Core-mantle Interaction Research Articles

The tungsten isotopic composition of Eoarchean rocks: Implications for early silicate differentiation and core–mantle interaction on Earth

We have measured 182W/ 184W for Eoarchean rocks from the Itsaq Gneiss Complex (3.8–3.7 Ga pillow meta-basalts, a meta-tonalite, and meta-sediments) and Acasta Gneiss Complex (4.0–3.6 Ga felsic orthogneisses) to assess possible W isotopic heterogeneity within the silicate Earth and to constrain W isotopic evolution of the mantle. The data reveal that 182W/ 184W values in the Eoarchean samples are uniform within the analytical error and indistinguishable from the modern accessible mantle signature, suggesting that the W isotopic composition of the upper mantle has not changed significantly since the Eoarchean era. The results imply either that chemical communication between the mantle and core has been insignificant in post-Hadean times, or that a lowermost mantle with a distinctive W isotope signature has been isolated from mantle convective cycling. Most terrestrial rock samples have a 0.2 ε 142Nd/ 144Nd higher than the chondrite average. This requires either the presence of a hidden enriched reservoir formed within the first 30 Ma of the Solar System, or the bulk Earth having a ∼ 5% higher Sm/Nd than the chondrite average. We explored the relevance of the 182Hf– 182W isotope system to the 146Sm– 142Nd isotope system during early silicate differentiation events on Earth. In this context, we demonstrate that the lack of resolvable 182W excesses in the Itsaq rocks, despite 142Nd excesses compared to the modern accessible mantle, is more consistent with the view that the bulk Earth has a non-chondritic Sm/Nd. In the non-chondritic Sm/Nd Earth model, the 182W– 142Nd chronometry constrains the age of the source mantle depletion for the Itsaq samples to more than ∼ 40 Ma after the Solar System origin. Our results cannot confirm the previous report of 182W anomalies in the Eoarchean Itsaq meta-sediments, which were interpreted as reflecting an impact-derived meteoritic component.

Tungsten isotopes as tracers of core–mantle interactions: The influence of subducted sediments

The postulated difference in W isotopic composition of the Earth’s core of ∼2 ε W units, compared to the bulk silicate earth (BSE) has previously been used to search for evidence of core–mantle interaction (CMI) in ocean island basalts (OIB). The absence of W isotope anomalies has thus been taken as evidence that CMI does not occur. However, the addition of subducted sediment with high W to the sources of OIB could obscure a core signature. This possibility brings into question the utility of W isotopes as tracers for CMI. To accurately consider the effects of sediment addition to mantle sources of OIB with respect to W requires improved constraints on the abundances of W in subducting sediment. Here, we present high-precision W abundance data (and other HFSE) for a suite of sediments from the Banda subduction regime in East Indonesia. Subducting East Indonesian sediments have trace element concentrations that resemble those of average upper continental crust (UCC), making these sediments valuable to consider as typical of subducted sediments. Average W abundances of 2.1 ppm, corrected for carbon content coupled with current models of 0.5% core addition and 1% sediment addition to EM1 or HIMU plume, suggest that a model hybrid source should exhibit values of ε W = −0.24 with ∼25 ppb W. Prior studies have not reported such low W isotopic compositions or high estimated W concentrations present in the sources of either Hawaiian or French Polynesian lavas, so such large additions of core material to these plume sources seems unlikely. Given these constraints, core contributions to these source, if present, can be no more than ∼0.1%.

Tungsten in Hawaiian picrites: A compositional model for the sources of Hawaiian lavas

