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The magnetic susceptibility has been used as a quantitative or semi‐quantitative proxy for reconstructing the summer monsoon intensity in the Chinese Loess Plateau based on extensive studies on climatic or/and environmental mechanisms producing the magnetic susceptibility signatures. However, the precise nature of the link between past climates and the susceptibility signatures has remained uncertain primarily due to lack of our understanding in the ?nalizing and preserving processes of the signatures. This paper attempts to examine the reliability or acceptability of this summer monsoon proxy from non‐magnetic perspectives of soil‐forming processes. We chose nine sections along two transects: one across the western part of the Chinese Loess Plateau and another across the eastern part. Several conclusions can be drawn from our analytical data. First, clay translocation within the S1 palaeosol pro?les, as indicated by ?eld‐observed clay coatings on ped faces in Bt and Bk horizons and demonstrated by laboratory‐analysed clay contents, must have moved some of the magnetic minerals downward so that the susceptibility re?ects only the post‐translocation distribution of the magnetic‐susceptibility‐producing minerals. Second, the best‐developed palaeosol S1S3 at most of the sections studied is not expressed by the magnetic susceptibility because this palaeosol developed in underlying coarse loess (L2) and coarse textures tend to lower the susceptibility. Third, carbonate concentration is normally negatively correlated with the magnetic susceptibility or simply suppresses the magnetic susceptibility peak when the susceptibility enhancement exceeds the carbonate dilution effect. To conclude, extreme caution must be observed when using magnetic susceptibility signatures to retrieve high‐resolution records of the last interglacial palaeoclimate in the Chinese Loess Plateau. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
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居里面是地球内部铁磁性物质向顺磁性物质转换的界面,在温度略低于居里点时物质磁化率会快速升高,这被称为Hopkinsin峰.对于地球内部而言Hopkinsin峰是只有几百米至几公里厚的薄层,由于其与居里温度的关系,因此其底界面的深度可以作为居里面深度的估计.传统的居里面深度探测方法包括谱分析方法、等层模型方法和温度－深度剖面法.这些方法是人们研究地球内部热结构和居里面深度的重要手段,但是谱分析和等层模型的结果均有一些固有的缺点,如横向分辨率太低等;而地热方程的结果则受地表因素影响十分严重,并且地球内部的热源分布也不是十分清楚,这导致了其结果是不可靠的.本文提出用MT方法探测居里面深度,通过对几种简单一维模型进行的正反演数值试验,论证了该方法的可行性.结果表明,用MT方法研究居里面性质,不但可以得到居里面深度,还可以得到居里面顶部Hopkinsin峰所对应介质的电学和磁学性质,但必须同时反演岩层的电阻率和磁导率,才能获得较为可靠的居里面深度估计. 相似文献
4.
High magnetic susceptibility produced by thermal decomposition of core samples from the Chelungpu fault in Taiwan 总被引：7，自引：0，他引：7
Wataru Tanikawa Toshiaki Mishima Tetsuro Hirono Wonn Soh Sheng-Rong Song 《Earth and Planetary Science Letters》2008,272(1-2):372-381
We carried out thermomagnetic susceptibility analyses of fault rocks from core samples from Hole B of the Taiwan Chelungpu Fault Drilling Project (TCDP) to investigate the cause of high magnetic susceptibilities in the fault core. Test samples were thermally and mechanically treated by heating to different maximum temperatures of up to 900 °C and by high-velocity frictional tests before magnetic analyses. Thermomagnetic susceptibility analyses of natural fault rocks revealed that magnetization increased at maximum heating temperatures above 400 °C in the heating cycle, and showed three step increases, at 600 to 550 °C and at 300 °C during the cooling cycle. These behaviors are consistent with the presence of pyrite, siderite and chlorite, suggesting that TCDP gouge originally included these minerals, which contributed to the generation the magnetic susceptibility by thermomechanical reactions. The change in magnetic susceptibility due to heating of siderite was 20 times that obtained by heating pyrite and chlorite, so that only a small fraction of siderite decomposition is enough to cause the slight increase of the susceptibility observed in the fault core. Color measurement results indicate that thermal decomposition by frictional heating took place under low-oxygen conditions at depth, which prevented the minerals from oxidizing to reddish hematite. This finding supports the inference that a mechanically driven chemical reaction partly accounts for the high magnetic susceptibility. A kinetic model analysis confirmed that frictional heating can cause thermal decomposition of siderite and pyrite. Our results show that decomposition of pyrite to pyrrhotite, siderite and, to some extent, chlorite to magnetite is the probable mechanism explaining the magnetic anomaly within the Chelungpu fault zone. 相似文献
5.
