Mechanism of magnetic property changes of serpentinites from ODP Holes 897D and 1070A

Serpentinites have great implications for the oceanic crust, subduction zones, island arc magmatism activity, and the formation of nickel ore deposits. To further determine the mechanism of magnetic property changes of serpentinites, samples from ODP Holes 897 D and 1070 A were investigated by integrating both magnetic and non-magnetic methods. Detailed rock magnetic results demonstrate that magnetite prevails in the entire serpentinite section, while maghemite is present in the upper and altered parts. The concentration of Fe in the fresh peridotite is inhomogeneous; nonetheless, the magnetic properties are generally determined by the serpentinization process. The formation and state of the magnetite depend on the fracture conditioning and fluid activities which are controlled by the serpentinization process. By comparing these two holes, we found that the production of magnetite is consistent with the serpentinization process; serpentinization is a multi-stage process which involves early high-temperature serpentinization and later low-temperature oxidation. As the serpentinization continues, the fine magnetic particles become coarser, combined with the formation of new SP particles, and the later low-temperature oxidation leads to the maghemitization of the magnetites. The duration of oxidation also contributes to the differences of remanent magnetization between these two holes. These results greatly improve our understanding of the magnetic enhancement during the serpentinization process, and provide constraints on the interpretation of the paleomagnetic and aeromagnetic anomalies in this area.     

Geochemical constraints on the origin and tectonic setting of the serpentinized peridotites from the Paleoproterozoic Nyong series,Eseka area,SW Cameroon

Serpentinized rocks closely associated with Paleoproterozoic eclogitic metabasites were recently discovered at Eseka area in the northwestern edge of the Congo craton in southern Cameroon.Here,we present new field data,petrography,and first comprehensible wholerock geochemistry data and discuss the protolith and tectonic significance of these serpentinites in the region.The studied rock samples are characterized by pseudomorphic textures,including mesh microstructure formed by serpentine intergrowths with cores of olivine,bastites after pyroxene.Antigorite constitutes almost the whole bulk of the rocks and is associated(to the less amount) with tremolite,talc,spinel,and magnetite.Whole-rock chemistry of the Eseka serpentinites led to the distinction of two types.Type 1 has high MgO( 40 wt%) content and high Mg#values(88.80) whereas Type 2 serpentinite samples display relatively low MgO concentration and Mg#values(40 and 82.88 wt%,respectively).Both types have low Al/Si and high Mg/Si ratios than the primitive mantle,reflecting a refractory abyssal mantle peridotite protolith.Partial melting modeling indicates that these rocks were derived from melting of spinel peridotite before serpentinization.Bulk rock high-Ti content is similar to the values of subducted serpentinites( 50 ppm).This similarity,associated with the high Cr contents,spinel-peridotite protolith compositions and Mg/Si and Al/Si ratios imply that the studied serpentinites were formed in a subductionrelated environment.The U-shaped chondrite normalizedREE patterns of serpentinized peridotites,coupled with similar enrichments in LREE and HFSE,suggest the refertilized nature due to melt/rock interaction prior to serpentinization.Based on the results,we suggest that the Eseka serpentinized peridotites are mantle residues that suffered a high degree of partial melting in a subductionrelated environment,especially in Supra Subduction Zone setting.These new findings suggest that the Nyong series in Cameroon represents an uncontested Paleoproterozoic suture zone between the Congo craton and the Sao Francisco craton in Brazil.     

The identification of magnetite in limestones using the low-temperature transition

The low-temperature (K₁) transition has been measured in prepared samples with a low concentration of magnetite to test the validity of the technique for identifying magnetite in weakly magnetized rocks. Using an astatic magnetometer, magnetite concentrations as low as 1 part in 100,000 can be satisfactorily detected.Measurements on natural samples show the presence of magnetite in a variety of limestones which are known to have a stable natural remanent magnetization.     

Ferrospinels and petromagnetism of Siberian traps: A case study of traps drilled by the Norilsk borehole

Magnetic characteristics and compositions of ferromagnetic species of trap samples from the Norilsk borehole are studied; the borehole penetrated the complete section of a trappean rock sequence (nearly 3 km) represented by eight magmatic formations. The lowest (Ivakinskian) formation consists of rocks of the subalkalic type, and the remaining formations generally belong to the tholeiitic series. Primary ferromagnetic phases are represented by titanomagnetites with various ulvospinel concentrations in a solid solution. Three modes characterized by TiO₂ concentrations of 14.6, 18, and 23.9 wt % are revealed. The Ivakinskian rocks are most magnetized, and magnetization is low in core samples from depths of 1700–2500 m. Various magnetic characteristics have polymodal distributions of their values. Plateau basalt samples of the Ivakinskian Formation from other regions of Siberia, the Lory Plateau (Armenia), and Kamchatka are additionally studied. On the whole, titanomagnetites of plateau basalts were crystallized under the quartz-fayalite-magnetite (QFM) buffer conditions.     