Concentrations of tungsten (W) and uranium (U), which represent two of the most highly incompatible elements during mantle melting, have been measured in a suite of Hawaiian picrites and primitive tholeiites from nine main-stage shield volcanoes. Tungsten abundances in the parental melts are estimated from correlations between sample W abundances and MgO contents, and/or by olivine correction calculations. From these parental melt determinations, along with independent estimates for the degree of partial melting at each volcanic center, we extrapolate the W content of the mantle sources for each shield volcano. The mantle sources of Hualalai, Mauna Loa, Kohala, Kilauea, Mauna Kea, Koolau and Loihi contain 9 ± 2 (2 σ), 11 ± 5, 10 ± 4, 12 ± 4, 10 ± 5, 8 ± 7 and 11 ± 5 ng/g, respectively. When combined, the mean Hawaiian source has an average of 10 ± 3 ng/g W, which is three-times as enriched as the Depleted MORB Mantle (DMM; 3.0 ± 2.3 ng/g). The relatively high abundances of W in the mantle sources that contribute to Hawaiian lavas may be explained as a consequence of the recycling of W-rich oceanic crust and sediment into a depleted mantle source, such as the depleted MORB mantle (DMM). However, this scenario requires varying proportions of recycled materials with different mean ages to account for the diversity of radiogenic isotope compositions observed between Kea- and Loa-trend volcanoes. Alternatively, the modeled W enrichments may also reflect a primary source component that is less depleted in incompatible trace elements than the DMM. Such a source would not necessarily require the addition of recycled materials, although the presence of some recycled crust is permitted within our model parameters and likely accounts for some of the isotopic variations between volcanic centers. The physical admixture of ⩽0.5 wt.% outer core material with the Hawaiian source region would not be resolvable via W source abundances or W/U ratios; however, W isotopes may provide a more sensitive to this mixing process. Recent W isotopic studies show no indication of core–mantle interaction, indicating that either such a process does not occur, or that mechanisms other than physical mixing may operate at the core–mantle boundary.

W isotope compositions of oceanic islands basalts from French Polynesia and their meaning for core–mantle interaction

In order to understand the behavior of W during magmatic differentiation and to examine possible W contributions from the core, the W abundances and isotope compositions were obtained for oceanic island basalts (OIB) from French Polynesia, where a whole-mantle–scale mantle upwelling has been documented using global seismic tomography. An additional reason for analyzing OIB in this particular region is the occurrence of HIMU (high time-integrated 238U/ 204Pb or high µ) OIB, which was proposed to tap the mantle source with minor contributions of recycled sediments with high W abundance. The W abundances of HIMU OIBs from Mangaia, Rurutu, and Tubuai and of EM1 OIBs from Rarotonga show five-fold variation from 0.2 ppm to 1.0 ppm, which is attributed to result from variation in degrees of melting, fractionation and accumulation of olivine and clinopyroxene phenocrysts, and source heterogeneities. The W abundances of HIMU OIBs lower than those of EM1 OIBs at a given MgO content is consistent with an idea that HIMU magma sources were contaminated by recycled sediments to lesser extent compared to EM1-magmas. The W abundance in an inferred primary HIMU magma and plausible degrees of melting (1–3%), as estimated based on incompatible element concentrations, enable us to deduce the W abundance in the sources of HIMU OIBs (2–15 ppb). The W abundance in the HIMU source is likely to be somewhere between that in the depleted mantle and that in primitive mantle, suggesting that severe contamination with recycled sediment is unlikely. All analyzed OIB samples have W isotopic composition identical to that of the bulk silicate Earth. The mixing calculation demonstrates that any contribution of core material to the source of HIMU OIBs is no greater than 0.6%.

Pt-Re-Os同位体系でコア-マントル相互作用を見ることができるか？

Pt-Re-Os同位体系でコア-マントル相互作用を見ることができるか？

Dynamos with weakly convecting outer layers: implications for core-mantle boundary interaction

Convection in the Earth's core is driven much harder at the bottom than the top. This is partly because the adiabatic gradient steepens towards the top, partly because the spherical geometry means the area involved increases towards the top, and partly because compositional convection is driven by light material released at the lower boundary and remixed uniformly throughout the outer core, providing a volumetric sink of buoyancy. We have therefore investigated dynamo action of thermal convection in a Boussinesq fluid contained within a rotating spherical shell driven by a combination of bottom and internal heating or cooling. We first apply a homogeneous temperature on the outer boundary in order to explore the effects of heat sinks on dynamo action; we then impose an inhomogeneous temperature proportional to a single spherical harmonic Y 2² in order to explore core-mantle interactions. With homogeneous boundary conditions and moderate Rayleigh numbers, a heat sink reduces the generated magnetic field appreciably; the magnetic Reynolds number remains high because the dominant toroidal component of flow is not reduced significantly. The dipolar structure of the field becomes more pronounced as found by other authors. Increasing the Rayleigh number yields a regime in which convection inside the tangent cylinder is strongly affected by the magnetic field. With inhomogeneous boundary conditions, a heat sink promotes boundary effects and locking of the magnetic field to boundary anomalies. We show that boundary locking is inhibited by advection of heat in the outer regions. With uniform heating, the boundary effects are only significant at low Rayleigh numbers, when dynamo action is only possible for artificially low magnetic diffusivity. With heat sinks, the boundary effects remain significant at higher Rayleigh numbers provided the convection remains weak or the fluid is stably stratified at the top. Dynamo action is driven by vigorous convection at depth while boundary thermal anomalies dominate in the upper regions. This is a likely regime for the Earth's core.

