Earth's Primitive Mantle Research Articles

Pyrrhotite–silicate melt partitioning of rhenium and the deep rhenium cycle in subduction zones

Abstract The Re-Os isotopic system serves as an important tracer of recycled crust in Earth's deep mantle because of the large Re/Os ratios and time-integrated enrichment of radiogenic Os in Earth's crust. However, the Re distribution in Earth's known reservoirs is mass imbalanced, and the behavior of Re during subduction remains little understood. We performed laboratory experiments to determine the partition coefficients of Re between pyrrhotite and silicate melt () at 950–1080°C, 1–3 GPa, and oxygen fugacities (in log units relative to the fayalite-magnetite-quartz [FMQ] buffer) of FMQ–1.3 to FMQ +2. The obtained values are 200–25,000, which increase with decreasing oxygen fugacity and the total iron content (FeOtot) of silicate melt but decrease with increasing temperature or decreasing pressure. Applying to constrain the behavior of Re during slab melting demonstrates that slab melts contribute minimal Re to the sub-arc mantle, with most Re dissolved in sulfides subducted into Earth's deep mantle. Deep storage of recycled oceanic basalts and sediments can explain the mass imbalance of Re in Earth's primitive mantle, depleted mantle, and crust.

Burying Earth's Primitive Mantle in the Slab Graveyard

AbstractThe evolution of mantle composition can be viewed as a process of destruction whereby the initial chemical state is overprinted and reworked with time. Analyses of ocean island basalts reveals that some portion of the mantle has survived this process, retaining a chemically “primitive” signature. A question that remains is how this primitive signature has survived four and a half billion years of vigorous convection. We hypothesize that some of Earth's primitive mantle is buried within a slab graveyard at the core‐mantle boundary. We explore this possibility using high‐resolution finite element models of mantle convection, in which oceanic lithosphere is produced at zones of plate spreading and subducted at zones of plate convergence. Upon subduction, dense oceanic crust sinks to the base of the mantle and gradually accumulates to form broad, robust thermochemical piles. Sinking oceanic crust entrains the surrounding mantle whose composition is predominantly primitive early in the model's evolution. As a result, thermochemical piles are initially supplied with relatively high concentrations of primitive material—summing up to ∼30% their total mass. The dense oceanic crust dominating the piles resists efficient mixing and preserves the primitive material that it is intermingled with. The significance of this process is shown to be proportional the rate of mantle processing through time and the excess density of oceanic crust at mantle pressures and temperatures. Unlike other theories for the survival of Earth's primitive mantle, this one does not require the early Earth to have large‐scale domains of anomalously high density and/or viscosity.

Zinc isotope composition of the Earth and its behaviour during planetary accretion

The terrestrial planets are depleted in volatile elements with respect to chondritic meteorites, their possible building blocks. However, the timing, extent and origin of volatile depletion is debated. Zinc is a moderately volatile element (MVE), whose stable isotopic composition can distinguish when and where this depletion took place. Here, we report data for 40 ultramafic rocks comprising pristine upper mantle peridotites from the Balmuccia orogenic lherzolite massif and Archean komatiites that together define the Zn isotope composition of the Earth's primitive mantle. Peridotites and komatiites are shown to have indistinguishable Zn isotopic compositions of δ66Zn=+0.16±0.06‰ (2SD), (with δ66Zn the per mille deviation of 66Zn/64Zn from the JMC-Lyon standard), implying a constant Zn isotope composition for the silicate Earth since 3.5Ga. After accounting for Zn sequestration during core formation, the Earth falls on the volatile-depleted end of a carbonaceous chondrite array in δ66Zn-Zn/Mg space, implying Earth avoided modification of its MVE budgets during late accretion (e.g. during a giant impact), in contrast to the Moon. The Moon deviates from the chondritic array in a manner consistent with evaporative loss of Zn, where its δ66Zn co-varies with Mn/Na, implying post-nebular volatile loss is more pronounced on smaller bodies. Should the giant impact deliver the Earth's volatile complement of Pb and Ag, it cannot account for the budget of lithophile MVEs (e.g. Zn, Rb, Mn), whose abundances reflect those of Earth's nebular building blocks. The Earth initially accreted from material that experienced chemical- and mass-dependent isotopic fractionation akin to carbonaceous chondrites, though volatile depletion was more pronounced on Earth.

