2014 Vol. 39, No. 2
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2014, 39(2): 125-140.
doi: 10.3799/dqkx.2014.013
Abstract:
Eolian deposits accumulated in the Chinese Loess Plateau (CLP) are valuable archive for reconstruction of paleoenvironmental change in East Asia. The source area of the eolian deposits in the CLP is correlated with the evolution of past atmospheric circulation of East Asia, so it is highly focused by researchers. However, the controversies are still being debated. During the past, methods of geochemistry (isotope geochemistry, element geochemistry, and detrital zircon U-Pb age), mineralogy (dolomite, heavy mineral composition), optically stimulated luminescence (OSL) sensitivity, electronic spine resonance (ESR) signal intensity, environmental magnetism, meteorological observation and modeling, and geomorphic methods were applied to track the source area of the eolian deposits by comparative study of the sediments in the CLP and the potential source areas (e.g., gobi and deserts in north China and the northeast Qinghai-Tibet Plateau) surrounding the CLP. The results show that the source area of eolian deposits in the CLP lies in the north and northwest of the CLP, and northeast Qinghai-Tibet Plateau, the Alashan Plateau (Badan Jaran desert, Tengger desert), Tarim basin and Mongolia would be the most important source areas. The controversies mostly focus on which detail area can be the source area of the eolian deposits in the CLP and whether the source area changed temporarily and spatially. Recent provenance studies of the eolian deposits in the CLP are mainly concentrated on the Quaternary loess deposits, but the studies of older deposits are relatively few. Thus, more methodologies of tracing the source area of eolian deposits and more studies for provenance of eolian deposits which accumulated before Quaternary would be the main task in the future.
Eolian deposits accumulated in the Chinese Loess Plateau (CLP) are valuable archive for reconstruction of paleoenvironmental change in East Asia. The source area of the eolian deposits in the CLP is correlated with the evolution of past atmospheric circulation of East Asia, so it is highly focused by researchers. However, the controversies are still being debated. During the past, methods of geochemistry (isotope geochemistry, element geochemistry, and detrital zircon U-Pb age), mineralogy (dolomite, heavy mineral composition), optically stimulated luminescence (OSL) sensitivity, electronic spine resonance (ESR) signal intensity, environmental magnetism, meteorological observation and modeling, and geomorphic methods were applied to track the source area of the eolian deposits by comparative study of the sediments in the CLP and the potential source areas (e.g., gobi and deserts in north China and the northeast Qinghai-Tibet Plateau) surrounding the CLP. The results show that the source area of eolian deposits in the CLP lies in the north and northwest of the CLP, and northeast Qinghai-Tibet Plateau, the Alashan Plateau (Badan Jaran desert, Tengger desert), Tarim basin and Mongolia would be the most important source areas. The controversies mostly focus on which detail area can be the source area of the eolian deposits in the CLP and whether the source area changed temporarily and spatially. Recent provenance studies of the eolian deposits in the CLP are mainly concentrated on the Quaternary loess deposits, but the studies of older deposits are relatively few. Thus, more methodologies of tracing the source area of eolian deposits and more studies for provenance of eolian deposits which accumulated before Quaternary would be the main task in the future.
2014, 39(2): 141-154.
