Stable Magnetization Components Research Articles

Thermal rock magnetic cycling (TRMC): a method to track thermal alteration details for palaeointensity interpretations

SUMMARY Accurate absolute palaeointensity is essential for understanding dynamo processes on the Earth and other planetary bodies. Although great efforts have been made to propose techniques to obtain magnetic field strength from rock samples, such as Thellier-series methods, the amount of high-fidelity palaeointensities remains limited. One primary reason for this is the thermal alteration of samples that pervasively occurred during palaeointensity experiments. In this study, we developed a comprehensive rock magnetic experiment, termed thermal rock magnetic cycling (TRMC), that can utilize measurements of critical rock magnetic properties at elevated temperatures during multiple heating-cooling cycles to track thermal changes in bulk samples and individual magnetic components with different Curie temperatures in samples for palaeointensity interpretations. We demonstrate this method on a Galapagos lava sample, GA 84.6. The results for this specimen revealed that GA 84.6v underwent thermophysical alteration throughout the TRMC experiment, resulting in changes in its remanence carrying capacity. These findings were then used to interpret the palaeointensity results of specimen GA 84.6c, which revealed that the two-slope Arai plot yielded two linear segments with distinct palaeointensity values that were both biased by thermophysical alteration. To further test the TRMC method, we selected another historical lava sample (HS 2) from Mt Lassen, detecting slight thermal-physical changes after heating the specimen HS 2–8C to a target temperature of 400 °C. We also isolated a stable magnetic component with a Curie temperature below 400 °C using the TRMC method, which may provide a more reliable palaeointensity estimate of 51 μT. By providing a method for tracking thermal alteration independent of palaeointensity experiments, the TRMC method can explore subtle, unrecognizable thermal alteration processes in less detailed palaeointensity measurements, which can help to assess the thermal stability of the measured samples and interpret the changes in the TRM unblocking spectrum and palaeointensity estimates, facilitating the acquisition of more reliable records for constrain the formation of the inner core and the evolution of Earth's magnetic field.

Paleomagnetic Constraints From 925 Ma Mafic Dykes in North China and Brazil: Implications for the Paleogeography of Rodinia

AbstractCoeval Neoproterozoic mafic dykes with a well‐constrained isotopic age of 925 Ma are distributed in the North China Craton (NCC) of China and São Francisco Craton (SFC) of Brazil. Several recent studies favor the hypothesis that these two cratons were connected during 925 Ma and constituted building cratons for the supercontinent Rodinia. However, the paleo‐positions of the two cratons in Rodinia have not yet been paleomagnetically resolved. This paper presents the paleomagnetic results of these dykes in both NCC and SFC. Detailed thermal and alternating‐field demagnetization revealed that these dykes record stable components of magnetization with unblocking temperatures below the Curie temperature of magnetite. Rock magnetic results and scanning electron microscope analysis further confirm that the major magnetic minerals in dyke samples are pseudo‐single domain titanomagnetites. The mean directions of the characteristic remanent magnetization (ChRM) of the dykes are distinct from those of rocks of different ages in the sampling localities, and are supported by positive baked contact tests, indicating primary remanent magnetization. The ChRM of the SFC dykes also exhibits dual polarity. Our results suggest that the NCC and SFC were in the moderate to high paleolatitudes of the Southern Hemisphere during approximately 925 Ma, suggesting a possible paleogeographic connection between the two cratons. A new paleogeographic reconstruction of both cratons during 925 Ma is proposed, which is supported by other geological evidence.

Paleomagnetism of the Ordovician section of the Mishina Gora (Pskov oblast): a record of the ancient geomagnetic field and restrictions on the age of the Mishinogorsk explosive structure

