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1.
Modeling of the seismic, thermal, and density structure of the Siberian craton lithospheric mantle at depths of 100-300 km has been performed along the superlong Meteorite and Rift seismic profiles. The 2D velocity sections reflect the specific features of the internal structure of the craton: lateral inhomogeneities, seismic-boundary relief at depths of ~ 100, 150, 240, and 300 km, velocities of 8.3-8.7 km/s, and the lack of low-velocity zone in the lower lithosphere. Mapping of the thermal state along the Meteorite and Rift profiles shows a significant temperature decrease in the cratonic mantle as compared with the average temperatures of the surrounding Phanerozoic mantle (> 300 °C) estimated from the global reference model AK135. Lateral temperature variations, reflecting the thermal anomalies in the cratonic keel, are observed at depths of < 200 km (with some decrease in temperature in the central part of the craton), whereas at depths of > 200 km, temperature variations are negligible. This suggests the preservation of residual thermal perturbations at the base of the lithosphere, which must lead to the temperature equalization in the transition zone between the lithosphere and the asthenosphere. Variations in chemical composition have a negligible effect on the thermal state but affect strongly the density structure of the mantle. The results of modeling admit a significant fertilization of matter at depths more than 180-200 km and stratification of the cratonic mantle by chemical composition. The thicknesses of chemical (petrologic) and thermal boundary layers beneath the Siberian craton are estimated. The petrologic lithosphere is localized at depths of ~ 200 km. The bottom of the thermal boundary layer is close to the 1450 °C isotherm and is localized at a depth of 300 km, which agrees with heat flow and seismic-tomography data.  相似文献   

2.
Despite the violent eruption of the Siberian Traps at ~ 250 Ma, the Siberian craton has an extremely low heat flow (18–25 mW/m2) and a very thick lithosphere (300–350 km), which makes it an ideal place to study the influence of mantle plumes on the long-term stability of cratons. Compared with seismic velocities of rocks, the lower crust of the Siberian craton is composed mainly of mafic granulites and could be rather heterogeneous in composition. The very high Vp (> 7.2 km/s) in the lowermost crust can be fit by a mixture of garnet granulites, two-pyroxene granulites, and garnet gabbro due to magma underplating. The high-velocity anomaly in the upper mantle (Vp = 8.3-8.6 km/s) can be interpreted by a mixture of eclogites and garnet peridotites. Combined with the study of lower crustal and mantle xenoliths, we recognized multistage magma underplating at the crust-mantle boundary beneath the Siberian craton, including the Neoarchean growth and Paleoproterozoic assembly of the Siberian craton beneath the Markha terrane, the Proterozoic collision along the Sayan-Taimyr suture zone, and the Triassic Siberian Trap event beneath the central Tunguska basin. The Moho becomes a metamorphism boundary of mafic rocks between granulite facies and eclogite facies rather than a chemical boundary that separates the mafic lower crust from the ultramafic upper mantle. Therefore, multistage magma underplating since the Neoarchean will result in a seismic Moho shallower than the petrologic Moho. Such magmatism-induced compositional change and dehydration will increase viscosity of the lithospheric mantle, and finally trigger lithospheric thickening after mantle plume activity. Hence, mantle plumes are not the key factor for craton destruction.  相似文献   

