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Accreting in the first few million years (Ma) of the Solar System, planetesimals record conditions in the protoplanetary disc and are the remnants of planetary formation processes. The meteorite paleomagnetic record carries key insights into the thermal history of planetesimals and their extent of differentiation. The current paradigm splits the meteorite paleomagnetic record into three magnetic field generation epochs: an early nebula field (≲5 Ma after CAI formation), followed by thermal dynamos (∼5–34 Ma after CAI formation), then a gap in dynamo generation, before the onset of core solidification and compositional dynamos. These epochs have been defined using current thermal evolution and dynamo generation models of planetesimals. Here, we demonstrate these epochs are not as distinct as previously thought based on refined thermal evolution models that include more realistic parametrisations for mantle convection, non-eutectic core solidification, and radiogenic 60Fe in the core. We find thermal dynamos can start earlier and last longer. Inclusion of appreciable 60Fe in the core brings forward the onset of dynamo generation to ∼1–2 Ma after CAI formation, which overlaps with the existence of the nebula field. The second epoch of dynamo generation begins prior to the onset of core solidification this epoch is not purely compositionally driven. Planetesimal radius is the dominant control on the strength and duration of dynamo generation, and the choice of reference viscosity can widen the gap between epochs of dynamo generation from 0–200 Ma. Overall, variations in planetesimal properties lead to more variable timings of different planetesimal magnetic field generation mechanisms than previously thought. This alters the information we can glean from the meteorite paleomagnetic record about the early Solar System. Evidence for the nebula field requires more careful interpretation, and late paleomagnetic remanences, for example in the pallasites, may not be evidence for planetesimal core solidification. • Planetesimal thermal dynamos can begin before the dissipation of the nebula field. • Core solidification is not required to trigger a second epoch of dynamo generation. • Planetesimal dynamos can last several 100 Ma longer than previously thought. • Planetesimal radius and reference viscosity have a strong effect on dynamo duration. [ABSTRACT FROM AUTHOR]

Paleomagnetic measurements of ancient terrestrial and extraterrestrial samples indicate that numerous planetary bodies generated magnetic fields through core dynamo activity during the early solar system. The existence, timing, intensity and stability of these fields are governed by the internal transfer of heat throughout their parent bodies. Thus, paleomagnetic records preserved in natural samples can contain key information regarding the accretion and thermochemical history of the rocky bodies in our solar system. However, models capable of predicting these field properties across the entire active lifetime of a planetary core that could relate the processes occurring within these bodies to features in these records and provide such information are limited. Here, we perform asteroid thermal evolution models across suites of radii, accretion times and thermal diffusivities with the aim of predicting when fully and partially differentiated asteroids generated magnetic fields. We find that dynamo activity in both types of asteroid is delayed until ∼4.5-5.5 Myr after calcium-aluminium-rich inclusion formation due to the partitioning of 26Al into the silicate portion of the body during differentiation and large early surface heat fluxes, followed by a brief period (<12.5 Myr for bodies with radii <500 km) of thermally-driven dynamo activity as heat is convected from the core across a partially-molten magma ocean. We also expect that gradual core solidification produced compositionally-driven dynamo activity in these bodies, the timing of which could vary by tens to hundreds of millions of years depending on the S concentration of the core and the radius of the body. There was likely a pause in core cooling and dynamo activity following the cessation of convection in the magma ocean. Our predicted periods of magnetic field generation and quiescence match eras of high and low paleointensities in the asteroid magnetic field record compiled from paleomagnetic measurements of multiple meteorites, providing the possible origins of the remanent magnetisations carried by these samples. We also compare our predictions to paleomagnetic results from different meteorite groups to constrain the radii of the angrite, CV chondrite, H chondrite, IIE iron meteorite and Bjürbole (L/LL chondrite) parent bodies and identify a likely nebula origin for the remanent magnetisation carried by the CM chondrites. • We predict the timing of asteroid magnetic field generation using thermal models. • We model suites of radii, accretion times, thermal diffusivities and structures. • Our models predict various periods of field generation and quiescence. • These periods match eras of high and low field intensities recovered from meteorites. • We use the timing of these eras to constrain the radii of meteorite parent bodies. [ABSTRACT FROM AUTHOR]

