (de)hydration reactions described by equilibrium thermodynamics. We couple our transport model with reactions through fluid pressure: the fluid pressure gradient governs porous flow and the fluid pressure magnitude controls the reaction boundary. Our model validates the hypothesized scenarios and shows that the change in solid density across the reaction boundary, from lower to higher pressure, dictates whether hydration or dehydration fronts propagate: decreasing solid density causes dehydration front propagation in the direction opposite to fluid flow while increasing solid density enables both hydration and dehydration front propagation in the same direction as fluid flow. Our models demonstrate that reactions can drive the propagation of (de)hydration fronts, characterized by sharp porosity fronts, into a viscous medium with zero porosity and permeability; such propagation is impossible without reactions, as porosity fronts become trapped. We apply our model to serpentinite dehydration reactions with positive and negative Clapeyron slopes and granulite hydration (eclogitization). We use the results of systematic numerical simulations to derive a new equation that allows estimating the transient, reaction‐induced permeability of natural (de)hydration zones.

Is the mantle transition zone uniform beneath Precambrian shields?

In the present study, we examine the mantle transition zone (MTZ) structure below the prominent Precambrian shields on the Earth to understand its thermal and compositional properties and depth distribution of MTZ discontinuities. For this study, we used data from Canadian, Brazilian, Baltic, African and Australian Shields. We migrate the receiver functions to the depth domain using 3D tomographic models to examine the topography of MTZ discontinuities that define the upper and lower boundaries of the MTZ. The depth migration results were obtained using two different 3D tomographic velocity models, LLNL_G3D_JPS and GyPSuM. The analysis shows that both models yield similar results. The findings indicate a thinner than usual MTZ beneath all the Precambrian shields with an average thickness of ∼238 ± 8 km. Notably, the present study reveals the upper boundary of the MTZ (410 km discontinuity) displays distinctive topography; in contrast, the lower boundary of the MTZ (660 km discontinuity) is situated at shallower depths. Therefore, the main factor causing the variation in MTZ thickness is the shallowing of the 660 km discontinuity, indicating that the post-spinel transition occurred at higher temperatures with a negative Clapeyron slope, supporting the theory of whole-mantle convection. This suggests that mantle plumes have some appreciable impact on the MTZ beneath Precambrian shields, which are limited to the base of the MTZ. Alternatively, the global mantle warming explanation could be invoked, as it best explains the similar characteristics of the 660 km discontinuity beneath the Precambrian shields. However, this phenomenon needs to be tested through numerical modelling.

Constraints on the Fate of Delaminated Lithosphere in the Upper and Mid‐Mantle

AbstractDelamination of lower continental lithosphere is known to have occurred under different tectonic settings. However, its fate in the mantle is poorly understood. By analyzing global seismic models, we find that most of likely lithosphere that delaminated during the Cenozoic and Mesozoic is preserved in the mantle transition zone, especially beneath North America and Africa. Numerical experiments indicates that delaminated lithosphere can remain stagnant in the mantle transition zone for tens of millions of years, followed by its potential sinking into the lower mantle or re‐rising to shallower depths depending on its density, the Clapeyron slope of the spinel‐to‐post‐spinel phase change and increase in mantle viscosity at ∼660–1,000 km depths. Re‐ascent occurs when delaminated lithosphere is reheated so that its effective density becomes lower than its surrounding ambient mantle after ∼100 Myr. Delaminated fragments can also potentially be mobilized by underlying global mantle flow to move horizontally away from plume regions.

Present day mantle structure from global mantle convection models since the Cretaceous