3.
Several thousand clinopyroxene, garnet, and phlogopite inclusions of mantle rocks from Jurassic and Triassic kimberlites in the northeastern Siberian craton have been analyzed and compared with their counterparts from Paleozoic kimberlites, including those rich in diamond. The new and published mineral chemistry data make a basis for an updated classification of kimberlite-hosted clinopyroxenes according to peridotitic and mafic (eclogite and pyroxenite) parageneses. The obtained results place constraints on the stability field of high-Na lherzolitic clinopyroxenes, which affect the coexisting garnet and decrease its Ca contents. As follows from analyses of the mantle minerals from Mesozoic kimberlites, the cratonic lithosphere contained more pyroxenite and eclogite in the Mesozoic than in the Paleozoic. It virtually lacked ultradepleted harzburgite-dunite lithologies and contained scarce eclogitic diamonds. On the other hand, both inclusions in diamond and individual eclogitic minerals from Mesozoic kimberlites differ from eclogitic inclusions in diamond from Triassic sediments in the northeastern Siberian craton. Xenocrystic phlogopites from the D’yanga pipe have 40Ar/39Ar ages of 384.6, 432.4, and 563.4 Ma, which record several stages of metasomatic impact on the lithosphere. These phlogopites are younger than most of Paleozoic phlogopites from the central part of the craton (Udachnaya kimberlite). Therefore, hydrous mantle metasomatism acted much later on the craton periphery than in the center. Monomineral clinopyroxene thermobarometry shows that Jurassic kimberlites from the northeastern craton part trapped lithospheric material from different maximum depths (170 km in the D’yanga pipe and mostly < 130 km in other pipes). The inferred thermal thickness of cratonic lithosphere decreased progressively from ~ 260 km in the Devonian-Carboniferous to ~ 225 km in the Triassic and to ~ 200 km in the Jurassic, while the heat flux (Hasterok-Chapman model) was 34.9, 36.7, and 39.0 mW/m2, respectively. Dissimilar PT patterns of samples from closely spaced coeval kimberlites suggest different emplacement scenarios, which influenced both the PT variations across the lithosphere and the diamond potential of kimberlites.  相似文献   

4.
《Gondwana Research》2016,29(4):1344-1360
Using free-board modeling, we examine a vertically-averaged mantle density beneath the Archean–Proterozoic Siberian Craton in the layer from the Moho down to base of the chemical boundary layer (CBL). Two models are tested: in Model 1 the base of the CBL coincides with the LAB, whereas in Model 2 the base of the CBL is at a 180 km depth. The uncertainty of density model is < 0.02 t/m3 or < 0.6% with respect to primitive mantle. The results, calculated at in situ and at room temperature (SPT) conditions, indicate a heterogeneous density structure of the Siberian lithospheric mantle with a strong correlation between mantle density variations and the tectonic setting. Three types of cratonic mantle are recognized from mantle density anomalies. ‘Pristine’ cratonic regions not sampled by kimberlites have the strongest depletion with density deficit of 1.8–3.0% (and SPT density of 3.29–3.33 t/m3 as compared to 3.39 t/m3 of primitive mantle). Cratonic mantle affected by magmatism (including the kimberlite provinces) has a typical density deficit of 1.0–1.5%, indicative of a metasomatic melt-enrichment. Intracratonic sedimentary basins have a high density mantle (3.38–3.40 t/m3 at SPT) which suggests, at least partial, eclogitization. Moderate density anomalies beneath the Tunguska Basin imply that the source of the Siberian LIP lies outside of the Craton. In situ mantle density is used to test the isopycnic condition of the Siberian Craton. Both CBL thickness models indicate significant lateral variations in the isopycnic state, correlated with mantle depletion and best achieved for the Anabar Shield region and other intracratonic domains with a strongly depleted mantle. A comparison of synthetic Mg# for the bulk lithospheric mantle calculated from density with Mg# from petrological studies of peridotite xenoliths from the Siberian kimberlites suggests that melt migration may produce local patches of metasomatic material in the overall depleted mantle.  相似文献   

