Evidence from Sardinian basalt geochemistry for recycling of plume heads into the Earth's mantle

Up to 10 per cent of the ocean floor consists of plateaux--regions of unusually thick oceanic crust thought to be formed by the heads of mantle plumes. Given the ubiquitous presence of recycled oceanic crust in the mantle source of hotspot basalts, it follows that plateau material should also be an important mantle constituent. Here we show that the geochemistry of the Pleistocene basalts from Logudoro, Sardinia, is compatible with the remelting of ancient ocean plateau material that has been recycled into the mantle. The Sr, Nd and Hf isotope compositions of these basalts do not show the signature of pelagic sediments. The basalts' low CaO/Al2O3 and Ce/Pb ratios, their unradiogenic 206Pb and 208Pb, and their Sr, Ba, Eu and Pb excesses indicate that their mantle source contains ancient gabbros formed initially by plagioclase accumulation, typical of plateau material. Also, the high Th/U ratios of the mantle source resemble those of plume magmas. Geochemically, the Logudoro basalts resemble those from Pitcairn Island, which contain the controversial EM-1 component that has been interpreted as arising from a mantle source sprinkled with remains of pelagic sediments. We argue, instead, that the EM-1 source from these two localities is essentially free of sedimentary material, the geochemical characteristics of these lavas being better explained by the presence of recycled oceanic plateaux. The storage of plume heads in the deep mantle through time offers a convenient explanation for the persistence of chemical and mineralogical layering in the mantle.     

Os isotopic evidence for a carbonaceous chondritic mantle source for the Nagqu ophiolite from Tibet and its implications

Early-crystallizing chromian spinel(Cr-spinel) in the Nagqu ophiolite has high Os and low Re contents,and it is resistant to alteration during serpentinization,weathering and metamorphism.The chemical composition of primitive magma is preserved in Cr-spinel,which makes it suitable for determining the initial Os-isotope composition of the mantle source.This study presents Cr-spinel Os isotopes and zircon U-Pb ages for cumulate dunite and gabbro,respectively,in the same cumulate section of the ophiolite at Nagqu in Tibet.The results shed light on the formation and evolution of lithospheric mantle.The Nagqu ophiolite is located in the central part of the Bangong-Nujiang suture zone.It is a remnant of the Neotethyan oceanic crust,and contains cumulate dunite and gabbro.Zircon from the gabbro yielded a weighted mean 206 Pb/238 U age of 183.7±1 Ma.Cr-spinel exhibits Os values of 0.2 to 0.3,suggesting that the mantle source for the dunite is similar to that of carbonaceous chondrites.Thus,the Tibetan lithosphere is primarily a relic of Tethyan oceanic lithosphere,which has formed by the transformation of the normal asthenospheric mantle in the Mesozoic.This is the first study to combine the spinel Os isotopes with accurate zircon U-Pb ages to constrain the geochemical characteristics of the mantle source for the ophiolite.     

Evolution of helium isotopes in the Earth's mantle

Degassing of the Earth's mantle through magmatism results in the irreversible loss of helium to space, and high (3)He/(4)He ratios observed in oceanic basalts have been considered the main evidence for a 'primordial' undegassed deep mantle reservoir. Here we present a new global data compilation of ocean island basalts, representing upwelling 'plumes' from the deep mantle, and show that island groups with the highest primordial signal (high (3)He/(4)He ratios) have striking chemical and isotopic similarities to mid-ocean-ridge basalts. We interpret this as indicating a common history of mantle trace element depletion through magmatism. The high (3)He/(4)He in plumes may thus reflect incomplete degassing of the deep Earth during continent and ocean crust formation. We infer that differences between plumes and the upper-mantle source of ocean-ridge basalts reflect isolation of plume sources from the convecting mantle for approximately 1-2 Gyr. An undegassed, primordial reservoir in the mantle would therefore not be required, thus reconciling a long-standing contradiction in mantle dynamics.     