Abstract The ilmenite-bearing cumulates (IBC) formed from the solidification of the lunar magma ocean are thought to have significantly affected the long-term evolution of the lunar interior and surface. Their high density is considered to trigger Rayleigh–Taylor instabilities which allow them to sink into the solidified cumulates below and drive a large-scale overturn in the lunar mantle. Knowledge of how the IBC participate in the overturn is important for studying the early lunar dynamo, chemistry of surface volcanism, and the existence of present-day partial melt at the lunar core–mantle boundary. Despite early efforts to study this process as Rayleigh–Taylor instabilities, no dynamical models have quantified the degree of IBC sinking systematically. We have performed quantitative 2-D geodynamical simulations to measure the extent to which IBC participate in the overturn after their solidification, and tested the effect of a range of physical and chemical parameters. Our results show that IBC overturn most likely happened when the magma ocean had not yet fully solidified, with the residual melt decoupling the crust and IBC, resulting in 50–70% IBC sinking. Participation of the last dregs of remaining magma ocean melt is unlikely, leaving its high concentrations of radiogenic elements close to the surface. Our simulations further indicate that foundered IBC can stay relatively stable at the core–mantle boundary until the present day, at temperatures consistent with the presence of a partially molten zone in the deep mantle as inferred from geophysical data. 30–50% of the primary IBC remain at shallow depths throughout lunar history, enabling their assimilation by rising magma to form high-Ti basalts. Highlights • We study the fate of lunar ilmenite-bearing cumulates (IBC) after they solidify. • Around 50–70% IBC sink to the mantle, when urKREEP is still partially molten. • 30–50% of the primary IBC remain at shallow depths throughout lunar history. • Participation of urKREEP in lunar mantle overturn is unlikely. • Foundered IBC may explain the present-day partial melt in deep lunar mantle. [ABSTRACT FROM AUTHOR]

Abstract The seismically-observed large low shear velocity provinces in the Earth's lowermost mantle have been hypothesized to be caused by thermochemical piles of compositionally distinct, more-primitive material which may be remnants of Earth's early differentiation. However, one critical question is how the Earth's thermal evolution is affected by the long-term presence of the large-scale compositional heterogeneity in the lowermost mantle. Here, we perform geodynamical calculations to investigate the time evolution of the morphology of large-scale compositional heterogeneity and its influence on the Earth's long-term thermal evolution. Our results show that a global layer of intrinsically dense material in the lowermost mantle significantly suppresses the CMB heat flux, which leads to faster cooling of the background mantle relative to an isochemical mantle. As the background mantle cools, the intrinsically dense material is gradually pushed into isolated thermochemical piles by cold downwellings. The size of the piles also decreases with time due to entraining of pile material into the background mantle. The morphologic change of the accumulations of intrinsic dense material eventually causes a gradual increase of CMB heat flux, which significantly reduces the cooling rate of Earth's mantle. Highlights • The morphology of compositional heterogeneity on the CMB affects mantle temperature. • A global layer of intrinsic dense material on the CMB leads to rapid mantle cooling. • The intrinsic dense material forms into isolated piles as the mantle cools. • The change from dense global layer to isolated piles reduces mantle cooling rate. [ABSTRACT FROM AUTHOR]

The conventional view of Earth's inner core is that it began to crystallize at Earth's center when the temperature dropped below the melting point of the iron alloy and has grown steadily since that time as the core continued to cool. However, this model neglects the energy barrier to the formation of the first stable crystal nucleus, which is commonly represented in terms of the critical supercooling required to overcome the barrier. Using constraints from experiments, simulations, and theory, we show that spontaneous crystallization in a homogeneous liquid iron alloy at Earth's core pressures requires a critical supercooling of order 1000 K, which is too large to be a plausible mechanism for the origin of Earth's inner core. We consider mechanisms that can lower the nucleation barrier substantially. Each has caveats, yet the inner core exists: this is the nucleation paradox. Heterogeneous nucleation on a solid metallic substrate tends to have a low energy barrier and offers the most straightforward solution to the paradox, but solid metal would probably have to be delivered from the mantle and such events are unlikely to have been common. A delay in nucleation, whether due to a substantial nucleation energy barrier, or late introduction of a low energy substrate, would lead to an initial phase of rapid inner core growth from a supercooled state. Such rapid growth may lead to distinctive crystallization texturing that might be observable seismically. It would also generate a spike in chemical and thermal buoyancy that could affect the geomagnetic field significantly. Solid metal introduced to Earth's center before it reached saturation could also provide a nucleation substrate, if large enough to escape complete dissolution. Inner core growth, in this case, could begin earlier and start more slowly than standard thermal models predict. [ABSTRACT FROM AUTHOR]