The composition of mantle plumes and the deep Earth

Determining the composition and geochemical diversity of Earth's deep mantle and subsequent ascending mantle plumes is vital so that we can better understand how the Earth's primitive mantle reservoirs initially formed and how they have evolved over the last 4.6 billion years. Further data on the composition of mantle plumes, which generate voluminous eruptions on the planet's surface, are also essential to fully understand the evolution of the Earth's hydrosphere and atmosphere with links to surface environmental changes that may have led to mass extinction events. Here we present new major and trace element and Sr–Nd–Pb–Hf isotope data on basalts from Curacao, part of the Caribbean large igneous province. From these and literature data, we calculate combined major and trace element compositions for the mantle plumes that generated the Caribbean and Ontong Java large igneous provinces and use mass balance to determine the composition of the Earth's lower mantle. Incompatible element and isotope results indicate that mantle plumes have broadly distinctive depleted and enriched compositions that, in addition to the numerous mantle reservoirs already proposed in the literature, represent large planetary-scale geochemical heterogeneity in the Earth's deep mantle that are similar to non-chondritic Bulk Silicate Earth compositions.

Water, fluorine, and sulfur concentrations in the lunar mantle

The concentrations of volatile elements in the moon have important implications for the formation of the earth–moon system. There is currently a debate regarding the water content of the lunar mantle: Authors studying H2O in lunar pyroclastic glass beads and in olivine-hosted melt inclusions in such pyroclastic samples and in plagioclase crystals in lunar highland anorthosites infer hundreds of ppm H2O in the lunar mantle. In contrast, authors studying Zn/Fe ratios infer that the H2O concentration in the lunar mantle is ≤1 ppm, and they argue that the glassy lunar basalts are a local anomaly. We contribute to a resolution of the debate by a broader examination of the concentrations of H2O and other volatile components in olivine-hosted melt inclusions in a wider range of lunar mare basalts, including crystalline melt inclusions that are homogenized by melting in the laboratory. We find that F, Cl, and S concentrations in various lunar melt inclusions (including those in glassy lunar basalts) are similar to one another, and previously studied glassy lunar basalts are not a local anomaly in terms of these volatile concentrations. Furthermore, we estimate the pre-degassing H2O/Ce, F/Nd, and S/Dy ratios of mare basaltic magmas to be at least 64, 4.0 and 100 respectively. These ratios are lower than those of primitive earth mantle by a factor of 3, 5, and 4 respectively. The depletion factors of these volatile elements relative to the earth's primitive mantle do not correlate strongly with volatility or bonding energy, and indeed they are roughly constant and similar to those of other volatile elements such as Li, Cs, Rb and K. This approximate constancy of volatile depletion in the moon relative to the earth can be explained by assuming that both the earth and the moon acquired volatiles from a similar source or by a similar mechanism but the earth was more efficient in acquiring the volatiles. We estimate the H2O, F and S concentrations in the primitive lunar mantle source to be at least 110, 5.3, and 70 ppm, respectively – similar to or slightly lower than those in terrestrial MORB mantle.

Lunar tungsten isotopic evidence for the late veneer.

According to the most widely accepted theory of lunar origin, a giant impact on the Earth led to the formation of the Moon, and also initiated the final stage of the formation of the Earth's core. Core formation should have removed the highly siderophile elements (HSE) from Earth's primitive mantle (that is, the bulk silicate Earth), yet HSE abundances are higher than expected. One explanation for this overabundance is that a 'late veneer' of primitive material was added to the bulk silicate Earth after the core formed. To test this hypothesis, tungsten isotopes are useful for two reasons: first, because the late veneer material had a different (182)W/(184)W ratio to that of the bulk silicate Earth, and second, proportionally more material was added to the Earth than to the Moon. Thus, if a late veneer did occur, the bulk silicate Earth and the Moon must have different (182)W/(184)W ratios. Moreover, the Moon-forming impact would also have created (182)W differences because the mantle and core material of the impactor with distinct (182)W/(184)W would have mixed with the proto-Earth during the giant impact. However the (182)W/(184)W of the Moon has not been determined precisely enough to identify signatures of a late veneer or the giant impact. Here, using more-precise measurement techniques, we show that the Moon exhibits a (182)W excess of 27±4 parts per million over the present-day bulk silicate Earth. This excess is consistent with the expected (182)W difference resulting from a late veneer with a total mass and composition inferred from HSE systematics. Thus, our data independently show that HSE abundances in the bulk silicate Earth were established after the giant impact and core formation, as predicted by the late veneer hypothesis. But, unexpectedly, we find that before the late veneer, no (182)W anomaly existed between the bulk silicate Earth and the Moon, even though one should have arisen through the giant impact. The origin of the homogeneous (182)W of the pre-late-veneer bulk silicate Earth and the Moon is enigmatic and constitutes a challenge to current models of lunar origin.