doi: 10.3799/dqkx.2014.014
Abstract:
The petrography and mineral chemical data of olivine-bearing clinopyroxenite xenolith entrained by the Early Cretaceous adakitic rocks in Xuhuai area, eastern China are reported in this paper. Olivine-bearing clinopyroxenite consists of clinopyroxene (~80%), orthopyroxene (~5%), olivine (~5%), and hornblende (~10%) and displays cumulate texture and massive structure. The orthopyroxene reaction rim can be found around olivine. The hornblendes occur among pyroxenes, and display poikilitic texture. The mineral chemical data indicate that olivines have forsterite (Fo)=77.7-79.3 and Ni=623×10-6-773×10-6, that orthopyroxenes have Mg#=75.6-80.2, Cr=161×10-6-684 ×10-6, and Ni=79×10-6-708×10-6, and that clinopyroxenes have Mg#=84.5-86.4, CaO=21.59%-23.13%, Al2O3=1.72%-2.44%. Chemically, these minerals are similar to the olivine, orthopyroxene, and clinopyroxene phenocrysts from the Mesozoic and Cenozoic basalts and those from the pyroxenites with cumulate origin. Additionally, the clinopyroxenes from the xenolith are characterized by convex upward rare earth element (REE) distribution patterns, low REE contents (∑REE=10.14×10-6-12.71×10-6), and no Eu anomalies (δEu=0.90-1.16), similar to the clinopyroxene phenocrysts in the Cenozoic basalts. Hornblendes in this xenolith have Mg#=74.0-80.4, SiO2=43.2%-44.5%, Na2O=2.04%-2.29%, and display flat REE patterns and depletion in high field strength elements (HFSEs, Nb, Ta, Zr and Hf) and Th as well as enrichment in Sr, Rb and Ba, different from those hornblende xenocrysts from Cenozoic basalts. Combined with its poikilitic texture, it is suggested that these hornblendes could be formed by injection of the host magma. Taken together, we conclude that the olivine-bearing clinopyroxenite xenolith could be formed by the high-pressure accumulation of basaltic melt. Combined with Early Cretaceous bimodal magmatism in eastern China, it is suggested that the olivine-bearing clinopyroxenite could be generated by the underplating of mantle-derived basaltic magma.
The petrography and mineral chemical data of olivine-bearing clinopyroxenite xenolith entrained by the Early Cretaceous adakitic rocks in Xuhuai area, eastern China are reported in this paper. Olivine-bearing clinopyroxenite consists of clinopyroxene (~80%), orthopyroxene (~5%), olivine (~5%), and hornblende (~10%) and displays cumulate texture and massive structure. The orthopyroxene reaction rim can be found around olivine. The hornblendes occur among pyroxenes, and display poikilitic texture. The mineral chemical data indicate that olivines have forsterite (Fo)=77.7-79.3 and Ni=623×10-6-773×10-6, that orthopyroxenes have Mg#=75.6-80.2, Cr=161×10-6-684 ×10-6, and Ni=79×10-6-708×10-6, and that clinopyroxenes have Mg#=84.5-86.4, CaO=21.59%-23.13%, Al2O3=1.72%-2.44%. Chemically, these minerals are similar to the olivine, orthopyroxene, and clinopyroxene phenocrysts from the Mesozoic and Cenozoic basalts and those from the pyroxenites with cumulate origin. Additionally, the clinopyroxenes from the xenolith are characterized by convex upward rare earth element (REE) distribution patterns, low REE contents (∑REE=10.14×10-6-12.71×10-6), and no Eu anomalies (δEu=0.90-1.16), similar to the clinopyroxene phenocrysts in the Cenozoic basalts. Hornblendes in this xenolith have Mg#=74.0-80.4, SiO2=43.2%-44.5%, Na2O=2.04%-2.29%, and display flat REE patterns and depletion in high field strength elements (HFSEs, Nb, Ta, Zr and Hf) and Th as well as enrichment in Sr, Rb and Ba, different from those hornblende xenocrysts from Cenozoic basalts. Combined with its poikilitic texture, it is suggested that these hornblendes could be formed by injection of the host magma. Taken together, we conclude that the olivine-bearing clinopyroxenite xenolith could be formed by the high-pressure accumulation of basaltic melt. Combined with Early Cretaceous bimodal magmatism in eastern China, it is suggested that the olivine-bearing clinopyroxenite could be generated by the underplating of mantle-derived basaltic magma.
2014, 39(2): 155-164.
doi: 10.3799/dqkx.2014.015
Abstract:
The Middle Permian of South China is a unique carbonate succession, which is rich in organic matter and chert nodule, and is one set of the most important marine source rocks. Limestone-marl alternations are widely distributed in this area. The limestones are rich in skeletal detritus, dominated by algae, foraminifera, and mollusks with subordinate amounts of brachiopods, echinoderms, and ostracods, and minor bryozoans and trilobites. Fragile skeletons such as calcareous algae were well preserved, indicating that cementation took place during early diagenesis with little diagenetic compaction. Marls are bioclastic wackestone or dark mudstone. Bioclasts are diverse, including remnants of brachiopods, ostracods and rare trilobites. Bioclastic flakes are oriented parallel to bedding planes, and thin-shelled fragments, e.g., ostracods, are broken due to compaction of the unlithified sediment. Between the particles are micrite and a large number of dissolution seams. As to the particular features of the limestone-marl alternations, it is inferred that differential diagenesis occurred between limestones and marls. Aragonite in marl layers was dissolved to cement limestone, which took place in shallow-burial realm. Mechanical compaction and then pressure dissolution were concentrated in the less cemented strata. The dissolution of aragonite in the shallow marine burial realm is thought to have been initiated by modification of the pore-water chemistry by bacterial oxidation of organic matter.
