Chondritic Mg isotope composition of the Earth

The processes of planetary accretion and differentiation have potentially been recorded as variations in the stable isotope ratios of the major elements between planetary objects. However, the magnitude of observed isotopic variations for several elements (Mg, Fe, Si) is at the limit of what current analytical precision and accuracy are able to resolve. Here, we present a comprehensive data set of Mg isotope ratios measured in ocean island and mid-ocean ridge basalts, peridotites and chondrites. The precision and accuracy were verified by isotopic standard addition for two samples, one carbonaceous chondrite (Murchison) and one continental flood basalt (BCR-1). In contrast with some previous studies, our data from terrestrial and chondritic materials have invariant Mg isotope ratios within the uncertainty of the method (0.1‰ for the ²⁶Mg/²⁴Mg ratio, 2SD). Although isotopic variations of less than about 0.1‰ could still be present, the data demonstrate that, at this level of uncertainty, the bulk silicate Earth and chondritic Mg reservoir have a homogeneous δ²⁶Mg = −0.23‰ (²⁶Mg/²⁴Mg ratio of the sample relative to the DSM3 standard set to zero by definition). This implies that neither planetary accretion processes nor partial mantle melting and subsequent shallow-level differentiation have fractionated Mg isotope ratios. These observations imply in particular that the formation of the Earth cannot stem from preferential sorting of chondrite constituents that would have been fractionated in their Mg isotope composition. It also implies that unlike oxygen isotopes, there was no zonation in Mg isotopes in the inner solar system.     

Magnesium isotopic composition of the Earth and chondrites

To constrain further the Mg isotopic composition of the Earth and chondrites, and investigate the behavior of Mg isotopes during planetary formation and magmatic processes, we report high-precision (±0.06‰ on δ²⁵Mg and ±0.07‰ on δ²⁶Mg, 2SD) analyses of Mg isotopes for (1) 47 mid-ocean ridge basalts covering global major ridge segments and spanning a broad range in latitudes, geochemical and radiogenic isotopic compositions; (2) 63 ocean island basalts from Hawaii (Kilauea, Koolau and Loihi) and French Polynesia (Society Island and Cook-Austral chain); (3) 29 peridotite xenoliths from Australia, China, France, Tanzania and USA; and (4) 38 carbonaceous, ordinary and enstatite chondrites including 9 chondrite groups (CI, CM, CO, CV, L, LL, H, EH and EL).Oceanic basalts and peridotite xenoliths have similar Mg isotopic compositions, with average values of δ²⁵Mg = −0.13 ± 0.05 (2SD) and δ²⁶Mg = −0.26 ± 0.07 (2SD) for global oceanic basalts (n = 110) and δ²⁵Mg = −0.13 ± 0.03 (2SD) and δ²⁶Mg = −0.25 ± 0.04 (2SD) for global peridotite xenoliths (n = 29). The identical Mg isotopic compositions in oceanic basalts and peridotites suggest that equilibrium Mg isotope fractionation during partial melting of peridotite mantle and magmatic differentiation of basaltic magma is negligible. Thirty-eight chondrites have indistinguishable Mg isotopic compositions, with δ²⁵Mg = −0.15 ± 0.04 (2SD) and δ²⁶Mg = −0.28 ± 0.06 (2SD). The constancy of Mg isotopic compositions in all major types of chondrites suggest that primary and secondary processes that affected the chemical and oxygen isotopic compositions of chondrites did not significantly fractionate Mg isotopes.Collectively, the Mg isotopic composition of the Earth’s mantle, based on oceanic basalts and peridotites, is estimated to be −0.13 ± 0.04 for δ²⁵Mg and −0.25 ± 0.07 for δ²⁶Mg (2SD, n = 139). The Mg isotopic composition of the Earth, as represented by the mantle, is similar to chondrites. The chondritic composition of the Earth implies that Mg isotopes were well mixed during accretion of the inner solar system.     

The underdetermined Earth

In the endeavour to achieve the goal of understanding the structure of the Earth in three dimensions, the limitations of the seismological probe for Earth structure have to be recognised. In geo-seismology there are high quality observations of a wide range of information on the seismic wavefield but only at a limited number of observation points around the globe. In contrast for the Sun, the other body for which seismological techniques have been routinely applied, there is the equivalent of a very high concentration of seismometers on one-hemisphere but very limited data quality. The restriction imposed by data quality is easily recognised but the restrictions imposed by the limited sampling of the globe are more subtle and have to be borne in mind in the interpretation of any images of seismic structure. A wide range of studies has demonstrated the presence of heterogeneity in Earth-structure on a wide range of scales from planet-wide variations in the degree 2 spherical harmonics to the intensity of variations on the micro-scale in intensely deformed rock belts. Whatever scale of structure is being examined, the potential influence of other classes of heterogeneity has to be recognised and so an effort made to minimise the transfer of information from different scales which may contaminate the picture. An important class of questions which must be faced in the development of threedimensional models is the nature of the reference model (or group of models) from which the model is to be derived, and also whether it is likely that limitations in the starting state will map into final images of structure. Further, are the various classes of data being used, compatible within the class of models which is being imposed?     