SUMMARY Using forward mantle convection models starting at 140 Ma, and assimilating plate reconstructions as surface velocity boundary condition, we predict present-day mantle structure and compare them with tomography models, using geoid as an additional constraint. We explore a wide model parameter space, such as different values of Clapeyron slope and density change across 660 km, density and viscosity of the thermochemical piles at the core–mantle boundary (CMB), internal heat generation rate, and model initiation age. We also investigate the effects of different strengths of a weak layer below 660 km and weaker asthenosphere and slabs. Our results suggest that slab structures at different subduction zones are sensitive to the viscosity of the asthenosphere, strength of slabs, values of Clapeyron slope and the density and viscosity of the thermochemical piles, while different internal heat generation rates do not affect the slab structures. We find that with a moderately weak asthenosphere ($10^{20}$ Pa·s) and strong slabs, the predicted slab structures are consistent with the tomography models, and the observed geoid is also matched well. Moreover, our models successfully reproduce the degree-2 structure of the lower mantle beneath Africa and the Pacific, also known as Large Low Shear Velocity provinces (LLSVPs). A moderate Clapeyron slope of −2.5 MPa K−1 at 660 km aids in slab stagnation while higher values result in massive slab accumulation at that depth, ultimately leading to slab avalanches. We also find that the convective patterns in the thermal and thermochemical cases with slightly denser LLSVPs are similar, although the geoid amplitudes are lower for the latter. However, with more dense LLSVPs, the slabs cannot perturb them and no plumes are generated. Plumes arise as thermal instabilities from the edges of the LLSVPs, when cold and viscous slabs perturb them. While our predicted plume locations are consistent with the observed hotspot locations, matching the plume structures in tomography models is difficult. These plumes are essential in fitting the finer features of the observed geoid. In longer-duration models, more voluminous subducted material reaches the CMB, which tends to erode the LLSVPs significantly, and yields a poor fit to the observed geoid. Our results suggest that with the presence of a thin, moderately weak layer below 660 km, a slightly dense LLSVP, and Clapeyron slope of −2.5 MPa K−1, the velocity anomalies in seismic tomography and the long-wavelength geoid can be matched well. One of the limitations of our models is that the assimilated plate motion history may be too short to overcome arbitrary initial conditions effects. Also, assimilated true plate velocities in our models may not represent the true convective vigour of the Earth.

Configurational entropy of ice XIX and its isotope effect

Ice XIX is a partly hydrogen-ordered polymorph related to disordered ice VI, similar to ice XV. We here investigate the order–order–disorder sequence ice XIX→ice XV→ice VI based on calorimetry at ambient pressure both for D2O and H2O-ice XIX. From these data we extract configurational entropy differences between ice XIX, ice XV and ice VI. This task is complex because, unlike for all other ices, the order–disorder transition from ice XIX to ice VI takes place in two steps via ice XV. Even more challenging, these two steps take place in an overlapping manner, so that careful separation of slow kinetics is necessary. This is evidenced best by changing the heating rate in calorimetry experiments: For fast heating experiments the second step, disordering of ice XV, is suppressed because the first step, formation of ice XV from ice XIX, is too slow. The transient state ice VI‡ that is initially produced upon ice XIX decay then does not have enough time to convert to ice XV, but remains disordered all along. In order to tackle the challenge to determine the entropy difference between ice XIX and VI as well as the entropy difference between ice XV and VI we employ two different approaches that allow assessing the impact of kinetics on the entropy change. “Single peak integration” defines a kinetically limited result, but “combined peak integration” allows estimation of the true thermodynamic values. Our best estimate for the true value shows ice XIX to be much more ordered than ice XV (25 ± 3% vs 9 ± 4% of the Pauling entropy). For D2Oice XIX samples we obtain 28% of order, but only when a small number of fast H-isotope defects are used. In the second part we use these results to estimate the location of the ice XIX phase boundary both for protiated and deuterated ice XIX. The initial Clapeyron slope at ambient pressure is determined from the combination of neutron powder diffraction volume differences and calorimetry entropy differences data to be 21 K GPa−1 with an order–disorder transition temperature To-d(0.0 GPa) = 103 ± 1 K. An in situ bracketing experiment at 1.8 GPa yields To-d(1.8 GPa) = 116 ± 3 K, i.e., the phase boundary slope flattens at higher pressures. These data allow us to determine the region of thermodynamic stability of ice XIX in the phase diagram and to explain the surprising isotope shift reversal at 1.6 GPa compared to 0.0 GPa, i.e., why D2O-ice XIX disorders at lower temperatures than H2O-ice XIX at 1.6 GPa, but at higher temperatures at ambient pressures.

