Magnetic Carrier Mineral Research Articles

Remagnetization of Jutal dykes in Gilgit area of the Kohistan Island Arc: Perspectives from the India–Asia collision

SUMMARY The Kohistan Island Arc (KIA) occupies the northwestern region of the Himalayan Mountains, sandwiched between Asia and India plates. Its formation, collision with plate boundaries, and evolution has been controversially discussed for a couple of decades. To better understand this, a palaeomagnetic study has been conducted on the Jutal dykes (ca. 75 Ma), intruded in the northeastern part of the KIA. Comprehensive rock magnetic investigations reveal that the magnetic carrier minerals are pyrrhotite and magnetite. An intermediate temperature component (ITC) predominates the natural remanent magnetization and shows good coincidence within-site; it is carried by pyrrhotite and is considered reliable, yielding a mean direction at Dg/Ig = 11.5°/39.9° (kg = 28.4, α95 = 3.5°) before and Ds/Is = 8.6°/12.1° (ks = 5.1, α95 = 9.1°) after tilt correction. A high-temperature component that is carried by magnetite exhibits random distribution within-site. The fold test for the ITC is negative, indicating a post-folding origin. Scanning electron microscopy combined with energy-dispersive X-ray spectroscopy indicates that the magnetic carrier minerals were influenced by metamorphism or thermochemical fluids. The comparison of mean palaeolatitude (22.6 ± 3.5°N) of the ITC with the collisional settings and thermal history of the study area implies that the remagnetization occurred at ∼50–35 Ma, consistent with the previous reported palaeomagnetic data of the KIA. We propose a tectonic model that shows the evolution of the Jutal dykes, supporting the concept that India collided with the KIA first, followed by a later collision with Asia.

Palaeo- and rock magnetic investigations of Late Quaternary sediments from the Upper Congo deep-sea fan: on the difficulty in obtaining palaeomagnetic secular variation records from low latitudes

We report here on results of palaeo- and rock magnetic investigations of two sediment cores from the Upper Congo deep-sea fan. The sediments have a high organic content and contain a heterogeneous Fe-mineral assemblage with biogenic magnetite and detrital (Ti-)magnetite as the main magnetic carrier minerals. Pyrite, hematite, and Fe-oxyhydroxides were identified by comparing high-temperature magnetic susceptibility curves with those from Fe-minerals of known composition. According to AMS 14C dates, the 6.8 m-long profile spans the last 37 kyr. Sediments older than 20 ka are affected by reductive diagenesis that has led to a loss of the fine-grained magnetic mineral fraction. Sediments younger than 20 ka have stable magnetizations. Characteristic remanent magnetization records of inclination and declination were obtained for each core. There is a little agreement between these records, modelled curves, and other sediment records from Equatorial Africa, so no composite record could be established. The cores are not ideal relative palaeointensity recorders and estimates using different normalizers did not yield consistent signals from both cores. Normalization methods used for relative palaeointensity estimation were not developed for sediments that contain large amounts of ultra-fine-grained biogenic magnetite; therefore, the relative palaeointensity estimates should be considered with caution. However, in view of the incoherent picture given by the scarce available palaeointensity information from the region off South-West Africa, the GeoB6517-2 record may provide a tentative relative palaeointensity record for comparison, at least for the past 10 kyr.

The relation between magnetite and silicate fabric in granitoids of the Adamello Batholith

The link between the macroscopic silicate fabric and the magnetite-controlled AMS (anisotropy of magnetic susceptibility) fabric in ferromagnetic rocks was investigated through a comprehensive comparison between different fabric measurement techniques. Sample lithologies include tonalites and granodiorites from the Lago della Vacca Complex, Adamello Batholith, Italy. The datasets used to assess the link between subfabrics and the coherence between methods include: 1) macroscopic silicate fabric measured directly in the field; 2) macroscopic silicate fabric derived from image analysis (IA) of outcrop pictures and sample pictures; 3) shape-preferred orientations (SPO) of mafic silicates, 4) SPO of magnetite, and 5) calculated distribution of magnetite grains from computer-assisted high-resolution X-ray tomography (X-ray CT) images; 6) fabrics derived from the AMS.Macroscopic mineral fabrics measured in the field agree with the IA results and with the SPO of mafic silicates obtained from the X-ray CT imaging. The X-ray CT results show that the SPO of the magnetite grains are consistent with the AMS data whereas the spatial distribution of the magnetite grains is less compatible with the AMS fabric. This implies that the AMS signal is mainly controlled by the shape of the magnetic carrier mineral rather than by the spatial arrangement of the magnetite grains. An exception is the presence of magnetite clusters. Furthermore, the SPO of mafic silicates and the SPO of the magnetite grains are consistent with the AMS data. Another finding of this study is that the magnetic susceptibility correlates linearly with the amount of magnetite in the samples. The coherent results obtained from a variety of methods reinforce the application of both AMS measurements and IA as robust tools to analyse fabrics in granitic intrusions.