5.
We performed thermodynamic calculations based on model and natural peridotitic compositions at pressure and temperature conditions relevant to the Earth’s upper mantle, using well-established free energy minimization techniques. The model is consistent with the available experimental data in Cr-bearing peridotitic systems and can therefore be used to predict phase relations and mineral compositions in a wide range of realistic mantle compositions. The generated phase diagrams for six different bulk compositions, representative of fertile, depleted and ultra-depleted peridotitic mantle, shown that the garnet + spinel stability field is always broad at low temperatures and progressively narrows with increasing temperatures. In lithospheric sections with hot geotherms (ca. 60 mW/m2), garnet coexists with spinel across an interval of 10–15 km, at ca. 50–70 km depths. In colder, cratonic, lithospheric sections (e.g. along a 40 mW/m2 geotherm), the width of the garnet–spinel transition strongly depends on bulk composition: In fertile mantle, spinel can coexist with garnet to about 120 km depth, while in an ultra-depleted harzburgitic mantle, it can be stable to over 180 km depth. The formation of chromian spinel inclusions in diamonds is restricted to pressures between 4.0 and 6.0 GPa. The modes of spinel decrease rapidly to less than 1 vol % when it coexists with garnet; hence, spinel grains can be easily overlooked during the petrographical characterization of small mantle xenoliths. The very Cr-rich nature of many spinels from xenoliths and diamonds from cratonic settings may be simply a consequence of their low modes in high-pressure assemblages; thus, their composition does not necessarily imply an extremely refractory composition of the source rock. The model also shows that large Ca and Cr variations in lherzolitic garnets in equilibrium with spinel can be explained by variations of pressure and temperature along a continental geotherm and do not necessarily imply variations of bulk composition. The slope of the Cr# [i.e. Cr/(Cr + Al)mol] isopleths in garnet in equilibrium with spinel changes significantly at high temperatures, posing serious limitations to the applicability of empirical geobarometric methods calibrated on cratonic mantle xenoliths in hotter, off-craton, lithospheric mantle sections.  相似文献   

6.
Heat has a close relation to cratonic stability. We reconstructed the thermal history of the eastern North China, Wyoming, and Siberian cratons by using xenolith and thermal indicator data, and then the thermal lithospheric thickness in geological time was calculated by combining the crustal structural and thermal parameters. Researches from three cratons demonstrate that heat-flow values of the eastern North China Craton (eNCC) and Wyoming Craton (WC) have increased exceeding 20 mW/m2 in the Meso-Cenozoic and the thermal lithospheric thickness has decreased by >100 km because of the cratonic destabilization. However, the thermal history variation of the Siberian Craton (SC) is small, ~5 mW/m2, the lithosphere has remained stable and it has experienced relatively little lithospheric thinning <100 km. The differences of the cratonic thermal regime are related to the breakup of the Pangea supercontinent. Destabilized cratons on active continental margin are directly affected by plate subduction. Stable cratons in the interior of the supercontinents are directly affected by the upwelling of superplumes and that may result in lithospheric thinning. Differences in the thermal evolutions of cratons provide new geothermal evidence for the different thermal regimes associated with the breakup of the Pangea supercontinent.  相似文献   

7.
The lithospheric structure of ancient cratons provides important constraints on models relating to tectonic evolution and mantle dynamics. Here we present the 3D lithospheric structure of the North China Craton (NCC) from a joint inversion of gravity, geoid and topography data. The NCC records a prolonged history of Archean and Paleoproterozoic accretion of crustal blocks through subduction and collision building the cratonic architecture, which was subsequently differentially destroyed during Mesozoic through extensive magmatism. The thermal structure obtained in our study is considered to define the lithosphere-asthenosphere boundary (LAB) of the NCC, and reflects the density variations within the mantle lithosphere. Employing the Moho depths from deep seismic sounding profiles for the inversion, and based on repeated computations using different parameters, we estimate the Moho depth, LAB depth and average crustal density of the craton. The Moho depth varies from 28 to 50 km and the LAB depth varies from 105 to 205 km. The LAB and Moho show concordant thinning from West to East of the NCC. The average crustal density is 2870 kg m− 3 in the western part of the NCC, higher than that in the eastern part (2750 kg m− 3). The results of joint inversion in our study yielded LAB depth and lithospheric thinning features similar to those estimated from thermal and seismic studies, although our results show different depth and variations in the thickness. The lithosphere gently thins from 145 to 105 km in the eastern NCC, where as the thinning is much less pronounced in the western NCC with average depth of about 175 km. The joint inversion results in this study provide another perspective on the lithospheric structure from the density properties and corresponding geophysical responses in an ancient craton.  相似文献   