Modeling the Global Water Cycle—The Effect of Mg‐Sursassite and Phase A on Deep Slab Dehydration and the Global Subduction Zone Water Budget

AbstractWe study the interplay of the thermal structure within subducted oceanic slabs together with the stability fields of hydrous phases that control slab dehydration and the amount of water transported into the deeper mantle. We implement different published thermodynamic data for phase A and Mg‐sursassite into a model of 56 subduction zones and evaluate the effect of these phases on the global water budget. We modeled vertical fluid fluxes within the slab such that dehydration‐derived fluid was allowed to react with fluid‐undersaturated rocks in other parts of the slab. The effect of Mg‐sursassite on the global water budget is limited, because the Clapeyron slope of this dehydration reaction is steeper than most pressure‐temperature trajectories in subduction zones. Two sets of published thermodynamic data for phase A yield significantly different values for the amount of deeply subducted water, ranging between 8 × 108 Tg/Ma and 1.4 × 109 Tg/Ma. In some subduction zones, the differences span several orders of magnitude. The absolute modeled amount of deeply subducted water strongly depends on the depth and intensity of slab mantle hydration, but a comparison of modeled and experimental data indicates that the thermodynamic dataset that yielded higher values is more reliable and should be implemented in future thermodynamic models. Our results show that the stable phases around the choke point as well as the slope and position of the phase A‐out reaction influence the deep water release from the slab, but the slope and position of the phase A dehydration reaction mainly control the recycling of water into the deep mantle.

Topography of the 660-km discontinuity within the Izu-Bonin subduction zone and evidence of slab penetration near the Bonin Super Deep Earthquake (∼680 km)

The Izu-Bonin subduction zone in the Northwest Pacific is an ideal location for understanding mantle dynamics such as cold lithosphere subduction. The slab produces a lateral thermal anomaly, inducing local topographic changes at the boundary of a post-spinel phase transformation, considered to be the origin of the ‘660-km discontinuity.’ In this study, the short-period (1–2 Hz) S-to-P conversion phase S660P was used to obtain the fine-scale structure of the discontinuity. More than 100 earthquakes that occurred from the 1980s to the 2020s and were recorded by high-quality seismic arrays in the United States and Europe were analyzed. A discontinuity in the ambient mantle with an average depth of ∼670 km was found beneath the 300–400-km event zone in the northern Bonin region near 33°N. Meanwhile, the ‘660-km discontinuity’ has been pushed upward, away from the slab, possibly because of a hot upwelling mantle plume. In the central part of the subduction zone, the 660-km discontinuity is depressed to an average depth of (690 ± 5) km within the slab at approximately 150 km below the coldest slab core, indicating a (300 ± 100) °C cold anomaly estimated using a post-spinel transformation Clapeyron slope of (−2.0 ± 1.0) MPa/K. In southern Bonin near 28°N, the discontinuity was found to be further depressed at an average depth of (695 ± 5) km below the deepest event and with a focal depth of ∼550 km. The discontinuity is located where the slab bends abruptly to become sub-horizontal toward the west-southwest. Near the zone of the isolated Bonin Super Deep Earthquake, which occurred at ∼680 km on May 30, 2015, the discontinuity is depressed to ∼700 km, suggesting a near-vertical penetrating slab and an S-to-P conversion in the coldest slab core, where a large low-temperature anomaly should exist.

The role of mantle viscosity heterogeneities on the development of secondary plumes in the upper mantle

SUMMARY Recent seismic observations have revealed diverse plume morphologies beneath hotspots that are characterized by multiple secondary plume branches with different sizes and temperatures. However, how the structures of these secondary plumes have developed and what controls the properties of these secondary plumes have not been well explored. Here, we performed 3-D numerical models to systematically investigate the effects of mantle viscosity structures and the strength of the 660-km phase change on the development of secondary plumes. We find that the characteristics of the secondary plumes, in terms of their numbers, excess temperatures and sizes, are controlled by the Clapeyron slope of the 660-km phase change (γ660), the properties of the low-viscosity layer beneath the mantle transition zone (viscosity reduction ratio, thickness, depth or phase change dependency of viscosity) and the presence of the weak asthenosphere. For γ660 between −2.0 and −3.0 MPa K−1, the weak layer viscosity reduction ratio plays a first-order role in controlling secondary plume properties, that is, a smaller viscosity reduction ratio results in more secondary plumes, higher plume excess temperature and larger plume size. The thickness and viscosity dependencies of the weak layer have a secondary effect on secondary plume properties. However, a relatively small γ660 (e.g. −1.0 MPa K−1) would result in a single secondary plume irrespective of the weak layer viscosity reduction ratio, the viscosity dependency, and whether with or without the weak asthenosphere. However, our models also show that the presence of the weak asthenosphere has a prominent influence in increasing the number of secondary plumes but reducing the size of the plumes. Our results provide new insight into plume dynamics and suggest that the diverse plume structures observed in the upper mantle likely reflect different mantle viscosity structures beneath the hotspots.

