Chemical differentiation of the Earth: the relationship between mantle, continental crust, and oceanic crust

The average chemical compositions of the continental crust and the oceanic crust (represented by MORB), normalized to primitive mantle values and plotted as functions of the apparent bulk partition coefficient of each element, form surprisingly simple, complementary concentration patterns. In the continental crust, the maximum concentrations are on the order of 50 to 100 times the primitive-mantle values, and these are attained by the most highly incompatible elements Cs, Rb, Ba, and Th. In the average oceanic crust, the maximum concentrations are only about 10 times the primitive mantle values, and they are attained by the moderately incompatible elements Na, Ti, Zr, Hf, Y and the intermediate to heavy REE.This relationship is explained by a simple, two-stage model of extracting first continental and then oceanic crust from the initially primitive mantle. This model reproduces the characteristic concentration maximum in MORB. It yields quantitative constraints about the effective aggregate melt fractions extracted during both stages. These amount to about 1.5% for the continental crust and about 8–10% for the oceanic crust.The comparatively low degrees of melting inferred for average MORB are consistent with the correlation of Na₂O concentration with depth of extrusion [1], and with the normalized concentrations of Ca, Sc, and Al ( 3) in MORB, which are much lower than those of Zr, Hf, and the HREE ( 10). Ca, Al and Sc are compatible with clinopyroxene and are preferentially retained in the residual mantle by this mineral. This is possible only if the aggregate melt fraction is low enough for the clinopyroxene not to be consumed.A sequence of increasing compatibility of lithophile elements may be defined in two independent ways: (1) the order of decreasing normalized concentrations in the continental crust; or (2) by concentration correlations in oceanic basalts. The results are surprisingly similar except for Nb, Ta, and Pb, which yield inconsistent bulk partition coefficients as well as anomalous concentrations and standard deviations.The anomalies can be explained if Nb and Ta have relatively large partition coefficients during continental crust production and smaller coefficients during oceanic crust production. In contrast, Pb has a very small coefficient during continental crust production and a larger coefficient during oceanic crust production. This is the reason why these elements are useful in geochemical discrimination diagrams for distinguishing MORB and OIB on the one hand from island arc and most intracontinental volcanics on the other.The results are consistent with the crust-mantle differentiation model proposed previously [2]. Nb and Ta are preferentially retained and enriched in the residual mantle during formation of continental crust. After separation of the bulk of the continental crust, the residual portion of the mantle was rehomogenized, and the present-day internal heterogeneities between MORB and OIB sources were generated subsequently by processes involving only oceanic crust and mantle. During this second stage, Nb and Ta are highly incompatible, and their abundances are anomalously high in both OIB and MORB.The anomalous behavior of Pb causes the so-called “lead paradox”, namely the elevated U/Pb and Th/Pb ratios (inferred from Pb isotopes) in the present-day, depleted mantle, even though U and Th are more incompatible than Pb in oceanic basalts. This is explained if Pb is in fact more incompatible than U and Th during formation of the continental crust, and less incompatible than U and Th during formation of oceanic crust.     

Earth and Mars: early thermal profiles

The extent of formation heating for the Earth and Mars has been evaluated assuming that the terrestrial planets accumulated from planetesimals. The main result is that, even if a long accumulation time is assumed (τ ≥ 100 Ma), it is possible to obtain a planetary structure with a large melted shell taking into account the role played by massive projectiles, which, upon reaching depths of several kilometres, are able to deposit heat significantly below the planetary surface. Internal temperatures, sufficient for the downward migration of the liquid iron alloy, have been obtained.     

The thickness and structural characteristics of the crust across Tibetan plateau from active-sources seismic profiles

The Tibetan plateau as one of the youngest orogen on the Earth was considered as the result of continent-continent collision between the Eurasian and Indian plates.The thickness and structure of the crust beneath Tibetan plateau is essential to understand deformation behavior of the plateau.Active-source seismic profiling is most available geo-physical method for imaging the structure of the continental crust.The results from more than 25 active-sources seismic profiles carried out in the past twenty years ...     