An MPI-based MoSST core dynamics model

Distributed systems are among the main cost-effective and expandable platforms for high-end scientific computing. Therefore scalable numerical models are important for effective use of such systems. In this paper, we present an MPI-based numerical core dynamics model for simulation of geodynamo and planetary dynamos, and for simulation of core–mantle interactions. The model is developed based on MPI libraries. Two algorithms are used for node–node communication: a “master–slave” architecture and a “divide-and-conquer” architecture. The former is easy to implement but not scalable in communication. The latter is scalable in both computation and communication. The model scalability is tested on Linux PC clusters with up to 128 nodes. This model is also benchmarked with a published numerical dynamo model solution.

Tungsten geochemistry and implications for understanding the Earth's interior

The concentration of tungsten (W) in basaltic melts provides a window into the behavior of this element during core–mantle separation, crust formation, silicate differentiation, and potentially core–mantle interaction. We have analyzed an extensive suite of modern basalts ( n = 86) for their trace element chemistry via laser ablation ICP-MS, with barium (Ba), thorium (Th), uranium (U), and W concentrations typically determined to ≤ 5% (2 σ) uncertainty. We find that the partitioning behavior of U mirrors that of W during basalt genesis, whereas Ba and Th both behave more incompatibly. The W/U ratio of our complete sample suite (0.65 ± 0.45, 2 σ) is representative of the mean modern mantle, and is indistinguishable from that of mid-ocean ridge basalts (W/U MORB = 0.65 ± 0.41, n = 52), ocean island basalts (W/U OIB = 0.63 ± 0.07, n = 10), and back-arc basin basalts (W/U BABB = 0.62 ± 0.09, n = 12). This ratio is also consistent with the W/U ratio of the continental crust, and thus represents the W/U ratio of the entire silicate portion of the Earth. Assuming a concentration of 20 ± 8 (2 σ) ng/g U in the bulk silicate Earth, the abundance of W in the silicate Earth is 13 ± 10 ng/g. Following mass balance, this implies a mean modern mantle and core composition of 8.3 ± 7.1 ng/g W and 500 ± 120 ng/g W, respectively. Additionally, the MORB source is modeled to contain approximately 3.0 ± 2.3 ng/g W, indicating a four-fold depletion of the highly incompatible elements in the MORB source relative to the silicate Earth. Although both the isotopic composition of W and the constancy of the silicate Earth W/U ratio allow for potential insight into core–mantle exchange, both of these proxies are extremely dependent on the chemical composition of the source. A case study of three Hawaiian picrites with enrichments in 186Os– 187Os but terrestrial ε 182W can be explained by: i) a lack of a core component in the Hawaiian “plume,” ii) crustal contamination, or iii) a source composition enriched in incompatible trace elements relative to the bulk silicate Earth.

Evidence of seismic anisotropy and a lower temperature condition in the D″ layer beneath Pacific Antarctic Ridge in the Antarctic Ocean

We examine shear waves of 23 deep earthquakes in south Pacific subduction zones recorded at the Syowa station in Antarctica to study the anisotropy and velocity structure of shear waves in the D″ layer beneath Pacific Antarctic Ridge (PAR) in the Antarctic Ocean. We observe earlier arrivals of transverse components (SH) than those of longitudinal components (SV) for S waves passing through the D″ layer and Scd phases due to the D″ discontinuity that has been reported beneath circum Pacific regions. We find that the optimum shear wave velocity structure in the D″ layer is transversely isotropic from waveform modeling and the differential travel time analysis among S, ScS and SKS phases. The SH velocity structure has a 2.0% velocity discontinuity at about 350 km above the core–mantle boundary. The SV structure is similar to PREM. The anisotropic D″ layer, V SV < V SH, beneath the Antarctic Ocean is significantly thicker than those beneath Alaska and the Caribbean Sea. We interpret that the amount of deposited paleo-slab material controls the variation in the thickness of the D″ layer. The difference of the thickness of the D″ layer, together with recent experimental and theoretical results of the phase transition from perovskite to post-perovskite, means that the D″ layer beneath PAR is 630–1000 K colder than that beneath Alaska. The cold mantle beneath PAR corresponds to the location of stationary patches of intense geomagnetic field in the southern hemisphere. This coincidence suggests the existence of the core–mantle interactions.