The Middle Permian of South China is a unique carbonate succession, which is rich in organic matter and chert nodule, and is one set of the most important marine source rocks. Limestone-marl alternations are widely distributed in this area. The limestones are rich in skeletal detritus, dominated by algae, foraminifera, and mollusks with subordinate amounts of brachiopods, echinoderms, and ostracods, and minor bryozoans and trilobites. Fragile skeletons such as calcareous algae were well preserved, indicating that cementation took place during early diagenesis with little diagenetic compaction. Marls are bioclastic wackestone or dark mudstone. Bioclasts are diverse, including remnants of brachiopods, ostracods and rare trilobites. Bioclastic flakes are oriented parallel to bedding planes, and thin-shelled fragments, e.g., ostracods, are broken due to compaction of the unlithified sediment. Between the particles are micrite and a large number of dissolution seams. As to the particular features of the limestone-marl alternations, it is inferred that differential diagenesis occurred between limestones and marls. Aragonite in marl layers was dissolved to cement limestone, which took place in shallow-burial realm. Mechanical compaction and then pressure dissolution were concentrated in the less cemented strata. The dissolution of aragonite in the shallow marine burial realm is thought to have been initiated by modification of the pore-water chemistry by bacterial oxidation of organic matter.
2014, 39(2): 165-173.
doi: 10.3799/dqkx.2014.016
Abstract:
Zhangdongkeng tungsten deposit has a special mineralization model, which is a quartz vein type mineralization in the upper portion while an altered granite type mineralization in the lower part. It also shows that tungsten was enriched in the upper part while molybdenum enriched in the lower. Molybdenites in fine-grained granite and quartz vein were separately used for Re-Os dating to test the temporal relation of the two types of mineralization. The isochron age of three molybdenite samples in fine-grained granite is 155.4±2.1 Ma (MSWD=1.12), with model ages of them between 154.9±2.6 Ma and 156.5±2.6 Ma, and the weighted average age 155.5±1.4 Ma (MSWD=0.44). Five molybdenite samples in quartz vein yield a Re-Os isochron age of 154.6±1.7 Ma (MSWD=0.030), with model ages 154.2±2.3 Ma to 154.7±2.7 Ma, and the weighted mean age is 154.29±0.98 Ma (MSWD=0.045). The results show that the ages of altered granite type mineralization and quartz vein type mineralization are basically almost same, which well indicates that molybdenites in both mineralization types are products of the same magmatic-hydrothermal event.
Zhangdongkeng tungsten deposit has a special mineralization model, which is a quartz vein type mineralization in the upper portion while an altered granite type mineralization in the lower part. It also shows that tungsten was enriched in the upper part while molybdenum enriched in the lower. Molybdenites in fine-grained granite and quartz vein were separately used for Re-Os dating to test the temporal relation of the two types of mineralization. The isochron age of three molybdenite samples in fine-grained granite is 155.4±2.1 Ma (MSWD=1.12), with model ages of them between 154.9±2.6 Ma and 156.5±2.6 Ma, and the weighted average age 155.5±1.4 Ma (MSWD=0.44). Five molybdenite samples in quartz vein yield a Re-Os isochron age of 154.6±1.7 Ma (MSWD=0.030), with model ages 154.2±2.3 Ma to 154.7±2.7 Ma, and the weighted mean age is 154.29±0.98 Ma (MSWD=0.045). The results show that the ages of altered granite type mineralization and quartz vein type mineralization are basically almost same, which well indicates that molybdenites in both mineralization types are products of the same magmatic-hydrothermal event.