In high Ti basanites from the Vogelsberg 1996 drillhole distinct variations in the field dependence of AC magnetic susceptibility correlate with compositional variations of titanomagnetite, as determined by temperature dependence of magnetic susceptibility. Curie temperatures for the basanites are in the range of 240 to 525°C. The field dependence reaches up to 20% for measurements in 30 A/m and 300 A/m AC field amplitude. It is demonstrated that two-field magnetic susceptibility measurements can prevail information about compositional changes of titanomagnetite and therefore support the interpretation of magnetic susceptibility logs from drillcores of basaltic rock suites. 相似文献
6.
Multiple origins and interpretations of the magnetic susceptibility signal in Chinese wind-blown sediments 总被引：16，自引：0，他引：16
In most Chinese loess–paleosol sequences, high magnetic susceptibility values are found in the soil horizons, with low values in the loess layers. The susceptibility signal has been widely used as a proxy climatic indicator. However, both the causes and mechanisms which control susceptibility still remain controversial. Our recent studies challenge some earlier interpretations of the magnetic susceptibility signal in Chinese wind-blown sediments. First, the prevailing hypothesis of pedogenic origin cannot totally account for the susceptibility variations in many sections. Second, in some cases, the principal carrier of the magnetization seems not to be the ultrafine pedogenic minerals, rather coarse lithogenic magnetic minerals derived from local sources can also contribute significantly to the susceptibility signal. Finally, not all the Chinese soils have higher susceptibility values than that of loess or sand layers, opposite relationships do exist. It is concluded that the source and causes of susceptibility may be different from site to site or even at different times at a given site. Any single hypothesis cannot completely account for the susceptibility variations in the Chinese Loess Plateau and its surrounding regions. Besides the previous explanations of carbonate leaching, pedogenic processes and organic decomposition, the contribution of source materials to magnetic susceptibility must also be taken into account. 相似文献
7.
湖泊沉积物的磁化率、频率磁化率及其古气候意义—以青海湖、岱海近代沉积为例 总被引：28，自引：9，他引：28
湖泊沉积物的磁性参数是古气候、古环境的重要代用指标。通过对青海湖QI5孔和岱海DH32孔短岩心的磁化率、频率磁化率的研究,并与地球化学参数等其它资料对比,探讨了以上两个湖区历史时期古气候变化规律;论证了用磁化率、频率磁化率作为古气候、古环境指标的可靠性和敏感性;并对其变化机制进行了初步探讨。 相似文献
8.
湖泊沉积物的磁化率、频率磁化率及其古气候意义—以青海湖、岱海近代沉积为例 总被引：15，自引：3，他引：12
湖泊沉积物的磁性参数是古气候、古环境的重要代用指标。通过对青海湖QI_5孔和岱海DH_(32)孔短岩心的磁化率、频率磁化率的研究,并与地球化学参数等其它资料对比,探讨了以上两个湖区历史时期古气候变化规律;论证了用磁化率、频率磁化率作为古气候、古环境指标的可靠性和敏感性;并对其变化机制进行了初步探讨。 相似文献
9.