N-isotope composition of the primitive mantle compared to diamonds

The nitrogen isotopic composition of the Earth's primitive mantle is controversial. Chromium-isotopic ratios of various terrestrial minerals and rocks, and chondritic meteorites are consistent with the silicate Earth being a mixture of enstatite and carbonaceous chondrites. From their relative proportions and N-isotope compositions we estimate that the bulk primitive mantle δ15N is −7±3‰. The negative value, as also evidenced by mantle-derived oceanic basalts and diamonds, is an intrinsic long-term feature of Earth's mantle. Some enstatite chondrite-like δ15N values down to −24‰ measured in very rare diamonds could be interpreted as a heterogeneous mantle. δ15N values in oceanic island basalts derived from the deep mantle have three components: deep mantle of ~−9‰ consistent with estimate, recycled sediments of about 15‰, and atmospheric N incorporated from groundwater and/or subducted atmospheric N (Mohapatra and Murty, 2000a; Chem. Geol. 164, 305–320). Some enriched δ15N values in MORB and OIB result from degassing fractionation. Shift of the upper mantle from an initial −7‰ to −5‰ by the Neoarchean can be explained by a combination of sediment recycling through subduction and upper mantle magma degassing processes.

Chemical composition of Earth's primitive mantle and its variance: 1. Method and results

We present a new statistical method to construct a model for the chemical composition of Earth's primitive mantle along with its variance. Earth's primitive mantle is located on the melting trend exhibited by the global compilation of mantle peridotites, using cosmochemical constraints on the relative abundances of refractory lithophile elements (RLE). This so‐called pyrolite approach involves the least amount of assumptions, thereby being probably most satisfactory compared to other approaches. Its previous implementations, however, suffer from questionable statistical treatment of noisy geochemical data, leaving the uncertainty of model composition poorly quantified. In order to properly take into account how scatters in peridotite data affect this geochemical inference, we combine the following statistical techniques: (1) modeling a nonlinear melting trend in the multidimensional compositional space through the principal component analysis, (2) determining the primitive mantle composition on the melting trend by simultaneously imposing all of cosmochemical constraints with least squares, and (3) mapping scatters in original data into the variance of the final model through the bootstrap resampling technique. Whereas our model is similar to previous models in terms of Mg, Si, and Fe abundances, the RLE contents are at ∼2.16 ± 0.37 times the CI chondrite concentration, which is lower than most of previous estimates. The new model is depleted by >20% in a number of incompatible elements including heat‐producing elements, U, Th, and K, and this depleted nature is further amplified (up to 60%) in terms of predicted composition for the present‐day mantle.

Chemical composition of Earth's primitive mantle and its variance: 2. Implications for global geodynamics

The global budgets of argon and heat‐producing elements have traditionally been used to argue for layered‐mantle convection because they require a large fraction of Earth's mantle to remain isolated from mantle convection. We revise these mass balance arguments using our new composition model of the primitive mantle and show that the global budgets of argon, heat‐producing elements, and rare earth elements are consistent with Earth's mantle almost entirely composed of the mid‐ocean ridge basalt source mantle, supporting the notion of whole mantle convection. Combined with a recent theory on the thermal evolution of Earth, our revised thermal budget implies inefficient mixing and processing in the past, explaining the survival of long‐lived geochemical heterogeneities in convecting mantle.

“New” lunar meteorites: Implications for composition of the global lunar surface, lunar crust, and the bulk Moon

Abstract— New data for lunar meteorites and a synthesis of literature data have significant implications for the interpretation of global Th data and for the Moon's bulk composition. As presently calibrated (Prettyman et al. 2002), the Lunar Prospector gamma‐ray data imply that the average global surface Th = 1.58 μg/g. However, that calibration yields implausibly high concentrations for the three most Th‐poor documented sampling sites, it extrapolates to a nonzero Lunar Prospector Th, ˜0.7 μg/g, at zero sample Th, and it results in a misfit toward too‐high Th when compared with the global regolith Th spectrum as constrained using mainly lunaite regolith breccias. Another problem is manifested by Th versus K systematics. Ground truth data plot consistently to the high‐Th/K side of the Prospector data trend, offset by a factor of 1.2. A new calibration is proposed that represents a compromise between the Th levels indicated by ground truth constraints and the Prettyman et al. (2002) calibration. Conservatively assuming that the Th versus K issue is mostly a K problem, the average global surface Th is estimated to be ˜1.35 μg/g. The Moon's remarkable global asymmetry in KREEP abundance is even more pronounced than previously supposed. The surface Th concentration ratio between the hemisphere antipodal to the Procellarum basin and the hemisphere centered on Procellarum is reduced to 0.24 in the new calibration. This extreme disparity is most simply interpreted as a consequence of Procellarum's origin at a time when the Moon still contained at least a thin residual layer of a global magma ocean. Allowing for diminution of Th with depth, the extrapolated bulk crustal Th is ˜0.73 μg/g. Further extrapolation to bulk Moon Th yields ˜0.07 μg/g, which is nearly identical to the consensus estimate for Earth's primitive mantle. Assuming chondritic proportionality among refractory lithophile elements implies Al2O3 of approximately 3.8 wt%. The Moon's bulk mantle mg ratio is only weakly constrained by seismic and mare‐basaltic data. KREEP‐and mare‐free lunaite regolith samples, other thoroughly polymict lunar meteorites, and a few KREEP‐free Apollo highland samples manifest a remarkable anticorrelation on a plot of Al2O3 versus mg. This trend implies that an important component of the Moon is highly magnesian. The bulk Moon is inferred to have an Earth‐like oxide mg ratio of ˜87–88 mol%. The close resemblance between the bulk Moon and Earth's primitive mantle extends to moderately volatile elements, most clearly Mn. Unless major proportions of Cr and V are sequestered into deep mantle spinel, remarkably Earth‐like depletions (versus chondrites) are also inferred for bulk Moon Cr and V.