The Fe/Mn ratio in MORB and OIB determined by ICP-MS

Variations in the abundance of iron in the mantle may have important consequences for mantle dynamics and geochemistry. The abundance of iron in lavas derived from mantle source regions varies during partial melting and subsequent fractionation, so that source heterogeneities are not easily resolved in iron abundances alone. However, manganese is a geochemically similar element, so that the planetary Fe/Mn ratio is approximately constant. Here, we report new Fe/Mn results for mid-oceanic ridge basalts (MORBs), oceanic island basalts (OIBs) and komatiites using a precise inductively coupled plasma mass spectrometric (ICP-MS) method to measure Fe/Mn to better than 0.5% (2 σ). As a measure of reproducibility of Fe/Mn, five olivine and five orthopyroxene grains from a Kilbourne Hole peridotite xenolith yielded Fe/Mn 69.8 ± 0.4 and 44.3 ± 0.2, respectively. To avoid ubiquitous secondary Fe–Mn oxides, Fe/Mn ratios in Pacific, Atlantic, and Indian MORBs were determined by Laser Ablation ICP-MS. MORB Fe/Mn (53–56) corrected for crystal fractionation yielded a value of 54.0 ± 1.2 (1 σ). Icelandic basalts and picrites (MgO 10–29%) had Fe/Mn ratios 56–61, with a single exception. Six relatively fresh komatiites from Belingwe (MgO 20–29%) yielded Fe/Mn values of 58.3 ± 0.2 (1 σ). Basalts from Tahiti and Reunion exhibited high Fe/Mn (>65), like Hawaii. This implies that the mantle source regions of Tahiti and Reunion lavas may have been enriched in Fe relative to other mantle reservoirs (e.g., MORBs, Iceland, Belingwe). Combined with previous results for Hawaii, we now find that Fe/Mn > 65 is characteristic of at least two plumes from the Pacific Superswell. It is conceivable that this is evidence for excess Fe due to core–mantle interaction in these mantle plumes, although partial melting of secondary pyroxenites may cause similar variations in Fe/Mn. Heterogeneity of Fe/Mn in mantle-derived lavas is now clearly documented.

A diffusion mechanism for core–mantle interaction

Understanding the geochemical behaviour of the siderophile elements--those tending to form alloys with iron in natural environments--is important in the search for a deep-mantle chemical 'fingerprint' in upper mantle rocks, and also in the evaluation of models of large-scale differentiation of the Earth and terrestrial planets. These elements are highly concentrated in the core relative to the silicate mantle, but their concentrations in upper mantle rocks are higher than predicted by most core-formation models. It has been suggested that mixing of outer-core material back into the mantle following core formation may be responsible for the siderophile element ratios observed in upper mantle rocks. Such re-mixing has been attributed to an unspecified metal-silicate interaction in the reactive D'' layer just above the core-mantle boundary. The siderophile elements are excellent candidates as indicators of an outer-core contribution to the mantle, but the nature and existence of possible core-mantle interactions is controversial. In light of the recent findings that grain-boundary diffusion of oxygen through a dry intergranular medium may be effective over geologically significant length scales and that grain boundaries can be primary storage sites for incompatible lithophile elements, the question arises as to whether siderophile elements might exhibit similar (or greater) grain-boundary mobility. Here we report experimental results from a study of grain-boundary diffusion of siderophile elements through polycrystalline MgO that were obtained by quantifying the extent of alloy formation between initially pure metals separated by approximately 1 mm of polycrystalline MgO. Grain-boundary diffusion resulted in significant alloying of sink and source particles, enabling calculation of grain-boundary fluxes. Our computed diffusivities were high enough to allow transport of a number of siderophile elements over geologically significant length scales (tens of kilometres) over the age of the Earth. This finding establishes grain-boundary diffusion as a potential fast pathway for chemical communication between the core and mantle.

