Track of fluid paleocirculation in dolomite host rock at regional scale by the Anisotropy of Magnetic Susceptibility (AMS): An example from Aptian carbonates of La Florida,Northern Spain

The present study aims to apply the AMS method (Anisotropy of Magnetic Susceptibility) at a regional scale to track the fluid circulation direction that has produced an iron metasomatism within pre-existing dolomite host rock. The Urgonian formations hosting the Zn–Pb mineralizations in La Florida (Cantabria, northern Spain) have been taken as target for this purpose. Sampling was carried out, in addition to ferroan dolomite host rock enclosing the Zn–Pb mineralizations, in dolomite host rock and limestone to make the comparison possible between magnetic signals from mineralized rocks, where fluid circulation occurred, and their surrounding formations. AMS study was coupled with petrofabric analysis carried out by texture goniometry, Scanning Electron Microscopy (SEM) observations and also Shape Preferred Orientation (SPO) statistics. SEM observations of ferroan dolomite host rock illustrate both bright and dark grey ribbons corresponding respectively to Fe enriched and pure dolomites. SPO statistics applied on four images from ferroan dolomite host rock give a well-defined orientation of ribbons related to the intermediate axis of magnetic susceptibility K₂. For AMS data, two magnetic fabrics are observed. The first one is observed in ferroan dolomite host rock and characterized by a prolate ellipsoid of magnetic susceptibility with a vertical magnetic lineation. The magnetic susceptibility carrier is Fe-rich dolomite. These features are probably acquired during metasomatic fluid circulations. In Fe-rich dolomite host rock, ?c? axes are vertical. As a rule, (0001) planes (i.e. planes perpendicular to ?c? axes) are isotropic with respect to crystallographic properties. So, the magnetic anisotropy measured in this plane should reflect crystallographic modification due to fluid circulation. This is confirmed by the texture observed using the SEM. Consequently, AMS results show a dominant NE–SW elongation interpreted as the global circulation direction and a NW–SE secondary elongation that we have considered as sinuosities of the fluid trajectory. The second type of magnetic fabric is essentially observed in the limestone and characterized by an oblate form of the ellipsoid of magnetic susceptibility, a horizontal magnetic foliation and mixed magnetic susceptibility carriers. It is interpreted as a sedimentary fabric.     

Magnetic fabrics of the Bloodgood Canyon and Shelley Peak Tuffs,southwestern New Mexico: implications for emplacement and alteration processes

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.     

Subaqueous sand blow deposits induced by the 1995 Hyogo-ken Nanbu Earthquake, Japan

Abstract The 1995 Hyogo-ken Nanbu Earthquake (Mw 6.9) occurred in the region around Kobe City and Awaji Island in south-west Japan. Co-seismic liquefaction caused subsidence of the land and damage to sea wall caissons on the man-made Port Island at Kobe City. A zone 2–3 m wide behind the caissons of the northern wharf on the island subsided into the intertidal zone and a sandy deposit settled into this subsided zone. The depos-it consists of upward-fining sequences that are subdivided into three parts, in ascending order: graded coarse- to medium-grained sand, parallel-laminated fine- to very-fine-grained sand, and massive mud. Grain fabric analysis (employing the anisotropy of magnetic susceptibility method and microscopic measurement) of these sequences shows that there is a remarkable contrast in grain fabric between the lowest portion of the graded sand division and the laminated sand division. The former has a high q -value (magnetic lineation/foliation) and a unimodal orientation of elongate grains in the horizontal plane, but random orientation in the vertical plane. Conversely, the latter is characterized by a low q -value and a grain fabric in which the long axes of the grains have random orientations and are nearly parallel to the plane of deposition. This result shows that the main depositional processes changed from a combination of flow and allied processes to the force of gravity. As still water is essential for gravity to be the dominant factor in deposition, this deposit is regarded as subaqueous sand blow deposits. If this interpretation is correct, the grain fabric produced by gravity alone is a useful criterion for distinguishing between subaqueous sand blow deposits and other liquefaction-induced deposits.     