Equation of state for Mg3Al2Si3O12 pyrope: Implications for post-garnet transitions and mantle dynamics

Mantle convection is the dominant process driving the upwelling of hot materials and subduction of cold slabs. These processes are density-dependent and influenced by post-spinel and post-garnet transitions in the mantle transition zone (MTZ)/ lower mantle (LM). Knowledge of the Clapeyron slope of these transformations is used to express the dynamics of mantle blocks and is necessary for understanding the deep Earth. Here, we provide the Kunc-Einstein equation of state (EoS) for the pyrope Mg3Al2Si3O12 (Prp) calculated from a joint analysis of the experimentally measured isobaric heat capacity, bulk moduli, thermal expansion, pressure (P), unit cell volume (V), temperature (T) data. Based on our model, the bulk modulus and its pressure derivatives were K0,T0 = 168.5 GPa, K′0,T = 4.77, and V0 = 1501.7 Å. The optimised parameters include two Einstein temperatures, i.e., θ1 = 331 and θ2 = 1093 K, Grüneisen parameter at ambient condition γ0 = 1.77, infinite compression γ∞ = 0 with β = 1.12 and an intrinsic anharmonicity parameter a0 = –20. The value for the thermal expansion coefficient was calculated to be α = 2.33·10−5 K−1, and the thermodynamic Grüneisen parameter was estimated as γth = 1.42. The obtained EoS for Prp and the preliminarily fitted EoS for Al-bearing akimotoite (Al-Aki), in combination with literature data on bridgmanite (Bdm) and corundum (Crn), allowed the calculation of the phase diagram of the system with 75 mol% MgSiO3 + 25 mol% Al2O3 under LM conditions. The transformation from Prp to Bdm + Crn was computed at 24 GPa and 1570 K and exhibited a slightly positive Clapeyron slope (dP/dT = 2.1 MPa/K). The stability field of Al-Aki was detected at T = 1250–1570 K and P = 23–27 GPa. At P values higher than 24–27 GPa, the Al-Aki transforms into a Bdm + Crn assemblage with a highly negative Clapeyron slope. Calculations of sound velocities for the studied phases showed that the transformation from Prp to Bdm + Crn increased Vp and Vs by up to 9 and 20%, respectively. Such a big jump in the sound velocities indicates that the post-garnet transition is a better candidate than the post-Aki transition for producing a double discontinuity at the base of the MTZ, in combination with the transformation of ringwoodite into the Bdm + ferropericlase assemblage. The increase in sound velocities associated with the formation and dissolution of Al-Aki is unlikely to be sensitive to the MTZ. The combination of post-garnet, post-spinel, and post-Aki transitions near 660–720 km depths may have sluggished both the upwelling hot mantle and the subducted cold plates. The stagnant, hot LM material at the base of MTZ warmed the harzburgite and eclogite layers stacked during subduction. The heated MTZ may have been involved in further upwelling.

