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1.
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
2.
Edgardo Cañón-Tapia 《Pure and Applied Geophysics》1994,142(2):365-382
Twenty-eight parameters used to characterize measurements of the anisotropy of magnetic susceptibility are compared theoretically in this work by introducing the concept of the field of susceptibility tensors, which allows the representation of parameters as families of lines in a plane. It is demonstrated that the foliation and lineation parameters are but a special case of the shape parameters, implying that the resolution of these two rock fabric elements using AMS measurements alone is more an artifact of the numerical range of definition of some parameters than a quantification of two physically independent features. Also, it is shown that parameters presumably of the same type do not necessarily yield equivalent interpretation of results in a qualitative sense, and therefore, caution should be strongly exercised when parameters are to be selected. Paramters quantifying the degree of anisotropy are, in general, equivalent to each other because of the very small departure observed in natural rocks from the isotropic case. However, a final consideration of the possible ability to differentiate rock types and a convenient range of values allowing expression of the degree of anisotropy in a well-defined percentage are pointed out as the main factors to be considered before selecting one parameter within this class. 相似文献
3.
J. Urrutia-Fucugauchi 《Bulletin of Volcanology》1982,45(1):1-8
Anisotrophy of magnetic susceptibility (AMS) results from 27 specimens drilled from the top and two sides of a single columnar basalt segment are presented. The magnetic foliation plane is nearly horizontal for all parts of the column, which is consistent with a primary magma flow pattern, without evidence of local convection or differentiative processes. The shape of AMS ellipsoids is however predominantly prolate, which may be indicative of increased magnetic grain elongation due to crystal growth or grain realignment normal to a vertical stress field (due to thermal contraction). Apparent systematic variations related to column shape are found in bulk susceptibility, anisotropy degree and degree of lineation and foliation; some of the variation may also be related to weathering effects. The results are consistent with a primary AMS pattern resulting from thermal contractive stresses during column formation. Comparison of results from previous studies of columnar basalts reveals that there is a relatively large variation in AMS properties. There appears to exist a number of factors which may locally control the magnetic anisotropy of columns and very likely some of their other characteristics. 相似文献
4.
The influence of magnetic interactions on the anisotropy of magnetic susceptibility (AMS) have been largely studied by several theoretical models or experiments. Numerical models have shown that when magnetostatic interactions occur, the distributions of particles over the volume rather than their individual orientations control the AMS. We have shown recently from a comprehensive rock magnetic study and from a theoretical 2-dimensional (2-D) model that single domain particles closely packed in globule aggregates could produce strong local random interaction magnetic fields which could influence the magnetic susceptibility and decrease the degree of anisotropy. In this paper, we first present in detail this 2-D theoretical model and then we extend it to the 3-D case. The possible distribution function of the magnetostatic interaction fields comprises two extreme states: it is either isotropic or ordered. The former case corresponds to the thermal-demagnetized state while the second case corresponds to the alternating field (AF) demagnetized state. We show that when easy axes of magnetization are not uniformly distributed, the degree of anisotropy decreases as the interaction field increases in both AF- and thermal-demagnetized states in 2-D and 3-D geometry. Thus we conclude that random magnetic fields generated by a random arrangement of magnetic particles over the sample volume decrease the degree of anisotropy of AMS and may alter the magnetic fabric. 相似文献
5.
6.
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
7.
We studied 93 samples from 8 basaltic dykes of the Koolau volcanic range on the island of Oahu,Hawaii,USA,to determine the influence of hydrothermal alteration on the magnetic fabric as determined by anisotropy of magnetic susceptibility (AMS) measurements. Rock magnetic as well as microscopic investigations show that only ≈25% of the samples have retained their original magnetomineralogical composition of unaltered Ti-poor titanomagnetite. The remaining samples have undergone hydrothermal alteration which transformed the primary magnetic phase into a granular intergrowth of titanomagnetite, titanomaghemite and hematite. In both sample groups,this magnetic phase occurs in coarse (tens of microns),irregularly shaped particles as well as interstitial clusters of smaller (< 5 μm) grains. Our investigations show that hydrothermal alteration does change the bulk susceptibility and the degree of anisotropy but not the directions of principal axes of the AMS ellipsoid which are predominantly corresponding to normal magnetic fabric. The stability of AMS directions, regardless of the degree of alteration, points towards the model of distribution anisotropy as the controlling factor for the observed magnetic fabric. 相似文献
8.