2014, 39(2): 174-186.
doi: 10.3799/dqkx.2014.017
Abstract:
Oil shales are present in the Nenjiang Formation of Songliao basin, representing excellent source rocks. Their biomarker compositions provide evidence for a major contribution of aquatic organism. Phytoplankton blooms were promoted by warm-humid climate, and lake eutrophication. Based on the parameters from petrology, element geochemistry and biomarker, a stratified water column with high salinity and anoxic bottom water conditions contributes to OM (organic matter) preservation in the lower oil shale layer. In contrast, under decreased salinity stratified water, high bioproductivity in combination with OM preservation favoured by enhanced algae sizes and telalginite concentrations are suggested as the OM enrichment mechanisms in the upper oil shale layer. In this study, a model of predominant preservation within the lower oil shale layer and a model of dominant productivity within the upper oil shale layer are established. This result can be used as an important indicator for probing into the genesis of OM enrichment in the large continental lake with partial oxygenation and fresh water.
Oil shales are present in the Nenjiang Formation of Songliao basin, representing excellent source rocks. Their biomarker compositions provide evidence for a major contribution of aquatic organism. Phytoplankton blooms were promoted by warm-humid climate, and lake eutrophication. Based on the parameters from petrology, element geochemistry and biomarker, a stratified water column with high salinity and anoxic bottom water conditions contributes to OM (organic matter) preservation in the lower oil shale layer. In contrast, under decreased salinity stratified water, high bioproductivity in combination with OM preservation favoured by enhanced algae sizes and telalginite concentrations are suggested as the OM enrichment mechanisms in the upper oil shale layer. In this study, a model of predominant preservation within the lower oil shale layer and a model of dominant productivity within the upper oil shale layer are established. This result can be used as an important indicator for probing into the genesis of OM enrichment in the large continental lake with partial oxygenation and fresh water.
2014, 39(2): 187-198.
doi: 10.3799/dqkx.2014.018
Abstract:
The pyroclastics grain size characteristics, the pyroclastics composition and the pyroclastics rocks facies of two types of pyroclastics rocks samples of Yingcheng Formation in the southeast edge of Songliao basin, tuffs and tuffaceous sandstone, had been studied. The results show that, the pyroclastics had been transported at the beginning by volcanism excitation, however, traction flow, gravity flow and the double mechanism of traction flow and gravity flow controlled the transportation process. The conclusion drawn from pyroclastic microscopic characteristics study, genetic type analysis and lithofacies analysis of those samples in study area calculates that, the majority volcanic debris accumulation was composed of hot base surge and pyroclastic flow deposition and a small portion had been composed of fallout. The characteristics of detrital composition were those, there were a large amount of crystal fragments, a small amount of shard and lithic fragments, and the lithic fragments existed only in coarse grain composition. Studies suggest that, the pyroclastics outcropped in this area were the reworked pyroclastics in the depositional environment. There were 4 types of pyroclastics facies, the hot base surge in the previous valley, the pyroclastics flow in the previous valley and the base surge and fallout on the alluvial plain had been distinguished in this area. Based on the member Ⅱ of Yingcheng Formation in the southeast edge of Songliao basin, facies models of the reworked pyroclastics in these fluvial-alluvial plain sedimentary environments were established.
The pyroclastics grain size characteristics, the pyroclastics composition and the pyroclastics rocks facies of two types of pyroclastics rocks samples of Yingcheng Formation in the southeast edge of Songliao basin, tuffs and tuffaceous sandstone, had been studied. The results show that, the pyroclastics had been transported at the beginning by volcanism excitation, however, traction flow, gravity flow and the double mechanism of traction flow and gravity flow controlled the transportation process. The conclusion drawn from pyroclastic microscopic characteristics study, genetic type analysis and lithofacies analysis of those samples in study area calculates that, the majority volcanic debris accumulation was composed of hot base surge and pyroclastic flow deposition and a small portion had been composed of fallout. The characteristics of detrital composition were those, there were a large amount of crystal fragments, a small amount of shard and lithic fragments, and the lithic fragments existed only in coarse grain composition. Studies suggest that, the pyroclastics outcropped in this area were the reworked pyroclastics in the depositional environment. There were 4 types of pyroclastics facies, the hot base surge in the previous valley, the pyroclastics flow in the previous valley and the base surge and fallout on the alluvial plain had been distinguished in this area. Based on the member Ⅱ of Yingcheng Formation in the southeast edge of Songliao basin, facies models of the reworked pyroclastics in these fluvial-alluvial plain sedimentary environments were established.