PBE-GGA predicts the B8↔B2 phase boundary of FeO at Earth’s core conditions

FeO is a crucial component of the Earth's core, and its thermodynamic properties are essential to developing more accurate core models. It is also a notorious correlated insulator in the NaCl-type (B1) phase at ambient conditions. It undergoes two polymorphic transitions at 300 K before it becomes metallic in the NiAs-type (B8) structure at ~100 GPa. Although its phase diagram is not fully mapped, it is well established that the B8 phase transforms to the CsCl-type (B2) phase at core pressures and temperatures. Here, we report a successful ab initio calculation of the B8↔B2 phase boundary in FeO at Earth's core pressures. We show that fully anharmonic free energies computed with the Perdew-Burke-Ernzerhof-generalized gradient approximation coupled with thermal electronic excitations reproduce the experimental phase boundary within uncertainties at P > 255 GPa, including the largely negative Clapeyron slope of -52 MPa/K. This study validates the applicability of a standard density functional theory functional to FeO under Earth's core conditions and demonstrates the theoretical framework that enables complex predictive studies of this region.

How the Indian Ocean Geoid Low Was Formed

AbstractThe origin of the Earth's lowest geoid, the Indian Ocean geoid low (IOGL) has been controversial. The geoid predicted from present‐day tomography models has shown that mid to upper mantle hot anomalies are integral in generating the IOGL. Here we assimilate plate reconstruction in global mantle convection models starting from 140 Ma and show that sinking Tethyan slabs perturbed the African Large Low Shear Velocity province and generated plumes beneath the Indian Ocean, which led to the formation of this negative geoid anomaly. We also show that this low can be reproduced by surrounding mantle density anomalies, without having them present directly beneath the geoid low. We tune the density and viscosity of thermochemical piles at core‐mantle boundary, Clapeyron slope and density jump at 660 km discontinuity, and the strength of slabs, to control the rise of plumes, which in turn determine the shape and amplitude of the geoid low.

The Post‐Perovskite Transition in Fe‐ and Al‐Bearing Bridgmanite: Effects on Seismic Observables

AbstractThe primary phase of the Earth’s lower mantle, (Al, Fe)‐bearing bridgmanite, transitions to the post‐perovskite (PPv) phase at Earth’s deep mantle conditions. Despite extensive experimental and ab initio investigations, there are still important aspects of this transformation that need clarification. Here, we address this transition in (Al3+, Fe3+)‐, (Al3+)‐, (Fe2+)‐, and (Fe3+)‐bearing bridgmanite using ab initio calculations and validate our results against experiments on similar compositions. Consistent with experiments, our results show that the onset transition pressure and the width of the two‐phase region depend distinctly on the chemical composition: (a) Fe3+‐, Al3+‐, or (Al3+, Fe3+)‐alloying increases the transition pressure, while Fe2+‐alloying has the opposite effect; (b) in the absence of coexisting phases, the pressure‐depth range of the Pv‐PPv transition is likely too broad to cause a sharp D” discontinuity (<30 km); (c) the average Clapeyron slope of the two‐phase regions are consistent with previous measurements, calculations in MgSiO3, and inferences from seismic data. In addition, (d) we observe a softening of the bulk modulus in the two‐phase region. The consistency between our results and experiments gives us the confidence to proceed and examine this transition in aggregates with different compositions computationally, which will be fundamental for resolving the most likely chemical composition of the D" region by analyses of tomographic images.

Penetrative Superplumes in the Mantle of Large Super‐Earth Planets: A Possible Mechanism for Active Tectonics in the Massive Super‐Earths

AbstractRecent theoretical studies suggest that the physical and rheological mantle properties in massive rocky planets fall outside conventional behaviors inferred for mantle properties at the Earth's mantle pressures. The vacancy diffusion occurring at low pressures is assumed to be followed by interstitial diffusion above ∼0.1 TPa resulting in viscosity reduction at higher pressures. In addition, the dissociation transition of MgSiO3 post‐perovskite (pPv) into new phases of minerals at 0.9 and 2.1 TPa, both with large negative Clapeyron slopes, has further impact(s) on the style of circulation in the mantle of super‐Earth planets. Further, the electronic contribution of conductivity increases exponentially with temperature at temperatures ∼5000 K and higher. We employ 3D‐controlled volume spherical convection models to explore the style of mantle circulation in large rocky super‐Earth planets with different core temperatures. Our numerical models resembling a GJ 876 d size super‐Earth reveal that due to the buffering influence of the pPv‐dissociation transition at ∼0.9 TPa, for deep mantle viscosities lower than ∼1022 Pa.s a small‐scale convective layer may develop at the top of the core‐mantle boundary (CMB). Penetrative superplumes originating from deep mantle‐layered regions can maintain the heat flux from the CMB required for the planet's geodynamo, and can survive for billions of years reaching shallow depths of the mantle without significant lateral migration. The strength of the focused penetrative superplumes that can potentially sustain surface volcanism and plate tectonics is enhanced with increasing CMB temperature, but diminished by higher rates of internal heating.