Paleomagnetic studies of the Ordovician rocks of the Mishina Gora explosive structure (Gdovsky district, Pskov region) have been carried out. Based on the results of thermal magnetic cleaning, two stable magnetization components S and P have been identified. These components have similar unblocking temperature ranges (~200–600°C), but their mean directions are statistically different the difference of their mean directions are statistically significant. Refional fold–test indicates the postfolding age of the component S and prefolding age of the component P. Prefolding age of the latter is supported by the positive result of the local fold-test which has been carried out in the scale of the studied section. Comparison of the paleomagnetic pole calculated from the S component with the Apparent Polar Wander Path for the East European Platform definitely indicates the Late Permian-Early Triassic age of this component, which, in combination with the available geological observations, determines the time range of the formation of the Mishina Gora structure as ~370-250 million years ago. The ancient component of the magnetization P has the same direction as the components, which have been identified earlier in a number of coeval sections of the North-West of Russia, Estonia, and Scandinavia and which have been considered to be primary. The corresponding paleomagnetic poles are widely used in Paleozoic geodynamic and tectonic paleoreconstructions with the participation of the Baltic paleocontinent. A positive fold test result for the P component validates the use of these poles for paleoreconstructions. For the stratigraphic interval, which includes the upper Floian, Dapinian, and lower 2/3 of the Darrivilian stages, a magnetostratigraphic section has been constructed containing a single magnetozone corresponding to the reverse polarity of the geomagnetic field. Thus, the magnetostratigraphic data do not support the hypothesis of the existence of intervals of normal polarity within the Ordovician geomagnetic superchron of reverse polarity (Moyero superchron).

Dating of Holocene fluvial deposits in the southern Sierras Pampeanas (Argentina) by matching paleomagnetic secular variation to a geomagnetic field model

A 2-m long Holocene sedimentary sequence collected from a fluvial system located in the eastern piedmont of Sierra de Las Quijadas (San Luis, Argentina) was dated by comparing its paleomagnetic secular variation (PSV) record with the SHA.DIF.14 k time-varying spherical harmonic geomagnetic field model. Rock magnetic data suggest that the magnetic signal is mainly carried by low-coercivity magnetic minerals like magnetite. Low field magnetic susceptibility, used as a magnetic concentration parameter, shows a variation of about a factor of two along the profile. Progressive demagnetization experiments of the natural remanent magnetization show the presence of a stable magnetic component, and the Characteristic Remanent Magnetization directions are well defined (MAD <5°). The obtained magnetic inclination, declination, and the relative paleointensity (RPI) records were then compared with previously published lacustrine Holocene records from Argentina. Several common features and similar millennial-scale fluctuations are observed among the records, supporting a common geomagnetic origin. The consistency and good match among natural archives on a broader scale in different Argentinian regions suggest that the PSV records of El Chimborazo site are reliable and can thus be used to better constrain the secular variation path of the geomagnetic field in this area.

Paleomagnetism of Rumuruti chondrites suggests a partially differentiated parent body

Different types of magnetic fields were at work in the early solar system: nebular fields generated within the protoplanetary nebula, solar fields, and dynamo fields generated within the solar system solid bodies. Paleomagnetic studies of extraterrestrial materials can help unravel both the history of these magnetic fields, and the evolution of solar system solid bodies. In this study we studied the paleomagnetism of two Rumuruti chondrites (PCA 91002 and LAP 03639). These chondrites could potentially bear the record of the different fields (solar, nebular, dynamo fields) present during the early solar system. The magnetic mineralogy consists of pseudo-single domain pyrrhotite in LAP 03639 and pyrrhotite plus magnetite in PCA 91002. Paleomagnetic analyses using thermal and alternating field demagnetization reveal a stable origin trending component of magnetization. Fields of 12 μT or higher are required to account for the magnetization in PCA 91002, but the timing and exact mechanism of the magnetization are unconstrained. In LAP 03639, considering various chronological constraints on the parent body evolution and on the evolution of the different sources of magnetic field in the early solar system, an internally-generated (dynamo) field of ∼5 μT recorded during retrograde metamorphism is the most likely explanation to account for the measured magnetization. This result indicates the existence of an advecting liquid core within the Rumuruti chondrite parent body, and implies that, as proposed for CV and H chondrites, this chondritic parent body is partially differentiated.

Comparison of counterstreaming suprathermal electron signatures of ICMEs with and without magnetic cloud: are all ICMEs flux ropes?

