板块下的构造及地幔动力学

最新的全球地幔地震层析资料揭示了岩石圈板片可以俯冲到核幔边界,超地幔羽可以从核幔边界上升到地壳上部形成热点。在大陆板块汇聚边界,地幔地震层析图像不仅显示了岩石圈板片的超深俯冲,还保存了拆沉的岩石圈“化石”残片的重要信息。从地幔深部所获取的新资料为全地幔“单层对流“的新模式提供了依据。在介绍上述全球构造研究新动向的基础上,本文强调了研究岩石圈板块必须了解板块下的构造,探索岩石圈板块的驱动力应该从“岩石圈动力学”升华到“地幔动力学”,并提出了大陆板块汇聚边界地幔动力学研究的新思考。     

Permo–Triassic intraplate magmatism and rifting in Eurasia: implications for mantle plumes and mantle dynamics

At the transition from the Permian to the Triassic, Eurasia was the site of voluminous flood-basalt extrusion and rifting. Major flood-basalt provinces occur in the Tunguska, Taymyr, Kuznetsk, Verkhoyansk–Vilyuy and Pechora areas, as well as in the South Chinese Emeishen area. Contemporaneous rift systems developed in the West Siberian, South Kara Sea and Pyasina–Khatanga areas, on the Scythian platform and in the West European and Arctic–North Atlantic domain. At the Permo–Triassic transition, major extensional stresses affected apparently Eurasia, and possibly also Pangea, as evidenced by the development of new rift systems. Contemporaneous flood-basalt activity, inducing a global environmental crisis, is interpreted as related to the impingement of major mantle plumes on the base of the Eurasian lithosphere. Moreover, the Permo–Triassic transition coincided with a period of regional uplift and erosion and a low-stand in sea level. Permo–Triassic rifting and mantle plume activity occurred together with a major reorganization of plate boundaries and plate kinematics that marked the transition from the assembly of Pangea to its break-up. This plate reorganization was possibly associated with a reorganization of the global mantle convection system. On the base of the geological record, we recognize short-lived and long-lived plumes with a duration of magmatic activity of some 10–20 million years and 100–150 million years, respectively. The Permo–Triassic Siberian and Emeishan flood-basalt provinces are good examples of “short-lived” plumes, which contrast with such “long lived” plumes as those of Iceland and Hawaii. The global record indicates that mantle plume activity occurred episodically. Purely empirical considerations indicate that times of major mantle plume activity are associated with periods of global mantle convection reorganization during which thermally driven mantle convection is not fully able to facilitate the necessary heat transfer from the core of the Earth to its surface. In this respect, we distinguish between two geodynamically different scenarios for major plume activity. The major Permo–Triassic plume event followed the assembly Pangea and the detachment of deep-seated subduction slabs from the lithosphere. The Early–Middle Cretaceous major plume event, as well as the terminal–Cretaceous–Paleocene plume event, followed a sharp acceleration of global sea-floor spreading rates and the insertion of new subduction zone slabs deep into the mantle. We conclude that global plate kinematics, driven by mantle convection, have a bearing on the development of major mantle plumes and, to a degree, also on the pattern of related flood-basalt magmatism.     

Phase transformations and inherited lattice preferred orientations: implications for seismic properties

In phase transitions via either the martensitic (diffusionless shear) or nucleation and growth mechanism a specific orientation relationship may exist between the two phases. In cases where the orientation relationship is known, the lattice preferred orientation (LPO) inherited by the new phase may be calculated from the LPO of the old phase. The method of calculation is presented in a form suitable for the spherical harmonic method of texture analysis using the orientation distribution function (ODF). Examples are presented for the -β-quartz, calcite-aragonite, orthopyroxene-clinopyroxene and olivine-spinel transformations.