Magma flow revealed by magnetic fabric in the Okavango giant dyke swarm, Karoo igneous province, northern Botswana

To determine the magma flow direction of the giant, 179 Ma Okavango dyke swarm of northern Botswana, we measured the anisotropy of magnetic susceptibility (AMS) of 23 dykes. Dykes are located in two sections (Shashe and Thune Rivers), which are about 300 km and 400 km from the presumed magma source respectively; the Nuanetsi triple point. We collected samples from the margins of the dykes in order to use the imbrication of magnetic foliation to determine magma flow direction. About half of the magnetic fabric in the dykes is inverse, i.e. with the magnetic foliation perpendicular to the dyke plane. Lateral flow to the west and vertical flow is in evidence in the Shashe section. However, the overall analysis of normal and inverse magnetic fabric data supports that lateral flow to the west was dominant in the Shashe section. Across the Thune section, a poorly defined imbricated magnetic foliation also suggests lateral flow to the west.     

Inverse magnetic fabric in carbonate-bearing rocks

The observation of a magnetic susceptibility ellipsoid whose maximum axis corresponds to the minimum axis of petrofabric (pole of bedding or schistosity) is referred to as an inverse magnetic fabric. The investigation of the magnetic properties of some ferroan carbonate monocrystals and paramagnetic limestones demonstrates thatc-axis preferred orientation of paramagnetic carbonates results in a maximum susceptibility parallel to the flattening direction.Inverse magnetic fabrics due to magnetite are also encountered in weakly deformed limestones. A mineralogical model based on the property of single-domain grains to have a zero susceptibility parallel to their long axis is proposed. However, more complex cases are also encountered.     

Magnetic anisotropy of limestones from Limhamn,Southern Sweden

Summary The magnetic fabric of the Limhamn limestones is characterized by a low initial magnetic susceptibility. Of 68 specimens with a positive susceptibility, 10 show a distinct anisotropy. In nine of these, the axes of minimum susceptibility are practically normal to the magnetic foliation plane. The observed anisotropy pattern has some resemblance to a magnetic fabric acquired by deposition under influence of currents. A post-depositional origin due to gravitational compaction seems, however, more probable.Presented at 2nd conference on New Trends in Geomagnetrsm, Castle of Bechyn, Czechoslovakia, September 24–29, 1990.     

Composite mesoscopic and magnetic fabrics of the Paleo-Proterozoic Wangtu Gneissic Complex,Himachal Himalaya,India: Implications for ductile deformation and superposed folding of the Himalayan basement rocks

The present study demonstrates how the Paleo-Proterozoic Wangtu Gneissic Complex (WGC) of the Lesser Himalayan Crystalline sequence experienced superposed folding and doming prior to its exhumation, with the help of integrated field, microstructural, magnetic fabric anisotropy and geochronological studies. The WGC forms the basement of the Lesser Himalaya and is bounded by Vaikrita Thrust (VT) to the northeast and Munsiari Thrust (MT) to the southwest. The regional structure consists of upright large scale early folds (D1) trending NW–SE. The mesoscopic fabric is related to axial plane foliation of the D1 folds and, to a lesser extent, late D2 folds. The axis of maximum compression for D1 and D2 folds are mutually orthogonal. The D1 folds have formed simultaneously with the major Himalayan thrusts whereas the D2 folds have developed during a later deformation event. The magnetic lineation at the hangingwall of the VT is sub-horizontal indicating stretching along the strike of the thrust. In the interior parts of the WGC, the magnetic fabric is of two types: (i) magnetic lineation demarks the intersection of mesoscopic and magnetic foliation indicating superposed deformation and (ii) scattered distribution of magnetic lineations due to D2 folding on initially curved and non-cylindrical D1 surface. ⁴⁰Ar–³⁹Ar dating of biotite from one site from the core of WGC gives an age of 9.3 ± 0.3 (2σ) Ma. It is inferred that the doming of the WGC took place at ∼9 Ma and, instead of large scale thrusting, it is characterized by superposed folding and strike-parallel stretching along the VT zone. It is suggested that the effect of superposed folding and ductile deformation of the Himalayan basement rocks has to be taken into account before cross-section balancing or any estimation of crustal shortening is attempted.     