8.
The North China Craton (NCC) provides one of the classic examples of craton destruction, although the mechanisms and processes of its decratonization are yet to be fully understood. Here we integrate petrological, geochemical, geochronological and geophysical information from the NCC and conclude that the destruction of the craton involved multiple events of circum-craton subduction, which provided the driving force that destabilized mantle convection and tectonically eroded the lithospheric mantle beneath the craton. Furthermore, subducted-slab-derived fluids/melts weakened the subcontinental lithospheric mantle and facilitated thermo-mechanical and chemical erosion of the lithosphere. The more intense destruction beneath the eastern part of the NCC reflects the crucial contribution of Pacific plate subduction from the east that overprinted the mantle lithosphere modified during the early subduction processes. Our study further establishes the close relationship between lithospheric modification via peridotite–melt reactions induced by oceanic plate subduction and cratonic destruction.  相似文献   

9.
The composition and thermal state of the lithospheric mantle under the North Atlantic craton was investigated using a suite of peridotite xenoliths from the diamond-bearing Sarfartoq kimberlite dike swarm of southwestern Greenland. Elevated olivine and whole-rock Mg# (>0.9) attest to the refractory nature of the Sarfartoq mantle showing comparable degrees of depletion to other cratonic roots. Modal analyses indicate that the Sarfartoq mantle is not typified by the orthopyroxene enrichment observed in the Kaapvaal root, but shows more affinity with the Canadian Arctic (Somerset Island), Tanzania, and East Greenland (Wiedemann Fjord) peridotites. The Sarfartoq peridotites have equilibrated at temperatures and pressures ranging from 660 to 1,280 °C and from 2.2 to 6.3 GPa, and define a relatively low mantle heat flow of 13.2±1 mW/m2. In addition, the lithospheric mantle underneath the Sarfartoq area is compositionally layered as follows: (1) an internally stratified upper layer (70 to 180 km) consisting of coarse, un-deformed, refractory garnet-bearing and garnet-free peridotites and, (2) a lower layer (180 to 225 km) characterized by fertile, CPX-bearing, porphyroclastic garnet lherzolites. The stratification observed in the upper refractory harzburgite layer (70–180 km) is reflected by an increase in fertility (e.g., decrease in olivine abundance and forsterite content) with depth. The sharp nature of the boundary between the upper and lower layers may indicate multistage growth of the lithospheric mantle.Editorial responsibility: T.L. Grove  相似文献   

10.
We studied diamonds from a 2.697–2.700 Ga Wawa metaconglomerate (Southern Superior craton) and identified mineral inclusions of high-Cr, low-Ca pyrope garnet, low-Ti Mg-chromite, olivine (Fo93), and orthopyroxene (En94). The diamonds have δ13C of ?2.5 to ?4.0 ‰ and derive from the spinel-garnet and garnet facia of harzburgite. Geothermobarometry on non-touching, coexisting garnet-olivine and garnet-orthopyroxene pairs constrains the maximum geothermal gradient of 41 mW/m2 for the Neoarchean and a minimum lithosphere thickness of 190 km. The depleted harzburgitic paragenesis equilibrated at a relatively cold geotherm suggests the presence of a pre-2.7 Ga diamondiferous cratonic root beneath the northern Wawa terrane or the Opatica terrane of the Southern Superior craton, i.e., beneath terranes identified as sources for the metaconglomerate diamonds. Geophysical surveys, geothermal data, and petrology of mantle xenoliths emplaced in the Proterozoic-Mesozoic trace evolution of the mantle thermal regime and composition from the Archean to present. The root was thinned down to 150 km by the Jurassic, when the mantle was heated to 41–42 mW/m2. The diamondiferous root destruction was accompanied by more significant heating and was complete by 1.1 Ga in areas adjacent to the Midcontinent Rift. The geometry of the current high-velocity root and spatial correlations with boundaries of crustal terranes that docked to the nuclei of the Superior protocraton in the Neoarchean suggest that the root destruction in the Southern Superior may have been associated with tectonic erosion, craton amalgamation, and ensuing ingress of asthenospheric fluids.  相似文献   