The seismic properties of the transformed and untransformed phases are calculated from the ODF and the single crystal elastic constants. In particular the -β quartz transformation is considered in detail. The quartz polycrystal is very anisotropic in the alpha field (V_p anisotropy coefficient, A = 8.1%) and almost isotropic in the β-field (A = 2.1%). The transition is accompanied by V_p velocity increase of 0.6 km/s. In the other example discussed, olivine-β-spinel, there is also a decrease in V_p anisotropy coefficient from 11.1% (olivine) to 4.0% (β-spinel). The estimate of the volume fraction of olivine at the 400 km discontinuity (associated with this phase transition) is shown to depend on the direction of wave propagation.     

Phase relations of a natural MARID composition and implications for MARID genesis,lithospheric melting and mantle metasomatism

Melting experiments were performed on a natural mica-amphibole-rutile-ilmenite-clinopyroxene (MARID) sample from the Kaapvaal mantle lithosphere (AJE137) at 20 to 35 kbar and 800 to 1450°C. A solidus was determined at 1260°C and 30 kbar above which phlogopite, clinopyroxene and olivine were stable with an alkali-rich silicate melt. Olivine is the only crystallizing phase just below the liquidus of the AJE137 bulk composition and K-richterite was only stable in the subsolidus region ( 1100°C at 30 kbar). These results are consistent with previous studies in more simple systems. In experiments with 10 wt% added water the solidus was depressed by ca. 300°C and K-richterite was stabilized above this solidus. MARIDs represent a potential lowtemperature component in the lithospheric mantle beneath the Kaapvaal Craton of southern Africa. The addition of > 10 wt% water (with less than a 120°C rise of temperature above the geotherm) to this mantle region would create conditions for the melting of this component. This may then be incorporated in any continental flood basalt parent magma that traverse this lithospheric mantle. The derivation of MARIDs from a silicate melt of their bulk composition, even if water saturated, is considered unlikely as such small degree melts could not sustain the elevated liquidus temperatures required (> 1200°C at 30 kbar) in a cold (< 800°C at 30 kbar) mantle lithosphere. MARID xenoliths may be produced by the interaction of an alkali-rich fluid with a peridotite or as the residue to a group II kimberlitic parent magma that has undergone fractionation of olivine and the exsolution of a carbonatite component.     

Apatite in the mantle: implications for metasomatic processes and high heat production in Phanerozoic mantle

The abundance of apatite in Phanerozoic mantle may be greatly underestimated. This study shows that apatite has a widespread occurrence in Phanerozoic lithospheric mantle and can be divided into two geochemically distinct types using halogen content, presence or absence of structural CO₂, Sr and trace element (especially U, Th, and light rare earth) ratios and abundances, and association with either metasomatised mantle wall-rock peridotites (Apatite A) or high-pressure magmatic crystallisation products (Apatite B). Apatite A is inferred to result from metasomatism by CO₂- and H₂O-rich fluids derived from a primitive mantle source region, while Apatite B compositions are consistent with crystallisation from magmas within the carbonate–silicate compositional spectrum.

The presence of significant apatite in the lithospheric mantle is important not only for the geochemical budget but also for assessing heat production and heat flow in the mantle. The measured U and Th contents of mantle apatite average 60 and 200 ppm, respectively and 0.5% apatite would dominate heat production. Metasomatised mantle may also contain amphibole and mica with K₂O and clinopyroxene with detectable U and Th. In lithospheric mantle with a thickness of 70 km, this abundance of apatite would result in mantle heat flow contribution of about 12 mW/m², a significant proportion of the total “normal” mantle heat flow of about 18 mW/m².     

Phase relations in the phlogopite-apatite system at 20 kbar; implications for the role of fluorine in mantle melting