184Os/ 188Os and 186Os/ 188Os measurements by Negative Thermal Ionisation Mass Spectrometry (N-TIMS): Effects of interfering element and mass fractionation corrections on data accuracy and precision

Precise and accurate measurements of 186Os/ 188Os and 184Os/ 188Os ratios are a key requirement in the search for evidence of interaction between the Earth's core and mantle [e.g., Brandon, A.D., Walker, R.J., 2005. The debate over core–mantle interaction. Earth Planet. Sci. Lett. 232, 211–225] and in understanding nucleosynthetic processes, in particular the P-process, occurring in stars [e.g., Liu, Y., Huang, M., Masuda, A., Inoue, M., 1998. High-precision determination of osmium and rhenium isotope ratios by in-situ oxygen isotope correction using negative thermal ionisation mass spectrometry. Int. J. Mass Spectrom. Ion Process 173, 163–175.; Brandon, A.D., Walker, R.J., Puchtel, I.S., 2006. Platinum–osmium isotope evolution of the Earth's mantle: constraints from chondrites and Os-rich alloys. Geochim. Cosmochim Acta 70, 2093–2103.]. Here we present a detailed study of the effects of polyatomic interferences, mass fractionation and correction procedures that influence and ultimately limit the quality of Os isotope data that can be obtained using the N-TIMS method. Among the polyatomic interferences monitored closely, O isotopic interferences can lead to a 21 ppm deviation of the 186Os/ 188Os depending on the O isotopic composition used for correction. We found that because of the variation of the O isotopic composition within and between analyses, O isotopic composition should be determined in-run. Significant PtO 2 − signals are generated during typical N-TIMS analyses of Os and yet these interferences have not been commonly corrected for. While oxide interferences from PtO 2 − are negligible on the 186Os/ 188Os ratio, they are very significant on the 184Os isotope (up to 1.2% deviation). Hence, particular care must be given to PtO 2 − monitoring if 184Os/ 188Os is to be used to investigate supernova nucleosynthetic p-processes or used as a measure of any potential WO 3 − interferences. On the basis of detailed scans using a secondary electron multiplier, we find no evidence for the presence of PtO 3 −, PtCl − and BaCl 3 − interferences. Similarly, experiments using Os runs doped with W indicate that WO 3 − is not a common interference during typical N-TIMS analyses of Os. Among the 3 Os isotopic ratios commonly used for mass fractionation correction, the use of 189Os/ 188Os (i.e. 1.21978) leads to the largest shift on the 186Os/ 188Os, lowering it by 115 ppm on average. To aid future consistency we recommend that mass fractionation corrections are made relative to a 192Os/ 188Os value of 3.083 [e.g. Brandon, A.D., Walker, R.J., Puchtel, I.S., 2006. Platinum–osmium isotope evolution of the Earth's mantle: constraints from chondrites and Os-rich alloys. Geochim. Cosmochim Acta 70, 2093–2103.]. We have analysed 3 different Os reference materials. For all reference materials, we obtain an excellent agreement and very good long-term reproducibility for 190Os/ 188Os, 189Os/ 188Os and 187Os/ 188Os ratios compared with previously published values, or with new values obtained by MC-ICP-MS using large Os signals [Nowell, G.M., Luguet, A., Pearson, D.G., Horstwood, M.A., 2008-this volume. Precise and accurate 186Os/188Os and 187Os/188Os measurements by Multi-Collector Plasma Ionisation Mass Spectrometry (MC- ICP-MS) part I: solution analyses. Special Issue on Highly Siderophile Element Geochemistry. Chem. Geol. doi:10.1016/j.chemgeo.2007.10.020.]. The disagreement for 186Os/ 188Os and 184Os/ 188Os between our values and either previously published values, or with the new MC-ICP-MS data is more significant. Persistence of a residual correlation between 186Os/ 188Os and 184Os/ 188Os for the 3 Os reference materials following O isotope, PtO 2 − interference, and mass fractionation corrections suggests the presence of an additional interference affecting both 184Os and 186Os. This is unlikely to be WO 3 − ions since their presence during Os analysis has been discounted from our W-doped experiments. Similarly, discrepancies (70–90 ppm) in the 186Os/ 188Os between MC-ICP-MS results and our data when a correction for the residual correlation is applied also suggest the presence of additional interference that may affect the mass 234 during N-TIMS measurements. If 186Os is to be used to trace core–mantle interactions, and nuclear and supernova nucleosynthetic processes, further detailed investigation is required to precisely characterise these interferences and a common measurement and data processing procedure must be adopted in order to allow inter-laboratory comparisons.