Characteristic of magnetic susceptibility anisotropy in and around Laji Shan and its geological significance

This paper deals with the anisotropy of magnetic susceptibility (AMS) of Laji Shan and its adjacent areas. Cenozoic sediment is a suite of mudstone and sandstone, which underwent weak deformation and no metamorphism. Most of the magnetic ellipsoids are oblate withK _max trending NW-SE, and the minimum axesK _min show a large deviation from the poles of bedding planes with an NE preferred direction. Taken together, they reflect that the magnetic fabric is mainly of tectonic origin. In the study area, the NE-SW orientation ofK _min represents the recent principal compression direction. This point is substantiated by the structures in sites where magnetic fabric data are also available. The stress field provides a strong support for the sustained uplift of Laji Shan and its adjacent areas in the late Tertiary-Quaternary period.

     

The magnetic anisotropy of the Yampi Sound hematite ores

Summary The magnetic fabric of the Yampi Sound hematite ore bodies of Cockatoo and Koolan Island in Western Australia has been determined with a low field torque meter in an attempt to investigate the origin of the ores. The ore bodies are conformable with a ferruginous sedimentary sequence, which has been folded into overturned synclines and anticlines. Each ore body shows a consistent distribution of the principal susceptibility axes due to a preferred alignment of the trigonal axes of hematite, but the magnetic fabric is not related to the sedimentary structure of the ores. For the Cockatoo Island ore body the magnetic fabric indicates the presence of an axial plane foliation within the ore, suggesting that the preferred crystalline alignment is caused by arecrystallization of hematite during the folding of the sediments. The magnetic fabric of the ore bodies on Koolan Island is not related to either bedding or axial planes. This can be explained by assuming that the hematite recrystallized during an earlier stage of the folding process and that then the limbs of the fold were rotated into their present position. High field torque measurements on several ore specimens indicate a preferred crystalline alignment of hematite of about 25%. The ferruginous sediments of Cockatoo Island show a composite fabric caused by the superposition of two foliations, one being the bedding plane and the other the axial plane foliation impressed during the folding of the sediments.     

Bioturbation: minimal effects on the magnetic fabric of some natural and experimental sediments

Magnetic fabric measurements have been performed on three suites of sediment samples to contrast biologically disturbed and undisturbed sediments. The first of these analyses includes samples from laminated, relatively undisturbed and bioturbated, totally disturbed horizons found in an Athabasca Oil Sand core. The second includes samples from biologically undisturbed sediments from Mammoth Cave in Kentucky. The third includes evaluation of the evolution of a magnetite horizon which was established by the author in December of 1982 in a biologically active tidal flat on Sapelo Island, Georgia. Changes in this magnetite horizon were monitored by periodic subsampling.Results from these suites of samples indicate that the measured magnetic fabric in bioturbated sediments may exhibit primary sedimentary characteristics similar to undisturbed sediments. Such primary indicators include the anisotropy of magnetic susceptibility (AMS) parametersV used by Graham, always greater than 45°,Q used by Rees and Hamilton, always less than 0.69, and a subhorizontal magnetic foliation plane. I infer that the presence of apparently primary fabric distributions in these bioturbated sediments indicates that physical, not biological factors are responsible for the observed fabric. These factors include dewatering and simple, low magnitude compaction. The data indicate that it cannot be assumed, a priori, that poor fabric distributions are the result of bioturbation. On the contrary, for the Sapelo Island experiments, the magnetic fabric actually improved with bioturbation and the Athabasca bioturbated magnetic fabric is very similar to that from the Athabasca laminated sediment.Data from the Sapelo Island, Georgia, tidal flat experiment are interpreted to indicate that long-axis AMS alignments develop within the sediment. Such alignments are the result of water movement through sediment which exhibits increased porosity due to high rates of burrowing. It is also observed that RM precision improves during the experiment in spite of bioturbation.     