Phase relations have been determined at 20 kb in the simple, Fe-, Ti-free systems hydroxyphlogopite-hydroxyapatite and hydroxyfluorphlogopite-hydroxyfluorapatite in order to determine distribution of fluorine between phlogopite, apatite and melt under mantle conditions. No excess H₂O was present in the hydroxyphlogopite-hydroxyapatite system and the F/(OH) ratio was unity in the F-bearing system. Both systems are pseudobinary and contain forsterite at phlogopite-rich compositions. In the F-absent system, the minimum melting occurs at 1225°C and Phl₈₅Ap₁₅, whereas in the F-bearing system this temperature is 1260°C and Phl₆₆Ap₃₄. Phlogopite in the F-absent system has lower Al than in the F-bearing system with both showing Si+Mg=^[IV]Al+^[VI]Al as the principal substitution. Increase in CaO in forsterite increases with increasing apatite in the bulk composition and is more pronounced in the F-absent system. Distribution of fluorine between phlogopite and liquid and apatite and liquid shows that D _F ^(Phl/glass) ranges from 2–1.25 depending on temperature and bulk composition, whereas the D _F ^(Apat/glass) is about unity. These results suggest that fluorine will tend to remain in the solid phases rather than the melt during partial melting in the mantle. Hence the enrichment of fluorine in ultrapotassic magmas and its role in their evolution are constrained.     

Multiscale seismic tomography and mantle dynamics

In this article we first introduce the methodology of multiscale seismic tomography and the way to interpret the obtained tomographic images, and then review the significant recent results of multiscale seismic tomography with emphasis on mantle plumes and subducting slabs. Global and regional tomography shows that most of the slab materials under Western Pacific and East Asia are stagnant in the mantle transition zone before finally collapsing down to the core–mantle boundary as a result of large gravitational instability from phase transitions. Local and teleseismic tomography studies have imaged clearly the subducting slabs and arc magma chambers in the upper-mantle wedge, indicating that geodynamic systems associated with arc magmatism and back-arc spreading are related to deep processes, such as convective circulation in the mantle wedge and dehydration reactions of the subducting slab. Because most hotspots are located in poorly instrumented continental and oceanic regions, 3-D crust and upper-mantle structure is determined for only a few hotspots such as Iceland, Yellowstone and Eifel which are covered by seismic networks, and plume-like slow anomalies are revealed under those hotspots. Global tomography has revealed deep mantle plumes under the major hotspots such as Hawaii, Iceland, Kerguelen, South Pacific and Africa. Strong lateral heterogeneities are revealed at the bottom of the mantle, which are associated with the deeply subducted slabs and the birth of mantle plumes. A thorough understanding of the deep Earth structure will only be achieved by a combination of more effective seismic imaging techniques and dense coverage of global seismic networks, particularly in the oceans.     

Phase transformations in condensate pool development

The phase behavior of a gas-condensate mixture in the pool depletion process and the influence of porosity on the amount of extracted concentrate and gas and on the gas phase composition have been studied experimentally. A possible approach to the differential condensation process has been determined. The substantial influence of the zone of condensate micronuclei formation in a porous medium on the parameters of phase condensation and on the technological parameters of condensate pool development have been established for the first time.     

Possible implications of modal mineralogy for melting in mantle lherzolites

The melting reaction at the solidus of mantle peridotite is commonly peritectic in nature, with liquid and one or more solid phases produced upon melting. In some situations, one of the phases participating on the reactant side of the reaction is present in low abundance. This article explores the possible effects of the low abundance of a reactant phase on the melting behavior of mantle peridotite.For example, spinel lherzolite begins to melt via the peritectic reaction, clinopyroxene + orthopyroxene + spinel = olivine + liquid in the ∼1- to 2-GPa pressure range. In natural spinel lherzolites, spinel is a modally minor mineral and may be infrequently in contact with both clinopyroxene and orthopyroxene. If these mutual contacts are insufficient to generate an interconnected melt, then significant melting may not occur until a combination of minerals that are modally abundant and in contact begin to melt. This scenario could have implications for the physical process of melting and for the timing of formation of an interconnected melt network and separation of the melt from the residue.To begin to investigate this possibility, the spatial relationships between the constituent minerals in two fertile spinel lherzolites were determined by elemental mapping with the electron microprobe. Olivine, orthopyroxene, and clinopyroxene are of similar size, whereas the spinel was smaller and interstitial. Spinel and clinopyroxene are frequently in contact, but mutual contacts of spinel, clinopyroxene, and orthopyroxene are rare. Because of the changes in modal mineralogy anticipated for these lherzolites with increasing temperature, these mutual contacts will be even less common at the solidus. Therefore, an interconnected, potentially extractable, melt may not occur by the solidus spinel + orthopyroxene + clinopyroxene melting reaction.     