František Hrouda Martin Chadima Josef Ježek Jaroslav Kadlec 《Studia Geophysica et Geodaetica》2018,62(2):272-290
The relationship between the anisotropy of frequency-dependent magnetic susceptibility (fdAMS) and the anisotropy of out-of-phase magnetic susceptibility (opAMS) was investigated theoretically and also empirically at three loess/palaoesol profiles in Prague and in Southern Moravia. The data treatment was made in terms of mean susceptibility, degree of AMS, and orientations of principal susceptibilities. It has shown that the fdAMS and opAMS can serve as indicators of the preferred orientations of ultrafine magnetic particles that are on transition between superparamagnetic and stable single domain states in rocks, soils and environmental materials. In loess/palaeosol sequences, the fdAMS and opAMS correlate reasonably, because they are due to magnetic particles of similar grain sizes. The fdAMS and opAMS can be both coaxial with standard AMS (i.e. anisotropy of in-phase susceptibility - ipAMS) or non-coaxial indicating slightly different orientations of viscous magnetic particles. 相似文献
9.
Consideration of published anisotropy of magnetic susceptibility (AMS) studies on welded ignimbrites suggests that AMS fabrics are controlled by groundmass microlites distributed within the existing tuff fabric, the sum result of directional fabrics imposed by primary flow lineation, welding, and (if relevant) rheomorphism. AMS is a more sensitive indicator of fabric elements within welded tuffs than conventional methods, and usually yields primary flow azimuth estimates. Detailed study of a single densely welded tuff sample demonstrates that the overall AMS fabric is insensitive to the relative abundances of fiamme, matrix and lithics within individual drilled cores. AMS determinations on a welded-tuff dyke occurring in a choked vent in the Trans-Pecos Texas volcanic field reveals a consistent fabric with a prolate element imbricated with respect to one wall of the dyke, while total magnetic susceptibility and density exhibit axially symmetric variations across the dyke width. The dyke is interpreted to have formed as a result of agglutination of the erupting mixture on a portion of the conduit wall as it failed and slid into the conduit, followed by residual squeezing between the failed block and in situ wallrock. Irrespective of the precise mechanism, widespread occurrence of both welded-tuff dykes and point-welded, aggregate pumices in pyroclastic deposits may imply that lining of conduit walls by agglutionation during explosive volcanic eruptions is a common process. 相似文献
10.
The role of single-domain (SD) magnetic particles in creation of inverse magnetic fabrics is investigated on simple mathematical models using a realistic estimate for SD intrinsic susceptibility. In contrast to the fraction created by multi-domain (MD) particles, in which the anisotropy of magnetic susceptibility (AMS) is controlled by both the grain AMS and intensity of the preferred orientation of the particles, the AMS of the SD fraction is controlled solely by the intensity of the preferred orientation. The degree of AMS of ensemble of SD grains with a preferred orientation is therefore much higher than that of the same ensemble of MD particles implying the existence of frequent inverse magnetic fabrics. However, the occurrence of inverse magnetic fabrics due to SD particles is more the exception than the rule. Consequently, the amounts of SD particles is probably in general low. Nevertheless, the presence of SD particles in amounts insufficient to create inverse magnetic fabrics may diminish the whole rock AMS substantially. This can be one of the reasons for relatively low AMS in volcanic rocks whose magnetic particles may be really small obeying the conditions for the existence of SD particles. 相似文献
11.