AMS investigation in the Pingluoba and Qiongxi anticlines, Sichuan, China: Implications for deformation mechanism of the Qiongxi structure

Stratigraphic attitude is one of the most important field data for structural geology research. However, it is difficult to measure directly sometimes, especially in weak deformed sediments with low dip angle. Pingluoba anticline, mainly consisting of medium-high dip angle strata, is located on the Longmen Shan fold-and-thrust belt front. Qiongxi anticline is located in the southwestern Sichuan Basin, and it is mainly composed of sub-horizontal to low dip angle strata. An investigation of the anisotropy of magnetic susceptibility (AMS) in 78 sites has been carried out in these two folds. Stepwise demagnetization of three orthogonal isothermal remanent magnetizations suggests that hematite is the main magnetic carrier mineral in the Qiongxi anticline, meanwhile, hematite and magnetite are both existing in the Pingluoba anticline. It reveals three types of magnetic fabrics: sedimentary magnetic fabric, initial deformation magnetic fabric and pencil structure magnetic fabric, which indicate weak deformation. Throughout AMS theoretical analysis and example test in the Pingluoba anticline, magnetic fabric is a suitable technique to confirm the stratigraphic attitude in weak compressional deformed sediments. For the sedimentary magnetic fabric, initial deformation magnetic fabric and one atypical initial deformation magnetic fabric developed in the weak deformed tectonic superposition region, dip direction of bedding plane = pitch direction of K 3 +(or −) 180°, and dip angle of bedding plane = 90°-pitch angle of K 3 . 38 stratigraphic attitudes with low dip angle in the Qiongxi anticline have been rectified by magnetic fabric results. Qiongxi structure is a weak tectonic superposition deformation anticline. The deformation mechanism of Qiongxi structure may contain two stages: NW–SE compression stage started from the latest Cretaceous–early Cenozoic and nearly E–W shortening stage in the Late Cenozoic. • AMS is reasonable to confirm the sedimentary stratigraphic attitude. • Qiongxi structure is a weak tectonic superposition deformation anticline. • Qiongxi structure was formed as two compressional stages.

Electronic and magnetic structures of the postperovskite-type Fe 2 O 3 and implications for planetary magnetic records and deep interiors

Recent studies have shown that high pressure (P) induces the metallization of the Fe(2+)-O bonding, the destruction of magnetic ordering in Fe, and the high-spin (HS) to low-spin (LS) transition of Fe in silicate and oxide phases at the deep planetary interiors. Hematite (Fe(2)O(3)) is an important magnetic carrier mineral for deciphering planetary magnetism and a proxy for Fe in the planetary interiors. Here, we present synchrotron Mössbauer spectroscopy and X-ray diffraction combined with ab initio calculations for Fe(2)O(3) revealing the destruction of magnetic ordering at the hematite --> Rh(2)O(3)-II type (RhII) transition at 70 GPa and 300 K, and then the revival of magnetic ordering at the RhII --> postperovskite (PPv) transition after laser heating at 73 GPa. At the latter transition, at least half of Fe(3+) ions transform from LS to HS and Fe(2)O(3) changes from a semiconductor to a metal. This result demonstrates that some magnetic carrier minerals may experience a complex sequence of magnetic ordering changes during impact rather than a monotonic demagnetization. Also local Fe enrichment at Earth's core-mantle boundary will lead to changes in the electronic structure and spin state of Fe in silicate PPv. If the ultra-low-velocity zones are composed of Fe-enriched silicate PPv and/or the basaltic materials are accumulated at the lowermost mantle, high electrical conductivity of these regions will play an important role for the electromagnetic coupling between the mantle and the core.