中国东部与全球大陆地壳化学成分的比较

鄢明才教授、迟清华博士和地球物理地球化学勘查研究所、中国地质大学的同事们从中国东部5个一级大地构造单元的大约500条标准地层剖面和800多个侵入岩体、变质杂岩体中系统地采集了28253个岩石样品,覆盖了东经105°以东大约330×104km2的面积。将这些样品组合成了2718个组合样品,分析了周期表中64种常量、微量和痕量元素。另外,还用这28253个岩石样品按照不同大地构造单元中的岩石类型制备了150个大的组合样,分析了Br、I、Te、Re、Os、Ir、Ru、Rh等难测定的痕量和次痕量元素。用INAA、XRF、ICP-MS、AAS、AFS等15种方法系统地定…     

The impact environment of the Hadean Earth

Impact bombardment in the first billion years of solar system history determined in large part the initial physical and chemical states of the inner planets and their potential to host biospheres. The range of physical states and thermal consequences of the impact epoch, however, are not well quantified. Here, we assess these effects on the young Earth's crust as well as the likelihood that a record of such effects could be preserved in the oldest terrestrial minerals and rocks. We place special emphasis on modeling the thermal effects of the late heavy bombardment (LHB) – a putative spike in the number of impacts at about 3.9 Gyr ago – using several different numerical modeling and analytical techniques. A comprehensive array of impact-produced heat sources was evaluated which includes shock heating, impact melt generation, uplift, and ejecta heating. Results indicate that ∼1.5–2.5 vol.% of the upper 20 km of Earth's crust was melted in the LHB, with only ∼0.3–1.5 vol.% in a molten state at any given time. The model predicts that approximately 5–10% of the planet's surface area was covered by >1 km deep impact melt sheets. A global average of ∼600–800 m of ejecta and ∼800–1000 m of condensed rock vapor is predicted to have been deposited in the LHB, with most of the condensed rock vapor produced by the largest (>100-km) projectiles. To explore for a record of such catastrophic events, we created two- and three-dimensional models of post-impact cooling of ejecta and craters, coupled to diffusion models of radiogenic Pb*-loss in zircons. We used this to estimate what the cumulative effects of putative LHB-induced age resetting would be of Hadean zircons on a global scale. Zircons entrained in ejecta are projected to have the following average global distribution after the end of the LHB: ∼59% with no impact-induced Pb*-loss, ∼26% with partial Pb*-loss and ∼15% with complete Pb*-loss or destruction of the grain. In addition to the relatively high erodibility of ejecta, our results show that if discordant ca. 3.9 Gyr old zones in the Jack Hills zircons are a signature of the LHB, they were most likely sourced from impact ejecta.     

The Heat Balance in the Earth

Doklady Earth Sciences - The observed heat flux from the Earth is created mainly by the heat released due to radioactive decay, secular cooling of the Earth, and solidification of the growing inner...     

The oceans and the physics of the Earth

The Earth is the only terrestrial planet to have seas, by which it is covered to the extent of seventy percent. The oceans determine the climate of the Earth and it is through them that life and civilization have evolved, but they also have major influence on physical processes in the solid Earth. Tidal friction leads to the Earth's spin slowing down with a consequent recession of the Moon from the Earth, while the presence of the oceans controls the tectonic evolution of the Earth through the formation of oceanic crust, the erosion of land and the accumulation of sediment, the formation of mountains and the establishment of isostatic balance.     

A method to estimate the composition of the bulk silicate Earth in the presence of a hidden geochemical reservoir

The possibility of a hidden geochemical reservoir in the deep mantle has long been debated in geophysics and geochemistry, because of its bearings on the structure of the core-mantle boundary region, the origin of hotspots, the style of mantle convection, the history of the geomagnetic field, and the thermal evolution of Earth. The presence of such hidden reservoir, however, may invalidate existing models for the composition of the bulk silicate Earth because these models invariably assume that major chemical differentiation in the mantle follows the compositional trend exhibited by upper-mantle rocks. This article presents a new method to estimate the composition of the bulk silicate Earth by explicitly taking into account the possibility of a hidden reservoir. This geochemical inference is formulated as a nonlinear inverse problem, for which an efficient Markov chain Monte Carlo algorithm is developed. Inversion results indicate that the formation of a hidden reservoir, if any, took place at low pressures probably within the first 10 Myr of the history of the solar system and was subsequently lost from the Earth by impact erosion. The global mass balance of the bulk silicate Earth is revisited with the inversion results, and the depletion of highly incompatible elements in the present-day Earth is suggested to be moderate.     