Melting of the Earth's lithospheric mantle inferred from protactinium-thorium-uranium isotopic data

The processes responsible for the generation of partial melt in the Earth's lithospheric mantle and the movement of this melt to the Earth's surface remain enigmatic, owing to the perceived difficulties in generating large-degree partial melts at depth and in transporting small-degree melts through a static lithosphere. Here we present a method of placing constraints on melting in the lithospheric mantle using 231Pa-235U data obtained from continental basalts in the southwestern United States and Mexico. Combined with 230Th-238U data, the 231Pa-235U data allow us to constrain the source mineralogy and thus the depth of melting of these basalts. Our analysis indicates that it is possible to transport small melt fractions--of the order of 0.1%--through the lithosphere, as might result from the coalescence of melt by compaction owing to melting-induced deformation. The large observed 231Pa excesses require that the timescale of melt generation and transport within the lithosphere is small compared to the half-life of 231Pa (approximately 32.7 kyr). The 231Pa-230Th data also constrain the thorium and uranium distribution coefficients for clinopyroxene in the source regions of these basalts to be within 2% of one another, indicating that in this setting 230Th excesses are not expected during melting at depths shallower than 85 km.     

Evidence for tritium production in the Earth's interior

We have made a new investigation on the vertical profiles of tritium and helium isotopes in Lakes Van and Nemrut(Eastern Turkey),using experimental data from the reference by Kipfer et al.for study of long-term vertical mixing and deep water renewal in Lake Van.Lakes Van and Nemrut are crater lakes.Lake Nemrut is at the western border of Lake Van.The 3He and 4He are injected at the bottom of Lakes Van and Nemrut,and the both helium isotopes are confirmed from the mantle source.From 3H(tritium) data in Lakes Van and Nemrut,we have observed "3H anomaly" at the vertical profile of 3H concentrations in Lake Nemrut.The 3H concentration at the lake bottom is 10% higher than at the surface.The difference of 3H concentrations between surface and bottom is about 3.7±1.2 TU.This excess 3H should be injected from the lake bottom.An investigation on the origin of the injected tritium has been made.The results show the conventional origins are excluded,such as residence of precipitation tritium from nuclear testing in the early 1950s-1960s and tritium from known nuclear reactions.Based on the correlation of excess 3H with 3He and heat flow in Lake Nemrut,we infer that the 3He and 3H might be all from the mantle source,and produced by the supposed natural-nuclear-fusion,which might occur in an environment rich in water(H) and(U Th) at high temperature and high pressure in the deep Earth.Detection of tritium in the Earth's interior is a key evidence for exploration of natural nuclear fusion in the deep Earth.Based on the published data,we have found that the excess 3He and 3H injected at the bottom of Lake Laacher(Germany) were also released from the mantle source.The present work will be helpful to the further study of mechanism of natural nuclear fusion in the Earth's interior.     

Redox freezing and melting in the Earth's deep mantle resulting from carbon-iron redox coupling

Very low seismic velocity anomalies in the Earth's mantle may reflect small amounts of melt present in the peridotite matrix, and the onset of melting in the Earth's upper mantle is likely to be triggered by the presence of small amounts of carbonate. Such carbonates stem from subducted oceanic lithosphere in part buried to depths below the 660-kilometre discontinuity and remixed into the mantle. Here we demonstrate that carbonate-induced melting may occur in deeply subducted lithosphere at near-adiabatic temperatures in the Earth's transition zone and lower mantle. We show experimentally that these carbonatite melts are unstable when infiltrating ambient mantle and are reduced to immobile diamond when recycled at depths greater than ～250?kilometres, where mantle redox conditions are determined by the presence of an (Fe,Ni) metal phase. This 'redox freezing' process leads to diamond-enriched mantle domains in which the Fe(0), resulting from Fe(2+) disproportionation in perovskites and garnet, is consumed but the Fe(3+) preserved. When such carbon-enriched mantle heterogeneities become part of the upwelling mantle, diamond will inevitably react with the Fe(3+) leading to true carbonatite redox melting at ～660 and ～250 kilometres depth to form deep-seated melts in the Earth's mantle.     