Stirring geochemistry in mantle convection models with stiff plates and slabs

Numerical models of mantle convection are presented that readily yield midocean ridge basalt (MORB) and oceanic island basalt (OIB) ages equaling or exceeding the apparent ∼1.8-Ga lead isotopic ages of trace-element heterogeneities in the mantle. These models feature high-viscosity surface plates and subducting lithosphere, and higher viscosities in the lower mantle. The formation and subduction of oceanic crust are simulated by means of tracers that represent a basaltic component. The models are run at the full mantle Rayleigh number and take account of faster mantle overturning and deeper melting in the past. More than 97% of the mantle is processed in these models. Including the expected excess density of former oceanic crust readily accounts for the depletion of MORB source relative to OIB sources. A novel finding is of gravitational settling of dense tracers within the low-viscosity upper mantle, as well as at the base of the mantle. The models suggest as well that the seismological observation of a change in tomographic character in the deep mantle might be explained without the need to postulate a separate layer in the deep mantle. These results expand the range of models with the potential to reconcile geochemical and geophysical observations of the mantle.     

Nd-isotopes in selected mantle-derived rocks and minerals and their implications for mantle evolution

The Sm-Nd systematics in a variety of mantle-derived samples including kimberlites, alnoite, carbonatite, pyroxene and amphibole inclusions in alkali basalts and xenolithic eclogites, granulites and a pyroxene megacryst in kimberlites are reported. The additional data on kimberlites strengthen our earlier conclusion that kimberlites are derived from a relatively undifferentiated chondritic mantle source. This conclusion is based on the observation that the _Nd values of most of the kimberlites are near zero. In contrast with the kimberlites, their garnet lherzolite inclusions show both time-averaged Nd enrichment and depletion with respect to Sm. Separated clinopyroxenes in eclogite xenoliths from the Roberts Victor kimberlite pipe show both positive and negative _Nd values suggesting different genetic history. A whole rock lower crustal scapolite granulite xenolith from the Matsoku kimberlite pipe shows a negative _Nd value of -4.2, possibly representative of the base of the crust in Lesotho. It appears that all inclusions, mafic and ultramafic, in kimberlites are unrelated to their kimberlite host.The above data and additional Sm-Nd data on xenoliths in alkali basalts, alpine peridotite and alnoite-carbonatites are used to construct a model for the upper 200 km of the earth's mantle — both oceanic and continental. The essential feature of this model is the increasing degree of fertility of the mantle with depth. The kimberlite's source at depths below 200 km in the subcontinental mantle is the most primitive in this model, and this primitive layer is also extended to the suboceanic mantle. However, it is clear from the Nd-isotopic data in the xenoliths of the continental kimberlites that above 200 km the continental mantle is distinctly different from their suboceanic counterpart.     

Chemical transformations of n-hexane and cyclohexane under the upper mantle conditions

Alkanes are an important part of petroleum,the stability of alkanes under extreme conditions is of great significance to explore the origin of petroleum and the carbon cycle in the deep Earth.Here,we performed Raman and infrared(IR)spectroscopy studies of n-hexane and cyclohexane under high pressure up to~42 GPa at room temperature(RT)and high temperature(HT).n-Hexane and cyclohexane undergo several phase transitions at RT around 1.8,8.5,18 GPa and 1.1,2.1,4.6,13,30 GPa,respectively,without any chemical reaction.By using resistive heating combined with diamond anvil cell at pressure up to 20 GPa and temperature up to 1000 K,both n-hexane and cyclohexane decompose to hydrogenated graphitic carbon and n-hexane exhibits higher stability than cyclohexane.Our results indicate that hydrocarbons tend to dehydrogenate in the upper mantle,and the extension of carbon chains may lead to the formation of some unsaturated compounds and eventually transfer into graphitic products.     