Origin of a magnetic lineation on Kyushu Island, Japan

Abstract Several linear magnetic anomalies over continental crust have been identified in and around the Japanese Islands. The anomalies are probably related to island arc tectonic structures, but identifying specific sources has been difficult. Several deep holes were drilled in and around Aso caldera, where a linear anomaly occurs along an active fault. One drillhole located on the linear anomaly encountered a zone of highly magnetized and altered basement rocks at least 100 m thick at a depth of ∼1000 m. The other hole was located away from the anomaly and did not encounter any high-magnetic zones. Rocks from the zone have exceptionally strong remanent magnetization (several tens of A/m) sub-parallel to the present field. AF demagnetization experiments indicated that the magnetization is hard and stable. Magnetic modeling indicates that the linear anomaly is caused mainly by this layer. Microscopic examination of core samples shows that the highly magnetized zone includes secondary magnetic minerals and abundant hydrothermal alterations. Temperatures determined by fluid inclusions and down-hole temperatures show that the temperature of the highly magnetized zone was elevated in the past relative to surrounding rocks. The high temperature could destroy primary magnetic minerals and replace them with secondary magnetic minerals. Thus, the past hydrothermal system may have enhanced thermo-chemical remanent magnetization. The results can produce a model indicating that there was a past hydrothermal system related to the tectonic structure.     

Magnetic properties of the Olivenza meteorite—possible implications for its evolution and an early solar system magnetic field

The magnetic properties of samples of the Olivenza chondrite (LL5) obtained from four collections have been investigated. The natural remanent magnetization (NRM) consists of a very stable primary component, which is randomly scattered in direction on a scale of 1 mm³ or less within the samples, and a secondary magnetization widely varying in intensity, and probably also in direction. The origin of the secondary NRM is not clear, and may be of terrestrial origin. It is concluded that the NRM is carried by the ordered nickel-iron mineral, tetrataenite. The origin of the primary NRM could be a magnetic field associated with the solar nebula, out of which the metal grains condensed and acquired a thermo-remanent magnetization (TRM), or Olivenza could be a fine-grained breccia, the constituent fragments possessing randomly directed magnetization. The implications for the origin and evolution of Olivenza and its parent body if the former magnetizing process has occurred are discussed.     

On the origin of El Chichón volcano and subduction of Tehuantepec Ridge: A geodynamical perspective

The origin of El Chichón volcano is poorly understood, and we attempt in this study to demonstrate that the Tehuantepec Ridge (TR), a major tectonic discontinuity on the Cocos plate, plays a key role in determining the location of the volcano by enhancing the slab dehydration budget beneath it. Using marine magnetic anomalies we show that the upper mantle beneath TR undergoes strong serpentinization, carrying significant amounts of water into subduction. Another key aspect of the magnetic anomaly over southern Mexico is a long-wavelength (∼ 150 km) high amplitude (∼ 500 nT) magnetic anomaly located between the trench and the coast. Using a 2D joint magnetic-gravity forward model, constrained by the subduction P–T structure, slab geometry and seismicity, we find a highly magnetic and low-density source located at 40–80 km depth that we interpret as a partially serpentinized mantle wedge formed by fluids expelled from the subducting Cocos plate. Using phase diagrams for sediments, basalt and peridotite, and the thermal structure of the subduction zone beneath El Chichón we find that ∼ 40% of sediments and basalt dehydrate at depths corresponding with the location of the serpentinized mantle wedge, whereas the serpentinized root beneath TR strongly dehydrates (∼90%) at depths of 180-200 km comparable with the slab depths beneath El Chichón (200-220 km). We conclude that this strong deserpentinization pulse of mantle lithosphere beneath TR at great depths is responsible for the unusual location, singularity and, probably, the geochemically distinct signature (adakitic-like) of El Chichón volcano.     

Lunar magnetic anomalies detected by the Apollo substatellite magnetometers

Properties of lunar crustal magnetization thus far deduced from Apollo subsatellite magnetometer data are reviewed using two of the most accurate presently available magnetic anomaly maps — one covering a portion of the lunar near side and the other a part of the far side.