Context. Magnetic clouds (MCs), as in large-scale interplanetary magnetic flux ropes, are usually still connected to the Sun at both ends near 1 AU. Many researchers believe that all nonMC interplanetary coronal mass ejections (ICMEs) also have magnetic flux rope structures, which are inconspicuous because the observing spacecraft crosses the flanks of the rope structures. If so, the field lines of nonMC ICMEs should also usually be connected to the Sun at both ends. Aims. We want to know whether or not the field lines of most nonMC ICMEs are still connected to the Sun at both ends. Methods. This study examined the counterstreaming suprathermal electron (CSE) signatures of 272 ICMEs observed by the Advanced Composition Explorer (ACE) spacecraft from 1998 to 2008 and compared the CSE signatures of MCs and nonMC ICMEs. Results. Results show that only 10 of the 101 MC events (9.9% ) and 75 of the 171 nonMC events (43.9%) have no CSEs. Moreover, 21 of the nonMC ICMEs have high CSE percentages (more than 70%) and show relatively stable magnetic field components with slight rotations, which are in line with the expectations that the observing spacecraft passes through the flank of magnetic flux ropes. Therefore, the 21 events may be magnetic flux ropes but the ACE spacecraft passes through their flanks of magnetic flux ropes. Conclusions. Considering that most other nonMC events have disordered magnetic fields, we suggest that some nonMC ICMEs inherently have disordered magnetic fields, and therefore no magnetic flux rope structures.

Paleomagnetism of the Mn wad deposit at Niimi hot springs, Hokkaido, Japan

The Niimi hot springs is located at the foot of Mt. Shyakunage in the active Niseko volcanic area, Hokkaido, Japan. The manganese (Mn) wad deposit was precipitated from the hot spring water by biomineralization processes. However, no precise direct age dates for the diagenesis of such Mn oxides have been reported. Here is reported the first paleomagnetic application to date Mn oxides on 144 disseminated Mn wad and 42 pyroclastic clay specimens. The Mn wad specimens were collected at 14 levels between ∼5.5 and ∼62 cm in thickness on a three-dimensional grid pattern, and the clay specimens were collected at seven levels between 85 and 118 cm in thickness. Alternating field step demagnetization isolates either one or two stable characteristic remanent magnetization (ChRM) components in the Mn wad specimens. The lower coercivity component resides in pseudosingle domain titanomagnetite and shows clustered ChRM directions at each sampling level. Conversely, the higher coercivity component resides in single domain titanomagnetite and shows more random directions. The higher coercivity component is likely carried by pyroclastic fragments that landed on the site. The remanent directions of the clay specimens reside in multidomain magnetite, are less stable, and show a scattered distribution. When the observed ChRM directions of the Mn wad specimens are compared with the paleosecular variation record from Lake Biwa in Japan, the Mn wad deposition records the secular variation and appears to have lasted at least ∼1600 years from ∼3.5 to ∼1.9 kyr BP.

Systematics of Early Cambrian Paleomagnetic Directions from the Northern and Eastern Regions of the Siberian Platform and the Problem of an Anomalous Geomagnetic Field in the Time Vicinity of the Proterozoic–Phanerozoic Boundary

Representative paleomagnetic collections of Lower Cambrian rocks from the northern and eastern regions of the Siberian platform are studied. New evidence demonstrating the anomalous character of the paleomagnetic record in these rocks is obtained. These data confidently support the hypothesis (Pavlov et al., 2004) that in the substantial part of the Lower Cambrian section of the Siberian platform there are two stable high-temperature magnetization components having significantly different directions, each of which is eligible for being a primary component that was formed, at the latest, in the Early Cambrian. The analysis of the world’s paleomagnetic data for this interval of the geological history shows that the peculiarities observed in Siberia in the paleomagnetic record for the Precambrian–Phanerozoic boundary are global, inconsistent with the traditional notion of a paleomagnetic record as reflecting the predominant axial dipole component of the geomagnetic field, and necessitates the assumption that the geomagnetic field at the Proterozoic–Phanerozoic boundary (Ediacaran–Lower Cambrian) substantially differed from the field of most of the other geological epochs. In order to explain the observed paleomagnetic record, we propose a hypothesis suggesting that the geomagnetic field at the Precambrian–Cambrian boundary had an anomalous character. This field was characterized by the presence of two alternating quasi-stable generation regimes. According to our hypothesis, the magnetic field at the Precambrian–Cambrian boundary can be described by the alternation of long periods dominated by an axial, mainly monopolar dipole field and relatively short epochs, lasting a few hundred kA, with the prevalence of the near-equatorial or midlatitude dipole. The proposed hypothesis agrees with the data obtained from studies of the transitional fields of Paleozoic reversals (Khramov and Iosifidi, 2012) and with the results of geodynamo numerical simulations (Aubert and Wicht, 2004; Glatzmayer and Olson, 2005; Gissinger et al., 2012).