Experimental estimate of a relation between sea wave energies and the Earth’s crust microdeformations

The experimental data in the microseismic frequency range obtained using the seismo-acoustic-hydrophysical measurement complex are analyzed. The emphasis is put on estimating the ratio between the energy of surface sea wind waves in the area of the Japan Sea where the complex was located and the Earth’s crust microdeformations in this frequency range. The experimental evaluate obtained allow us to estimate the energy re-distribution at the hydrosphere-lithosphere boundary.     

中国大陆地壳与上地幔精细结构探测十条第一剖面和其作用与科学意义

地球内部物理学和地球动力学在地球科学研究领域里占有重要地位.为了研究和探索我国大陆不同大地构造单元和板内特异块体与其界带的深部介质属性、结构和深层物理-力学过程,已在我国最先构思、设计、布署并实施了十条人工源深部地震探测剖面.沿这十条第一剖面进行了高精度的地震反射、宽角反射和折射波场观测,进行了数据采集,获得了深部高分辨率的信息.通过数据处理和反演求得了各剖面辖区的沉积建造、结晶基底、地壳与土地幔的差异和层、块精细速度结构,发现、研究和提出了一系列的重要科学问题并取得了一系列新认识.为研究成山、成盆、成岩、成矿、成灾,圈层耦合和深化认识地球本体与动力学响应打下了坚实的基础,明确指出了其深入研究的内涵和发展导向.     

Crustal model based on aeromagnetic profiles,gravity studies and wave velocities in rocks and crust

Summary Regional airborne magnetic profiles from India and U.S.A. are analyzed. Profiles are i) 130 km offshore Manglore to 60 km offshore Madras (India) along 13th parallel; ii) Washington to San Francisco (U.S.A.): iii) Brownsville (Texas) to Guatemala City (Mexico). Depth to the sources of magnetic anomalies along Manglore-Madras profile and Washington-San Francisco profiles is calculated either by elementary approximation ofSmellie or Prism model method ofVacquier et al. It is significant that depth values for some of the anomalies obtained by these methods are in very good agreement with those based on drilling data. The magnetic pictures along these profiles are compared with Bouguer gravity anomaly maps and it is shown that in almost all cases where magnetic bodies lie below 5 km (approximately) from sea level they are not reflected in gravity maps whereas all the magnetic bodies which are above 5 km (approximately) produce a markable feature in Bouguer gravity anomaly. This indicates that density of material below this level is almost equal to that of normal basic rocks (2.80 gm/cm³) and those above 5 km have a density less than this. Based on these results the top most layer in crust is considered to be metasedimentary including intrusive rocks and below this it is tentatively taken as Quartz-diorite accounting for the quartz rich Archean formations. Curves representing the variation of compressional wave velocity in i) granite; ii) quartz-diorite; iii) gabbro and iv) dunite, with pressure and temperature as reported from measurements in laboratory, are studied in the light of the general variation of P-wave velocity in the earth's crust reported from seismic sounding studies. It is found that a change in composition from metasedimentary zone to quartz diorite at about 5 km below sea level is supported by this study. It is found that further increase in compressional wave velocity in earth's crust can be explained by a compositional change from quartz diorite to gabbro. At certain places an unusual high velocity for compressional wave at the base of the crust is reported. This can be explained by considering that gabbro merges to Dunite in those areas. Based on this crustal model a probable explanation for the origin of granite masses is attempted.     