Recently formed elastic anisotropy and petrological models for the continental subcrustal lithosphere in southern Germany

This paper advances new evidence for elastic anisotropy in the continental subcrustal lithosphere in southern Germany. The range of petrological models compatible with the observed azimuthal variation of seismic P-wave velocity is explored. The azimuthal distribution of amplitudes of mantle phases and the observed increase of P velocity with depth both indicate a continuation of anisotropy with depth together with an increase of preferred orientation. Even depletion of the upper mantle in basaltic components, as suggested by mantle xenoliths from various parts of Germany, cannot explain the velocity-depth and azimuthal amplitude observations without an increase of anisotropy with depth.Preferred orientation of olivine is the most likely mechanism for the observed phenomena. Its fast a-axis at the Moho level is directed towards N22.5°E. The b-axis is also required to be horizontal; i.e., the b-plane, one of the preferred glide planes of olivine, is vertical, with a strike of N22.5°E. Therefore, this preferred glide plane of olivine practically coincides with the plane of maximum horizontal shear stress deduced from fault-plane solutions of earthquakes in western Germany. This is a strong indication that the preferred orientation of olivine is formed in the recent West European crustal stress field leaking into the upper mantle. The distribution of velocities to a depth of at least 50 km requires slight horizontal rotation of the a-axis with depth by ～ 10° towards N32°E, and a change in the modal composition towards a depletion increasing with depth compatible with the composition of mantle xenoliths from western Germany. Further experiments are needed to substantiate this suggestion, which could lead to a better understanding of the interaction of crustal and upper-mantle stress-strain fields.     

The distribution of split shear-wave parameters in the backarc region of the South Kuril subduction zone

This paper reports a study in the distribution of parameters of split shear waves excited by deep earthquakes in the Sakhalin and Hokkaido area, with the orientation of the axes of symmetry in the earth being estimated assuming a viscoelastic anisotropic model for the mantle. A mantle flow along NW 310° ± 20° has been identified beneath the Japan Sea. The flow is inclined at an angle of 20–30° relative to the horizontal plane and is consistent with the motion of the Pacific plate. Beneath the southern Sea of Okhotsk the fast a-axis [100] of olivine is oriented NE 30 ± 15°, nearly parallel to the trend of the Kuril arc, while the c-axis [001] is inclined at ～35° relative to the horizontal plane. Bearing in mind the increased heat flow in the region, we assume the development of a mantle flow along the olivine c-axis at NW 300° ± 20° and with the shear plane [010] in the conditions of partial mantle melting (the B-type LPO). The lowest anisotropy (1–2.5%) was identified beneath Sakhalin and the greatest (3–5%) beneath the Japan Sea. An increasing degree of anisotropy is also noted to occur with an increasing depth of focus (down to 350 km).     

On the topographic coupling at the core-mantle interface

The mean tangential stresses at a corrugated interface between a solid, electrically insulating mantle and a liquid core of magnetic diffusivity λ are calculated for uniform rotation of both mantle and core at an angular velocity Ω in the presence of a corotating magnetic field B. The core and mantle are assumed to extend indefinitely in the horizontal plane. The interface has the form z = η(x, y), where z is the upward vertical distance and x, y are the zonal and latitudinal distances respectively. The function η(x, y) has a planetary horizontal length scale (i.e. of the order of the radius of the Earth) and small amplitude and vertical gradient. The liquid core flows with uniform mean zonal velocity U₀ relative to the mantle. Ω and B possess vertical and horizontal components.The vertical (poloidal) component B_p is uniform and has a value of 5 G while the horizontal (toroidal) field B_T = B_pαz, where α is a constant. When |α| ? 1, the mean horizontal stresses are found to have the same order of magnitude (10^?2 N m^?2) as those inferred from variations in the decade fluctuations in the length of the day, although the exact numerical values depend on the orientation of Ω as well as on the wavenumbers in the zonal and latitudinal directions.The influence of the steepness (as measured by α) of the toroidal field on the stresses is investigated to examine whether the constraint that the mean horizontal stresses at the core-mantle interface be of the order of 10^?2 N m^?2 might provide a selection mechanism for the behaviour of the toroidal field in the upper reaches of the outer core of the Earth. The results indicate that the restriction imposed on α is related to the value assigned to the toroidal field deep into the core. For example, if |α| ? 1 then the tangential stresses are of the right order of magnitude only if the toroidal field is comparable with the poloidal field deep in the core.     