11.
An updated analysis of geothermal data from the highland area of eastern Brazil has been carried out and the characteristics of regional variations in geothermal gradients and heat flow examined. The database employed includes results of geothermal measurements at 45 localities. The results indicate that the Salvador craton and the adjacent metamorphic fold belts northeastern parts of the study area are characterized by geothermal gradients in the range of 6–17°C/km. The estimated heat flow values fall in the range of 28–53 mW/m2, with low values in the cratonic area relative to the fold belts. On the other hand, the São Francisco craton and the intracratonic São Francisco sedimentary basin in the southwestern parts are characterized by relatively higher gradient values, in the range of 14–42°C/km, with the corresponding heat flow values falling in the range of 36–89 mW/m2. Maps of regional variations indicate that high heat flow anomaly in the São Francisco craton is limited to areas of sedimentary cover, to the west of the Espinhaço mountain belt. Crustal thermal models have been developed to examine the implications of the observed intracratonic variations in heat flow. The thermal models take into consideration variation of thermal conductivity with temperature as well as change of radiogenic heat generation with depth. Vertical distributions of seismic velocities were used in obtaining estimates of radiogenic heat production in crustal layers. Crustal temperatures are calculated based on a procedure that makes simultaneous use of the Kirchoff and Generalized Integral Transforms, providing thereby analytical solutions in 2D and 3D geometry. The results point to temperature variations of up to 300°C at the Moho depth, between the northern Salvador and southern São Francisco cratons. There are indications that differences in rheological properties, related to thermal field, are responsible for the contrasting styles of deformation patterns in the adjacent metamorphic fold belts.  相似文献   

12.
根据地质和地球物理特征表现出的岩石圈不连续，华北地区可区分出鄂尔多斯克拉通型、燕山-太行造山带型和华北平原裂谷型三类岩石圈。依据岩石学方法、壳幔演化模型、造山带形成过程以及地震波速与岩石化学成分之间的关系，建立了华北地区三类型岩石圈的壳幔岩石学结构和化学结构，讨论了不同岩石圈类型的壳幔物质结构、地壳和岩石圈地幔厚度的地质含义、岩石圈不连续在划分岩石圈单元中的作用及不同类型岩石圈形成的大陆动力学意义。  相似文献   

13.
This article discusses the Meso–Cenozoic thermal history, thermal lithospheric thinning, and thermal structure of the lithosphere of the Bohai Bay Basin, North China. The present-day thermal regime of the basin features an average heat flow of 64.5 ± 8.1 mW m–2, a lithospheric thickness of 76–102 km, and a ‘hot mantle but cold crust’-type lithospheric thermal structure. The Meso–Cenozoic thermal history experienced two heat flow peaks in the late Early Cretaceous and in the middle to late Palaeogene, with heat flow values of 82–86 mW m?2 and 81–88 mW m?2, respectively. Corresponding to these peaks, the thermal lithosphere experienced two thinning stages during the Cretaceous and Palaeogene, reaching a minimum thickness of 43–61 km. The lithospheric thermal structure transformed from the ‘hot crust but cold mantle’ type in the Triassic–Jurassic to the ‘cold crust but hot mantle’ type in the Cretaceous–Cenozoic, according to the ratio of mantle to surface heat flow (qm/qs). The research on the thermal history and lithospheric thermal structure of sedimentary basins can effectively reveal the thermal regime at depth in the sedimentary basins and provide significance for the study of the basin dynamics during the Meso–Cenozoic.  相似文献   