Lithium isotopic composition of Central American volcanic arc lavas: implications for modification of subarc mantle by slab-derived fluids: correction

Our previously reported lithium isotope data for the least enriched members of the Central American Volcanic Arc were in error due to problems in ion extraction from high MgO rocks. This paper presents reanalyses of the entire suite of the CAVA lavas after a systematic investigation of the elution behavior of Li as a function of the rock composition. The most significant correction pertains to the mantle end members of the Nicaragua and Costa Rica series, which now display MORB-like δ 6Li values (−4.5‰) and not much lighter (+1‰ to +3‰) as previously reported. Also revised is the composition of a peridotite from Zabargad Island (Red Sea), considered to represent undepleted upper mantle, whose corrected δ 6Li also resembles MORB. These new data therefore remove the speculation that Earth's primitive mantle has extremely light Li isotopic composition and that the mantle beneath CAVA contains such isotopically light domains. The δ 6Li range for the arc segment from Costa Rica to Guatemala has now been reset to −4.5‰ to −6.4‰. Despite the narrower range, Li isotopic values remain well correlated with other subduction-related properties, including LILE and fluid-mobile elements. Model calculations show that the isotopic compositions of the lavas are consistent with small additions of slab-derived fluids to the enriched and depleted domains of the subarc mantle.

Niobium and tantalum in carbonaceous chondrites: Constraints on the solar system and primitive mantle niobium/tantalum, zirconium/niobium, and niobium/uranium ratio

Abstract— We have determined Nb, Y, and Zr abundances in the carbonaceous chondrites Orgueil (CI), Murray (CM2), Murchison (CM2), Allende (CV3), and Karoonda (CK4), and in the eucrites, Pasamonte and Juvinas, by a recently developed spark source mass spectrometric technique using multiple ion counting (MIC‐SSMS). The abundance of Ta was determined in the same meteorites by radiochemical neutron activation analysis (RNAA). Precision of the MIC‐SSMS and RNAA techniques is ∼3% and ≤ 5%, respectively.The new abundances for CI chondrites are: Nb = 0.247, Ta = 0.0142, Zr = 3.86, Y = 1.56 μg/g; or 0.699, 0.0202, 11.2, and 4.64 atoms/106 Si atoms, respectively. The values agree with earlier compilations, but they are a factor of 2 more precise than earlier analyses. Trace element concentrations in the CM, CV, and CK chondrites are higher than in the CI chondrite Orgueil by about 37, 86, and 120%, respectively, in agreement with the variable absolute contents of refractory lithophile elements in different groups of carbonaceous chondrites.Of particular interest are the chondritic Nb/Ta, Zr/Nb, and Nb/U ratios, because these ratios are important tools for interpreting the chemical evolution of planetary bodies. We obtained Nb/Ta = 17.4 ± 0.5 for the carbonaceous chondrites and the Juvinas‐type eucrites investigated. Though this value is similar to previous estimates, it is much more precise. The same is true for Zr/Nb (15.5 ± 0.2) and Zr/Y (2.32 ± 0.12). In combination with recently published MIC‐SSMS U data for carbonaceous chondrites, we obtained a chondritic Nb/U ratio of 29 ± 2.Because Nb, Ta, Zr, Y, and U are refractory lithophile elements and presumably partitioned into the silicate phase of the Earth during core formation, the elemental ratios may also be used to constrain evolution of the Earth's primitive mantle and, with the more precise determinations fractionation of Nb and Ta during magmatic processes and mantle‐crust interactions, can now be interpreted with greater confidence.