A paleomagnetic record in loess\u2013paleosol sequences since late Pleistocene in the arid Central Asia

Geomagnetic excursions during Brunhes epoch have been brought to the forefront topic in paleomagnetic study, as they provide key information about Earth’s interior dynamics and could serve as another tool for stratigraphic correlation among different lithology. Loess–paleosol sequences provide good archives for decoding geomagnetic excursions. However, the detailed pattern of these excursions was not sufficiently clarified due to pedogenic influence. In this study, paleomagnetic analysis was performed in loess–paleosol sequences on the northern piedmont of the Tianshan Mountains (northwestern China). By radiocarbon and luminance dating, the loess section was chronologically constrained to mainly the last c.130 ka, a period when several distinct geomagnetic excursions were involved. The rock magnetic properties in this loess section are dominated by magnetite and maghemite in a pseudo-single-domain state. The rock magnetic properties and magnetic anisotropy indicate weakly pedogenic influence for magnetic record. The stable component of remanent magnetization derived from thermal demagnetization revealed the presence of two intervals of directional anomalies with corresponding intensity lows in the Brunhes epoch. The age control in the key layers indicates these anomalies are likely associated with the Laschamp and Blake excursions, respectively. In addition, relative paleointensity in the loess section is basically compatible with other regional and global relative paleointensity records and indicates two low-paleointensity zones, possibly corresponding to the Blake and Laschamp excursions, respectively. As a result, this study suggests that the loess section may have the potential to record short-lived excursions, which largely reflect the variation of dipole components in the global archives.

Paleomagnetism of Precambrian dikes in the Kola part of northeastern Fennoscandia and its relation to the Svecofennian orogeny

Based on the results of the preliminary paleomagnetic investigation of 57 Precambrian dikes of the Kola Peninsula, in 31 of them a stable monopolar component of natural remanent magnetization (NRM) is revealed (D = 353.2°, I = 53.0°, K = 58, and α95 = 3.4°). The peculiarities of the distribution of this magnetization component within the Kola Peninsula and the rock magnetic characteristics of the dikes in which this component is isolated suggest its secondary nature and relate the mechanism and formation time to the remagnetization processes which took place in the northwest of Fennoscandia about 1.8 billion years ago during the Svecofennian orogeny. The corresponding geomagnetic pole of Fennoscandia has the coordinates Plat = 54.5°, Plong = 224.0°, and A95 = 3.9° and is located in the immediate vicinity of the known Paleoproterozoic (1.9–1.7 Ga) poles of Baltica (Khramov et al., 1997; Veikkolainen et al., 2014).

Kinematics of the crust around the Tanggula Shan in North–Central Tibet: Constraints from paleomagnetic data

We have conducted a paleomagnetic investigation on the Middle–Upper Jurassic marine strata exposed in the hanging wall of the Tanggula Thrust system near the Yanshiping area, northern Tibet. Progressive demagnetization experiments successfully isolated stable magnetization over a broad spectrum of demagnetization temperatures. The mean direction of the characteristic remanent magnetizations for the Middle–Late Jurassic Yanshiping Group in stratigraphic coordinates (D/I (Declination/Inclination)=5.6°/60.3°, k=22.9, α95=12.9°, N=7s) is much more clustered than the mean direction in geographic coordinates (D/I=345.5°/37.2°, k=2.5, α95=48.4°), indicating magnetization was not acquired after folding. Although the conventional fold test is positive, incremental untilting test on the characteristic remanent magnetization reveals that a maximum value of precision parameter k occurs at 82.1±4.6% untilting (D/I=3.3°/57.8°, k=43.9, α95=9.2°), which indicates the ChRMs are probably acquired during Late Cretaceous folding. This synfolding magnetization component is therefore secondary. The corresponding pole position (84.4°N, 119.4°E with dp/dm=13.5/9.9°) is inconsistent with Jurassic–Early Cretaceous paleopoles of the region, but the paleolatitude is consistent with the Late Cretaceous paleolatitude observed in the Qiangtang terrane and its periphery. The synfolding component is carried by both magnetite and hematite, which were identified by isothermal remnant magnetization acquisition experiments, unblocking temperatures of stable magnetic components, and Curie temperature determination and correlated with observed hydrothermal veins. Available geological evidences indicate that the synfolding magnetization is probably the result of chemical remagnetization caused by orogenic fluids or hydrothermal sources during the early uplift of the Tibetan Plateau.