Sheila J Seaman William C McIntosh John W Geissman Michael L Williams Wolfgan E Elston 《Bulletin of Volcanology》1991,53(6):460-476
Anisotropy of magnetic susceptibility (AMS) of the middle Tertiary Bloodgood Canyon and Shelley Peak Tuffs of the Mogollon-Datil volcanic field has been used to (1) evaluate the ability of AMS to constrain flow lineations in low-susceptibility ash-flow tuffs; (2) establish a correlation between magnetic fabric, magnetic mineralogy, tuff facies, and characteristics of the depositional setting; and (3) constrain source locations of the tuffs. The tuffs are associated with the overlapping Bursum caldera and Gila Cliff Dwellings basin. The high-silica Bloodgood Canyon Tuff fills the Gila Cliff Dwellings basin and occurs as thin outcrops outside of the basin. The older Shelley Peak Tuff occurs as thin outcrops both along the boundary between the two structures, and outside of the complex. AMS data were collected from 16 sites of Bloodgood Canyon Tuff basin fill, 19 sites of Bloodgood Canyon Tuff outflow, and 11 sites of Shelley Peak Tuff. Sites were classified on the basis of within-site clustering of orientations of principal susceptibility axes, based on the categories of Knight et al. (1986). Most microscopically visible oxide minerals in the Bloodgood Canyon Tuff outflow and basin fill, and in the Shelley Peak Tuff are members of the hematite-ilmenite solid solution series. However, IRM acquisition data indicate that Bloodgood Canyon Tuff basin fill and Shelley Peak Tuff have magnetic mineralogy dominated by single- or pseudo-single-domain magnetite, and that the magnetic mineralogy of the Bloodgood Canyon Tuff outflow is dominated by hematite. Hematite in Bloodgood Canyon Tuff outflow is likely to be the result of deuteric and/or low-temperature alteration of magnetite and iron silicate minerals. Bulk magnetic susceptibility is higher in magnetite-dominated ash-flow tuff (Bloodgood Canyon Tuff basin fill and Shelley Peak Tuff) than it is in hematite-dominated ash-flow tuff (Bloodgood Canyon Tuff outflow). Bloodgood Canyon Tuff outflow has the highest total anisotropy (H) of the three units, followed by Shelley Peak Tuff and Bloodgood Canyon Tuff basin fill. All three ash-flow tuffs are genearlly characterized by oblate susceptibility ellipsoids, with those of the Bloodgood Canyon Tuff basin fill nearest to spherical. At high values of total anisotropy, Shelley Peak Tuff susceptibility ellipsoids attain a prolate shape; those of Bloodgood Canyon Tuff outflow attain an increasingly oblate shape. Three factors may influence differences in total anisotropy and susceptibility ellipsoid shape: (1) ash which travelled the greatest distance before deposition may show the best development of magnetic fabric, particularly of magnetic lineation; (2) deposition of ash in a closed basin may inhibit laminar flow throughout the sheet and the resulting development of flow textures; and (3) replacement of magnetite and iron silicates preferentially oriented within the foliation plane by hematite with strong crystalline anisotropy may enhance the magnetic susceptibility within that plane. Scatter in AMS axis orientation within sites may result from: (1) greater orientation inaccuracy in block-sampled than in fielddrilled samples; (2) rheomorphism; and (3) low accuracy of AMS measurement in low-susceptibility ashflow tuffs. Evaluation of flow lineation based on AMS of sites with well-clustered K
1 axes indicates that (1) Bloodgood Canyon Tuff basin fill flowed along a generally northwest-southeast azimuth; (2) Shelley Peak Tuff located on the boundary of the Bursum caldera and the Gila Cliff Dwellings basin flowed along a nearly east-west azimuth; and (3) Bloodgood Canyon Tuff outflow sites have K
1 susceptibility axes generally radial to the Bursum-Gila Cliff Dwellings complex, but within-site scatter of K
1 orientations is generally too large to draw conclusions about flow lineation orientation. Limited petrographic work on pilot thin sections adds flow direction information to AMS-derived flow lineation information. 相似文献
12.