Indium and tin in basalts, sulfides, and the mantle

The geochemistry of In and Sn are poorly understood, in part, because of difficulties in obtaining accurate concentrations for these elements in geological materials. Furthermore, In Sn ratios in sulfides could be sufficiently high to facilitate the use of 115In 115Sn geochronology, if the separation and precise measurement techniques were available. In this paper we describe methods for the separation of In and Sn from silicates and sulfides. Indium can be measured by thermal ionization mass spectrometry (TIMS) at very high sensitivity ( > 13%). However, its mass fractionation is difficult to correct reliably. Tin is more difficult to measure by TIMS because of its higher ionization potential. Both elements can be measured effectively using the new technique of MC-ICPMS, since the ionization efficiency is extremely high, molecular interferences are negligible, and mass fractionation in spiked In and Sn can be corrected by monitoring the mass bias in admixed Pd and Sb, respectively. Using these techniques, it is demonstrated that In and Sn concentrations can be measured reliably for silicates and sulfides. Indium and Sn data for international silicate rock standards are in excellent agreement with recommended values. The Sn Sm ratios determined for ocean island basalts (0113) are within the same range as those recently reported, where Sn was measured by spark source mass spectrometry. Indium is very uniform in OIB and behaves as a slightly incompatible trace element, comparable in bulk distribution coefficient to the heavy rare earths or Y. In Y in 0113 is very uniform, averaging 0.0028 ± 0.0005 (Iσ), but is weakly related with Pb Ce , implying that these ratios may be partly controlled by sulfide at small degrees of partial melting. The similarity in average In Y between OIB, N-MORB (0.0025) and the continental crust (0.0025), together with the similarity in Sn Sm in MORB, OIB, and continental crust contrasts with chalcophile/lithophile and siderophile/lithophile element ratios such as Pb Ce and W Ba , which are high in the continental crust because of decoupling in the subduction environment. The overall behavior of both In and Sn within the silicate Earth is dominated by lithophile affinity. The primitive mantle is estimated to have In Y = 0.003 ±0.001, both higher and lower than previous estimates and corresponding to an In concentration of 14 ppb. Ignoring any In that may have been partitioned into the core, the corresponding total Earth concentration of >10 ppb corresponds to <85% depletion relative to CI chondrites. This is less depleted than anticipated by at least a factor of 2, given the supposed volatility of In based on assumed condensation temperatures and depletions in volatile lithophile elements. There is no evidence that In has been segregated into the Earth's core. This can be explained if, during the earlier stages of accretion, under reducing conditions, In was too volatile to be transferred into the core. During the later stages of accretion, conditions may have been relatively oxidising such that In behaved as a lithophile element with higher condensation temperature rather than as a volatile chalcophile element. Hence, the In Y ratio of Earth's primitive mantle may be representative of the mixture of volatile depleted and undepleted material that accreted in the inner solar system. SNC meteorites have a similar range of In Y to the silicate Earth, suggesting Mars accreted from a similar mixture of material already depleted in In, and presumably other moderately volatile elements. In contrast, the In Y ratio in lunar basalts ranges through four orders of magnitude from silicate Earth values in lunar soils to extremely In-depleted compositions. This is unlikely to be caused by heterogeneous distributions of extreme In depletion on the Moon as a result of volatile depletion. Rather, the more reducing conditions appear to result in In behaving as a relatively compatible trace element during lunar melting and differentiation. Although their behavior on Earth is strongly lithophile, In and Sn are sometimes enriched in sulfides and In Sn can be sufficiently high in some sphalerite, chalcopyite, and tetrahedrite that the predicted 115Sn excess caused by decay of 115In in ancient sulfide deposits should be measurable with MC-ICPMS.

The influence of pressure and temperature on the metal-silicate partition coefficients of nickel and cobalt in a model C1 chondrite and implications for metal segregation in a deep magma ocean