The geological history and geodynamics of the Earth

The Earth formed through a hot accretion process. Almost simultaneously, the core and the mantle were separated from each other. At the final stages of the accretion process, the outer layer approximately 2000 km thick was molten, thus representing a magma ocean. This magma ocean produced the primary crust of the Earth. Surface waters were precipitated from the atmosphere and released from the crystallizing magma ocean. The plate tectonic processes started at around 4.3 to 4 Ga BP. In the Archean, the overall tectonic mechanism was quite specific, due to substantially higher mantle temperature and thicker oceanic crust. The normal plate tectonics acted during the Proterozoic and Phanerozoic with the periodic assembly of continents, which are known as supercontinent cycles.     

The age of the Earth and the oldest rocks

The ages of rocks and minerals can be determined by measuring the accumulation of the products of radioactive decay within them. The results provide a time–scale for the Earth and for the major geological events which have shaped the Earth's continental crust since it first started to separate from the mantle at least 3700 million years ago. The Earth itself is about 4600 million years old.     

Metal-silicate partitioning and constraints on core composition and oxygen fugacity during Earth accretion

We present the results of new partitioning experiments between metal and silicate melts for a series of elements normally regarded as refractory lithophile and moderately siderophile and volatile. These include Si, Ti, Ni, Cr, Mn, Ga, Nb, Ta, Cu and Zn. Our new data obtained at 3.6 and 7.7 GPa and between 2123 and 2473 K are combined with literature data to parameterize the individual effects of oxygen fugacity, temperature, pressure and composition on partitioning. We find that Ni, Cu and Zn become less siderophile with increasing temperature. In contrast, Mn, Cr, Si, Ta, Nb, Ga and Ti become more siderophile with increasing temperature, with the highly charged cations (Nb, Ta, Si and Ti) being the most sensitive to variations of temperature. We also find that Ni, Cr, Nb, Ta and Ga become less siderophile with increasing pressure, while Mn becomes more siderophile with increasing pressure. Pressure effects on the partitioning of Si, Ti, Cu and Zn appear to be negligible, as are the effects of silicate melt composition on the partitioning of divalent cations. From the derived parameterization, we predict that the silicate Earth abundances of the elements mentioned above are best explained if core formation in a magma ocean took place under increasing conditions of oxygen fugacity, starting from moderately reduced conditions and finishing at the current mantle-core equilibrium value.     

The chemical composition including the Rare Earth Elements of the three major glass types of Europe and the Orient used in late antiquity and the Middle Ages

Sets of 20 soda ash glasses, 16 soda lime glasses and 23 wood ash glasses mainly from excavations in Europe (additional soda ash glasses from Egypt) were analysed on 61 chemical elements. Average SiO₂ is about 62% in soda glasses and 50% in wood ash glasses. The three groups of glasses contain on average 13% Na₂O, 18% Na₂O and 13% K₂O as fluxes to lower the melting temperature of quartz at their production. The starting materials beside quartz were halophytic plant ash for soda ash glass, trona (Na₃H(CO₃)₂·2H₂O) and lime (clamshells) for soda lime glass and beech ash for wood ash glass. Each of the three major glass types contains specific Rare Earth Element (REE) concentrations mainly contained in quartz and its intergrown minerals. 50 Paleozoic and Mesozoic sandstones from Central Europe represent the quartz composition. The REE pattern of these glasses apparently indicates major compositional stages of the Continental Earth's Crust. The boron to lithium and sodium to potassium ratios as in seawater suggest reactions of materials for soda glass with seawater. Negative Ce anomalies in the three glasses are caused by reactions of quartz with seawater.     