Intense bombardment is considered characteristic of the Hadean and early Archean eons, yet some detrital zircons indicate that near-surface water was present and thus at least intervals of clement conditions may have existed. We investigate the habitability of the top few kilometers of the subsurface by updating a prior approach to thermal evolution of the crust due to impact heating, using a revised bombardment history, a more accurate thermal model, and treatment of melt sheets from large projectiles (>100 km diameter). We find that subsurface habitable volume grows nearly continuously throughout the Hadean and early Archean (4.5–3.5 Ga) because impact heat is dissipated rapidly compared to the total duration and waning strength of the bombardment. Global sterilization was only achieved using an order of magnitude more projectiles in 1/10 the time. Melt sheets from large projectiles can completely resurface the Earth several times prior to ∼4.2 Ga but at most once since then. Even in the Hadean, melt sheets have little effect on habitability because cooling times are short compared to resurfacing intervals, allowing subsurface biospheres to be locally re-established by groundwater infiltration between major impacts. Therefore the subsurface is always habitable somewhere, and production of global steam or silicate-vapor atmospheres are the only remaining avenues to early surface sterilization by bombardment. [ABSTRACT FROM AUTHOR]

We have studied the thermal and magnetic field evolution of planet Mercury with a core of Fe–Si alloy to assess whether an Fe–Si core matches its present-day partially molten state, Mercury's magnetic field strength, and the observed ancient crustal magnetization. The main advantages of an Fe–Si core, opposed to a previously assumed Fe–S core, are that a Si-bearing core is consistent with the highly reduced nature of Mercury and that no compositional convection is generated upon core solidification, in agreement with magnetic field indications of a stable layer at the top of Mercury's core. This study also present the first implementation of a conductive temperature profile in the core where heat fluxes are sub-adiabatic in a global thermal evolution model. We show that heat migrates from the deep core to the outer part of the core as soon as heat fluxes at the outer core become sub-adiabatic. As a result, the deep core cools throughout Mercury's evolution independent of the temperature evolution at the core-mantle boundary, causing an early start of inner core solidification and magnetic field generation. The conductive layer at the outer core suppresses the rate of core growth after temperature differences between the deep and shallow core are relaxed, such that a magnetic field can be generated until the present. Also, the outer core and mantle operate at higher temperatures than previously thought, which prolongs mantle melting and mantle convection. The results indicate that S is not a necessary ingredient of Mercury's core, bringing bulk compositional models of Mercury more in line with reduced meteorite analogues. [ABSTRACT FROM AUTHOR]

Density and moment of inertia of icy moons and dwarf planets suggest the presence of a low-density carbonaceous component in their rocky cores. This hypothesis was tested using inner density structure and thermal models. Rocky core densities in dwarf planets and icy moons are found to consist of a mixture of chondritic silicate-sulfide rocks and carbonaceous matter. Carbonaceous matter was originally mixed with ice in a rock-free precursor. In a homogeneous accretion scenario where these components are mixed in solar proportions, ices then differentiated from the carbon-rich refractory core, while hydration of silicates could take place. Thermal models taking into account the presence of carbonaceous matter suggest that originally hydrated silicates are only partially dehydrated in the refractory cores of most moons. Viable scenarios point to a difference in formation or evolution between Ganymede and Titan in spite of their similar size and mass. Fully dehydrated mineralogies, inferred in Europa and possibly the densest dwarf planet Eris, require heterogeneous accretion near the water snow line of the solar or circumplanetary nebula. Progressive gas release from slowly warming carbonaceous matter-rich cores may sustain up to present-day the replenishment of ice-oceanic layers in organics and volatiles. It accounts for the observation of nitrogen, light hydrocarbons and complex organic molecules at the surface, in the atmospheres, or in plumes emanating from moons and dwarf planets. The formation of large carbon-rich bodies in the outer solar system suggests that carbon-rich planets could form at the outskirts of extrasolar systems. • Rocky cores of dwarf planets and icy moons are rich in coal-like carbonaceous matter. • Homogeneous or heterogeneous accretion scenario may be distinguished. • Heterogeneous accretion would take place near the water snow line of the nebula. • Warming rocky cores may release complex organic molecules to the surface and plumes. • Carbon-rich moons and planets may form at the outskirts of extrasolar systems. [ABSTRACT FROM AUTHOR]