The metal-silicate partition coefficients of Ni and Co in a model C1 chondrite were determined at pressures ranging from 1.2 to 12.0 GPa and temperatures between 2123 and 2750 K. At 5.0 GPa and 2500 K, the effect of variable oxygen contents on the partitioning of Ni and Co was also investigated. Graphite was chosen as the sample container. Carbon is an integral part of the system because about 5 wt% C dissolved in the metal liquid. The slopes obtained for the relation between log D M∗ met/sil (M = Ni or Co) and log f O 2 (ΔIW) are compatible with Ni and Co, having valencies of 2 in the silicate melt. This allows the partitioning of Ni and Co as a function of pressure and temperature to be expressed in terms of exchange partition coefficients K DM/Fe met/sil = ( X M met/X MO sil)( X FeO sil/ X Fe met), which are essentially independent of oxygen fugacity. In the pressure-temperature interval investigated, increasing temperature and pressure both result in lowering K DNi/Fe met/sil and K D Co/Fe met¡l . In the context of reasonable geothermal gradients, the pressure effect is significantly more important, especially for Ni. The results obtained agree with recent studies on the partitioning of Ni and Co at high pressure and high temperature in that they all record much lower metal-silicate partition coefficients for these elements than those obtained at lower temperature and atmospheric pressure. The significant differences in the experimental conditions between these recent high-pressure/high-temperature studies can probably explain the small but significant discrepancies observed in the results. The abundance of Ni and Co that would be observed in the primitive mantle for pressures of equilibration up to 12.0 GPa were calculated assuming simple core-mantle equilibrium in a magma ocean. The resulting abundances of Ni and Co do not reach the values estimated for Earth's primitive mantle. Nevertheless, the significant decrease in the partition coefficients of Ni and Co with increasing pressure indicates that experiments at higher pressures (> 12.0 GPa) are necessary to get a more critical evaluation of high-pressure/high-temperature core-mantle equilibrium models.

The influence of pressure and temperature on the metal-silicate partition coefficients of nickel and cobalt in a model C1 chondrite and implications for metal segregation in a deep magma ocean

The metal-silicate partition coefficients of Ni and Co in a model C1 chondrite were determined at pressures ranging from 1.2 to 12.0 GPa and temperatures between 2123 and 2750 K. At 5.0 GPa and 2500 K, the effect of variable oxygen contents on the partitioning of Ni and Co was also investigated. Graphite was chosen as the sample container. Carbon is an integral part of the system because about 5 wt% C dissolved in the metal liquid. The slopes obtained for the relation between log D M∗ met/sil (M = Ni or Co) and log f O 2 (ΔIW) are compatible with Ni and Co, having valencies of 2 in the silicate melt. This allows the partitioning of Ni and Co as a function of pressure and temperature to be expressed in terms of exchange partition coefficients K D M/Fe met/sil = ( X M met/X MO sil )( X FeO sil / X Fe met ), which are essentially independent of oxygen fugacity. In the pressure-temperature interval investigated, increasing temperature and pressure both result in lowering K D Ni/Fe met/sil and K D Co/Fe met¡l . In the context of reasonable geothermal gradients, the pressure effect is significantly more important, especially for Ni. The results obtained agree with recent studies on the partitioning of Ni and Co at high pressure and high temperature in that they all record much lower metal-silicate partition coefficients for these elements than those obtained at lower temperature and atmospheric pressure. The significant differences in the experimental conditions between these recent high-pressure/high-temperature studies can probably explain the small but significant discrepancies observed in the results. The abundance of Ni and Co that would be observed in the primitive mantle for pressures of equilibration up to 12.0 GPa were calculated assuming simple core-mantle equilibrium in a magma ocean. The resulting abundances of Ni and Co do not reach the values estimated for Earth's primitive mantle. Nevertheless, the significant decrease in the partition coefficients of Ni and Co with increasing pressure indicates that experiments at higher pressures (> 12.0 GPa) are necessary to get a more critical evaluation of high-pressure/high-temperature core-mantle equilibrium models.