地球的层圈结构与穿越层圈构造

从1906年发现地核到20世纪60年代,地球物理学、地质学和矿物物理学的研究揭示了地球具有物理化学性质截然不同的层圈结构,并根据全球地震波速度和密度的变化建立了初始参考地球模型。1967年提出的板块构造理论假定刚性的岩石圈板块在塑性的软流圈之上发生运动,在洋中脊不断形成的洋壳逐渐在海沟俯冲,由于板块是刚性的,变形将主要集中在板块边界。板块构造理论成功地解释了大洋岩石圈的形成和消亡、火山和地震活动带的分布以及全球构造格局,给地球科学带来了一场革命。但是,经典的板块构造理论尚未解决板块运动的起源和驱动力、大陆岩石圈的弥散性变形、大陆深俯冲等问题,因此大陆动力学成为对板块构造理论的重要补充。近年来的研究表明:在板块汇聚边界,大洋岩石圈可以俯冲至地幔过渡带、下地幔,乃至核幔边界;而大陆岩石圈可以俯冲至150~300 km深度,然后相对低密度的陆壳物质快速折返形成含柯石英和微粒金刚石的超高压变质带。地幔柱活动是是俯冲板块再循环的产物,不仅可以形成大火成岩省和洋岛玄武岩,还可以把俯冲到地幔过渡带的物质带回浅部,导致蛇绿岩中保留金刚石和深地幔矿物。因此,俯冲带和地幔柱不仅提供了穿越层圈的物质和能量交换的通道,也驱动了对地球宜居性至关重要的水循环和碳循环,是研究地球物质组成和动力学演化的重要窗口。     

深部地幔的富水储库

地幔矿物中微量的水影响着地幔的物理和化学性质,从而深刻影响着板块构造和地表宜居性.长久以来,对岩石圈以下地幔的水含量估计,主要来自大洋中脊玄武岩和洋岛玄武岩的水含量分析结果表明,其源区水含量分别为50× 10-6～200×10-6和300× 10-6～900× 10-6.近十年来,通过对超深金刚石中的林伍德石、冰Ⅶ等矿物包裹体的研究,揭示了深部地幔的很有可能存在更富水的区域.而对超慢速扩张洋中脊玄武岩、科马提岩、苦橄岩等幔源岩浆含水性的分析,则从更宏观尺度上证实地球深部存在着与俯冲带释放流体无关的富水区域,同时揭示了古老地幔楔残余在浅部软流圈的滞留也是大洋板块内部富水的重要途径.尽管如此,对地幔深部富水储库形成机制、水来源及储库所在圈层位置等还没有清晰的认识,在将来的研究中这些问题应受到关注.     

Loess: The Yellow Earth

A wind-deposited silt forming large deposits in China and middle America, loess is the basis of much grade-one agricultural land.     

Mineralogical and chemical composition of some Bauchi soils and their relationship to passive Earth movement

Field and chemical data show that soils in some parts of Bauchi State, Nigeria, are rich in illite (20–35%), montmorillonite (60–75%), and kaolinite (45–73%). These expansive clays cause the soils to shrink and swell alternatively in response to the seasonal supply of moisture; resulting in observed damaging cracks. Plasticity index (PI) determinations on these soils are high (7–13.4%) suggesting that they are potentially hazardous. The attendant hazards and huge losses to the State are blamed on the expansive nature of these soils. The hazardous conditions can be mitigated by adopting proper construction precautions as well as by using chemical additives such as lime and phosphates, to lower the PI and help to increase the strength of the soils.     

The major element composition of the upper mantle estimated from the composition of lherzolites

The compilation of analyses of continental and oceanic spinel Iherzolites show that these two types of Iherzolites have very similar compositions. Their composition range differ from that of African garnet Iherzolites, and the data suggest that the mantle beneath Africa has an anomalous composition. If the composition of the upper mantle may be estimated from that of Iherzolites, the compositions of spinel Iherzolite should form the basis for this estimate. It is suggested that the compositions of spinel Iherzolite represent both undepleted and depleted compositions, and a representative composition for the primitive mantle is proposed on this basis.     

世界稀土产业格局变化与中国稀土产业面临的问题

稀土元素在永磁材料等新材料领域有着广泛的应用,被世界各主要国家列为战略资源.从资源、生产格局上看,近年来随着世界范围内稀土探矿热潮的出现以及中国稀土资源的大量开发,中国稀土资源优势地位正逐渐降低;美国等西方国家大力推动中国之外稀土矿山和冶炼分离项目的建设,全球稀土生产格局或将发生变化.从消费方面看,稀土消费可被归为永磁材料、催化剂等8种最终用途;永磁材料和催化剂占稀土消费量的60%,永磁材料占稀土消费价值的91%;中国、日本是全球最大的稀土消费国,美国和欧洲国家次之.稀土产业经过多年的发展,中国成为了全球唯一具有稀土全产业链的国家,但在在发展的过程中也面临缺乏稀土产品定价权、资源快速消耗、高端材料和应用技术受制于人等方面的问题.针对中国稀土产业面临的问题,提出了加强稀土产品研发和应用研究、构建稀土产业矿业航母等建议.