14.
We present a new regional model for the depth-averaged density structure of the cratonic lithospheric mantle in southern Africa constrained on a 30′ × 30′ grid and discuss it in relation to regional seismic models for the crust and upper mantle, geochemical data on kimberlite-hosted mantle xenoliths, and data on kimberlite ages and distribution. Our calculations of mantle density are based on free-board constraints, account for mantle contribution to surface topography of ca. 0.5–1.0 km, and have uncertainty ranging from ca. 0.01 g/cm3 for the Archean terrains to ca. 0.03 g/cm3 for the adjacent fold belts. We demonstrate that in southern Africa, the lithospheric mantle has a general trend in mantle density increase from Archean to younger lithospheric terranes. Density of the Kaapvaal mantle is typically cratonic, with a subtle difference between the eastern, more depleted, (3.31–3.33 g/cm3) and the western (3.32–3.34 g/cm3) blocks. The Witwatersrand basin and the Bushveld Intrusion Complex appear as distinct blocks with an increased mantle density (3.34–3.35 g/cm3) with values typical of Proterozoic rather than Archean mantle. We attribute a significantly increased mantle density in these tectonic units and beneath the Archean Limpopo belt (3.34–3.37 g/cm3) to melt-metasomatism with an addition of a basaltic component. The Proterozoic Kheis, Okwa, and Namaqua–Natal belts and the Western Cape Fold Belt with the late Proterozoic basement have an overall fertile mantle (ca. 3.37 g/cm3) with local (100–300 km across) low-density (down to 3.34 g/cm3) and high-density (up to 3.41 g/cm3) anomalies. High (3.40–3.42 g/cm3) mantle densities beneath the Eastern Cape Fold belt require the presence of a significant amount of eclogite in the mantle, such as associated with subducted oceanic slabs.We find a strong correlation between the calculated density of the lithospheric mantle, the crustal structure, the spatial pattern of kimberlites, and their emplacement ages. (1) Blocks with the lowest values of mantle density (ca. 3.30 g/cm3) are not sampled by kimberlites and may represent the “pristine” Archean mantle. (2) Young (< 90 Ma) Group I kimberlites sample mantle with higher density (3.35 ± 0.03 g/cm3) than the older Group II kimberlites (3.33 ± 0.01 g/cm3), but the results may be biased by incomplete information on kimberlite ages. (3) Diamondiferous kimberlites are characteristic of regions with a low-density cratonic mantle (3.32–3.35 g/cm3), while non-diamondiferous kimberlites sample mantle with a broad range of density values. (4) Kimberlite-rich regions have a strong seismic velocity contrast at the Moho, thin crust (35–40 km) and low-density (3.32–3.33 g/cm3) mantle, while kimberlite-poor regions have a transitional Moho, thick crust (40–50 km), and denser mantle (3.34–3.36 g/cm3). We explain this pattern by a lithosphere-scale (presumably, pre-kimberlite) magmatic event in kimberlite-poor regions, which affected the Moho sharpness and the crustal thickness through magmatic underplating and modified the composition and rheology of the lithospheric mantle to make it unfavorable for consequent kimberlite eruptions. (5) Density anomalies in the lithospheric mantle show inverse correlation with seismic Vp, Vs velocities at 100–150 km depth. However, this correlation is weaker than reported in experimental studies and indicates that density-velocity relationship in the cratonic mantle is strongly non-unique.  相似文献   

15.
The origin of high topography in southern Africa is enigmatic. By comparing topography in different cratons, we demonstrate that in southern Africa both the Archean and Proterozoic blocks have surface elevation 500–700 m higher than in any other craton worldwide, except for the Tanzanian Craton. An unusually high topography may be caused by a low density (high depletion) of the cratonic lithospheric mantle and/or by the dynamic support of the mantle with origin below the depth of isostatic compensation (assumed here to be at the lithosphere base). We use free-board constraints to examine the relative contributions of the both factors to surface topography in the cratons of southern Africa. Our analysis takes advantage of the SASE seismic experiment which provided high resolution regional models of the crustal thickness.We calculate the model of density structure of the lithospheric mantle in southern Africa and show that it has an overall agreement with xenolith-based data for lithospheric terranes of different ages. Density of lithospheric mantle has significant short-wavelength variations in all tectonic blocks of southern Africa and has typical SPT values of ca. 3.37–3.41 g/cm3 in the Cape Fold and Namaqua–Natal fold belts, ca. 3.34–3.35 g/cm3 in the Proterozoic Okwa block and the Bushveld Intrusion Complex, ca. 3.34–3.37 g/cm3 in the Limpopo Belt, and ca. 3.32–3.33 g/cm3 in the Kaapvaal and southern Zimbabwe cratons.The results indicate that 0.5–1.0 km of surface topography, with the most likely value of ca. 0.5 km, cannot be explained by the lithosphere structure within the petrologically permitted range of mantle densities and requires the dynamic (or static) contribution from the sublithospheric mantle. Given a low amplitude of regional free air gravity anomalies (ca. + 20 mGal on average), we propose that mantle residual (dynamic) topography may be associated with the low-density region below the depth of isostatic compensation. A possible candidate is the low velocity layer between the lithospheric base and the mantle transition zone, where a temperature anomaly of 100–200 °C in a ca. 100–150 km thick layer may explain the observed reduction in Vs velocity and may produce ca. 0.5–1.0 km to the regional topographic uplift.  相似文献   