The Composition and Early Evolution of Earth

A weighted average composition is suggestd as the "Best Bulk Silicate Earth" (BBSE). BBSE, thought to represent Earth's primitive mantle (modern mantle + crust), is a concept based on analyses of oceanic theoleiites and continental flood basalts, oceanic and continental alkali basalts, ultramafic ophiolites, Archean komatiites, and spinel- and garnet-bearing xenoliths from basalts and kimberlites. BBSE shows the combined effects of core formation and volatility, presumably including condensation and sublimation in the solar nebula and volatile loss during the accretion and early evolution of Earth. The abundances of volatile lithophile elements show a volatility trend (VT) when plotted against condensation temperature. The slope of VT suggests that Earth accreted matter that had condensed over a wide range of temperatures, consistent with planet-formation scenarios of G. W. Wetherill (1988, Mercury, pp. 670-691, Univ. of Arizona Press, Tucson; 1990, Annu. Rev. Earth Planet. Sci. 18, 205-256). Using VT, BBSE, and long-standing seismic constraints, we infer the composition of the entire Earth and its core. The bulk Earth's VT closely resembles the average composition of H-chondrites (H3-H6). The 10 most abundant elements in the core nominally include ∼85.55 mass% Fe, 5.18% O, 4.88% Ni, 2.69% S, 0.45% Cr, 0.41% Mn, 0.35% P, 0.22% Co, 0.07% Cl, and possibly 0.02% K. The core may contain most of Earth's heavy alkali and halogen elements, in addition to chalcophiles and siderophiles; alternatively, depletions of alkali halide components in BBSE may have resulted from impact blow-off of an early saline ocean. It is unclear whether the core contains a significant fraction of Earth's K and Pb. Possible partitioning of substantial quantities of O, Rb, and Cs into the core and the partitioning behavior of fourth-period transition metals suggests that metal-silicate equilibration occurred at characteristic pressures of several hundred kilobars or more; if correct, this implies core formation during or after accretion of Earth rather than during the planetesimal stage. The inferred partitioning between core and primitive mantle reflects periodic elemental properties, such as covalent radius; elements that possess covalent radii similar to those of iron are most strongly partitioned into the core. The periodic nature of inferred core/mantle partioning conflicts with models involving simple addition of siderophile elements and volatiles in a late chondritic veneer; however, this aspect is in accord with conceptual models involving an approach toward equilibrium partitioning during core formation. In one model, equilibrium melting and segregation of metal occurs continuously over a range of interior pressures and temperatures as fresh material accretes and mixes into Earth. BBSE may reflect retention of a small amount of the last dregs of metal whose composition was controlled by this "dynamic equilibrium."

Tin in mantle-derived rocks: Constraints on Earth evolution

We have analyzed tin together with other trace elements in a variety of mantle-derived rocks by isotope dilution-spark source mass spectrometry. Tin concentrations in oceanic basalts (mid-ocean ridge basalts, or MORB; oceanic island basalts, or OIB; island arc volcanics, or IAV) vary from 0.4 to 6 ppm. Tin is a moderately siderophile element which behaves like the moderately incompatible and lithophile element Sm during igneous processes in the mantle. The Sn Sm ratio is therefore similar in various reservoirs of the silicate portion of the Earth at the present day. To investigate possible secular variations of siderophile/chalcophile element abundances, we have also analyzed komatiites and basalts with ages ranging from Archean to Tertiary. Rocks having ages up to 3.4 Ga yield Sn Sm ratios comparable to those of the oceanic basalts. These results indicate that core growth during the last 3.4 Ga was negligibly small. In addition, the Sn Sm ratios in different mantle reservoirs are identical within the uncertainty of the data scatter. This indicates that tin was homogenized within the mantle after core formation. We conclude that the value of Sn Sm = 0.32 found in mantlederived rocks is representative of the primitive mantle because the estimated continental-crustal abundances of Sn and Sm yield a similar Sn Sm ratio. This results in Sn = 0.12 ± 0.01 ppm for the primitive mantle abundance, in agreement with that determined from fertile peridotite xenoliths (0.15 ± 0.02 ppm). To determine an absolute depletion factor for the volatile siderophile element tin in the primitive mantle, we have also analyzed four carbonaceous chondrites, including the CI chondrites Orgueil and Ivuna. We obtain a CI-value of tin of 1.62 ± 0.03 ppm. Tin is depleted in the Earth's primitive mantle by a factor of 33 ± 3; the volatility corrected depletion factor is similar to other moderately siderophile elements ( Newsom, 1990).

Mantle heterogeneities: Isotopic and trace element characterisation of a major OIB source component interpreted as representative of oceanic crust segments recycled into the mantle through its historical evolution

We have analyzed tin together with other trace elements in a variety of mantle-derived rocks by isotope dilution-spark source mass spectrometry. Tin concentrations in oceanic basalts (mid-ocean ridge basalts, or MORB; oceanic island basalts, or OIB; island arc volcanics, or IAV) vary from 0.4 to 6 ppm. Tin is a moderately siderophile element which behaves like the moderately incompatible and lithophile element Sm during igneous processes in the mantle. The SnSmratio is therefore similar in various reservoirs of the silicate portion of the Earth at the present day.To investigate possible secular variations of siderophile/chalcophile element abundances, we have also analyzed komatiites and basalts with ages ranging from Archean to Tertiary. Rocks having ages up to 3.4 Ga yield SnSm ratios comparable to those of the oceanic basalts. These results indicate that core growth during the last 3.4 Ga was negligibly small. In addition, the SnSm ratios in different mantle reservoirs are identical within the uncertainty of the data scatter. This indicates that tin was homogenized within the mantle after core formation. We conclude that the value of SnSm = 0.32found in mantlederived rocks is representative of the primitive mantle because the estimated continental-crustal abundances of Sn and Sm yield a similar SnSm ratio. This results in Sn = 0.12 ± 0.01 ppm for the primitive mantle abundance, in agreement with that determined from fertile peridotite xenoliths (0.15 ± 0.02 ppm).To determine an absolute depletion factor for the volatile siderophile element tin in the primitive mantle, we have also analyzed four carbonaceous chondrites, including the CI chondrites Orgueil and Ivuna. We obtain a CI-value of tin of 1.62 ± 0.03 ppm. Tin is depleted in the Earth's primitive mantle by a factor of 33 ± 3; the volatility corrected depletion factor is similar to other moderately siderophile elements (Newsom, 1990).