An early solar system magnetic field recorded in CM chondrites

Abstract We present a paleomagnetic study of seven CM carbonaceous chondrites. CM chondrites are believed to be some of the most chemically primitive materials available in our solar system and may sample the continuum of transitional objects between asteroids and comets formed in the outer solar system. As such, CM chondrites can help us to understand primordial aspects of the history of the early solar system including protoplanetary disk and planetesimal magnetism. The ferromagnetic assemblage of CM chondrites is composed of a mixture of primary metallic iron, pyrrhotite, and magnetite. The remanent properties are usually dominated by secondary pyrrhotite. Paleomagnetic analyses using thermal and alternating field demagnetization identified a stable origin-trending component of magnetization in the seven studied CM chondrites. In each meteorite, this component is homogeneous in direction at least at the cm scale and is therefore post-accretional. We interpret this stable component as a pre-terrestrial chemical remanent magnetization acquired during crystallization of magnetite and pyrrhotite during parent body aqueous alteration in a field of at least a few μ T ( 2 ± 1.5 μ T ) . Considering the timescale and intensities of primordial magnetic fields, both internally generated fields from a putative dynamo and external fields, generated in the protoplanetary disk, may have been recorded by CM chondrites. It is presently difficult to discriminate between the two hypotheses. Regardless, CM chondrites likely contain the oldest paleomagnetic record yet identified.

Palaeomagnetism of Palaeozoic glacial sediments of Northern Ethiopia: a contribution towards African Permian palaeogeography

One hundred fourteen oriented palaeomagnetic core samples were collected from 13 palaeomagnetic sites on subhorizontal to tilted glacial sediments at five localities of Northern Ethiopia. Combined alternating field (AF) and stepwise thermal demagnetization techniques were successfully applied to resolve the complete directional spectrum. A viscous remagnetization (VRM) and one stable component of magnetization were identified in most of the specimens. The VRM is removed between a temperature range of 120–350 °C and AF of up to 30 mT. Further heating until ∼650 °C results in smooth decay of the natural remanent magnetization (NRM) intensity to about 50 per cent and the rest of the NRM is efficiently removed by heating to 690 °C, while only 30–50 per cent of NRM is removed by the maximum AF available suggesting haematite as remanence carrier. Results of the magnetization decay curve plots and rock magnetic analyses using the variable field translation balance indicated the presence of magnetite with minor goethite, pyrrhotite as well. The high stability component defining a straightline segment, starting 350 °C and/or 30 mT is mostly directed towards the origin and interpreted as the characteristic remanent magnetizations (ChRMs). The direction of magnetization is determined both by best-fitting line using the least-square technique of Kirschvink and remagnetization circles of Halls for few unresolved overlapping components. The site mean directions of the sediments from two sites are normal polarity and are close to present-day field directions at the sample site. The site mean directions from 11 sites, on the other hand, are reversed in polarity with better grouping in the tilt-corrected coordinate and pass the McFadden fold test. This overall site mean direction is Dec = 143.4º, Inc = 58.8º (N = 11, α95 = 9.7º) with a corresponding mean pole position of Lat = 26.0º, Lon = 249.5º (N = 11, A95 = 13.1º). This geomagnetic pole position is later rotated into West Africa coordinates to allow for extensional rifting in the Benue Trough about an Euler pole position, at 19.2ºN, 352.6ºE through an angle −6.3º (clockwise). The resulting pole position is located at ϕs = 246.6ºE, λs = 31.8ºS (N = 11, A95 = 13.1º), this pole with its 95 per cent confidence circle intersects the 270–310 Ma, segment of the APW path for West Africa consistent with ages of between late Carboniferous and early Permian. The result also implies that the Late Carboniferous Dwyka land ice sheet had probably extended more than 1000 km further north to Ethiopia than previously known.