Dating the lower crust by ion microprobe

Ion microprobe UThPb ages of zircons from granulite facies lower crustal xenoliths from north Queensland, Australia, correlate well with the ages of major orogenic episodes manifest at the earth's surface. About half of the xenoliths contain Proterozoic zircons which are similar in age to the episodes of high-grade metamorphism of the older surface rocks. All the xenoliths contain late Paleozoic zircons which show a real 100 Ma range in²⁰⁶Pb²³⁸/U ages (from 320 to 220 Ma), which is attributed to granulite facies metamorphism followed by slow cooling in the deep crust. The Paleozoic zircon ages coincide in time with the prolonged episode of eruption of voluminous felsic ash-flows and intrusion of high-level granites in this region (320-270 Ma). Mineral and melt inclusions in the zircons provide clues to the origin of some of the xenoliths, and coupled with the age information, can be used to infer the geological processes operating in the lower crust. The zircons from two mafic xenoliths contain felsic and intermediate melt inclusions implying at least a two-stage history for these rocks, involving either partial melting of a more felsic protolith or crystal accumulation from an evolved melt. Some of the zircons from the felsic xenoliths contain CO₂-rich fluid inclusions, indicating that those zircons grew during high-grade metamorphism. The isotopic and chemical data for the whole rock xenoliths show that they originate from a segment of the lower crust which is a heterogeneous mixture of supracrustal and mafic, mantle-derived, lithologies. The major orogenic event responsible for the formation of that crust occurred in the late Paleozoic, when Proterozoic supracrustal rocks were emplaced into the lower crust, possibly along thin-skinned thrust slices. This was accompanied by intrusion of high-temperature, mantle-derived melts which caused partial melting of pre-existing crust. The most likely setting for such tectonism is a continental margin subduction zone.     

用最小二乘配置法构建局部重力场模型

基于非均匀分布的陆地重力观测数据,重构局部重力场模型是区域重力资料处理与解释的重要环节。本文对比了多种局部重力场建模方法,并以EGM2008模型提供的自由空气重力异常模型重采样数据进行测试,综合比较了不同噪声条件下不同建模方法的实际效果。结果表明:在不同噪声水平下,优选出适合重力位场问题的协方差函数后,最小二乘配置法的建模效果优于其它方法。      

Precise estimation of covariance parameters in least-squares collocation by restricted maximum likelihood

Precise estimates of the covariance parameters are essential in least-squares collocation (LSC) in the case of increased accuracy requirements. This paper implements restricted maximum likelihood (REML) method for the estimation of three covariance parameters in LSC with the Gauss-Markov second-order function (GM2), which is often used in interpolation of gravity anomalies. The estimates are then validated with the use of an independent technique, which has been often omitted in the previous works that are confined to covariance parameters errors based on the information matrix. The crossvalidation of REML estimates with the use of hold-out method (HO) helps in understanding of REML estimation errors. We analyzed in detail the global minimum of negative log-likelihood function (NLLF) in the estimation of covariance parameters, as well, as the accuracy of the estimates. We found that the correlation between covariance parameters may critically contribute to the errors of their estimation. It was also found that knowing some intrinsic properties of the covariance function may help in the scoring process.     

最小二乘配置下的天山地区应变场特征分布

利用已有的GPS观测数据,借助球面最小二乘配置方法对天山地区的GPS速度场进行研究,得到了研究区域应变场的空间分布特征．其最大主压应变表明,大地震多发生在主压应变快速交替变化的地带,主压应变最大值主要分布于西南天山与帕米尔弧及塔里木西北交汇的地区,强地震(M7.0—8.0)基本发生在该区域．面膨胀值表明天山地区应变呈挤压收缩的特征．     

The structure of the crust in TESZ area by kriging interpolation

A precise 3D model of the crust is necessary to start any tectonic or geodynamic interpretation. It is also essential for seismic interpretations of structures lying below as well as for correct analysis of shallow structures using reflection seismics. During the last decades, a number of wide-angle refraction experiments were performed on the territory of central and eastern Europe (POLONAISE’97, CELEBRATION 2000, SUDETES 2003), resulting in many high quality 2D models. It is an interesting and complicated transition zone between Precambrian and Palaeozoic Platforms. This paper presents 3D model of the velocity distribution in the crust and upper mantle interpolated from 2D models of the structure along 33 profiles. The obtained model extends to a depth of 50 km and accurately describes the main features of the crustal structures of Poland and surrounding areas. Different interpolation techniques (Kriging, linear) are compared to assure maximum precision. The final model with estimated uncertainty is an interesting reference of the area for other studies.     