The trapped fluid phase in upper mantle xenoliths from Victoria,Australia: implications for mantle metasomatism

Mantle-derived xenoliths of spinel lherzolite, spinel pyroxenite, garnet pyroxenite and wehrlite from Bullenmerri and Gnotuk maars, southwestern Victoria, Australia contain up to 3 vol.% of fluids trapped at high pressures. The fluid-filled cavities range in size from fluid inclusions (1–100 m) up to vugs 11/2 cm across, lined with euhedral high-pressure phases. The larger cavities form an integral part of the mosaic microstructure. Microthermometry and Raman laser microprobe analysis show that the fluids are dominantly CO₂. Small isolated inclusions may have densities 1.19 g/cm³, but most inclusions show microstructural evidence of partial decrepitation during eruption, and these have lower fluid densities. Mass-spectrometric analysis of gases released by crushing or heating shows the presence of He, N₂, Ar, H₂S, CO_s and SO₂ in small quantities; these may explain the small freezing-point depressions observed in some inclusions. Petrographic, SEM and microprobe studies show that the trapped fluids have reacted with the cavity walls (in clinopyroxene grains) to produce secondary amphiboles and carbonates. The trapped CO₂ thus represents only a small residual proportion of an original volatile phase, which has undergone at least two stages of modification — first by equilibration with spinel lherzolite to form amphibole (±mica±apatite), then by limited reaction with the walls of the fluid inclusions. The inferred original fluid was a CO₂-H₂O mixture, with significant contents of (at least) Cl and sulfur species. Generation of this fluid phase in the garnet-peridotite stability field, followed by its migration to the spinel peridotite stability field, would provide an efficient mechanism for metasomatic enrichment of the upper mantle in LIL elements. This migration could involve either a volatile flux or transport in small volumes of silicate melt that crystallize in the spinel peridotite field. These observations suggest that some portions of the subcontinental upper mantle contain large reservoirs of free fluid CO₂, which may be liberated during episodes of rifting or magmatism, to induce granulite-facies metamorphism of the lower crust.     

Nitrogen recycling in subducted mantle rocks and implications for the global nitrogen cycle

The nitrogen concentrations [N] and isotopic compositions of ultramafic mantle rocks that represent various dehydration stages and metamorphic conditions during the subduction cycle were investigated to assess the role of such rocks in deep-Earth N cycling. The samples analyzed record low-grade serpentinization on the seafloor and/or in the forearc wedge (low-grade serpentinites from Monte Nero/Italy and Erro Tobbio/Italy) and two successive stages of metamorphic dehydration at increasing pressures and temperatures (high-pressure (HP) serpentinites from Erro Tobbio/Italy and chlorite harzburgites from Cerro del Almirez/Spain) to allow for the determination of dehydration effects in ultramafic rocks on the N budget. In low-grade serpentinites, δ¹⁵N_air values (?3.8 to +3.5 ‰) and [N] (1.3–4.5 μg/g) are elevated compared to the pristine depleted MORB mantle (δ¹⁵N_air ~ ?5 ‰, [N] = 0.27 ± 0.16 μg/g), indicating input from sedimentary organic sources, at the outer rise during slab bending and/or in the forearc mantle wedge during hydration by slab-derived fluids. Both HP serpentinites and chlorite harzburgites have δ¹⁵N_air values and [N] overlapping with low-grade serpentinites, indicating no significant loss of N during metamorphic dehydration and retention of N to depths of 60–70 km. The best estimate for the δ¹⁵N_air of ultramafic rocks recycled into the mantle is +3 ± 2 ‰. The global N subduction input flux in serpentinized oceanic mantle rocks was calculated as 2.3 × 10⁸ mol N₂/year, assuming a thickness of serpentinized slab mantle of 500 m. This is at least one order of magnitude smaller than the N fluxes calculated for sediments and altered oceanic crust. Calculated global input fluxes for a range of representative subducting sections of unmetamorphosed and HP-metamorphosed slabs, all incorporating serpentinized slab mantle, range from 1.1 × 10¹⁰ to 3.9 × 10¹⁰ mol N₂/year. The best estimate for the δ¹⁵N_air of the subducting slab is +4 ± 1 ‰, supporting models that invoke recycling of subducted N in mantle plumes and consistent with general models for the volatile evolution on Earth. Estimates of the efficiency of arc return of subducted N are complicated further by the possibility that mantle wedge hydrated in forearcs, then dragged to beneath volcanic fronts, is capable of conveying significant amounts of N to subarc depths.     