A switch from horizontal compression to vertical extension in the Vrancea slab explained by the volume reduction of serpentine dehydration

The Vrancea slab, Romania, is a subducted remnant of the Tethyan lithosphere characterized by a significant intermediate-depth seismicity (60–170 km). A recent study showed a correlation between this seismicity and major dehydration reactions, involving serpentine minerals up to 130 km depth, and high-pressure hydrated talc deeper. Here we investigate the potential link between the triggering mechanisms and the retrieved focal mechanisms of 940 earthquakes, which allows interpreting the depth distribution of the stress field. We observe a switch from horizontal compression to vertical extension between 100 and 130 km depth, where the Clapeyron slope of serpentine dehydration is negative. The negative volume change within dehydrating serpentinized faults, expected mostly sub-horizontal in the verticalized slab, could well explain the vertical extension recorded by the intermediate-depth seismicity. This apparent slab pull is accompanied with a rotation of the main compressive stress, which could favour slab detachments in active subduction zones.

Temporal variations in plume flux: characterizing pulsations from tilted plume conduits in a rheologically complex mantle

SUMMARY Along age-progressive hotspot volcano chains, the emplacement rate of igneous material varies through time. Time-series analysis of changing emplacement rates at a range of hotspots finds that these rates vary regularly at periods of a few to several tens of millions of years, indicative of changing melt production within underlying mantle plumes. Many hotspots exhibit at least one period between ∼2 and 10 Myr, consistent with several proposed mechanisms for changing near-surface plume flux, and thus melting rate, such as small-scale convection, solitary waves and instability formation in tilted plume conduits. Here, we focus on quantifying instability growth within plumes tilted by overlying plate motion. Previous studies using fluids with constant or temperature-dependent viscosity suggest that such instabilities should not form under mantle conditions. To test this assertion, we use a modified version of the finite element code ASPECT to simulate 400 Myr of evolution of a whole-depth mantle plume rising through the transition zone and spreading beneath a moving plate. In a 2-D spherical shell geometry, ASPECT solves the conservation equations for a compressible mantle with a thermodynamically consistent treatment of phase changes in the mantle transition zone and subject to either a temperature- and depth-dependent linear rheology or a temperature-, depth- and strain-rate dependent non-linear rheology. Additionally, we examine plume evolution in a mantle subject to a range of Clapeyron slopes for the 410 km (1–4 MPa K–1) phase transitions. Results suggest that plume conduits tilted by >67° become unstable and develop instabilities that lead to initial pulses in the transition zone followed by repeated plume pulsing in the uppermost mantle. In these cases, pulse size and frequency depend strongly on the viscosity ratio between the plume and ambient upper mantle. Based upon our results and comparison with other studies, we find that the range of statistically significant periods of plume pulsing in our models (∼2–7 Myr), the predicted increase in melt flux due to each pulse (3.8–26 × 10−5 km3 km−1 yr−1), and the time estimated for a plume to tilt beyond 67° in the upper mantle (10–50 Myr) are consistent with observations at numerous hotspot tracks across the globe. We suggest that pulsing due to destabilization of tilted plume conduits may be one of several mechanisms responsible for modulating the melting rate of mantle plumes as they spread beneath the moving lithosphere.