A new insight into the problem of annual periodicity of the electric resistivity of the Earth’s crust in earthquake prediction research

As a result of the analysis of both the monitoring data of the electric resistivity of the Earth’s crust at the Garm test site using a modified method of dipole sounding and the earthquake catalogue of this test site it is shown that annual periodicity exists not only in the variations of the electric resistivity of the Earth’s crust but also in seismicity. It is not clear yet whether the appearance of the annual periodicity in the variations of electric resistivity is related to the influence of the periodic processes of earthquake nucleation on this process or both processes are synchronized by a third, unknown process. The results of the investigation show that the annual component of time variations in the electric resistivity contains important information about the processes in the Earth’s crust and should not be filtered out but, in contrast, studied in detail. The joint analysis of periodic components in the compared time series of the electric resistivity of the Earth’s crust and earthquakes opens new possibilities for deeper understanding of the processes of seismicity genesis and for improving the methods of strong earthquakes prediction.     

Recycling of the continental crust

In order to understand the evolution of the crust-mantle system, it is important to recognize the role played by the recycling of continental crust. Crustal recycling can be considered as two fundamentally distinct processes: 1) intracrustal recycling and 2) crust-mantle recycling. Intracrustal recycling is the turnover of crustal material by processes taking place wholly within the crust and includes most sedimentary recycling, isotopic resetting (metamorphism), intracrustal melting and assimilation. Crust-mantle recycling is the transfer of crustal material to the mantle with possible subsequent return to the crust. Intracrustal recycling is important in interpreting secular changes in sediment composition through time. It also explains differences found in crustal area-age patterns measured by different isotopic systems and may also play a role in modeling crustal growth curves based on Nd-model ages. Crustal-mantle recycling, for the most part, is a subduction process and may be considered on three levels. The first is recycling with only short periods of time in the mantle (<10 m.y.). This may be important in explaining the origin of island-arc and related igneous rocks; there is growing agreement that 1–3% recycled sediment is involved in their origin. Components of recycled crustal material, with long-term storage (up to 2.5 b.y.) in the mantle as distinct entities, has been suggested for the origin of ocean island and ultrapotassic volcanics but there is considerably less agreement on this interpretation. A third proposal calls for the return of crustal material to the mantle with efficient remixing in order to swamp the geochemical and isotopic signature of the recycled component by the mantle. This type of recycling is required for steady-state models of crustal evolution where the mass of the continents remains constant over geological time. It is unlikely if crust-mantle recycling has exceeded 0.75 km³/yr over the past 1–2 Ga.Good evidence exists that selective recycling is an important process. Sedimentary rocks preserved in different tectonic settings are apparently recycled at different rates, resulting in a bias in the sediment types preserved in the geologic record. Selective recycling has important implications for the interpretation of Nd model ages of old sedimentary rocks and in the analysis of accreted terranes. Although there is evidence that continental crust was formed prior to 3.8 Ga, the oldest preserved rocks do not exceed this age. It is likely that the intense meteorite bombardment, which affected the earth during the period 4.56–3.8 Ga, coupled with rapid mantle convection, which resulted from greater heat production, caused the destruction and probable recycling into the mantle of any early formed crust.Although crust-mantle recycling is seen as a viable process, it is concluded that crustal growth has exceeded crust-mantle recycling since at least 3.8 Ga. Intracrustal recycling has not been given adequate consideration in models of crustal growth based on isotopic data (particularly Nd model ages). It is concluded that crustal growth curves based on Nd model ages, while vastly superior to those based on K/Ar or Rb/Sr, tend to underestimate the volume of old crust, due to crust-mantle and/or intracrustal recycling.     