Magdalena Kądziałko-Hofmokl Maria Jeleńska Katarzyna Delura Paweł Bylina 《Acta Geophysica》2010,58(2):269-299
Complex paleomagnetic, rock-magnetic and mineralogical studies were performed on serpentinized utramafic rocks from Braszowice-Brze?nica massif (BB) situated at the southern extremity of the Niemcza Shear Zone, close to the Sudetic Marginal Fault. Studies of magnetic minerals revealed presence of several varieties of magnetite grains, and partly altered Cr-spinels. Paleomagnetic studies revealed stable component HS of natural remanence carried by magnetite formed probably during the initial serpentinization. The directions of HS have good grouping within each locality, but differ between localities. Studies of anisotropy of magnetic susceptibility (AMS) have shown that directions of anisotropy axes are steep instead of horizontal as is common within the Niemcza Shear Zone. Taking advantage of the directions of AMS we rotated the directions of HS and brought them to the reference direction for the Sudetes for 372 Ma. We suggest that after acquiring AMS and HS during the Upper Devonian, the BB became divided into multiple units due to tectonic activity in the region. 相似文献
13.
根据含多畴磁铁矿岩石的热剩磁(TRM)及其磁化率各向异性(AMS)张量间的特征值的比值关系,推导出一种因AMS 而引起的TRM 方向偏离外磁场方向的具体校正公式,并将其成功地应用到中国月山闪长岩体及法国Flamanville 花岗岩体的古地磁研究中. 相似文献
14.
Summary The changes of the anisotropy of magnetic susceptibility of igneous rocks, induced by a magnetic field, are studied. It is proved that changes in the degree of anisotropy of susceptibility and of the orientation of the susceptibility ellispoid of specimens occur due to the configuration of the domain structure under the effect of the magnetic field. The influence of this effect on the total anisotropy of rocks depends on the degree of anisotropy due to the shape factor and on the stability of the domain structure. A model concept is presented, explaining the qualitatively different pattern of the changes of the anisotropy of susceptibility under the effect of the magnetic field in various directions of the specimens. 相似文献
15.
岩石的磁化率张量的统计处理，传统的处理方法是用费歇尔法，但对于火成岩等磁化率各向异性较弱的岩石，这种方法存在着局限性。本文介绍了“自益再采样法”，并与传统的方法进行了对比，指出，自益再采样法不仅有助于消除测量过程中的误差，而且它是建立在数据的实际分布之上的，能更准确地反映磁化率轴的实际分布方位以及方位的分散程度。； 相似文献
16.
F. Hrouda M. Chlupáčová K. Schulmann J. Šmíd P. Závada 《Studia Geophysica et Geodaetica》2005,49(2):191-212
The degree of the anisotropy of magnetic susceptibility (AMS) of basaltic rocks, as is known from the large AMS database of these rocks, is generally very low, while in more acidic volcanic rocks such as andesites, trachytes and phonolites, which have been investigated much less frequently, it is in general much higher. In the present study, the AMS of various volcanic rocks including trachytic and phonolitic rocks was investigated in the Tertiary volcanic region of the eské stedohoí Mts. Viscosities of the respective lavas were calculated from the chemical composition using the KWARE program. A rough correlation was found between the degree of AMS and lava viscosities, probably resulting from different mechanisms orienting the magnetic minerals. In basaltic lava flows this mechanism is traditionally considered to be of a hydrodynamic nature, in trachytic and phonolitic bodies it can also be represented by quasi-intrusive flows resembling, at least partially, ductile flow deformation. This is in agreement with the AMS data predicted by the viscous (liquid flow) and line/plane (ductile flow) models. 相似文献
17.