Xiuming Liu Zhisheng An Tim Rolph Xiaoke Qiang Paul Hesse Huayu Lu Jie Zhou Yanjun Cai 《中国科学D辑(英文版)》2001,44(7):635-651
The study on magnetic properties of the red clay indicates that the red clay and loesspaleosol sequence have a common magnetic
mineralogy, with magnetite, maghemite, hematite (and possibly goethite) contributing to the magnetic behavior. The red clay
magnetic susceptibility is also found to have a positive relation with extrafine superparamagnetic grains. This suggests that,
like the Quaternary loess-paleosols, an ultrafine ferrimagnetic component produced during pedogenesis in the red clay under
humid conditions also plays an important role in susceptibility enhancement in the soil units. This is supported by the correlation
between Rb/Sr ratio and magnetic susceptibility. This signifies that, like the above loess-paleosol sequence, the magnetic
susceptibility of the red clay can be used as a general proxy paleoclimatic indicator, although whether its susceptibility
in the red clay is comparable to pedogenesis intensity and requires further investigation. Magnetic susceptibility variation
in the red clay thus also provides an eolian/pedogenic record of paleoclimatic evolution. Study of the background susceptibility
indicates that, on average, the absolute scale of the paleoclimatic shift from red clay development to Quaternary loess deposition
is similar to the climatic shift from stage 5 (S1) to stage 2–4 (L1). This may suggest that during the Quaternary there is
an evident strengthening of the absolute wind intensity to bring more (about double) coarser and less weathered (non-SP fraction)
eolian magnetic input from the source regions to the Loess Plateau than during the Pliocene. The presence of eolian red clay
since 7.5 Ma BP in central-northern China implies an important environmental change from the underlying Cretaceous red sandstone.
The red clay development was closely related to global drying and climate cooling since the Cretaceous and closely associated
with the abrupt uplift of the Qinghai-Xizang Plateau at about that time. This uplift of the plateau intensified the East Asia
monsoon system and started red clay deposition. 相似文献
10.
F. Hrouda 《Studia Geophysica et Geodaetica》2007,51(3):423-438
The anisotropy of magnetic susceptibility (AMS) within the Rayleigh Law range was investigated theoretically, using mathematical
modelling. It was revealed that the orientations of the principal susceptibilities and the shape parameter vary with field
so weakly that these variations can be regarded as negligible from the practical point of view. The degree of AMS increases
with field according to the degree of anisotropy of the initial susceptibility used and according to the intensity of susceptibility
change with field of the mineral considered. The degree of AMS calculated using linear theory is very near to the degree of
AMS following from the analysis of AMS within the Rayleigh Law range. If it is desirable to correct the field-dependent degree
of AMS, a simple technique is suggested based on measurement of the AMS in two fields.
fhrouda@agico.cz 相似文献
11.
Laboratory measurements of ‘porosity‐free’ intrinsic Vp and Vs in an olivine gabbro of the Oman ophiolite: Implication for interpretation of the seismic structure of lower oceanic crust 
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下载免费PDF全文 In order to determine ‘porosity‐free’ intrinsic ultrasonic compressional (Vp) and shear wave (Vs) velocities and Vp/Vs of an olivine gabbro from the Oman ophiolite, we developed a new experimental system using a piston‐cylinder type high‐pressure apparatus. The new system allowed us to measure velocities at pressures ranging from 0.20 to 1.00 GPa and at temperatures up to 300°C for Vp and 400°C for Vs. At room temperature, the Vp and Vp/Vs increase rapidly with pressure up to 0.40 GPa, while between 0.45 and 1.00 GPa the increase is more gradual. The change in increasing rate is attributed to closure of porosity at pressures above 0.45 GPa. Based on the linear regression of data obtained at higher pressures (0.45–1.00 GPa) and extrapolation to the lower pressures, combined with temperature derivatives of velocities of the sample measured at 1.00 GPa, we determined the intrinsic Vp and Vs of the sample as a function of pressure (P, in GPa) and temperature (T, in °C). The intrinsic velocities can be expressed as Vp (km/s) = 7.004 + 0.096 × P ? 0.00015 × T, and Vs (km/s) = 3.827 + 0.007 × P ? 0.00008 × T. We evaluated the intrinsic Vp and Vs of the olivine gabbro at oceanic crustal conditions and compared them with a velocity depth‐profile of the borehole seismic observatory WP‐2 area in the northwestern Pacific Basin. Although the intrinsic Vp (～7.0 km/s) and Vs (～3.8 km/s) for the olivine gabbro studied are comparable to those of seismic layer 3 in the WP‐2 area, the estimated vertical gradients of intrinsic velocities are significantly smaller than those reported from layer 3. These results suggest that velocity profiles of layer 3 in the WP‐2 area may reflect the presence of a minor porosity in lower oceanic crust, which closes with increasing depth and/or continuous changes in mineralogy of layer 3 rocks. 相似文献
12.