Magnetic fabric investigation in the northwestern Sichuan Basin and its regional inference

An investigation of the anisotropy of magnetic susceptibility (AMS) in 39 sites has been carried out in the northwestern Sichuan Basin which is adjacent to the northeastern Tibetan plateau, discussing the deformation pattern of the basin and the orogenic process of the corresponding orogens. Stepwise demagnetization of three orthogonal isothermal remanent magnetization suggests that hematite is the main magnetic carrier mineral. Combined with the low value of magnetic susceptibility, it is considered that the AMS is some combination of hematite and paramagnetic matrix minerals. Different types of magnetic fabrics from sedimentary to pencil structure magnetic fabric and sharps of susceptibility ellipsoids from oblate to prolate were observed. The existence of sedimentary magnetic fabric and the accordance of magnetic lineation and bedding strike indicate the magnetic fabrics along the front of Longmen Shan belt were the result of tectonic deformation. Thermochronology researches in the orogens and our apatite fission track analysis in the northwestern Sichuan Basin suggest that magnetic fabric mainly reflects the Cenozoic deformation. The magnetic lineation which is consistent with the fault and fold strikes in the Longmen Shan was induced by Mesozoic and Cenozoic deformation, and the latter is the primary one. Besides the typical weak deformation magnetic fabrics, atypical magnetic fabrics characterized by a magnetic lineation that is oblique to the bedding strike are shown in 53% sites along the front of Micang Shan. It is proposed that such a type of atypical magnetic fabric is the result of two competing sub-fabrics in the weakly deformed tectonic superposition region.

Rock magnetic studies on sediments from Erlongwan maar lake, Long Gang Volcanic Field, Jilin province, NE China

SUMMARY Detailed rock magnetic investigations were carried out on two 23-m-long sediment cores from Erlongwan maar lake, NE China. The completely laminated sediment sequence of the lake is interrupted by 410 graded layers with thicknesses between 0.1 and 150 cm. Magnetite of PSD-size was identified as the main magnetic carrier mineral by temperature-dependent measurements of the saturation magnetization and determination of hysteresis parameters. The minerogenic components in the laminated sediments and the graded layers are nearly identical, and their rock magnetic characteristics reflect the prevailing conditions, anoxic or oxic, during deposition. The most reliable criteria for estimating the availability of oxygen is whether an increase in minerogenic influx is linked to a shift in the magnetic grain size spectrum to coarser (oxic) or finer (anoxic) grains. Comparison of different rock magnetic parameters indicative for magnetic grain size and coercitivity revealed, that the S-ratio which is known to reflect the presence of high coercive minerals, is grain size indicative in sediments with a monomineralic magnetic composition.

Sedimentology, clay mineralogy and grain-size as indicators of 65 ka of climate change from El’gygytgyn Crater Lake, Northeastern Siberia

El’gygytgyn Crater Lake, NE Siberia was investigated for sedimentological proxies for regional climate change with a focus on the past 65 ka. Sedimentological parameters assessed relative to magnetic susceptibility include stratigraphy, grain size, clay mineralogy and crystallinity. Earlier work suggests that intervals of high susceptibility in these sediments are coincident with warmer (interglacial-like) conditions and well-mixed oxygenated bottom waters. In contrast, low susceptibility intervals correlate with cold (glacial-like) conditions when perennial ice-cover resulted in anoxia and the dissolution of magnetic carrier minerals.

Results of rock magnetic investigations and relative paleointensity determinations on lacustrine sediments from Birkat Ram, Golan Heights (Israel)

Detailed rock magnetic investigations, including the analysis of the laboratory installed magnetizations, the measurement of hysteresis parameters, and the high temperature‐dependency of the saturation magnetization, were carried out on sediments from Birkat Ram, Golan Heights, Israel. The 550 cm long composite profile, composed of three single cores, is supposed to span the Late Holocene, i.e., the last 4400 years, at least. Titanomagnetites in the pseudo single‐domain range were identified as the main magnetic carrier minerals in the homogenous sediments. The variations in concentration and grain size of the magnetic fraction are only partly reflected in the sediment lithology. The relative paleointensity was estimated by normalizing the intensity of the natural remanent magnetization (NRM) after demagnetization at 20 mT (JNRM(20 mT)) by different concentration parameters. The median destructive field of the NRM (MDFNRM) is coherent with the relative paleointensity estimates, indicating that a sedimentary effect has not sufficiently been removed by the normalization process. The morphologies of these estimates, however, correspond to the archeomagnetic records from Bulgaria and Greece. Therefore a second normalization, based on the linear relationship between the paleointensity estimates and the MDFNRM, was applied, resulting in an improvement of the records.