Experimental evidence for the existence of iron-rich metal in the Earth's lower mantle

The oxidation state recorded by rocks from the Earth's upper mantle can be calculated from measurements of the distribution of Fe3+ and Fe2+ between the constituent minerals. The capacity for minerals to incorporate Fe3+ may also be a significant factor controlling the oxidation state of the mantle, and high-pressure experimental measurements of this property might provide important insights into the redox state of the more inaccessible deeper mantle. Here we show experimentally that the Fe3+ content of aluminous silicate perovskite, the dominant lower-mantle mineral, is independent of oxygen fugacity. High levels of Fe3+ are present in perovskite even when it is in chemical equilibrium with metallic iron. Silicate perovskite in the lower mantle will, therefore, have an Fe3+/total Fe ratio of at least 0.6, resulting in a whole-rock ratio of over ten times that of the upper mantle. Consequently, the lower mantle must either be enriched in Fe3+ or Fe3+ must form by the disproportionation of Fe2+ to produce Fe3+ plus iron metal. We argue that the lower mantle contains approximately 1 wt% of a metallic iron-rich alloy. The mantle's oxidation state and siderophile element budget have probably been influenced by the presence of this alloy.     

Neon isotopes constrain convection and volatile origin in the Earth's mantle

Identifying the origin of primordial volatiles in the Earth's mantle provides a critical test between models that advocate magma-ocean equilibration with an early massive solar-nebula atmosphere and those that require subduction of volatiles implanted in late accreting material. Here we show that neon isotopes in the convecting mantle, resolved in magmatic CO2 well gases, are consistent with a volatile source related to solar corpuscular irradiation of accreting material. This contrasts with recent results that indicated a solar-nebula origin for neon in mantle plume material, which is thought to be sampling the deep mantle. Neon isotope heterogeneity in different mantle sources suggests that models in which the plume source supplies the convecting mantle with its volatile inventory require revision. Although higher than accepted noble gas concentrations in the convecting mantle may reduce the need for a deep mantle volatile flux, any such flux must be dominated by the neon (and helium) isotopic signature of late accreting material.     

Spin crossover and iron-rich silicate melt in the Earth's deep mantle

A melt has greater volume than a silicate solid of the same composition. But this difference diminishes at high pressure, and the possibility that a melt sufficiently enriched in the heavy element iron might then become more dense than solids at the pressures in the interior of the Earth (and other terrestrial bodies) has long been a source of considerable speculation. The occurrence of such dense silicate melts in the Earth's lowermost mantle would carry important consequences for its physical and chemical evolution and could provide a unifying model for explaining a variety of observed features in the core-mantle boundary region. Recent theoretical calculations combined with estimates of iron partitioning between (Mg,Fe)SiO(3) perovskite and melt at shallower mantle conditions suggest that melt is more dense than solids at pressures in the Earth's deepest mantle, consistent with analysis of shockwave experiments. Here we extend measurements of iron partitioning over the entire mantle pressure range, and find a precipitous change at pressures greater than ～76?GPa, resulting in strong iron enrichment in melts. Additional X-ray emission spectroscopy measurements on (Mg(0.95)Fe(0.05))SiO(3) glass indicate a spin collapse around 70?GPa, suggesting that the observed change in iron partitioning could be explained by a spin crossover of iron (from high-spin to low-spin) in silicate melt. These results imply that (Mg,Fe)SiO(3) liquid becomes more dense than coexisting solid at ～1,800?km depth in the lower mantle. Soon after the Earth's formation, the heat dissipated by accretion and internal differentiation could have produced a dense melt layer up to ～1,000?km in thickness underneath the solid mantle. We also infer that (Mg,Fe)SiO(3) perovskite is on the liquidus at deep mantle conditions, and predict that fractional crystallization of dense magma would have evolved towards an iron-rich and silicon-poor composition, consistent with seismic inferences of structures in the core-mantle boundary region.     