Siderophile and chalcophile element abundances in oceanic basalts, Pb isotope evolution and growth of the Earth's core

We have investigated the hypothesis that mantle Pb isotope ratios reflect continued extraction of Pb into the Earth's core over geologic time. The Pb, Sr and Nd isotopic compositions, and the abundance of siderophile and chalcophile elements (W, Mo and Pb) and incompatible lithophile elements have been determined for a suite of ocean island and mid-ocean ridge basalt samples. Over the observed range in Pb isotopic compositions for oceanic rocks, we found no systematic variation of siderophile or chalcophile element abundances relative to abundances of similarly incompatible, but lithophile, elements. The high sensitivity of theMo/Pr ratio to segregation of Fe-metal or S-rich metallic liquid (sulfide) and the observed constantMo/Pr ratio rules out the core formation model as an explanation for the Pb paradox. The mantle and crust have the sameMo/Pr and the sameW/Ba ratios, suggesting that these ratios reflect the ratio in the Earth's primitive mantle. Our data also indicate that thePb/Ce ratio of the mantle is essentially constant, but the presentPb/Ce ratio in the mantle (≅ 0.036) is too low to represent the primitive value (≅ 0.1) derived from Pb isotope systematics. HigherPb/Ce ratios in the crust balance the lowPb/Ce of the mantle, and crust and mantle appear to sum to a reasonable terrestrialPb/Ce ratio. The constancy of thePb/Ce ratio in a wide variety of oceanic magma types from diverse mantle reservoirs indicates this ratio is not fractionated by magmatic processes. This suggests crust formation must have involved non-magmatic as well as magmatic processes. Hydrothermal activity at mid-ocean ridges may result in significant non-magmatic transport of Pb from mantle to crust and of U from crust to mantle, producing a higherU/Pb ratio in the mantle than in the total crust. We suggest that the lower crust is highly depleted in U and has unradiogenic Pb isotope ratios which balance the radiogenic Pb of upper crust and upper mantle. The differences between thePb/Ce ratio in sediments, this ratio in primitive mantle, and the observed ratio in oceanic basalts preclude both sediment recycling and mixing of primitive and depleted reservoirs from being important sources of chemical heterogeneities in the mantle.

Chemical composition and origin of the earth's primitive mantle

An attempt has been made to estimate the chemical composition of the earth's primitive mantle by a critical evaluation of data derived from ultramafic mantle samples and partial melting model calculations for mafic and ultramafic magmas of various ages. Compatible (Al, Ca, Si, Mg, Fe) and moderately incompatible (Ti, Zr, heavy and middle rare earth) elements in basaltic magma sources have not changed significantly since the early Archaean (~3.5 Byr). Estimated abundances for refractory lithophile elements (such as Al, Ca, Ti, Zr, Y, Se, REE etc.) in the primitive mantle are about 2.0 times ordinary chondrites (~ 1.1 times Cl chondrites relative to Mg). Highly incompatible volatile elements (K, Rb, Cs, Tl, Pb etc.) are depleted in the mantle throughout geological time. Abundances of Fe, Ni and Co are obtained on the basis of values for ultramafic nodules and model calculations using komatiites of various ages. The results show little (⩽ 20%?) dispersion and there is no obvious secular variation since 3.5 Byr. Noble metals show similar effects. These data permit constraints to be placed on the timing of core formation. The estimated elemental abundances for the primitive mantle are normalized to Cl chondrites relative to Mg and plotted against the solar condensation temperature at 10 −4 atm. Above 700 K there are two parallel trends which are defined by lithophile elements (Al, Ca, REE, Ti, Mg, Si, Cr, Mn, Na, K, Rb, F, Zn etc.) and siderophile elements (W, Ni, Co, P, As, Ag, Sb and Ge) respectively. The depletion factor for the siderophile trend relative to the lithophile trend is about 0.085. Within each trend there is a continuous depletion towards lower temperature. A third trend is defined by noble metals (Ir, Os, Re, Pd, Pt and Au) with a depletion factor of about 0.003 relative to Cl chondrites. These trends are interpreted in terms of core-mantle differentiation and volatility-controlled processes operating before and during earth accretion.