Several episodes of complete melting have probably occurred during the first stages of the Earth's evolution. We have developed a numerical model to monitor the thermal and melt fraction evolutions of a cooling and crystallizing magma ocean from an initially fully molten mantle. For this purpose, we numerically solve the heat equation in 1D spherical geometry, accounting for turbulent heat transfer, and integrating recent and strong experimental constraints from mineral physics. We have explored different initial magma ocean viscosities, compositions, thermal boundary layer thicknesses and initial core temperatures. We show that the cooling of a thick terrestrial magma ocean is a fast process, with the entire mantle becoming significantly more viscous within 20 kyr. Due to the slope difference between the adiabats and the melting curves, the solidification of the molten mantle occurs from the bottom up. In the meantime, a crust forms due to the high surface radiative heat flow, the last drop of fully molten silicate is restricted to the upper mantle. Among the studied parameters, the magma ocean lifetime is primarily governed by its viscosity. Depending on the thermal boundary layer thickness at the core–mantle boundary, the thermal coupling between the core and magma ocean can either insulate the core during the magma ocean solidification and favor a hot core or drain the heat out of the core simultaneously with the cooling of the magma ocean. Reasonable thickness for the thermal boundary layer, however, suggests rapid core cooling until the core–mantle boundary temperature results in a sluggish lowermost mantle. Once the crystallization of the lowermost mantle becomes significant, the efficiency of the core heat loss decreases. Since a hotter liquidus favors crystallization at hotter temperatures, a hotter deep mantle liquidus favors heat retention within the core. In the context of an initially fully molten mantle, it is difficult to envision the formation of a basal magma ocean or to prevent a major heat depletion of the core. As a consequence, an Earth's geodynamo sustained only by core cooling during 4 Gyr seems unlikely and other sources of motion need to be invoked. [ABSTRACT FROM AUTHOR]

Subduction zones are first-order features of plate tectonics on Earth, yet the mechanisms by which subduction initiates remain enigmatic and controversial. Here, we reappraise the timing of metamorphism of the rock units first detached from the leading edge of the downgoing slab during initiation of the Neotethys subduction, now preserved in the metamorphic sole of the Semail ophiolite (Oman–United Arab Emirates). Using petrochronology and phase equilibrium modeling, we demonstrate that subduction initiated prior to 102–100 Ma at a slow rate (< 1 cm/yr). Subduction stagnated at relatively warm conditions (15–20 °C/km) for > 5 Myr before evolving into a faster (≥ 2–5 cm/yr) and colder (∼7 °C/km) self-sustained regime. Subduction acceleration (i.e. "unlocking" stage) triggered the onset of slab retreat, large-scale corner flow and fast ocean spreading in the overriding plate at 96–95 Ma, through the progressive change of thermo-mechanical structure of the plate interface. This study reconciles conflicting analogue and numerical subduction initiation models, shedding light on the thermal, mechanical and kinematic complexity of subduction initiation. • Titanite and monazite in metamorphic soles record subduction initiation dynamics. • Initiation of the Neotethys subduction (Oman-UAE) was tectonically-induced prior to 102-100 Ma. • Subduction first stagnated for 5-10 Myr at warm conditions (15-20 °C/km). • Subduction evolved into a faster and colder self-sustained regime after 96-95 Ma. [ABSTRACT FROM AUTHOR]

Abstract: We have investigated the influence of post-perovskite (ppv) viscosity on mantle convective dynamics and stirring efficiency, using numerical modeling and simple analytical theory. Our results show that the strength of ppv has a dramatic influence on convective dynamics. The presence of a weak ppv enhances heat transfer across the bottom thermal boundary layer, resulting in higher temperatures, lower mantle viscosities and considerably larger convective velocities. This leads to a significant increase in stirring efficiencies with decreasing the ppv strength, by at least one order of magnitude. In addition, using a simple parameterized convection evolution that includes the influence of ppv, coupled to a mixing model, we show that during the long term history of the Earth''s mantle, the presence of ppv yields systematically hotter thermal evolution and more efficient convective stirring. Such a strong effect of ppv strength on mantle stirring efficiency suggests that the influence of ppv phase must be considered when interpreting both geochemical and geophysical observations. [Copyright &y& Elsevier]