146Sm-142Nd evidence from Isua metamorphosed sediments for early differentiation of the Earth's mantle

Application of the 147Sm-143Nd chronometer (half-life of 106 Gyr) suggests that large-scale differentiation of the Earth's mantle may have occurred during the first few hundred million years of its history. However, the signature of mantle depletion found in early Archaean rocks is often obscured by uncertainties resulting from open-system behaviour of the rocks during later high-grade metamorphic events. Hence, although strong hints exist regarding the presence of differentiated silicate reservoirs before 4.0 Gyr ago, both the nature and age of early mantle differentiation processes remain largely speculative. Here we apply short-lived 146Sm-142Nd chronometry (half-life of 103 Myr) to early Archaean rocks using ultraprecise measurement of Nd isotope ratios. The analysed samples are well-preserved metamorphosed sedimentary rocks from the 3.7-3.8-Gyr Isua greenstone belt of West Greenland. Our coupled isotopic calculations, combined with an initial epsilon 143Nd value from ref. 6, constrain the mean age of mantle differentiation to 4,460 +/- 115 Myr. This early Sm/Nd fractionation probably reflects differentiation of the Earth's mantle during the final stage of terrestrial accretion.     

Re-Os isotopic evidence for long-lived heterogeneity and equilibration processes in the Earth's upper mantle

The geochemical composition of the Earth's upper mantle is thought to reflect 4.5 billion years of melt extraction, as well as the recycling of crustal materials. The fractionation of rhenium and osmium during partial melting in the upper mantle makes the Re-Os isotopic system well suited for tracing the extraction of melt and recycling of the resulting mid-ocean-ridge basalt. Here we report osmium isotope compositions of more than 700 osmium-rich platinum-group element alloys derived from the upper mantle. The osmium isotopic data form a wide, essentially gaussian distribution, demonstrating that, with respect to Re-Os isotope systematics, the upper mantle is extremely heterogeneous. As depleted and enriched domains can apparently remain unequilibrated on a timescale of billions of years, effective equilibration seems to require high degrees of partial melting, such as occur under mid-ocean ridges or in back-arc settings, where percolating melts enhance the mobility of both osmium and rhenium. We infer that the gaussian shape of the osmium isotope distribution is the signature of a random mixing process between depleted and enriched domains, resulting from a 'plum pudding' distribution in the upper mantle, rather than from individual melt depletion events.     

Grain boundaries as reservoirs of incompatible elements in the Earth's mantle

The concentrations and locations of elements that strongly partition into the fluid phase in rocks provide essential constraints on geochemical and geodynamical processes in Earth's interior. A fundamental question remains, however, as to where these incompatible elements reside before formation of the fluid phase. Here we show that partitioning of calcium between the grain interiors and grain boundaries of olivine in natural and synthetic olivine-rich aggregates follows a thermodynamic model for equilibrium grain-boundary segregation. The model predicts that grain boundaries can be the primary storage sites for elements with large ionic radius--that is, incompatible elements in the Earth's mantle. This observation provides a mechanism for the selective extraction of these elements and gives a framework for interpreting geochemical signatures in mantle rocks.     

Polybaric partial melting in the continental mantle: Evidence from mantle xenoliths from Qilin Guangdong Province

The extremely low Ti content (160–245 μg/g) in clinopyroxene in some spinel peridotites from Qilin, South China is indicative of high degree of partial melting, inconsistent with their relatively high clinopyroxene modes (7.4%–12.4%). These clinopyroxenes show fractionated HREE patterns ((Gd/Yb)_n<0.2), suggesting the involvement of garnet in the melting regime. These REE patterns can be modeled as residues of 22%–23% fractional melting from a primitive mantle, first in garnet stability field (12%) then continuing in spinel stability field (10%–11%) after breakdown of garnet to pyroxenes and spinel. Such a polybaric melting suggests the lithospheric thinning and rapid mantle upwelling in south China during the Cenozoic. This is consistent with the dominant MORB-OIB isotopic signature and high thermal gradient of the lithospheric mantle in this region, and supports the contention that the formation of South China Sea basin is related to southward migration of continental lithosphere extension, rather than passive back-arc basin.     