The largest single anomaly found within the region of coverage on the near-side map correlates exactly with a conspicuous, light-colored marking in western Oceanus Procellarum called Reiner Gamma. This feature is interpreted as an unusual deposit of ejecta from secondary craters of the large nearby primary impact crater Cavalerius. An age for Cavalerius (and, by implication, for Reiner Gamma) of 3.2 ± 0.2 × 10⁹ y is estimated. The main (30 × 60 km) Reiner Gamma deposit is nearly uniformly magnetized in a single direction, with a minimum mean magnetization intensity of 7 × 10⁻² G cm³/g (assuming a density of 3 g/cm³), or about 700 times the stable magnetization component of the most magnetic returned samples. Additional medium-amplitude anomalies exist over the Fra Mauro Formation (Imbrium basin ejecta emplaced 3.9 × 10⁹ y ago) where it has not been flooded by mare basalt flows, but are nearly absent over the maria and over the craters Copernicus, Kepler, and Reiner and their encircling ejecta mantles.

The mean altitude of the far-side anomaly gap is much higher than that of the near-side map and the surface geology is more complex, so individual anomaly sources have not yet been identified. However, it is clear that a concentration of especially strong sources exists in the vicinity of the craters Van de Graaff and Aitken. Numerical modeling of the associated fields reveals that the source locations do not correspond with the larger primary impact craters of the region and, by analogy with Reiner Gamma, may be less conspicuous secondary crater ejecta deposits. The reason for a special concentration of strong sources in the Van de Graaff-Aitken region is unknown, but may be indirectly related to the existence of strongly modified crustal terrain which also occurs in the same region. The inferred directions of magnetization for the several sources of the largest anomalies are highly inclined with respect to one another, but are generally depleted in the north-south direction. The north-south depletion of magnetization intensity appears to continue across the far-side within the region of coverage.

The mechanism of magnetization and the origin of the magnetizing field remain unresolved, but the uniformity with which the Reiner Gamma deposit is apparently magnetized, and the north-south depletion of magnetization intensity across a substantial portion of the far side, seem to require the existence of an ambient field, perhaps of global or larger extent. The very different inferred directions of magnetization possessed by nearly adjacent sources of the Van de Graaff-Aitken anomalies, and the depletion in their north-south component of magnetization, do not favor an internally generated dipolar field oriented parallel to the present spin axis. A variably oriented interplanetary magnetizing field that was intrinsically strong or locally amplified by unknown surface processes is least inconsistent with the data.     

FACTOR ANALYSIS FOR INTERPRETING PETROPHYSICAL DATA ON RORO ULTRAMAFICS,SINGHBHUM DISTRICT,INDIA*

The physical properties of the serpentinized pyroxenitic and saxonitic rocks from stratiform ultramafic complexes of Roro, Singhbhum distrct, India, are examined using principal factor (R-mode) analysis technique. The variations and inter-relations of these properties reflect the compound effects of two processes—(a) degree of serpentinization, and (b) mineralogical and other changes attendant to serpentinization. Factor analysis is thus shown to be an effective tool for petrophysical inferences.     

Magnetic mineralogy and paleomagnetism of serpentinized ultramafic rocks from the Braszowice-Brzeźnica fragment of sudetic paleozoic ophiolite

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.     

Palaeomagnetic directions in Precambrian basic intrusives of the Mount Isa Province,Australia

Palaeomagnetic investigation of basic intrusives in the Proterozoic Mount Isa Province yields three groups of directions of stable components of NRM after magnetic cleaning in fields up to 50 mT (1 mT= 10 Oe). The youngest group (IA) includes results from the Lakeview Dolerite, and yields a palaeomagnetic pole at 12°S, 124°E (A₉₅ = 11°). The second group (IB) has a palaeomagnetic pole 53°S, 102°E (A₉₅ = 11°). The third group (IC) is derived from the Lunch Creek Gabbro and contains normal and reversed polarities of magnetization with a palaeomagnetic pole at 63°S, 201°E (A₉₅ = 9°). Some samples from the gabbro have anomalously low intensities of remanent magnetization in obscure directions attributed to the relative enhancement of the non-dipole component of the palaeomagnetic field during polarity reversal. The present attitude of the igneous lamination is probably of primary, not tectonic origin.     