Solidus melting of pyrolite and bridgmanite: Implication for the thermochemical state of the Earth's interior

Melting properties of the deep mantle remain controversial due to experimental difficulties; e.g., reports of solidus temperatures of mantle-relevant compositions span over ∼700 K at 2000 km depth. This situation limits our understanding of the thermochemical state of the Earth's interior. Using the laser heated diamond anvil cell (LH-DAC), we performed new experimental determination of the solidus profile of ultra-dry pyrolite and the solidus of two compositions of (Mg,Fe)(Si,Al)O3 bridgmanite (Bg). Melting was detected (i) from -the correlation between laser power and sample temperature, -changes of sample texture and -the level of visible light absorption, for all samples, (ii) using X-ray diffraction, for the MgSiO3 composition and (iii) after scanning electron microscope observations, for selected Fe-bearing samples. Special care was given to using ultra-dry experimental chambers and to determination of sample temperature. In particular, we discuss the wavelength-dependent thermal emission of silicate samples, which lowers the solidus by 100 to 300 K, compared to the grey-body assumption.The solidus of MgSiO3-Bg is in good agreement with previous reports using ab initio calculations and shock wave experiments. We observe a net decrease in the solid-liquid Clapeyron slope at 60(3) GPa and 4400(200) K, which can be related to rapid pressure-induced coordination change of Si in the melt. (Mg0.955,Fe0.045)(Si0.993,Al0.007)O3 Bg melts 600–800 K lower than MgSiO3-Bg. Its solidus evolves smoothly with pressure, suggesting progressive Si coordination change in the melt. In the pressure range investigated (24–135 GPa) Clapeyron slopes suggest rapid decrease of the volume of fusion, from 14 to 2% for MgSiO3 and from 9 to 3% for (Fe,Al)-bearing Bg, assuming congruent melting. By comparing the solidii of various silicates, it appears that the higher the number of cations, the less pronounced is the curvature of the solidus. This observation suggests that the relatively ordered structure of simple liquid compositions with a limited number of distinct network-modifying cations frustrates the coexistence of tetrahedrally and octahedrally coordinated Si polyhedral.The solidus of pyrolite presents a smooth evolution from 2200(100) K to 3950(200) K in the same pressure interval. This is very similar to our previous work on chondritic-type mantle. The new solidus is 200–300 K lower than that of KLB-1 peridotite, which can be related to more incompatible elements in pyrolite. It remains problematic that our solidus plots several 100 K higher than other recent measurements performed on pyrolite; we discuss the possibility of a higher water content in previous samples, compared to our experiments. Assuming a dry lowermost mantle, our results imply a core-mantle boundary temperature lower than 3950(200) K. Modeling the melting diagram at the core-mantle boundary suggests a pseudo-eutectic melt significantly depleted in SiO2, compared to the composition of the mean mantle.

Anharmonic thermodynamic properties and phase boundary across the postperovskite transition in MgSiO3

To address the effects of lattice anharmonicity across the perovskite to post-perovskite transition in MgSiO$_3$, we conduct calculations using the phonon quasiparticle (PHQ) approach. The PHQ is based on \textit{ab initio} molecular dynamics and, in principle, captures full anharmonicity. Free energies in the thermodynamic limit ($N \rightarrow \infty$) are computed using temperature-dependent quasiparticle dispersions within the phonon gas model. Systematic results on anharmonic thermodynamic properties and phase boundary are reported. Both the local density approximation (LDA) and the generalized gradient approximation (GGA) calculations are performed to provide confident constraints on these properties. Anharmonic effects are demonstrated by comparing results with those obtained using the quasiharmonic approximation (QHA). The inadequacy of the QHA is indicated by its overestimation of thermal expansivity and thermodynamic Gr\"{u}neisen parameter and its converged isochoric heat capacity in the high-temperature limit. The PHQ phase boundary has a Clapeyron slope ($dP/dT$) that increases with temperature. This result contrasts with the nearly zero curvature of the QHA phase boundary. Anharmonicity bends the phase boundary to lower temperatures at high pressures. Implications for the double-crossing of the phase boundary by the mantle geotherm are discussed.