Geochemical data are presented for a suite of ultramafic and related rocks from the Dras ophiolitic melange of the Indus suture zone in the western Himalaya. Harzburgites from the suite have highly refractory chemistry. Lherzolites from the suite represent modified mantle material and are comparable to potential source rocks for MORB and to the lherzolites of the Bay of Islands ophiolites. Cumulus dunite, pyroxenite and gabbro units contain olivine, chromite, clinopyroxene and plagioclase as the major cumulus phases. Orthopyroxene is absent as a cumulus phase and in this respect the Dras ophiolite differs from the Marum, Betts Cove and Troodos ophiolites. However, the Dras cumulates are similar to the Vourinos and Bay of Islands cumulate sequence and are consistent with accumulation of low-pressure liquidus phases of mid-oceanic ridge-type magmas. Magmas parental to the Dras cumulate rocks contained high 100 Mg/(Mg + Fe2+) ratios of 77–79, high Ni, Cr and possibly Ca, low Ti and depleted LREE. Parent magmas were probably similar to those of normal MORB formed by two-stage (or dynamic) melting processes in the mantle. Peridotite fabrics suggest high-temperature plastic (mantle) deformation. Disruption, serpentinisation and melange formation were probably produced during emplacement in the Indus suture zone. 相似文献
13.
Edgardo Can-Tapia George P. L. Walker Emilio Herrero-Bervera 《Journal of Volcanology and Geothermal Research》1997,76(1-2)
We studied the anisotropy of magnetic susceptibility (AMS) of 22 basaltic flow units, including S-type pahoehoe, P-type pahoehoe, toothpaste lava and 'a'
emplaced over different slopes in two Hawaiian islands. Systematic differences occur in several aspects of AMS (mean susceptibility, degree of anisotropy, magnetic fabric and orientation of the principal susceptibilities) among the morphological types that can be related to different modes of lava emplacement. AMS also detects systematic changes in the rate of shear with position in a unit, allowing us to infer local flow direction and some other aspects of the velocity field of each unit. 'A'
flows are subject to stronger deformation than pahoehoe, and also their internal parts behave more like a unit. According to AMS, the central part of pahoehoe commonly reveals a different deformation history than the upper and lower extremes, probably resulting from endogenous growth. 相似文献
14.
In the Bay of Islands ophiolite complex both the basal part of the cumulate gabbro sequence and the underlying peridotites have been plastically deformed. Higher in the series the cumulate gabbros are undeformed except in localized shear zones slightly to strongly oblique to the layering. The resulting shear bands are narrow and composed either of mylonitic amphibole-bearing gabbros or of highly foliated amphibolites.The amphibole composition and nature, and the chemistry of coexisting minerals combined with structural evidence imply that a first episode of shearing subhorizontal in the crust occurred at decreasing temperatures ranging from 750 to 450°C. This shearing probably represent the response of the lithosphere to stresses also responsible of asthenospheric flow in the upper mantle. A second episode of shearing oblique to the first one and probably related to cooling of the crust away from the ridge, occurred at lower temperatures ranging from 500 to 300°C. The surrounding gabbros have undergone static hydrothermal metamorphism at temperatures decreasing from 650 to 450°C. All metamorphic events occurred in hydrous conditions in the vicinity of the ridge axis. 相似文献
15.