Magnetic model of the central uplift of the Vredefort impact structure, South Africa

The Vredefort structure is the largest known impact structure on earth, with an original diameter estimated at—at least—250 km. The central uplift of this structure is about 80 km wide and contains a 40- to 50-km wide core of crystalline basement surrounded by a so-called collar of supracrustal sequences. Previous investigations of the structure of this large impact crater, based on integrated geophysical modeling, have shown that the central rise structure is an upper crustal feature with a presently recognized uplift of some 12 km, decreasing to about 4 km at the depth of the Moho. This study concentrates on the structure of the central uplift region (the Vredefort Dome), using aeromagnetic data along a SSW–NNE profile across its center. Available rock magnetic data were used to constrain the magnetic modeling. On the basis of this modeling, it is concluded that a 60- to 70-km wide region in the center of the structure experienced a magnetic overprint in connection with the impact event. This overprint is related to the occurrence of high-coercive magnetite and its subsequent thermal magnetization. Three sources for this thermal event are envisaged—the potential occurrence of a now eroded impact melt complex, the impact-related rise of hot crustal material of the crystalline core, and the thermal energy deposited by the shock wave. Thermal demagnetization of rocks from the crystalline core has shown that magnetite, most likely formed by shock dissociation of Mg–Fe silicates, is the dominant carrier mineral of the remanent magnetization. The thermal overprint has also included those parts of the upturned metasedimentary collar rocks, which contained suitable magnetic carrier minerals. Its present manifestation is the generally negative magnetic anomaly centered on, and encircling, the central uplift of the impact structure. Within the crystalline core, the magnetic models indicate structures of outward collapse along low-angle thrust surfaces. The impact event has, thus, caused an increased magnetic complexity both by drastic changes of the magnetic properties and by the structural perturbation of the upper crystalline crust.

High-resolution magnetostratigraphy of four sediment cores from the Greenland Sea-II. Rock magnetic and relative palaeointensity data

SUMMARY High-resolution magnetostratigraphic analysis of three sediment cores from the base of the volcanic seamount Vesteris Banken in the Greenland Basin and one from the Jan Mayen Fracture Zone revealed records of three pronounced geomagnetic events: the Mono Lake excursion (28–27 ka), the Laschamp event (37–33 ka) and the Biwa I*** Jamaica event (189–179 ka). Rock magnetic investigations, measurements of magnetic susceptibility, and ARM and IRM acquisition/demagnetization experiments show that only fine-grained (titano) magnetite (≤ 0.5 μm) is the dominant magnetic carrier mineral in the hemipelagic muds. There is no evidence for high amounts of high-coercivity minerals like haematite or goethite. Volcanic (basaltic) ash layers in sediments around Vesteris Banken seamount are characterized by extremely high values of magnetic susceptibility, reaching 16 000 times 10-6 (SI), as well as by low median destructive fields of the anhysteretic remanent magnetization (MDFARM), indicating magnetite of larger grain sizes (≥ 1 μm). Besides this, no significant variation in rock magnetic parameters across intervals of intermediate to reversed ChRM inclinations are observed. Therefore, these are interpreted as true records of geomagnetic field variations. The recovered sediments, characterized by only moderate fluctuations in concentration as well as grain-size-related rock magnetic parameters, provided good conditions for the reconstruction of the relative palaeointensity of the geomagnetic field. The recorded geomagnetic events are clearly linked to a drastic reduction in the relative palaeointensity of the field. The lowermost values were obtained for the time of the polarity transition with a trend to higher relative intensities in the course of the polarity events. Event ages and the relative palaeointensity variations, based on different methods, derived from the Greenland Sea sediments investigated are in good agreement with data published in the literature.

Magnetostratigraphy of Early Cretaceous Maiolica sections of the Southern Alps (Cismon and Pra da Stua, Italy)

The bottom part of the Cretaceous Cismon section in the Southern Alps was sampled for high-resolution magnetostratigraphy. Although the almost pure pelagic nannofossil limestones (c. 90 per cent CaCO3) of the Maiolica/Biancone Formation are extremely weakly magnetized, stepwise thermal and alternating-field demagnetization removed overprints and isolated a characteristic remanent magnetization which is interpreted as a primary magnetization. The dominant magnetic carrier mineral is magnetite; a small fraction of haematite may be present. A clear reversal pattern can be correlated unambiguously with Mesozoic polarity chrons CM10N to CM8. A less well-constrained magnetostratigraphy from the Pra da Stua section could not be directly correlated with the global polarity scale, but biostratigraphic information allows its assignment to the interval CM10-CM5. A counterclockwise rotation of 56° and a northward translation of 28° latitude for the Cismon locality since the Early Cretaceous are derived from the palaeomagnetic data, consistent with previous results from the Southern Alps. The high-resolution magnetostratigraphy of the Cismon section is used in an effort to refine the Cretaceous timescale by the combination of magnetostratigraphic and cyclostratigraphic results from the same section. The cyclostratigraphic duration estimates of chrons CM10N to CM8 are compared to their equivalents in a number of traditional timescales and found to be shorter by a factor of 1.26–2.58.