Abstract: Volcanism plays an important role in the formation and thermal evolution of the crusts of all terrestrial planets. Martian volcanoes have been extensively studied, and it has been suggested that the volcanism on Mars that created the visible volcanic features was initiated in the Noachian (>3.8Ga) and continued to the Late Amazonian (<0.1Ga). However, styles of ancient volcanism, their links with the earliest volcanic constructions, and the thermal evolution of the planet are still not well understood. Here we show that numerous Early Noachian (>4.0Ga) volcanoes are preserved in the heavily cratered southern highlands. Most of these are central volcanoes with diameters ranging from 50 to 100km and heights of 2–3km. Most of them are spatially adjacent to and temporally continuous with the Tharsis and circum-Hellas volcanic provinces, suggesting that these two volcanic provinces have experienced more extensive and longer duration volcanism than previously thought. These edifices are heavily cut by radial channels, suggesting that an early phase of aqueous erosion occurred and ended prior to the emplacement of the encircling Hesperian lava fields. [Copyright &y& Elsevier]

Abstract: The dynamics of a planet and its evolution are controlled to a large extent by its viscosity. In this study, we demonstrate that the dependence of mantle viscosity on temperature and water concentration introduces strong dynamic feedbacks. We derive a dimensionless parameter to quantitatively evaluate the relative strength of those feedbacks, and show that water and heat transport are equally important in controlling present-day dynamics for the Earth. A simple parameterized evolution model illustrates the strong feedbacks and behavior of the system and agrees well with our analytic results. The analysis identifies characteristic times for changes of viscosity, temperature, and water concentration and demonstrates, for time scales greater than a few hundred million years, that the system should either be degassing while warming or regassing while cooling. This yields a characteristic evolution in which, after an initial period of rapid adjustment, the mantle warms while degassing, and subsequently cools rapidly while regassing. As the planet continues to cool, the entire surface ocean may eventually return to the mantle. Our results suggest that a simple relationship may exist between the rate of change of water concentration and the rate of change of temperature in the mantle. This connection is extended by deriving an explicit equation for the Urey ratio that depends on both heat and water transport. [Copyright &y& Elsevier]

Abstract: The thermal structure and evolution of the oceanic lithosphere is revisited with the help of a global shear velocity model of the upper mantle. Seismic velocities of the lithospheric mantle are converted to temperatures, with a particular care dedicated to the estimation of final uncertainties. These are evaluated from a Monte Carlo error propagation, taking into account the uncertainties on velocities and those on the parameters related to the velocity–temperature relationship (mantle composition, thermoelastic properties and attenuation factor). The seismically derived temperature, averaged by age interval, serves to constrain the thermal structure of the lithosphere, together with surface heat flow and ocean depth. Using an experimentally determined thermal expansivity α, the Chablis model, which prescribes a constant heat flow at some isotherms, provides a much better fit to all data than the plate model, which imposes a constant basal temperature. Only a strongly reduced α (30% reduction) allows the latter model to achieve a joint fitting comparable to the Chablis model, and then with a fit to seismically derived temperature that remains inferior the latter model. The good fit of the plate model thus depends on a reduction of α down to the lower possible limit and relies mostly on ocean depth, whose behavior at old ages is considerably obscured by anomalous crust. The Chablis model therefore appears favored by this study, which should give new perspectives on various processes related to mantle convection and to the dynamics of the lithosphere. [Copyright &y& Elsevier]

Abstract: The bending of tectonic plates as they subduct causes resistance to plate motions and mantle convection. It has been proposed that this effect could keep plate velocities relatively constant with time, and it would imply relatively high mantle temperatures through much of Earth history and relatively rapid cooling at present. It also implies a low Urey ratio, compatible with that inferred from cosmochemistry. Here it is confirmed that bending resistance only plays a significant role if plate thickness is determined mainly by dehydration stiffening accompanying melting, rather than by conductive cooling. Even then the bending resistance is quite sensitive to the radius of curvature of the subducting plate. Observed radii are generally larger than the 200km assumed in some studies, ranging up to 600km or more. Furthermore radii of curvature tend to adjust so as to prevent bending resistance from becoming large. When these factors are accounted for, calculations show that bending resistance is unlikely to have been a large factor through Earth history, and the thermal evolution of the mantle is unlikely to have been affected very much. The resolution of the Urey ratio problem should then be sought elsewhere. [Copyright &y& Elsevier]