秦祁接合带造山缝合带磁组构特征及其构造意义

结合构造及磁化率各向异性研究详细解剖了秦祁接合带唐藏—关子镇—武山和新阳—元龙造山缝合带的应变及岩组特征.41个采点168个构造岩样品的平均磁化率全部较低,磁化率椭球形态分析表明其以平面和压扁应变为主,磁化率各向异性度普遍较高,属强变形岩石组构类型,结合野外观察认为其与变形强度明显正相关.此外,磁化率各向异性参数T、P′可能受岩石类型一定程度的影响.磁化率椭球主轴方位与变形密切相关,提供了丰富的岩组信息.两构造带具有类似的岩组特征,磁面理大致分为呈共轭形态的两组,暗示高应变剪切带在平面上可能以网格状形态出露;高倾伏角磁面理与占优势的低倾伏角、近水平磁线理表明了构造带明显的走滑特征,部分高角度磁线理可能与构造带的挤压和（或）转换挤压相关;磁组方法不能简单用于判别复杂强变形带的运动指向,糜棱面理的复杂变化及Kmin与构造带夹角过高使其判别结果意义不明,而野外及显微构造观察都表明了构造带的右行走滑特征.上述结果表明,沿缝合带大规模的右行转换挤压形成了秦祁接合带反“S”型的平面构造形态,暗示在南北板块拼合过程中,西秦岭诸中、小块体一定程度的向西挤逸.     

Variation of magnetic properties in sedimentary rocks hosting the Foum Zguid dyke (southern Morocco): Combined effects of re-crystallization and Fe-metasomatism

The effects of dyke intrusion on the magnetic properties of host sedimentary rocks are still poorly understood. Therefore, we have evaluated bulk magnetic parameters of standard palaeomagnetic samples collected along several sections across the sediments hosting the Foum Zguid dyke in southern Morocco. The study has been completed with the evaluation of the magnetic fabric after laboratory application of sequential heating experiments.The present study shows that: (1) close to Foum Zguid dykes, the variations of the bulk magnetic parameters and of the magnetic fabric is strongly related with re-crystallization and Fe-metasomatism intensity. (2) The thermal experiments on AMS of samples collected farther from the dyke and, thus, less affected by heating during dyke emplacement, indicate that 300–400 °C is the minimum experimental temperature necessary to trigger appreciable transformations of the pre-existing magnetic fabrics. For temperatures higher than ca. 580 °C, the magnetic fabric transformations are fully realized, with complete transposition of the initial fabric to a fabric similar to that of samples collected close to the dyke. Therefore, measured variations of the magnetic fabric can be used to evaluate re-crystallization temperatures experienced by the host sedimentary rock during dyke emplacement. The distinct magnetic behaviour observed along the cross-sections strongly suggests that samples collected farther from the dyke margins did not experience thermal episodes with temperatures higher than 300 °C after dyke emplacement. (3) AMS data shows a gradual variation of the magnetic fabric with distance from the dyke margin, from sub-horizontal K₃ away from the dyke to vertical K₃ close to the dyke. Experimental heating shows that heat alone can be responsible for this strong variation. Therefore, such orientation changes should not be unequivocally interpreted as the result of a stress field (resulting from the emplacement of the dyke, for instance). (4) Magnetic studies prove to be a very sensitive tool to assess rock magnetic transformations, thermally and chemically induced by dyke intrusion in hosting sediments.     

Magnetic anisotropy study of a columnar basalt from San Anton,Morelos, Mexico

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.     

Iron ions Fe<Superscript>2+</Superscript> and Fe<Superscript>3+</Superscript> in a magnetic model of the sedimentary medium

The concentration of rock-forming elements, the static magnetic susceptibility κ, spectra of electron paramagnetic resonance, and their relative intensities I are studied in samples from a borehole drilled in Cenozoic sedimentary deposits of southern Western Siberia. All measured values experience appreciable irregular variations with depth. A linear dependence exists between κ and I within the range of their medium and large values; κ and I have maximum values in the same sample, and κ_max = 1920 × 10^?6SI, κ_min = 210 × 10^?6 SI, and κ_av = 630 × 10^?6 SI. The magnetic properties of the samples are controlled by Fe²⁺ ions present in clastic material and by microphases (clusters) with Fe³⁺ ions of the goethite and lepidocrocite type present in the cement. The theoretically possible magnetic susceptibility of the Fe²⁺ ion system (provided that all iron exists in this form) is quite comparable with κ_min but, even with very high concentrations of Fe²⁺, does not reach half of κ_av: (154 < κ(Fe²⁺) < 254) × 10^?6 SI. Anomalously high values of κ are due to a large number of clusters with Fe³⁺ ions if structural units FeOOH do not dissociate and the interaction of the clusters with hydroxides of aluminum and precipitation medium impedes the process of their coagulation. Otherwise, the cluster sizes gradually increase, an antiferromagnetic structure develops in clusters, and the magnetic susceptibility decreases.     