Thermal core–mantle interaction: Exploring regimes for ‘locked’ dynamo action

Possible effects on the geodynamo of lateral variations in heat flux from the core are explored using two different patterns of heat flow from the core. One is based on lower mantle shear wave velocity and the other is the single spherical harmonic Y 2 2 . The self-consistent dynamo equations driven by thermal convection in a Boussinesq fluid are solved. Our choice of parameters is guided by earlier work on non-magnetic convection. We have already found a nearly steady solution locked to the tomographic boundary condition that bears a remarkable resemblance to the present day field; here we seek to understand this locked regime. Numerical considerations demand an artificially high-Ekman number; we choose a low-Rayleigh number and a Prandtl number of order 1. In this regime locking occurs when the underlying convection has an azimuthal wavelength similar to that of the boundary conditions, as in the non-magnetic case. This is demonstrated where a drifting non-magnetic flow dominated by m = 8 rolls is converted to locked large scale-flow by the presence of a self-generated magnetic field. Large but geophysically reasonable lateral variations in the flux are required for locking. Dynamo action can fail at very large variation, where strong thermal winds disrupt the mechanism in this regime. If the Rayleigh number is too close to critical the dynamo can fail for low lateral variation, where the flow has a larger component driven by the boundary condition. Similarly, increased azimuthal flow at lower Prandtl numbers blurs the effect of the boundary condition.

186Os and 187Os enrichments and high- 3He/ 4He sources in the Earth’s mantle: Evidence from Icelandic picrites

Picrites from the neovolcanic zones in Iceland display a range in 187Os/ 188Os from 0.1297 to 0.1381 (γ Os = + 2.1 to +8.7) and uniform 186Os/ 188Os of 0.1198375 ± 32 (2 σ). The value for 186Os/ 188Os is within uncertainty of the present-day value for the primitive upper mantle of 0.1198398 ± 16. These Os isotope systematics are best explained by ancient recycled crust or melt enrichment in the mantle source region. If so, then the coupled enrichments displayed in 186Os/ 188Os and 187Os/ 188Os from lavas of other plume systems must result from an independent process, the most viable candidate at present remains core–mantle interaction. While some plumes with high 3He/ 4He, such as Hawaii, appear to have been subjected to detectable addition of Os (and possibly He) from the outer core, others such as Iceland do not. A positive correlation between 187Os/ 188Os and 3He/ 4He from 9.6 to 19 Ra in Iceland picrites is best modeled as mixtures of 1 Ga or older ancient recycled crust mixed with primitive mantle or incompletely degassed depleted mantle isolated since 1–1.5 Ga, which preserves the high 3He/ 4He of the depleted mantle at the time. These mixtures create a hybrid source region that subsequently mixes with the present-day convecting MORB mantle during ascent and melting. This multistage mixing scenario requires convective isolation in the deep mantle for hundreds of million years or more to maintain these compositionally distinct hybrid sources. The 3He/ 4He of lavas derived from the Iceland plume changed over time, from a maximum of 50 Ra at 60 Ma, to approximately 25–27 Ra at present. The changes are coupled with distinct compositional gaps between the different aged lavas when 3He/ 4He is plotted versus various geochemical parameters such as 143Nd/ 144Nd and La/Sm. These relationships can be interpreted as an increase in the proportion of ancient recycled crust in the upwelling plume over this time period. The positive correlation between 187Os/ 188Os and 3He/ 4He demonstrates that the Iceland lava He isotopic compositions do not result from simple melt depletion histories and consequent removal of U and Th in their mantle sources. Instead their He isotopic compositions reflect mixtures of heterogeneous materials formed at different times with different U and Th concentrations. This hybridization is likely prevalent in all ocean island lavas derived from deep mantle sources.