地幔转换带中的水及其地球动力学意义

综述了近20年国际上地幔转换带中水的研究进展。前人研究表明,地球深部的水主要以OH-(hy-droxyl)形式存储在名义上无水矿物(NAMs)中。高温高压实验研究表明,地幔转换带中的主要矿物均具有较高的储水能力,且在转换带的温压条件下,其储水能力随着温度的升高而降低,其中瓦兹利石(β-Ol)和林伍德石(γ-Ol)的储水能力为2%～3%,超硅石榴子石(Mj)的储水能力为0.1%左右,据此估算地幔转换带的储水能力约为1.2%～1.91%,是地表水总量的3.9～6.2倍;而转换带除外的上地幔和下地幔主要矿物的含水量或储水能力均小于0.1%,因此与上、下地幔相比,地幔转换带可能是地幔的主要储水库。尽管地幔转换带具有较强的储水能力,但对地幔转换带的实际含水量还存在干、湿两方面的地质和地球物理证据和争议。地幔转换带中的水会对转换带中一系列的过程产生重要影响,当水含量增加时,橄榄石(Ol)向β-Ol、γ-Ol分解以及超硅石榴石的分解反应分别向低压、高压和低压方向迁移,从而由橄榄石向β-Ol和γ-Ol分解两个相变反应界定的转换带宽度也会增加;水还会使地幔深部的部分熔融温度降低,熔体的密度降低;同时,水的加入可以很好地解释地幔岩"pyrolite"模型在410km不连续面处产生的与地震波测量不相符突变,也可以解决全地幔对流模式所不能解释的地幔成分分层问题。因此,深入研究和探讨转换带中的水对地球深部动力学过程的影响,包括中国东部地区受太平洋板块深俯冲作用的影响,均具有重要的约束和研究意义。     

Phase transformations of enstatite in spherical shock waves

The analysis of available theoretical evaluations and experimental data reveals discrepancies and makes it possible to formulate the goals for the comprehensive study of the behavior of enstatite MgSiO₃ in shock isentropic waves of various scale and intensity. The paper presents the layout and results of an explosion experiment on the compression of an enstatite sphere with spherical shock waves and the subsequent recovery of the experimental material and its examination in discrete zones (along the sphere radius) that were produced by shock waves in the material. The latter were examined with the application of scanning electron microscopy, Raman spectroscopy, and X-ray diffraction analysis. The comparison of the systematic variations in the texture, chemistry, and phase composition of enstatite along the sphere radius with calculated pressure P(R, t) and temperature T(R, t) values led us to the following conclusions: enstatite starts melting on an isentrope upon pressure relief after shock wave compression at ?? _xx ?? 80 GPa and melts on the front of the spherically converging shock wave at ?? _xx ?? 160 GPa and T ?? 6300 K. Our laboratory experiments with shock waves were the world??s first in which enstatite was loaded with spherical converging shock isentropic waves and which provided evidence that shock wave-loaded MgSiO₃ shows certain morphological and mineralogical features never before detected in this mineral loaded with plane shock wave of smaller amplitude and duration. Goals are formulated for the further studying of shock wave-loaded materials, and the necessity is discussed for conducting an explosion experiment with a five to seven times greater spherical system in order to increase the duration of the shock wave loading impulse.     