2014, 39(2): 199-209.
doi: 10.3799/dqkx.2014.019
Abstract:
How to evaluate the marine source rocks with high maturity is of crucial importance. Element geochemical indexes are used to build a new evaluating method for source rock in this study. In this paper, the samples are from the Lower Cambrian in Yangba section of Nanjiang County, Sichuan. We used the measured Xxs (a relative excess of X compared to titanium in post-Archean Australian shale), the authigenetic elements from the the same environment are treated as a class. Then, illustrate the linear relation between each of TOC, δ13Ccarb and δ13Corg and the authigenetic elements from the same class, optimize the authigenetic elements that have closely relationship with enriched organic matter. Finally, the factor formula is built to indicate the changing of the quantity and quality of organic matter during the process of formation, deposition and burial according to the factor analysis. This formula is used to divide the rank of source rock quality of the Lower Cambrian in Yangba section. Thus, this geochemistry evaluation method would provide some new ideas for identifying and evaluating the marine high-maturity source rocks.
How to evaluate the marine source rocks with high maturity is of crucial importance. Element geochemical indexes are used to build a new evaluating method for source rock in this study. In this paper, the samples are from the Lower Cambrian in Yangba section of Nanjiang County, Sichuan. We used the measured Xxs (a relative excess of X compared to titanium in post-Archean Australian shale), the authigenetic elements from the the same environment are treated as a class. Then, illustrate the linear relation between each of TOC, δ13Ccarb and δ13Corg and the authigenetic elements from the same class, optimize the authigenetic elements that have closely relationship with enriched organic matter. Finally, the factor formula is built to indicate the changing of the quantity and quality of organic matter during the process of formation, deposition and burial according to the factor analysis. This formula is used to divide the rank of source rock quality of the Lower Cambrian in Yangba section. Thus, this geochemistry evaluation method would provide some new ideas for identifying and evaluating the marine high-maturity source rocks.
2014, 39(2): 210-220.
doi: 10.3799/dqkx.2014.020
Abstract:
Material balance equation is an effective approach to estimate the OGIP, water influx, and gas recharge capacity in condensate gas reservoirs. The condensate gas reservoir with gas recharge capacity, abnormally high pressure, and edge and bottom water-drive is one of the most complicated reservoirs which include the features of condensate gas, pressure-dependent parameters and water influx. In this study, a new material balance equation for this reservoir was derived which considering retrograde condensation, the expansion of rock, bounded water and condensate oil, the phase change of vapor and dissolved gas, water and gas influx. The theories of Haiming Distance and relevance is used to analyze the mean square error of drive index, deviation index between pseudo pressure ratio and recovery, and generalized objective function. Based on the analysis of relative similarity degree and closeness coefficient, we chose the optimization scheme, and calculated the OGIP, gas recharge, water influx, and the aquifer volume. Based on the case study in condensate gas reservoir in Northwest China, a calculated reserve is 66.80×108 m3. We investigated and analyzed the parameters sensitivity by calculating the errors in the assumption of those parameters not included. The error of reserve estimation is up to 91.77% if oil condensate is not considered. Other parameters also have some effect such as water influx, the expansion and gas recharge. Respectively, the errors are 75.75%, 23.95% and 12.87%. The effect of gas escape from the dissolution and vapor is not significant but cannot be neglected, the errors are 2.69% of escaped gas and 1.65% of vapor.
Material balance equation is an effective approach to estimate the OGIP, water influx, and gas recharge capacity in condensate gas reservoirs. The condensate gas reservoir with gas recharge capacity, abnormally high pressure, and edge and bottom water-drive is one of the most complicated reservoirs which include the features of condensate gas, pressure-dependent parameters and water influx. In this study, a new material balance equation for this reservoir was derived which considering retrograde condensation, the expansion of rock, bounded water and condensate oil, the phase change of vapor and dissolved gas, water and gas influx. The theories of Haiming Distance and relevance is used to analyze the mean square error of drive index, deviation index between pseudo pressure ratio and recovery, and generalized objective function. Based on the analysis of relative similarity degree and closeness coefficient, we chose the optimization scheme, and calculated the OGIP, gas recharge, water influx, and the aquifer volume. Based on the case study in condensate gas reservoir in Northwest China, a calculated reserve is 66.80×108 m3. We investigated and analyzed the parameters sensitivity by calculating the errors in the assumption of those parameters not included. The error of reserve estimation is up to 91.77% if oil condensate is not considered. Other parameters also have some effect such as water influx, the expansion and gas recharge. Respectively, the errors are 75.75%, 23.95% and 12.87%. The effect of gas escape from the dissolution and vapor is not significant but cannot be neglected, the errors are 2.69% of escaped gas and 1.65% of vapor.