16.
Internally consistent thermodynamic computation of equilibria in the FeO-MgO-SiO2 system up to 300 kbar is carried out and phase diagrams and profiles of the elastic properties and density are constructed at the depths of 300–800 km. Comparisons of calculated thermodynamic properties for different petrological models with seismic velocity profiles have been used to constrain the mineralogy of the mantle discontinuities. The 400-km discontinuity may represent the univariant or divariant transition in the olivine component of pyrolite as well as a chemical boundary. For the pyrolite composition at the depth of 650 km there are two different spinel + perovskite + stishovite (640 km) and magnesiowustite + spinel + perovskite (650 km) divariant loops (1–2 km wide) separated by a Invariant zone spinel + perovskite (4–6 km wide). The results indicate that phase changes in pyrolite do not explain the 650-km discontinuity. It is also shown that it is impossible to match the seismic properties observed at the depths of 600–800 km and through the discontinuity with any isochemical petrological model considered in the FMS system. However, increasing the iron content or silica and iron contents across the 650-km discontinuity can produce thermodynamic properties in the lower mantle that are more consistent with those inferred from seismic observations. Constraints on the SiO2 and iron contents in the mantle are inferred from the comparison of thermodynamic and seismological data.  相似文献   

17.
A method is proposed for determining the temperature of the Earth’s upper mantle from geochemical and seismic data. The data are made consistent by physicochemical simulations, which enable one to derive physical characteristics from geochemical compositional models (direct problem) and to convert seismic velocity profiles into model for the temperature distribution (inverse problem). The methods were used to simulate temperature distribution profiles in the “normal” and “cold” mantle on the basis of profiles for the velocities of P and S waves in the IASP91 model and regional models for the Kaapvaal craton. The constraints assumed for the chemical composition included the depleted material of garnet peridotites and the fertile primitive mantle. The conversion of seismic into thermal profiles was conducted by minimizing the Gibbs free energy with the use of equations of state for the mantle material with regard for anharmonicity and the effects of inelasticity. The sensitivity of the model to the chemical composition and its importance in application to the solution of inverse problems is demonstrated. Temperature profiles derived from the IASP91 and some regional models for depths of 200–210 km display an inflection on geotherms toward decreasing temperatures, which is physically senseless. This anomaly cannot be related to either the presence of volatiles or the occurrence of partial melting, because both of them should have resulted in a decrease, but not an increase, in the seismic velocities. Temperature inversion can be ruled out by the gradual fertilization of the mantle with depth. In this situation, the upper mantle material at depths of 200–300 km should be enriched in FeO, Al2O3, and CaO relative to garnet peridotites and be simultaneously depleted in these oxides relative to the pyrolite material of the primitive mantle. It can be generally concluded that both the lithosphere and sublithospheric mantle of the Kaapvaal craton, as well as the normal mantle, should be chemically stratified.  相似文献   