Stability of remanent magnetization of some dykes from Mysore State,India

Summary The magnetic properties of some dykes from Mysore State, India, have been studied in detail. The rocks were found to have aQ _n ratio varying from 1.5 to 11.6, a remanent coercive force varying from 100 to 250 Oersteds, Curie temperature varying from 250 to 480°C and were found to have lamellae of ilmenite oriented in (111) plane of magnetite. The stable natural remanent magnetization of the rock seems to be of TRM origin with titanomagnetite and low grade titanomaghemite being the main carrier of remanent magnetization.N.G.R.I. Contribution No. 70-215.     

A paleomagnetic conglomerate test using the Abee E4 meteorite

The meteorite Abee is a type 4 enstatite chondrite with many centimeter-size clasts. The paleomagnetic conglomerate test was applied to these clasts, to study the thermal and magnetic history of the meteorite. The directions of magnetization in mutually oriented clasts are significantly different, suggesting the meteorite was not reheated to temperatures much above 100°C during or after accretion. Paleointensity estimates were made using Thellier's method. Interior samples which were probably not reheated during entry into the earth's atmosphere show paleointensities of several oersteds. The fusion crust is also strongly magnetized, showing paleointensities up to 60 Oe.     

磁赤道处化极方法

化向地磁极(化极)是最基本的磁测资料处理方法之一,化极能消除或减少斜磁化影响,提高对磁测资料的认识程度和解释水平,对研究地壳产生的磁异常具有重要意义.但低纬度地区特别是磁赤道处,化极处理很不稳定甚至奇异,一直是位场研究的难点.针对地磁纬度较低特别是磁赤道地区磁异常化极的困难,利用从磁北极处垂直磁化向低纬度地区水平磁化方向转换稳定的特点,提出"狭义化赤"概念,并将其与低纬度磁异常"倒相"解释方法结合,提出专门用于磁赤道处化极的方法.该方法扩展了现有的化极理论,实现了磁赤道处的稳定化极.区别于目前任何方法,专门用于(近)水平磁化条件下的化极计算,具有原理简单,实现方便,收敛速度快等特点.对理论模型和实际资料计算表明这种针对磁赤道地区磁异常的化极处理方法是稳定、可靠的.     

Monodomain behaviour in multiphase oxidized titanomagnetite

Large grains, of the order of tens of microns in size, of synthetic titanomagnetite have been oxidized to produce an intergrowth of phases, essentially similar to that produced by the natural process of deuteric oxidation. The scale of the intergrowth is at the limit of the optical range but the electron microscope reveals the characteristic lamella microstructure. The magnetic hysteresis properties of the most highly oxidized material, having probably about 10% of residual spinel phase, are typical of dispersed monodomain magnetite rods. The investigation therefore supports the model in which it is proposed that titanomagnetite grains in a slowly cooled basalt may carry a highly stable component of natural remanent magnetization.     

Rock-magnetic properties of topsoils and urban dust from Morelia (>800,000 inhabitants), Mexico: Implications for anthropogenic pollution monitoring in Mexico’s medium size cities

In this work, we investigate the correlation between some magnetic parameters and the level of contamination by heavy metals in urban soils from Morelia city, western Mexico. The magnetic study was carried out on 98 urban soils samples belonging to distinct land uses. Most of analyzed samples contain ferrimagnetic minerals as the responsible for magnetization, most probably corresponding to the titanomagnetites/ titanomaghemites solid solutions. This is inferred from the susceptibility vs. temperature measurements and the isothermal remanent magnetization (IRM) experiments. These measurements also indicate that most of samples are almost completely saturated before 300 mT. Additionally, the S_-200 values (S_-200 = IRM_-200/ SIRM, where IRM_-200= Back-field of 200 mT after magnetic saturation) are between 0.7 and 1.0, characteristic of low coercivity magnetic minerals. The averaged saturation isothermal remanent magnetization (SIRM) curves can be used as an indicator of pollution level, as these curves show different saturation values according to the level of contamination by heavy metals: Cu, Ni, Cr and Sr. These associations of (titano)magnetite with heavy metals were observed by Scanning Electron Microscope revealing some complex aggregates rather than commonly detected spherules.     

Estimation of depth and magnetization of magnetic sources from magnetic data: The linearized continuous inverse problem for 21/2D structures

The estimation of the depth to the top and bottom of a magnetic source from magnetic data defines a nonlinear inverse problem, while the evaluation of the distribution of magnetization determines a linear inverse problem. In this paper, these interpretation problems are resolved in the continuous case of 2_1/2D magnetized bodies with lateral magnetization variations. A formulation of the magnetic problem accounting for different directions of remanent and total magnetization vectors and including a more general definition of apparent susceptibility is presented. Differences between 2D and 2_1/2D formulations are stressed, as regards the anomaly amplitude, shape and zero-level.In order to utilize well-known continuous linear inverse methods, Fréchet derivatives of the data functionals with respect to the depth of the source top and bottom, are analytically described. Thus, using the spectral expansion inverse method (Parker, 1977) and linearizing the problem at several steps of an iterative process, the source depth is obtained within a few iterations, although the starting model is distant from the final solution. The interpretation of an anomaly in the Italian region shows the usefulness of the method.     