Measurements of the anisotropy of magnetic susceptibility (AMS) of natural lavas have shown that AMS varies with depth within a lava flow. We have investigated the reasons for such variation by studying the effects of temperature and strain rate on the AMS of recent lava in the laboratory. Samples of lava from Kilauea were melted and subjected to a range of strain rate and cooling histories. The results show that the degree of anisotropy is a function of both the thermal and shearing history of a sample. High degrees of anisotropy were found only in samples that were deformed at temperatures close to those encountered during eruption and then rapidly quenched. Lavas subjected to similar shear stresses at high temperatures had low degrees of anisotropy if allowed to cool down slowly without further deformation. Additionally, lava subjected to complex shearing yield a lower degree of anisotropy even when high strain rates were imposed on it. These results lead to the conclusion that only the last phase of deformation is detectable using AMS and that high strain rates will not result in high degrees of anisotropy if either deformation ends while lava is still fluid or if the orientation of the maximum shear stress varies with time. The relation between the orientation of the principal susceptibilities and that of shear is less sensitive to variation on shear with time. Consequently, flow directions can be inferred confidently with this type of measurements. 相似文献
16.
核磁共振测井在火成岩地层应用的适应性分析 总被引：1，自引：0，他引：1
火成岩岩石具有高磁化率特征,与沉积岩差别明显,岩石的高磁化率会对核磁共振信号产生显著影响,开展核磁共振测井在火成岩地层应用的适应性分析研究,为广泛使用核磁共振测井用于火成岩储层评价提供参考.采用理论模拟、实验分析及实际测井资料相结合的方法,分析了高磁化率岩石的核磁共振响应特征.分析结果表明,火成岩岩石具有很高的磁化率,从酸性火成岩到基性火城岩,岩石的磁化率一般是逐渐增大的.高磁化率岩石孔隙内部会产生强梯度磁场,孔隙内部的磁场梯度越强,核磁共振的T2谱前移越明显,核磁信号衰减幅度也越大,核磁分析孔隙度与常规分析的孔隙度误差也越大;相同岩石磁化率情况下,岩石孔径越小,岩石孔隙内部磁场梯度越强,核磁信号衰减越大,核磁共振测井计算的孔隙度也越低.因此,核磁共振测井与岩性有关系,核磁共振测井会受到高磁化率岩石的显著影响.核磁共振测井在部分火成岩,尤其在中基性火成岩和小孔径火成岩储层中应用具有很大的局限性. 相似文献
17.
Akira Ishiwatari Yuki Yanagida Yi‐Bing Li Teruaki Ishii Satoru Haraguchi Kazuto Koizumi Yuji Ichiyama Masaru Umeka 《Island Arc》2006,15(1):102-118
Abstract During the Hakuho‐Maru KH03‐3 cruise and the Tansei‐Maru KT04‐28 cruise, more than 1000 rock samples were dredged from several localities over the Hahajima Seamount, a northwest–southeast elongated, rectangular massif, 60 km × 30 km in size, with a flat top approximately 1100 m deep. The rocks included almost every lithology commonly observed among the on‐land ophiolite outcrops. Volcanic rocks included mid‐oceanic ridge basalt (MORB)‐like tholeiitic basalt and dolerite, calc‐alkaline basalt and andesite, boninite, high‐Mg adakitic andesite, dacite, and minor rhyolite. Gabbroic rocks included troctolite, olivine gabbro, olivine gabbronorite (with inverted pigeonite), gabbro, gabbronorite, norite, and hornblende gabbro, and showed both MORB‐type and island arc‐type mineralogies. Ultramafic rocks were mainly depleted mantle harzburgite (spinel Cr? 50–80) and its serpentinized varieties, with some cumulate dunite, wehrlite and pyroxenites. This rock assemblage suggests a supra‐subduction zone origin for the Hahajima Seamount. Compilation of the available dredge data indicated that the ultramafic rocks occur in the two northeast–southwest‐oriented belts on the seamount, where serpentinite breccia and gabbro breccia have also developed, but the other areas are free from ultramafic rocks. Although many conical serpentinite seamounts 10 km in size are aligned along the Izu–Ogasawara (Bonin)–Mariana forearc, the Hahajima Seamount may be better interpreted as a fault‐bounded, uplifted massif composed of ophiolitic thrust sheets, resembling the Izki block of the Oman ophiolite in its shape and size. The ubiquitous roundness of the dredged rocks and their thin Mn coating (<2 mm) suggest that the Hahajima Seamount was uplifted above sealevel and wave‐eroded, like the present Macquarie Is., a rare example of ophiolite exposure in an oceanic setting. The Ogasawara Plateau on the Pacific Plate is adjacent to the east of the Hahajima Seamount, and collision and subduction of the plateau may have caused uplift of the forearc ophiolite body. 相似文献
18.