The development of magnetic susceptibility anisotropy through crystallographic preferred orientation in a calcite rock

High-field torque-meter measurements of diamagnetic susceptibility anisotropy of a suite of samples of Carrara marble, axially shortened by amounts up to 50% at (1.5–3.0) · 10⁸ Pa confining pressure and at 20–500°C (mainly 400°C), have been compared with optical measurements of preferred crystallographic orientation. A revised value for the susceptibility anisotropy of calcite has been obtained from studies of single crystals, and it has been shown to be almost independent of the state of intracrystalline plastic strain. From the measured anisotropy of calcite, quantitative comparison of optical and magnetic fabric measurements is possible. It is found that these measurements agree and the implications of the observed progressive development of fabric intensity with strain are discussed.     

Correlation between the horizontal wind direction and orientation of cross-field anisotropy of small-scale irregularities in the <Emphasis Type="Italic">F</Emphasis> region of midlatitude ionosphere

Radio sounding of midlatitude ionosphere shows that natural small-scale electron density irregularities in the F region are cross-field anisotropic. The orientation of the cross-field anisotropy is different under different geophysical conditions. The cross-field anisotropy orientation is matched with the horizontal wind direction calculated within the HWM07 model for each event. It is ascertained that natural irregularities in a plane perpendicular to the magnetic field are stretched along the horizontal wind direction under different geophysical conditions.     

Attenuation of UVR and PAR in a clear and deep lake: Spatial distribution and affecting factors

Light attenuation is considered as a sentinel for environmental change in lakes and has a profound influence on aquatic ecosystems. However, the spatial distribution of ultraviolet radiation (UVR) and photosynthetically active radiation (PAR) attenuation, and the underlying mechanisms are still not fully understood. We carried out a field investigation with 60 sampling sites covering the entire Lake Qiandaohu from November 29 to December 1, 2013, during the weak stratification period to elucidate the spatial pattern and driving mechanisms. The diffuse attenuation coefficient of UVB (K_d(313)), UVA (K_d(340)) and PAR (K_d(PAR)) varied from 1.48 to 4.63 m⁻¹, 1.09 to 3.43 m⁻¹, and 0.26 to 0.94 m⁻¹, respectively. The corresponding ranges for the 1% attenuation depths were from 0.10 to 3.11 m, 1.34–4.21 m and 4.87–17.58 m, respectively. Total suspended matter (TSM) concentration was highly significantly correlated with K_d(313), K_d(340) and K_d(PAR) indicating that TSM was the main driver of UVR and PAR attenuation in Lake Qiandaohu in the late autumn and early winter. TSM concentration, K_d(313), K_d(340) and K_d(PAR) had obvious horizontal spatial heterogeneity presenting a decreasing trend from the estuary area to the center area in the lake. These results suggested that the spatial distribution of TSM from the inflow drived the spatial distribution of UVR and PAR attenuation. Significantly positive correlations were also observed between the chromophoric dissolved organic matter (CDOM) absorption coefficient and K_d(313). TSM and CDOM absorption spectra showed that in the UVR waveband (350–400 nm), the mean relative contribution rates of CDOM (a_g(λ)), non-algal particles (a_nap(λ)), phytoplankton (a_ph(λ)) and pure water (a_w(λ)) to the total absorption were 67.5 %, 24.0 %, 5.0 % and 3.5 %, respectively. In the PAR waveband, the mean relative contribution rates of a_g(λ), a_nap(λ), a_ph(λ) and a_w(λ) to the total absorption were 25.4 %, 18.6 %, 9.4 % and 46.6 %, respectively. Our findings could provide support for ecological environment protection in Lake Qiandaohu considering the importance of UVR and PAR attenuation in aquatic ecosystems.