18.
The Archean lithospheric mantle beneath the Kaapvaal–Zimbabwe craton of Southern Africa shows ±1% variations in seismic P-wave velocity at depths within the diamond stability field (150–250 km) that correlate regionally with differences in the composition of diamonds and their syngenetic inclusions. Seismically slower mantle trends from the mantle below Swaziland to that below southeastern Botswana, roughly following the surface outcrop pattern of the Bushveld-Molopo Farms Complex. Seismically slower mantle also is evident under the southwestern side of the Zimbabwe craton below crust metamorphosed around 2 Ga. Individual eclogitic sulfide inclusions in diamonds from the Kimberley area kimberlites, Koffiefontein, Orapa, and Jwaneng have Re–Os isotopic ages that range from circa 2.9 Ga to the Proterozoic and show little correspondence with these lithospheric variations. However, silicate inclusions in diamonds and their host diamond compositions for the above kimberlites, Finsch, Jagersfontein, Roberts Victor, Premier, Venetia, and Letlhakane do show some regional relationship to the seismic velocity of the lithosphere. Mantle lithosphere with slower P-wave velocity correlates with a greater proportion of eclogitic versus peridotitic silicate inclusions in diamond, a greater incidence of younger Sm–Nd ages of silicate inclusions, a greater proportion of diamonds with lighter C isotopic composition, and a lower percentage of low-N diamonds whereas the converse is true for diamonds from higher velocity mantle. The oldest formation ages of diamonds indicate that the mantle keels which became continental nuclei were created by middle Archean (3.2–3.3 Ga) mantle depletion events with high degrees of melting and early harzburgite formation. The predominance of sulfide inclusions that are eclogitic in the 2.9 Ga age population links late Archean (2.9 Ga) subduction-accretion events involving an oceanic lithosphere component to craton stabilization. These events resulted in a widely distributed younger Archean generation of eclogitic diamonds in the lithospheric mantle. Subsequent Proterozoic tectonic and magmatic events altered the composition of the continental lithosphere and added new lherzolitic and eclogitic diamonds to the already extensive Archean diamond suite.  相似文献   

19.
《Gondwana Research》2014,25(3-4):849-864
We have imaged the lithospheric structure beneath the central and western North China Craton (NCC) with Rayleigh wave tomography. The Rayleigh waveforms of 100 teleseismic events recorded by 208 broadband stations are used to yield high-resolution phase velocity maps at 13 periods from 20 s to 143 s. A 3-D S-wave velocity model is constructed based on the phase velocity maps. Our S-wave velocity model is broadly consistent with the results of previous tomography studies, but shows more detailed variations within the lithosphere. The Trans-North China Orogen (TNCO) is generally characterized by low-velocity anomalies but exhibits great heterogeneities. Two major low-velocity zones (LVZs) are observed in the north and south, respectively. The northern LVZ laterally coincides with sites of Cenozoic magmatism and extends to depths greater than 200 km. We propose that a small-scale mantle upwelling is present, confined to the north of the TNCO. A high-velocity patch in the uppermost mantle is also observed between the two LVZs adjacent to the narrow transtensional zone of the Cenozoic Shanxi–Shaanxi Rift (SSR). We interpret this as the remnant of a cratonic mantle root. The Ordos Block in the western NCC is associated with high-velocity anomalies, similarly reflecting the existence of cratonic mantle root, but a discernible low-velocity layer is observed at depths of 100–150 km in this location. We interpret that this mid-lithospheric structure was probably formed by metasomatic processes during the early formation of the NCC. Based on the observations from our S-wave velocity model, we conclude that the current highly heterogeneous lithospheric structure beneath the TNCO is the result of multiphase reworking of pre-existing mechanically weak zones since the amalgamation of the craton. The latest Cenozoic lithospheric reworking is dominated by the far-field effects of both Pacific plate subduction and the India–Eurasia collision.  相似文献   

20.
中国东部岩石圈热状态与流变学强度特征   总被引：10，自引：1，他引：9  
根据均衡原理制约的地热计算得到中国东部岩石圈的温度分布状态,以40、70、100km和莫霍面深度等温线图以及600°C、1100°C等温面深度的形式表示.同时计算了以1350°C等温面深度表示的中国东部的热岩石圈厚度.结果显示:在扬子克拉通西部四川盆地之下存在160～200km厚的岩石圈根,但在整个华北克拉通之下缺失岩...  相似文献   

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