Five samples from historical lava flows on Mt. Etna, which had previously been used in a palaeointensity study, were examined using a combination of rock magnetic and microscopic techniques to elucidate the causes of failure of palaeointensity determination. The samples were characterised using a combination of low-temperature susceptibility (LT-), Bitter pattern imaging and scanning electron microscope (SEM). High-temperature susceptibility curves and hysteresis loops had been previously measured by Calvo et al. (2001). Of the five samples only one gave an accurate palaeointensity. This sample was deuterically oxidised and consisted of large exsolved ore grains. It was only possible to distinguish this sample either optically or magnetically from a similar sample by the LT- warming curves; the unsuccessful sample displayed alteration in the LT- warming curves, which was not readily observed in the high temperature susceptibility curves. It is proposed that the measurement of LT- curves before and after heating could be a more sensitive method of determining suitability for palaeointensity determination than previous rock magnetic pre-selection techniques. 相似文献
19.
对苏北盆地兴化1#钻孔岩心沉积物进行了质量磁化率、频率磁化率、变化曲线、等温剩磁和磁滞回线等参数的测试分析，结果表明，粘土质沉积物中的主要载磁矿物为磁铁矿和赤铁矿；砂质沉积物中的磁性矿物除了磁铁矿和赤铁矿外，还含有少量的磁赤铁矿和针铁矿.针铁矿普遍存在于钻孔下部的样品中.整个钻孔沉积物中的磁性矿物颗粒都是介于单畴和多畴之间的准单畴颗粒，但粘土质沉积物中的磁性矿物颗粒更趋近于单畴颗粒，而砂质沉积物中的磁性矿物颗粒更趋近于多畴.根据不同的磁性矿物组合特征，选择合理的温度和磁场强度区间对古地磁退磁结果进行分析，得到沉积物可靠的特征剩磁方向，为古地磁年代学提供依据. 相似文献
20.
Liu Xiuming AN Zhisheng Tim Rolph QIANG Xiaoke Paul Hesse Lu Huayu ZHOU Jie CAI Yanjun 《中国科学D辑(英文版)》2001,44(7)
The study on magnetic properties of the red clay indicates that the red clay and loess- paleosol sequence have a common magnetic mineralogy, with magnetite, maghemite, hematite (and possibly goethite) contributing to the magnetic behavior. The red clay magnetic susceptibility is also found to have a positive relation with extrafine superparamagnetic grains. This suggests that, like the Quaternary loess-paleosols, an ultrafine ferrimagnetic component produced during pe-dogenesis in the red clay under humid conditions also plays an important role in susceptibility enhancement in the soil units. This is supported by the correlation between Rb/Sr ratio and magnetic susceptibility. This signifies that, like the above loess-paleosol sequence, the magnetic susceptibility of the red clay can be used as a general proxy paleoclimatic indicator, although whether its susceptibility in the red clay is comparable to pedogenesis intensity and requires further investigation. Magnetic susceptibility variation in the red clay thus also provides an eo-lian/pedogenic record of paleoclimatic evolution. Study of the background susceptibility indicates that, on average, the absolute scale of the paleoclimatic shift from red clay development to Quaternary loess deposition is similar to the climatic shift from stage 5 (S1) to stage 2-4 (L1). This may suggest that during the Quaternary there is an evident strengthening of the absolute wind intensity to bring more (about double) coarser and less weathered (non-SP fraction) eolian magnetic input from the source regions to the Loess Plateau than during the Pliocene. The presence of eolian red clay since 7.5 Ma BP in central-northern China implies an important envi-ronmental change from the underlying Cretaceous red sandstone. The red clay development was closely related to global drying and climate cooling since the Cretaceous and closely associated with the abrupt uplift of the Qinghai-Xizang Plateau at about that time. This uplift of the plateau intensified the East Asia monsoon system and started red clay deposition. 相似文献