Stability of magnesite and its high-pressure form in the lowermost mantle

Carbonates are important constituents of marine sediments and play a fundamental role in the recycling of carbon into the Earth's deep interior via subduction of oceanic crust and sediments. Study of the stability of carbonates under high pressure and temperature is thus important for modelling the carbon budget in the entire Earth system. Such studies, however, have rarely been performed under appropriate lower-mantle conditions and no experimental data exist at pressures greater than 80 GPa (refs 3-6). Here we report an in situ X-ray diffraction study of the stability of magnesite (MgCO(3)), which is the major component of subducted carbonates, at pressure and temperature conditions approaching those of the core-mantle boundary. We found that magnesite transforms to an unknown form at pressures above approximately 115 GPa and temperatures of 2,100-2,200 K (depths of approximately 2,600 km) without any dissociation, suggesting that magnesite and its high-pressure form may be the major hosts for carbon throughout most parts of the Earth's lower mantle.     

Effect of phase transitions on compressional-wave velocities in the Earth's mantle

The velocities of seismic waves in the Earth are governed by the response of the constituent mineral assemblage to perturbations in pressure and stress. The effective bulk modulus is significantly lowered if the pressure of the seismic wave drives a volume-reducing phase transformation. A comparison between the amount of time required by phase transitions to reach equilibrium and the sampling period thus becomes crucial in defining the softening and attenuation of compressional waves within such a two-phase zone. These phenomena are difficult to assess experimentally, however, because data at conditions appropriate to the Earth's deep interior are required. Here we present synchrotron-based experimental data that demonstrate softening of the bulk modulus within the two-phase loop of olivine-ringwoodite on a timescale of 100 s. If the amplitude of the pressure perturbation and the grain size are scaled to those expected in the Earth, the compressional-wave velocities within the discontinuities at 410, 520 and, possibly, 660 km are likely to be significantly lower than otherwise expected. The generalization of these observations to aluminium-controlled phase transitions raises the possibility of large velocity perturbations throughout the upper 1,000 km of the mantle.     

Carbon solubility in olivine and the mode of carbon storage in the Earth's mantle

The total amount of carbon in the atmosphere, oceans and other near-surface reservoirs is thought to be negligible compared to that stored in the Earth's mantle. Although the mode of carbon storage in the mantle is largely unknown, observations of microbubbles on dislocations in minerals from mantle xenoliths has led to the suggestion that carbon may be soluble in silicates at high pressure. Here we report measurements of carbon solubility in olivine, the major constituent of the upper mantle, at pressures up to 3.5 GPa. We have found that, contrary to previous expectations, carbon solubility in olivine is exceedingly low--of the order of 0.1 to 1 parts per million by weight. Together with similar data for pyroxenes, garnet and spinel, we interpret this to imply that most carbon must be present as a separate phase in the deeper parts of the upper mantle, probably as a carbonate phase. Large-scale volcanic eruptions tapping such a carbonate-bearing mantle reservoir might therefore rapidly transfer large amounts of carbon dioxide into the atmosphere, consistent with models that link global mass extinctions to flood basalt eruptions via a sudden increase in atmospheric carbon dioxide levels.     

Pressure-induced changes in the compression mechanism of aluminous perovskite in the Earth's mantle

Although aluminium is the fifth most abundant element in the Earth's mantle, its effect on the physical properties of perovskite, the main mineral phase in the lower mantle, has largely been ignored. It is becoming clear, however, that many properties of MgSiO3 perovskites are remarkably sensitive to small amounts of aluminium. In particular, perovskite with only 5 wt% Al2O3 has a bulk modulus 10% lower than that of the pure magnesian end-member. The increased compressibility may be due to the high concentrations of oxygen vacancies required to balance the charge of the aluminium; if so, this would have important consequences for the mantle, as aluminous perovskites could be weaker, have lower seismic velocities and be hosts for water. To test whether oxygen vacancies exist in aluminous perovskites, I have calculated the compressibility of end-member defect-bearing perovskites using ab initio methods. The results show that perovskites with oxygen vacancies do have significantly greater compressibilities than those without such vacancies. But the results also suggest that oxygen vacancies become unfavourable at high pressures, in which case only the physical properties of the shallow lower mantle would be affected by aluminium-with the deeper mantle retaining properties similar to those of aluminium-free perovskite.     