Abstract: The secular cooling of the Earth''s mantle and the growth of the continental crust together imply changes in the isostatic balance between continents and oceans, in the oceanic bathymetry and in the area of emerged continental crust. The evolution of these variables is of fundamental importance to the geochemical coupling of mantle, continental crust, atmosphere and ocean. To explore this further, we developed a model that evaluates the area of emerged continental crust as a function of mantle temperature, continental area and hypsometry. In this paper, we investigate the continental freeboard predicted using different models for the cooling of the Earth. We show that constancy of the continental freeboard (±200 m) is possible throughout the history of the planet as long as the potential temperature of the upper mantle was never more than 110–210 °C hotter than present. Such numbers imply either a very limited cooling of the planet or, most likely, a change in continental freeboard since the Archaean. During the Archaean a greater radiogenic crustal heat production and a greater mantle heat flow would have reduced the strength of the continental lithosphere, thus limiting crustal thickening due to mountain building processes and the maximum elevation in the Earth''s topography [Rey, P. F., Coltice, N., Neoarchean strengthening of the lithosphere and the coupling of the Earth''s geochemical reservoirs, Geology 36, 635–638 (2008)]. Taking this into account, we show that the continents were mostly flooded until the end of the Archaean and that only 2–3% of the Earth''s area consisted of emerged continental crust by around 2.5 Ga. These results are consistent with widespread Archaean submarine continental flood basalts, and with the appearance and strengthening of the geochemical fingerprint of felsic sources in the sedimentary record from ∼2.5 Ga. The progressive emergence of the continents as shown by our models from the late-Archaean onward had major implications for the Earth''s environment, particularly by contributing to the rise of atmospheric oxygen and to the geochemical coupling between the Earth''s deep and surface reservoirs. [Copyright &y& Elsevier]

Abstract: The Eastern Ghats Belt (EGB) is a deeply eroded Proterozoic orogenic belt juxtaposed against the Archaean Dharwar–Bastar–Singhbhum cratons in the Indian Shield. The tectono-metamorphic history of this belt is broadly similar to the Grenvillian orogenic belts. The EGB exposes dominantly the granulite-facies rocks such as charnockites, khondalites and migmatitic gneisses with less abundant intermediate to mafic granulites. The granulites are intruded by small plutons of granites, syenites and anorthosites. The northern and southern parts of the belt (NEGB and SEGB) have distinct crustal histories, but 1600–1000–550 Ma old tectono-metamorphic events are common to both the belts. In this research, we attempt to understand the present-day and past thermal state of this orogen on the basis of our radioelement (K, U and Th) measurements at 562 sites using in-situ gamma-ray spectrometry, with other geological and geophysical constraints. The dominant rock types of the NEGB and SEGB have similar radioelement abundances (>3% K, 3 ppm U and 30 ppm Th) and heat production. Heat production of charnockites, gneisses, khondalites and intermediate granulites of the NEGB is 2.9, 2.8, 2.9 and 1.1 μW m−3, respectively. In the SEGB, they are 2.7, 2.5, 2.8 and 0.6 μW m−3, respectively, and the mafic granulites are the lowest in heat production (0.3 μW m−3). On the basis of the heat production data and crustal petrologic models, we show that the crustal contribution of the NEGB and SEGB is 54 and 45 mW m−2, respectively, which are broadly in good agreement with the available surface heat flow data. Thermal models of the present-day EGB crust indicate that the temperature at the Moho is ∼550 °C. Radioelements and heat production of the Eastern Ghats granulites are higher than the other granulite belts in the Indian shield. The crustal radiogenic heat contribution and Moho temperatures of the EGB are higher than the adjoining Archaean cratons. This could also be responsible for the development of inverted metamorphic isograds in the tectonic boundary between the EGB and the adjoining cratons during a collisional orogenic event at about 550 Ma ago. [Copyright &y& Elsevier]