F. JANK 《Geophysical Prospecting》1972,20(2):375-384
In the interpretation of magnetic anomalies and in paleomagnetism, the anisotropy of magnetic susceptibility is commonly neglected. Nevertheless, this property has basic significance, because, owing to susceptibility anisotropy, the directions of the vectors of induced and remanent magnetization are deflected from the direction of the Earth's magnetic field. Almost all rock types investigated possess higher or lower degree of the susceptibility anisotropy. Effusive and sedimentary rocks have the lowest degree of anisotropy. For the latter, the “masking effect” of the paramagnetic mineral components has some influence on the anisotropy degree due to the low mean susceptibility of sedimentary rocks. Metamorphic and plutonic rocks usually exhibit a considerable degree of anisotropy. The highest degree of anisotropy has been found in the rocks containing ferromagnetic minerals with mimetic fabric. The dependence of the degree of the susceptibility anisotropy on the degree of metamorphism proved to be very complicated; of the rock sequence from slates to gneisses, the transient rocks (roofing slates and mica-schist-gneisses) showed the highest degree of anisotropy. This result can be used in geology for reliable determination of these rock types. 相似文献
18.
Graham J. Borradaile 《Pure and Applied Geophysics》1991,135(1):15-29
Existing correlations between strain and anisotropy of low-field magnetic susceptibility (AMS) have been re-assessed using a single parameter to express both anisotropies. TheP parameter (Hrouda, 1982) shows potential as a powerful single expression of the intensity of strain and of AMS. Previous correlations are improved by use of this parameter. Cautious optimism is justified for correlations between strain and susceptibility in a certain strain window between a lower limit (excluding the incomplete overprint of predeformation anisotropy) and an upper limit (excluding the effects of saturation anisotropy). For successful correlations the influence of stress-controlled recrystallisation should be minimal and the mineralogical sources of susceptibility must predate deformation. 相似文献
19.
Mike Jackson 《Pure and Applied Geophysics》1991,136(1):1-28
The magnetic fabric of rocks and sediments is most commonly characterized in terms of the anisotropy of low-field magnetic susceptibility (AMS). However, alternative methods based on remanent magnetization (measured in the absence of a magnetic field) rather than induced magnetization (measured in the applied field) have distinct advantages for certain geological applications. This is particularly true for; (1) adjunct studies in paleomagnetism, in order to assess the fidelity with which a natural remanence records the paleofield orientation; (2) studies of weakly magnetic or weakly deformed rocks, for which susceptibility anisotropy is very difficult to measure precisely; and (3) quantitative applications such as strain estimation. The fundamental differences between susceptibility and remanence (and their respective anisotropies) are due to several factors: (1) susceptibility arises from all of the minerals present in a sample, whereas remanence is carried exclusively by a relatively small number of ferromagnetic minerals; (2) ferromagnetic minerals are generally more anisotropic than para- and diamagnetic minerals; (3) for ferromagnetic minerals, remanence is inevitably more anisotropic than susceptibility; and (4) a number of common minerals, including single-domain magnetites, possess an inverse anisotropy of susceptibility, i.e., they tend to have minimum susceptibility parallel to the long axis of an individual particle; remanence is immune to this phenomenon. As a consequence of all these factors, remanence anisotropy may generally provide a better quantitative estimate of the actual distribution of particle orientations in a rock sample.Contribution number 9102 of the Institute for Rock Magnetism, University of Minnesota. 相似文献
20.
The frequency-dependent anisotropy of magnetic susceptibility (AMS) can be most easily determined through subtracting directional susceptibilities measured at two operating frequencies along each of the measuring directions and the differences subsequently processed using standard methods for AMS computation. The effect of the measurement precision on the accuracy of the determination of the frequency-dependent AMS is investigated through mathematical simulation of the measurement process using the statistical theory of measuring the AMS of rocks. The accuracy of the AMS determination is presented in terms of the errors in determining the principal susceptibilities and principal directions. By modelling the measuring errors, the limits are discovered for the investigation of the frequency-dependent AMS. The modelling shows extremely high requirements for measurement accuracy met by the most sensitive instruments, only. 相似文献