The control of alkalies and uranium in seawater by ocean crust alteration

Alteration of the oceanic crust during hydrothermal circulation of seawater produces fluxes of K, Rb and Cs between these reservoirs which are significant compared to the river input of these elements. The ocean crust U flux, on the other hand, is probably not significant. The upper crust, altered at low temperature, is a sink for all of these elements (as shown by direct analysis of upper crustal materials). The lower crust is a source for K, Rb, and Cs, based on the observation that high-temperature fluids exiting the crust as “host springs” are enhanced over seawater in K, Rb and Cs concentration. While the sign of the hot spring fluxes may be correct, the absolute magnitudes cannot be, as the calculated yearly hot spring flux of Rb and Cs significantly exceeds the total Rb and Cs inventory of newly formed unaltered crust. By modelling the crust as a melt/cumulate combination, we show that the crust as a whole is a sink for K, Rb, Cs, and probably U, with yearly fluxes of1.1 × 10¹³, 2.6 ×10¹⁰, 6.0 × 10⁸ and 1.0 × 10⁹g, respectively (com to yearly river dissolved fluxes of7.4 × 10¹³, 3.5 × 10¹⁰, 6.4 × 10⁸and1.0 × 10¹⁰g, respectively). The alteratio oceanic crust appears capable of quantitatively balancing the river inputs of Rb and Cs. For K, an additional sink comprising～ 85% of the river input is necessary. Because this missing K sink cannot be arbitrarily manipulated without destroying the Rb and Cs balances, a sink with K/Rb higher than the continental crust is required, and may possible be found in the sediments of the continental shelves.     

The MT inversion for conductivity anisotropy and EDA precursor, stress field and deformation band in the Earth’s deep crust

The conductivity anisotropy behaviour is described for certain environment in the Earth’s crust and the MT inversion method for a layered symmetrically anisotropic model is presented. The inversion interpretations of the anisotropic model from the observational data are helpful to identify the earthquake precusors as indicated by the deep conductivity anisotropic variations, and also provide some useful information to investigate the stress states and deformation bands in the deep crust of the Earth. This project is sponsored by the Chinese Joint Seismological Science Foundation.     

The current strain distribution in the North China Basin of eastern China by least-squares collocation

In this paper, the velocities of 154 stations obtained in 2001 and 2003 GPS survey campaigns are applied to formulate a continuous velocity field by the least-squares collocation method. The strain rate field obtained by the least-squares collocation method shows more clear deformation patterns than that of the conventional discrete triangle method. The significant deformation zones obtained are mainly located in three places, to the north of Tangshan, between Tianjing and Shijiazhuang, and to the north of Datong, which agree with the places of the Holocene active deformation zones obtained by geological investigations. The maximum shear strain rate is located at latitude 38.6°N and longitude 116.8°E, with a magnitude of 0.13 ppm/a. The strain rate field obtained can be used for earthquake prediction research in the North China Basin.     

Analysis on monitoring crust movement by using GPS in Qinghai-Xiqang Plateau

Based on the analysis of the 1993 and 1995 GPS data acquired from crust movement and deformation monitoring in thc Qinghai-Xizang Plateau, the following preliminary conclusions could be drawn: the levelly moving rate and direction of the land massifs in the Qinghai-Xizang Plateau tally with the viewpoint generally held by geologists and geophysicists irr recent years; the accuracy of monitoring crust movement has reached the world advanced level; the result has provided valuable and reliable information to the quantitative analysis of the modern crust movement and deformation in the plateau. Project supported by the Nntional Natural Science Foundation of China.     

Monitoring Urban Land Use by Earth Observation

This paper discuses the use of high resolution spaceborne Earth observation data in studies of the urban environment. Adopting a rigorous, mathematical approach, it introduces the Spatial Re-classification Kernel (SPARK) algorithm; this is appliedto a suburban area in the south-east of London. It also introduces a region-based, structural pattern recognition approach to the same problem through which urban land use might be inferred from an analysis of the morphological properties and structural relations of discrete land cover regions identified in fine spatial resolution images.