Abstract: The possibility that some mantle plumes may carry a geochemical signature of core/mantle interaction has rightly generated considerable interest and attention in recent years. Correlated 186Os–187Os enrichments in some plume-derived lavas (Hawaii, Gorgona, Kostomuksha) have been interpreted as deriving from an outer core with elevated Pt/Os and Re/Os ratios due to the solidification of the Earth''s inner core (c.f., [A.D. Brandon, R.J. Walker, The debate over core–mantle interaction, Earth Planet. Sci. Lett. 232 (2005) 211–225.] and references therein). Conclusive identification of a “core signal” in plume-derived lavas would profoundly influence our understanding of mantle convection and evolution. This paper reevaluates the Os-isotope evidence for core/mantle interaction by examining other geochemical constraints on core/mantle interaction, geophysical constraints on the thermal evolution of the outer core, and geochemical and cosmochemical constraints on the abundance of heat-producing elements in the core. Additional study of metal/silicate and sulfide/silicate partitioning of K, Pb, and other trace elements is needed to more tightly constrain the likely starting composition of the Earth''s core. However, available data suggest that the observed 186Os enrichments in Hawaiian and other plume-derived lavas are unlikely to derive from core/mantle interaction. 1) Core/mantle interaction sufficient to produce the observed 186Os enrichments would likely have significant effects on other tracers such as Pb- and W-isotopes that are not observed. 2) Significant partitioning of K or other heat-producing elements into the core would produce a “core depletion” pattern in the Silicate Earth very different from that observed. 3) In the absence of heat-producing elements in the core, core/mantle heat flow of ∼6–15 TW estimated from several independent geophysical constraints suggests an inner core age (< ∼2.5 Ga) too young for the outer core to have developed a significant 186Os enrichment. Core/mantle thermal and chemical interaction remains an important problem that warrants future research. However, Os-isotopes may have only limited utility in this area due to the relatively young age of the Earth''s inner core. [Copyright &y& Elsevier]

Abstract: We discuss evidence for, and evaluate the consequences of, the growth of magma reservoirs by small increments of thin (⋍ 1–2 m) sills. For such thin units, cooling proceeds faster than the nucleation and growth of crystals, which only allows a small amount of crystallization and leads to the formation of large quantities of glass. The heat balance equation for kinetic-controlled crystallization is solved numerically for a range of sill thicknesses, magma injection rates and crustal emplacement depths. Successive injections lead to the accumulation of poorly crystallized chilled magma with the properties of a solid. Temperatures increase gradually with each injection until they become large enough to allow a late phase of crystal nucleation and growth. Crystallization and latent heat release work in a positive feedback loop, leading to catastrophic heating of the magma pile, typically by 200 °C in a few decades. Large volumes of evolved melt are made available in a short time. The time for the catastrophic heating event varies as Q −2, where Q is the average magma injection rate, and takes values in a range of 105–106 yr for typical geological magma production rates. With this mechanism, storage of large quantities of magma beneath an active volcanic center may escape detection by seismic methods. [Copyright &y& Elsevier]

A coupled core-mantle evolution model that combines a global heat balance in the core with a fully dynamic thermo-chemical mantle convection model is developed to investigate the thermal evolution of the core over the 4.5 Gyr of Earth history. The heat balance in the core includes gravitational energy release, latent heat release and compositional convection associated with inner core growth. In the mantle convection model, compositional variations, plate-like behavior, phase changes and melting-induced differentiation are included. For mantle compositional variations, three idealized situations are considered: no variations (isochemical), variations resulting from a layered initial condition, and variations resulting from melting-induced differentiation from a homogeneous start. Only models whose thermal evolution satisfies three criteria are judged to be ‘successful’, with the criteria based on: (1) the radius of the inner core, (2) the heat flux through the core-mantle boundary (CMB), and (3) the heat flux through the surface. The radius of the inner core is the strictest criterion of these three. Models with an isochemical mantle fail because the inner core becomes much larger than the current size of the inner core. The final inner core radius is quite sensitive to mantle chemical buoyancy ratio. Models that fully satisfy all three criteria have a 1.5–2% compositional density difference, and either initial layering or compositional layering generated from melt-induced differentiation. These results imply that the heat flux buffering effect of a compositionally-dense layer in the deep mantle may be required to explain the thermal evolution of the core, when the heat flux through the CMB is calculated using a fully dynamical mantle convection model. Considering geochemical constraints, the compositional layering could be generated a combination of melt-induced differentiation and primordial layering. However, while the observed trends are robust, the models include various approximations and uncertainties, with the core model not including the effects of heat generated by radioactive element, so further investigations are warranted. [Copyright &y& Elsevier]