Melting Phase Equilibria from 4 to 7 GPa in the System CaO-MgO-Al2O3-SiO2-CO2, the Persistence of the “Ledge” in Carbonated Basalt with Excess Silica, and the Most Likely Limits on the Depths of Termination of Carbon Cycle at Subduction Zones

Abstract Melting phase relations involving model carbonated basalt with excess silica were studied in experiments over the pressure range of 4–7 GPa in the system CaO-MgO-Al2O3-SiO2-CO2 to determine if there is a sharp decrease in the melting temperatures along the transition from carbon dioxide vapor (vapor) to dolomite. The phase assemblages of clinopyroxene + garnet + coesite + vapor + carbon dioxide-bearing silicate liquid (silicate liquid) and clinopyroxene + garnet + coesite + dolomite + carbonate liquid, exist over 4–5 and 5.8–7 GPa, respectively. These two distinct phase assemblages form the two, vapor + silicate liquid and dolomite + carbonate liquid-bearing divariant surfaces. The dissolved carbon dioxide and the molar calcium number [Ca# 100*(Ca/Ca + Mg)] of the silicate and carbonate liquids are approximately 4–8 wt% and between 50–55 and 35–40 wt% and 69–71, respectively. The compositions of phases vary little, implying minimal topography along the two surfaces, and the temperatures rise linearly along the silicate liquid-bearing divariant surface over 4–5 GPa. Between 5.2 and 5.6 GPa, however, the temperatures decrease precipitously by ~200–250°C and, along with this steep decline, the liquid changes from silicate to carbonate, with the rest of the phase assemblage of clinopyroxene + garnet + coesite + vapor, persisting. Hence, and this is important to emphasize, this liquid, coexisting with vapor, is carbonate in composition in the absence of dolomite. Isobaric invariance, at 5.4 GPa/1250°C, 5.6 GPa/1150°C, and 5.8 GPa/1100°C, consists of the six-phase assemblage of clinopyroxene + garnet + coesite + vapor + dolomite + carbonate liquid. Melting phase relations are thus univariant, and correspond to that of a solidus ‘ledge’, i.e. with a negative Clapeyron slope, in this part of the composition space. The melting reaction along the ledge is clinopyroxene + vapor = garnet + coesite + dolomite + carbonate liquid, and the ledge separates the two divariant surfaces. The Ca# of the coexisting carbonate liquid and dolomite here are opposite to those of the carbonate liquid and dolomite on the calcite-magnesite join at similar pressures as in this study. This is most likely a consequence of the combined effects of (a) observations from experiments and theory that the fusion curve of calcite starts to diverge from that of magnesite toward lower temperatures at pressures in excess of ~5 GPa, and (b) the pressure, where ultrabasic silicate–carbonate (~2.5–3 GPa) and excess-silica carbonate-basalt (>4 GPa, as inhere) systems undergo carbonation. These, in turn, cause the liquid and dolomite in experiments here to become more calcic and more magnesian than observed in experiments on the calcite-magnesite join. The solidus ledge, here, has a profound effect because the most plausible modern-day model ocean crust subduction zone geotherms in Earth will, in all likelihood, intersect it and cause fusion of dolomite, thereby, in effect, liberating all carbon from what once was a carbonate-basalt mixture. Thereafter, little exists to suggest that there is anything ‘deep’ to the carbon cycle, through recycling, with most of it likely confined to less than ~200 km in Earth.

A New Approach Determining a Phase Transition Boundary Strictly Following a Definition of Phase Equilibrium: An Example of the Post-Spinel Transition in Mg2SiO4 System

The Clapeyron slope is the slope of a phase boundary in P–T space and is essential for understanding mantle dynamics and evolution. The phase boundary is delineating instead of balancing a phase transition’s normal and reverse reactions. Many previous high pressure–temperature experiments determining the phase boundaries of major mantle minerals experienced severe problems due to instantaneous pressure increase by thermal pressure, pressure drop during heating, and sluggish transition kinetics. These complex pressure changes underestimate the transition pressure, while the sluggish kinetics require excess pressures to initiate or proceed with the transition, misinterpreting the phase stability and preventing tight bracketing of the phase boundary. Our recent study developed a novel approach to strictly determine phase stability based on the phase equilibrium definition. Here, we explain the details of this technique, using the post-spinel transition in Mg2SiO4 determined by our recent work as an example. An essential technique is to observe the change in X-ray diffraction intensity between ringwoodite and bridgmanite + periclase during the spontaneous pressure drop at a constant temperature and press load with the coexistence of both phases. This observation removes the complicated pressure change upon heating and kinetic problem, providing an accurate and precise phase boundary.