Inertial core-mantle coupling and libration of Mercury

The internal structure of Mercury is the most puzzling among the terrestrial planets. The space missions MESSENGER and the upcoming BepiColombo as well as ground-based radar measurements will play an important role in constraining our understanding of the structure, formation, and evolution of Mercury. The development of a complete theory of the coupled spin-orbit motion of Mercury within the Solar System is an essential complement to observational data and will improve significantly our knowledge of the planet. Prior work concerning the effect of core-mantle couplings on the rotation of Mercury has assumed that the obliquity of Mercury is equal to zero and that its orbit is Keplerian. This work deals with the Hermean core-mantle interactions in a realistic model of the orbital and rotational motions of Mercury. To this aim, we have used the SONYR model of the Solar System including Mercury’s spin-orbit motion (SONYR is the acronym of Spin-Orbit N-bodY Relativistic model). We studied the dynamical behavior of the rotational motion of Mercury considered as a solid body including either a solid core or a liquid core. The liquid core and the mantle are assumed to be coupled through an inertial torque on the ellipsoidal core-mantle boundary. We determined Mercury’s rotation for a large set of interior structure models of Mercury to be able to identify and to clarify the impact of the core motion on the librations. In this paper, we present a comparative study of the librations resulting from different models of the internal structure. The geophysical models have been calculated for a three-layer planet composed of a solid mantle, a liquid outer core, and a solid inner core. We find that (i) the influence of inertial coupling is of the order of a milliarcsecond for a core ellipticity of the order of 10 −4 ; (ii) the amplitude of the 88-day libration depends essentially on the radius of the core or, equivalently, on the concentration of sulfur in the core; and (iii) the range of amplitude values is 19 arcsec, indicating the possibility to discriminate between models of internal structure by using accurate libration measurements.

Constraints on the coupled thermal evolution of the Earth's core and mantle, the age of the inner core, and the origin of the 186Os/ 188Os “core signal” in plume-derived lavas

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.

186Os– 187Os signatures of pyroxenites and the core-mantle interaction debate

186Os– 187Os signatures of pyroxenites and the core-mantle interaction debate

Deformation-induced metal melt networks in silicates: Implications for core–mantle interactions in planetary bodies

Abstract Previous analyses of texturally equilibrated, hydrostatically annealed olivine rocks containing metal and metal–sulfide melts have determined that these melts do not form interconnected networks at low volume fractions. To characterize the change in connectivity of metal melt in response to deformation, we used optical microscopy and electron microprobe analyses to investigate the microstructures of samples of olivine + gold sheared to shear strains of > 200%. Deformation causes the development of a strong melt-preferred orientation 13–15° to the shear plane and antithetic to the shear direction. This melt-preferred orientation consists of previously isolated pockets of gold melt that have connected into networks extending for many grain lengths, as well as linear “trails” of isolated pockets. Melt was observed to have migrated 125–200 μm in response to pressure gradients induced by serrations in the pistons in contact with the sample, proving that finite permeability exists during deformation well below the critical melt fraction for connectivity. Consideration of the balance between differential and interfacial stresses on melt pockets indicates that melt channels formed in response to deformation can drain rocks to a very small melt fraction in a direction subparallel to the shear direction, stranding less melt than drainage under hydrostatic conditions. Therefore, core formation by percolative flow of non-wetting melt in actively deforming planets and planetesimals may be accomplished more efficiently than previously believed. In addition, core material entrained in the mantle at the core–mantle boundary will be strongly aligned sub-horizontally in regions of horizontal shear and have little to no permeability in the vertical direction, providing a mechanism to trap core material in Dʺ.

消滅核種182Hf，107Pdを用いたコア－マントル相互作用の検証の試み

消滅核種182Hf，107Pdを用いたコア－マントル相互作用の検証の試み

Thallium isotopic evidence for ferromanganese sediments in the mantle source of Hawaiian basalts

Ocean island basalts are generally thought to be the surface expression of mantle plumes, but the nature of the components in the source regions of such mantle plumes is a subject of long-standing debate. The lavas erupted at Hawaii have attracted particular attention, as it has been proposed that coupled 186Os and 187Os anomalies reflect interaction with the Earth's metallic core. It has recently been suggested, however, that such variations could also result from addition of oceanic ferromanganese sediments to the mantle source of these lavas. Here we show that Hawaiian picrites with osmium isotope anomalies also exhibit pronounced thallium isotope variations, which are coupled with caesium/thallium ratios that extend to values much lower than commonly observed for mantle-derived rocks. This correlation cannot be created by admixing of core material, and is best explained by the addition of ferromanganese sediments into the Hawaii mantle source region. However, the lack of correlation between thallium and osmium isotopes and the high thallium/osmium ratios of ferromanganese sediments preclude a sedimentary origin for the osmium isotope anomalies, and leaves core-mantle interaction as a viable explanation for the osmium isotope variations of the Hawaiian picrites.