南海复蘑菇状地幔低速柱结构及其地幔动力学

运用高分辨率天然地震面波层析成像和体波层析成像技术,研究东亚西太平洋地区及全球地幔三维速度结构时发现南海地区地幔存在深达2000km以上的巨型复蘑菇状地幔低速柱,结合地质、地球化学和地球物理相关标志,将复蘑菇状地幔低速柱称为南海复蘑菇状地幔柱。本文在论述南海复蘑菇状地幔柱的地质地球物理特征基础上,将地幔柱划分为柱头、柱体、柱尾、幔枝和热点等部分,建立起地幔柱三维几何结构模型,探讨了复蘑菇状地幔柱在南海海盆扩张过程中的主导作用以及欧亚板块、菲律宾海板块和印度洋板块相互作用对南海演化过程的影响。     

The buffering capacity of lithospheric mantle: implications for diamond formation

Current models for the formation of natural diamond involve either oxidation of a methane-bearing fluid by reaction with oxidized mantle, or reduction of a carbonate-bearing fluid (or melt) by reaction with reduced mantle. Implicit in both models is the ability of the mantle with which the fluid equilibrates to act as an oxidizing or reducing agent, or more simply, to act as a source or sink of O₂. If only redox reactions involving iron are operating, the ability of mantle peridotite to fulfill this role in diamond formation may not be sufficient for either model to be viable. Using the recent experimental recalibration of olivine–orthopyroxene–garnet oxybarometers of Stagno et al. (2013), we re-evaluated the global database of ~200 garnet peridotite samples for which the requisite Fe³⁺/Fe²⁺ data for garnet exist. Relative to the previous calibration of Gudmundsson and Wood (1995), the new calibration yields somewhat more oxidized values of Δlog fO₂ (FMQ), with the divergence increasing from <0.5 units of log fO₂ at ~3 GPa to as much as 1.5 units at 5–6.5 GPa. Globally, there is a range of ~4 log units fO₂ for samples from the diamond stability field at any given pressure. Most samples are sufficiently reduced such that diamond, rather than carbonate, would be stable, and CHO fluids at these conditions would be H₂O-rich (>60 mol%), with CH₄ being the next most abundant species. To ascertain the capacity for mantle peridotite to act as a source or sink of O₂, we developed a new model to calculate the fO₂ for a peridotite at a given P, T, and Fe³⁺/Fe²⁺. The results from this model predict 50 ppm or less O₂ is required to shift a depleted mantle peridotite the observed four log units of fO₂. Coupled with the observed distribution of samples at values of fO₂ intermediate between the most reduced (metal-saturated) and most oxidized (carbonate-saturated) possible values for diamond stability, these results demonstrate that peridotites are very poor sinks or sources of O₂ for possible redox reactions to form diamond. A corollary of the poor redox buffering capacity of cratonic peridotites is that they can be employed as faithful indicators of the redox state of the last metasomatic fluid that passed through them. We propose that diamond formation from CHO fluids is a predictable consequence either of isobaric cooling or of combined cooling and decompression of the fluid as it migrates upward in the lithosphere. This establishes a petrological basis for the observed close connection between subcalcic garnet and diamond: based on high solidus temperatures of harzburgite and dunite effectively precluding dilution of CHO fluids through incipient melts, such highly depleted cratonic peridotites are the preferred locus of diamond formation. Due to a rapid increase in solidus temperature with increasing CH₄ content of the fluid, diamond formation related to reduced CHO fluids may also occur in some cratonic lherzolites.     