Development of anisotropic structure in the Earth's lower mantle by solid-state convection

Seismological observations reveal highly anisotropic patches at the bottom of the Earth's lower mantle, whereas the bulk of the mantle has been observed to be largely isotropic. These patches have been interpreted to correspond to areas where subduction has taken place in the past or to areas where mantle plumes are upwelling, but the underlying cause for the anisotropy is unknown-both shape-preferred orientation of elastically heterogeneous materials and lattice-preferred orientation of a homogeneous material have been proposed. Both of these mechanisms imply that large-strain deformation occurs within the anisotropic regions, but the geodynamic implications of the mechanisms differ. Shape-preferred orientation would imply the presence of large elastic (and hence chemical) heterogeneity whereas lattice-preferred orientation requires deformation at high stresses. Here we show, on the basis of numerical modelling incorporating mineral physics of elasticity and development of lattice-preferred orientation, that slab deformation in the deep lower mantle can account for the presence of strong anisotropy in the circum-Pacific region. In this model-where development of the mineral fabric (the alignment of mineral grains) is caused solely by solid-state deformation of chemically homogeneous mantle material-anisotropy is caused by large-strain deformation at high stresses, due to the collision of subducted slabs with the core-mantle boundary.     

The elasticity of the MgSiO3 post-perovskite phase in the Earth's lowermost mantle

MgSiO3 perovskite has been assumed to be the dominant component of the Earth's lower mantle, although this phase alone cannot explain the discontinuity in seismic velocities observed 200-300 km above the core-mantle boundary (the D" discontinuity) or the polarization anisotropy observed in the lowermost mantle. Experimental and theoretical studies that have attempted to attribute these phenomena to a phase transition in the perovskite phase have tended to simply confirm the stability of the perovskite phase. However, recent in situ X-ray diffraction measurements have revealed a transition to a 'post-perovskite' phase above 125 GPa and 2,500 K--conditions close to those at the D" discontinuity. Here we show the results of first-principles calculations of the structure, stability and elasticity of both phases at zero temperature. We find that the post-perovskite phase becomes the stable phase above 98 GPa, and may be responsible for the observed seismic discontinuity and anisotropy in the lowermost mantle. Although our ground-state calculations of the unit cell do not include the effects of temperature and minor elements, they do provide a consistent explanation for a number of properties of the D" layer.     

Vapour undersaturation in primitive mid-ocean-ridge basalt and the volatile content of Earth's upper mantle

The analysis of volatiles in magmatic systems can be used to constrain the volatile content of the Earth's mantle and the influence that magmatic degassing has on the chemistry of the oceans and the atmosphere. But most volatile elements have very low solubilities in magmas at atmospheric pressure, and therefore virtually all erupted lavas are degassed and do not retain their primary volatile signatures. Here we report the undersaturated pre-eruptive volatile content for a suite of mid-ocean-ridge basalts from the Siqueiros intra-transform spreading centre. The undersaturation leads to correlations between volatiles and refractory trace elements that provide new constraints on volatile abundances and their behaviour in the upper mantle. Our data generate improved limits on the abundances of carbon dioxide, water, fluorine, sulphur and chlorine in the source of normal mid-ocean-ridge basalt. The incompatible behaviour of carbon dioxide, together with the CO(2)/Nb and CO(2)/Cl ratios, permit estimates of primitive carbon dioxide and chlorine to be made for degassed and chlorine-contaminated mid-ocean-ridge basalt magmas, and hence constrain degassing and contamination histories of mid-ocean ridges.