2014, 39(2): 221-226.
doi: 10.3799/dqkx.2014.021
Abstract:
With the emergence and deepening of GIS cloud concept, after the client/server structure (C/S) and browser/server structure (B/S), this paper presents a new generation software architecture named T-C-V (terminal-cloud-virtual), which is suitable for cloud computing and cloud service, and elaborates its basic concept, general structure and core technologies. In this structure, V-tier based on the virtualization software and hardware equipment in bottom tier, shielding different structural characteristics among different computers, different networks, different storage devices, provides a unified and efficient operating environment for the upper tier; C-tier is a framework for massive GIS data, service and resource management and service system; T-tier faced to the information users such as the government, business, the public et al., provides standard visiting interface and builds all kinds of terminal applications. In a word, T-C-V structure will change geographic information service model, calculation model and business model, which can create an environment for better interaction and more transparent creation of public and enterprise applications.
With the emergence and deepening of GIS cloud concept, after the client/server structure (C/S) and browser/server structure (B/S), this paper presents a new generation software architecture named T-C-V (terminal-cloud-virtual), which is suitable for cloud computing and cloud service, and elaborates its basic concept, general structure and core technologies. In this structure, V-tier based on the virtualization software and hardware equipment in bottom tier, shielding different structural characteristics among different computers, different networks, different storage devices, provides a unified and efficient operating environment for the upper tier; C-tier is a framework for massive GIS data, service and resource management and service system; T-tier faced to the information users such as the government, business, the public et al., provides standard visiting interface and builds all kinds of terminal applications. In a word, T-C-V structure will change geographic information service model, calculation model and business model, which can create an environment for better interaction and more transparent creation of public and enterprise applications.
2014, 39(2): 227-239.
doi: 10.3799/dqkx.2014.022
Abstract:
Birnessite, one of the most common Mn oxide minerals in soils and sediments, has two types of substructures, hexagonal and triclinic, on the basis of the MnO6 layer symmetrical features. In the surface environment, hexagonal birnessite is formed through the chemical or biological oxidation of Mn2+, but the formation pathway of triclinic birnessite and further transformation into todorokite are still not clear. In the simulated surface environment, the chemical conditions and mineralogy mechanism of hexagonal birnessite (acid birnessite and vernadite) transformation to triclinic birnessite, and then into todorokite were investigated by EXAFS, XRD, FESEM/TEM and chemical composition analyses. The results show that hexagonal birnessite can be converted into triclinic birnessite under appropriate Mn(Ⅱ) concentration and weak alkaline conditions (pH≥8), and triclinic birnessite can be further converted into todorokite which consists of long fibers after Mg2+ exchanged and refluxed under the atmospheric pressure. Long fibers of todorokite form mainly through a dissolution-recrystallization process. Reaction of aqueous Mn(Ⅱ) with Mn(Ⅳ) in the hexagonal birnessite MnO6 octahedral layers causes transformation of hexagonal birnessite into triclinic birnessite via filling of yielded Mn(Ⅲ) into vacancy sites in the layers. Compared with acid birnessite, the transformation of vernadite into triclinic birnessite was much easier due to weak crystallization and turbostratic structure of vernadite. Higher pH facilitates adsorption of Mn(Ⅱ) and reaction of Mn(Ⅱ) with Mn(Ⅳ), thus speeds up the transformation of hexagonal birnessite into triclinic birnessite. Therefore, one of the important sources of triclinic birnessite in nature can be denoted as: hexagonal birnessite → triclinic birnessite, which may be one of the important chemical formation mechanisms of todorokite in the surface environment.