Structure, mineralogy and dynamics of the lowermost mantle

The 2004-discovery of the post-perovskite transition initiated a vigorous effort in high-pressure, high-temperature mineralogy and mineral physics, seismology and geodynamics aimed at an improved understanding of the structure and dynamics of the D"-zone. The phase transitions in basaltic and peridotitic lithologies under pT-conditions of the lowermost mantle can explain a series of previously enigmatic seismic discontinuities. Some of the other seismic properties of the lowermost mantle are also consistent with the changes in physical properties related to the perovskite (pv) to post-perovskite (ppv) transition. After more than 25 years of seismic tomography, the lowermost mantle structure involving the sub-Pacific and sub-African Large Low Shear-Velocity Provinces (LLSVPs) has become a robust feature. The two large antipodal LLSVPs are surrounded by wide zones of high Vs under the regions characterized by Mesozoic to recent subduction. The D" is further characterized by a negative correlation between shear and bulk sound velocity which could be partly related to an uneven distribution of pv and ppv. Ppv has higher VS and lower $ V_{\Phi } $ (bulk sound speed) than pv and may be present in thicker layers in the colder regions of D". Seismic observations and geodynamic modelling indicate relatively steep and sharp boundaries of the 200-500 km thick LLSVPs. These features, as well as independent evidence for their long-term stability, indicate that they are intrinsically denser than the surrounding mantle. Mineral physics data demonstrate that basaltic lithologies are denser than peridotite throughout the lowermost mantle and undergo incremental densification due to the pvppv- transition at slightly shallower levels than peridotite. The density contrasts may facilitate the partial separation and accumulation of basaltic patches and slivers at the margins of the thermochemical piles (LLSVPs). The slopes of these relatively steep margins towards the adjacent horizontal core-mantle boundary (CMB) constitute a curved (concave) thermal boundary layer, favourable for the episodic generation of large mantle plumes. Reconstruction of the original positions of large igneous provinces formed during the last 300 Ma, using a paleomagnetic global reference frame, indicates that nearly all of them erupted above the margins of the LLSVPs. Fe/Mg-partitioning between pv, ppv and ferropericlase (fp) is important for the phase and density relations of the lower mantle. Electronic spin transition of Fe2+ and Fe3+ in the different phases may influence the Fe/Mg-partitioning and the radiative thermal conductivity in the lowermost mantle. The experimental determination of the $ {K_D}{^{Fe/Mg}_{pv/fp}}\left[ { = {{\left( {Fe/Mg} \right)}_{pv}}/{{\left( {Fe/Mg} \right)}_{fp}}} \right] $ and $ {K_D}{^{Fe/Mg}_{ppv/fp}} $ is technologically challenging. Most studies have found a $ {K_D}{^{Fe/Mg}_{pv/fp}} $ of 0.1-0.3 and a higher Fe/Mg-ratio in ppv than in pv. The experimental temperature is important, with the partitioning approaching unity with increasing temperature. Although charge-coupled substitutions of the trivalent cations Al and Fe³⁺ seem to be important in both pv and ppv (especially in basaltic compositions), the complicating crystal-chemistry effects of these cations are not fully clarified. The two anti-podal thermochemical piles as well as the thin ultra-low velocity zones next to the CMB may represent geochemically enriched reservoirs that have remained largely isolated from the convecting mantle through a major part of Earth history. The existence of such “hidden” reservoirs have previously been suggested in order to account for the imbalance between the inferred composition of the geochemically accessible convecting mantle and the observed heat flow from the Earth and chondritic models for the bulk Earth.     

East Asia: Seismotectonics,magmatism and mantle dynamics

In this article, we review the significant recent results of geophysical studies and discuss their implications on seismotectonics, magmatism, and mantle dynamics in East Asia. High-resolution geophysical imaging revealed structural heterogeneities in the source areas of large crustal earthquakes, which may reflect magma and fluids that affected the rupture nucleation of large earthquakes. In subduction zone regions, the crustal fluids originate from the dehydration of the subducting slab. Magmatism in arc and back-arc areas is caused by the corner flow in the mantle wedge and dehydration of the subducting slab. The intraplate magmatism has different origins. The continental volcanoes in Northeast Asia (such as Changbai and Wudalianchi) seem to be caused by the corner flow in the big mantle wedge (BMW) above the stagnant slab in the mantle transition zone and the deep dehydration of the stagnant slab as well. The Tengchong volcano in Southwest China is possibly caused by a similar process in BMW above the subducting Burma microplate (or Indian plate). The Hainan volcano in southernmost China seems to be a hotspot fed by a lower-mantle plume associated with the Pacific and Philippine Sea slabs’ deep subduction in the east and the Indian slab’s deep subduction in the west down to the lower mantle. The occurrence of deep earthquakes under the Japan Sea and the East Asia margin may be related to a metastable olivine wedge in the subducting Pacific slab. The stagnant slab finally collapses down to the bottom of the mantle, which may trigger upwelling of hot mantle materials from the lower mantle to the shallow mantle beneath the subducting slabs and cause the slab–plume interactions. Some of these issues, such as the origin of intraplate magmatism, are still controversial, and so further detailed studies are needed from now.