Magnetic properties of Antarctic shergottite meteorites EETA 79001 and ALHA 77005: possible relevance to a Martian magnetic field

The possibility that the parent body of the SNC meteorites is Mars implies that the magnetic properties of these meteorites may provide evidence concerning ancient Martian magnetic fields. EETA 79001 possesses a weak, very stable primary magnetization, the properties of which are consistent with its acquisition in an ambient magnetic field either during the meteorite's formation or during the severe shock event later in its history. The samples of ALHA 77005 studied possessed no measurable primary magnetization: the observed remanence appears to be a viscous magnetization acquired in local laboratory fields. The magnetic carriers in the meteorites are fine-grained magnetite and a lower Curie point mineral, probably titanomagnetite or pyrrhotite, present to the extent of less than 0.1% by weight. Estimates of the strength of the magnetizing field for EETA 79001 are in the range 1–10 μT.     

First archaeomagnetic results and dating of Neolithic structures in northern Greece

Archaeomagnetism in Greece has continuously developed during the last decades. Numerous studies have provided high quality data and accurate secular variation curves for the direction and intensity of the geomagnetic field have been constructed. The Greek Secular Variation Curves (SVCs) cover the last 8 millennia for intensity and 6 millennia for direction. The coverage of the archaeological periods remains uneven, with several gaps, mostly in the directional dataset, with only two results for periods older than 2500 B.C. In the present contribution, the first archaeomagnetic results from Neolithic settlements in northern Greece are presented. For the present study, samples were collected from three different archaeological sites: burnt structures in Avgi (Kastoria) and Vasili (Farsala) and one oven from Sosandra (Aridaia). The natural remanent magnetization (NRM) grouping of all specimens indicated that the majority of the samples were burnt in situ, providing thus a reliable direction of the ancient field. Magnetic cleaning (both alternating-field and thermal) revealed the presence of one stable component of magnetisation. Rock magnetic experiments (acquisition of isothermal remanent magnetization (IRM), thermal demagnetisation of the IRM, thermomagnetic curves) have been performed on pilot samples indicating that low coercivity magnetic minerals such as magnetite or Timagnetite are prevailing. The mean directions (declination D, inclination I and parameters of the Fisherian statistics), which arose from the three sites are as follows: Sosandra: D = 343°, I = 55.6°, ??₉₅ = 4.8°; Avgi: D = 10.1°, I = 53.4°, ??₉₅ = 4.2° and Vasili: D = 357.5°, I = 43.1°, ??₉₅ = 4.1°. The obtained data are in a very good agreement with results from Neolithic Bulgaria. This study represents the beginning of an effort to fill the gaps of the Greek secular variation curves and their extension to the Neolithic period.     

Serpentinite textural evolution related to tectonically controlled solid-state intrusion along the Kurosegawa Belt, northwestern Kanto Mountains, central Japan

Abstract   Serpentinite bodies in the Kurosegawa Belt are mapped along fault boundaries between the Cretaceous Sanchu Group (forearc basin-fill sediments) and the rocks of the Southern Chichibu Belt (Jurassic to Early Cretaceous accretionary prism) in the northwestern Kanto Mountains, central Japan. The serpentinites were divided into three types based on microtextures and combinations of serpentine minerals: massive, antigorite and chrysotile serpentinites. Massive serpentinite retains initial pseudomorphic textures without any deformation after serpentinization. Antigorite serpentinite exhibits shape-preferred orientation of antigorite replacing the original lizardite and/or chrysotile to form pseudomorphs. It has porphyroclasts of chromian spinel, and is characterized by ductile deformation under relatively high-pressure–temperature conditions. Chrysotile serpentinite shows evidence for overprinting of pre-existing serpentinite features under shallow, low-temperature conditions. It exhibits unidirectional development of chrysotile fibers. Foliations in antigorite and chrysotile serpentinites strike parallel to the elongate direction of the serpentinite bodies, suggesting a continuous deformation during solid-state intrusion along the fault zones after undergoing complete serpentinization at deeper levels (lower crust and upper mantle).