The past several years have seen dramatic developments in the study of planetary magnetic fields, including a wealth of new data, mainly from the Galilean satellites and Mars, together with major improvements in our theoretical modeling effort of the dynamo process believed responsible for large planetary fields. These dynamos arise from thermal or compositional convection in fluid regions of large radial extent. The relevant electrical conductivities range from metallic values to values that may be only about 1% or less that of a typical metal, appropriate to ionic fluids and semiconductors. In all planets, the Coriolis force is dynamically important, but slow rotation may be more favorable for a dynamo than fast rotation. The maintenance and persistence of convection appears to be easy in gas giants and ice-rich giants, but is not assured in terrestrial planets because the quite high electrical conductivity of iron-rich cores guarantees a high thermal conductivity (through the Wiedemann–Franz law), which allows for a large core heat flow by conduction alone. In this sense, high electrical conductivity is unfavorable for a dynamo in a metallic core. Planetary dynamos mostly appear to operate with an internal field ∼(2ρΩ/σ)1/2 where ρ is the fluid density, Ω is the planetary rotation rate and σ is the conductivity (SI units). Earth, Ganymede, Jupiter, Saturn, Uranus, Neptune, and maybe Mercury have dynamos, Mars has large remanent magnetism from an ancient dynamo, and the Moon might also require an ancient dynamo. Venus is devoid of a detectable global field but may have had a dynamo in the past. The presence or absence of a dynamo in a terrestrial body (including Ganymede) appears to depend mainly on the thermal histories and energy sources of these bodies, especially the convective state of the silicate mantle and the existence and history of a growing inner solid core. Induced fields observed in Europa and Callisto indicate the strong likelihood of water oceans in these bodies. [Copyright &y& Elsevier]

• Earthquakes beneath continents and oceans occur in material cooler than 600 C ∘. • After 50 Ma old oceanic lithosphere remains cooler than this under 25 km of sediment. • Subduction of such lithosphere at 5 cm/a can produce seismicity to a depth of 300 km. Subduction of oceanic lithosphere is now occurring beneath the Aegean and central Caspian Seas, and beneath the Indo-Burman Ranges in NE India, generating zones of earthquakes that reach depths of more than 150 km. Their existence is surprising. In general, where the temperature of the mantle part of the oceanic lithosphere can be estimated, earthquakes are confined to material whose temperature is below 600 C ∘. In these three regions the oceanic lithosphere is overlain by sediments with low thermal conductivity whose thickness is probably 10–25 km. The temperature of the oceanic crust and upper half of the lithosphere should be considerably increased by such a thick sedimentary cover, yet there is no obvious difference between these subduction zones and those where there is little sediment on the subducting plate. Detailed thermal modelling of the temperature shows that, for the first 40 Ma, the deposition of even a thick sedimentary layer has little effect on the thickness of the region of lithosphere whose temperature is below 600 C ∘ , principally because little heat is conducted across the Moho. The modest effect of sediment blanketing over a period as long as 40 Ma was quite unexpected. Over the next 200 Ma the lithospheric temperature increases, until the Moho temperature reaches ∼ 600 C ∘. This behaviour has important implications for oil and gas generation in the sediments, and for the existence of large earthquakes at depths of 150 km or more beneath the Hindu Kush, Pamirs and Romania. [ABSTRACT FROM AUTHOR]

• Determination of the thermal maturity and evolution of the SE China continent-ocean transition. • IODP Expeditions 367–368 enable us to study the thermal imprint of breakup processes. • Approach combining organic petrography, Raman spectroscopy and biomarkers analysis. • Breakup related magmatism affects the thermal maturity of the rift sediments. The stretching of continental lithosphere results in asthenospheric upwelling, raising of isotherms, melting during decompression and eventually seafloor spreading. The thermal maturity of overlying sedimentary organic matter from these settings would be expected to be distinctly altered by these processes, however this is still poorly constrained and quantitatively unexplored. International Ocean Discovery Program (IODP) Expeditions 367–368 cored sediments at the Continent Ocean Transition (COT) on the Northern Margin of the South China Sea (SCS). From two settings at the South East China COT we measured and modelled thermal maturity in pre-/syn- to post-rift sediments making use of a range of thermal maturity parameters. Various heat-flow evolutionary scenarios were investigated, with notable jumps in thermal maturity for sediments corresponding to different depositional packages. In order to match observations of thermal maturity, it was found that the deeper and likely pre-rift sediments were heated to temperatures as high as 200 °C during initial breakup. Achieving this temperature for the deeper sediments requires that significant additional heat be imparted at shallow depths (e.g. exposure to at least the far-field effects of a magmatic intrusion or subsurface expressions of volcanism). The post-rift sediments have lower thermal maturities which are likely due to limited burial and the absence of late post-rift magmatism. The comparison of the SE China COT with other margin examples highlights some parameters controlling the thermal evolution and its record. [ABSTRACT FROM AUTHOR]