Birnessite, one of the most common Mn oxide minerals in soils and sediments, has two types of substructures, hexagonal and triclinic, on the basis of the MnO6 layer symmetrical features. In the surface environment, hexagonal birnessite is formed through the chemical or biological oxidation of Mn2+, but the formation pathway of triclinic birnessite and further transformation into todorokite are still not clear. In the simulated surface environment, the chemical conditions and mineralogy mechanism of hexagonal birnessite (acid birnessite and vernadite) transformation to triclinic birnessite, and then into todorokite were investigated by EXAFS, XRD, FESEM/TEM and chemical composition analyses. The results show that hexagonal birnessite can be converted into triclinic birnessite under appropriate Mn(Ⅱ) concentration and weak alkaline conditions (pH≥8), and triclinic birnessite can be further converted into todorokite which consists of long fibers after Mg2+ exchanged and refluxed under the atmospheric pressure. Long fibers of todorokite form mainly through a dissolution-recrystallization process. Reaction of aqueous Mn(Ⅱ) with Mn(Ⅳ) in the hexagonal birnessite MnO6 octahedral layers causes transformation of hexagonal birnessite into triclinic birnessite via filling of yielded Mn(Ⅲ) into vacancy sites in the layers. Compared with acid birnessite, the transformation of vernadite into triclinic birnessite was much easier due to weak crystallization and turbostratic structure of vernadite. Higher pH facilitates adsorption of Mn(Ⅱ) and reaction of Mn(Ⅱ) with Mn(Ⅳ), thus speeds up the transformation of hexagonal birnessite into triclinic birnessite. Therefore, one of the important sources of triclinic birnessite in nature can be denoted as: hexagonal birnessite → triclinic birnessite, which may be one of the important chemical formation mechanisms of todorokite in the surface environment.
2014, 39(2): 240-250.
doi: 10.3799/dqkx.2014.023
Abstract:
In this paper, the multi-scale wavelet decomposition method was applied to separate the Bouguer gravity anomalies of Nanling region to obtain the local anomalies of granites at different depth scales. The granite distribution is delineated at 5 km, 15 km and 25 km respectively by 3D density mapping technique. We discuss some hot issues of Nanling granitic intrusions, including their geological and geophysical model, induced heat source and genetic model, by integrating and analyzing the seismic imaging and magnetotelluric data. The results show that the gravity anomalies and granitic intrusions have apparent partition features marked by Chaling-Chenzhou fault. The local gravity anomalies in southeast region with lower amplitude indicate shallow emplacement depth of granitic intrusions, and the magma intrude into the near-surface along the small channel to form rock cover, whereas the local gravity anomalies in northwest region with higher amplitude suggest deeper emplacement depth. Most of granites' emplacement depths are no more than 25 km. The large-scale low-density and low-speed region in Zhuguangshan and Maoerling area exits near the depth range of 15-25 km. The low-speed and low-resistance region also appears at the top of mantle in Zhuguangshan area, suggesting the upwelling channel of high-temperature asthenosphere lava. Therefore, the basaltic magma underplating may cause the protolith partial melting in middle-lower crust, which also provides the heat and material sources for the large-scale mineralization in Nanling region.
In this paper, the multi-scale wavelet decomposition method was applied to separate the Bouguer gravity anomalies of Nanling region to obtain the local anomalies of granites at different depth scales. The granite distribution is delineated at 5 km, 15 km and 25 km respectively by 3D density mapping technique. We discuss some hot issues of Nanling granitic intrusions, including their geological and geophysical model, induced heat source and genetic model, by integrating and analyzing the seismic imaging and magnetotelluric data. The results show that the gravity anomalies and granitic intrusions have apparent partition features marked by Chaling-Chenzhou fault. The local gravity anomalies in southeast region with lower amplitude indicate shallow emplacement depth of granitic intrusions, and the magma intrude into the near-surface along the small channel to form rock cover, whereas the local gravity anomalies in northwest region with higher amplitude suggest deeper emplacement depth. Most of granites' emplacement depths are no more than 25 km. The large-scale low-density and low-speed region in Zhuguangshan and Maoerling area exits near the depth range of 15-25 km. The low-speed and low-resistance region also appears at the top of mantle in Zhuguangshan area, suggesting the upwelling channel of high-temperature asthenosphere lava. Therefore, the basaltic magma underplating may cause the protolith partial melting in middle-lower crust, which also provides the heat and material sources for the large-scale mineralization in Nanling region.