Instantaneous paleomagnetic record from the Miocene Kozagawa Dike of the Kumano Acidic Rocks, Kii Peninsula, Southwest Japan: cautionary note on tectonic interpretation

We have conducted a paleomagnetic investigation of the Kozagawa Dike, which is part of the Kumano Acidic Rocks that are the product of a Middle Miocene volcano- plutonic complex cropping out on the Kii Peninsula of Southwest Japan. The granite porphyry facies of the dike yields stable remanent magnetization components. Precisely determined site-mean directions of characteristic components, as revealed by principal component analysis of stepwise alternating-field and thermal demagnetization results, form a tight cluster. The overall mean direction exhibits an approximately 40° clockwise declination shift from the south and a steep negative inclination (D = 217.3°, I =- 59.8°, a95 = 2.8°; 10 sites). Rock magnetic analyses suggest that magnetite is the main stable component carrier. The overall mean direction is interpreted to be a geologically instan- taneous record of the paleomagnetic field that was acquired in a relatively short time interval relative to paleomagnetic secular variations. The mean direction in geographic (in situ) coordinates obtained for the Kozagawa Dike is distinct from previously published data for laccolithic granite porphyry intrusions of the Kumano Acidic Rocks. This differ- ence may result from a time lag during acquisition of remanent magnetization by the different units of the Kumano Acidic Rocks. The southwest and up paleomagnetic direction has previously been interpreted as representing clockwise rotation of Southwest Japan associated with back-arc opening of the Japan Sea. However, we contend that these directions actually reflect the transient capture of an extraordinary paleomagnetic field at ca 14.3 Ma, such as a transitional or excursional state, and are not of tectonic origin.

Paleomagnetic studies of the Lower Devonian gray deposits of Podolia

New results of paleomagnetic studies of Lower Devonian sediments from the southwest of Ukraine (the Podolia area, the Dniester river basin) have been presented. The samples are grey limestone and dolomite of Tiverian series comparable with the Lochkovian stage of the Geological time scale. Stable component of magnetization of the most samples indicate to remagnetization of rocks (the Permian pole, ϕ=45qS, λ=340qE, dp/dm=2q/5q). In some samples of the Ivanev suite the primary component of magnetization (Lower Devonian direction) has been distinguished. Paleomag-netic pole (ϕ=0q, λ=329qE, dp/dm=10q/12q) is in a good agreement with Lower Devonian fragment of apparent wander path for the Baltica. The results are in agreement with earlier data of authors for Silurian gray sedimentary rocks of Podolia.

Magnetic study of large Apollo samples: Possible evidence for an ancient centered dipolar field on the Moon

We present new rock magnetic and paleomagnetic results obtained from seventeen Apollo samples. Measurements of the magnetic properties of the samples confirm that the magnetic properties of lunar rocks are dominated by about 0.1wt.% of multidomain FeNi grains. The ferromagnetic mineral assemblage is to the first order the same in all lunar lithologies, with higher concentration in basalts than in anorthosites (some of which are diamagnetic). Impact processing leads to an increase of the ferromagnetic content. Out of the seventeen samples, eleven show at least one stable component of natural remanent magnetization. In some cases, this magnetization may be a magnetic contamination. However, five basalt samples have a component of magnetization that may have recorded a lunar paleofield acquired on the Moon. For these samples (plus two similar samples from a previous study), using the rock magnetic fabric as a paleohorizontal proxy, paleoinclinations can be estimated from the paleomagnetic data. These paleoinclinations are best explained with a dipolar field and a magnetic paleopole located at 75 N. Therefore our results suggest the existence on the Moon, at least between 3.8 and 3.3Ga, of a dipolar field whose axis is close to the present-day rotation axis of the Moon. The estimated surface field intensities of several tens of μT support previous result that the Moon had a dynamo field in the past, and the proposed paleofield geometry suggests that this dynamo was centered on the lunar present-day rotation axis.

The 1420 Ma Indiavaí Mafic Intrusion (SW Amazonian Craton): Paleomagnetic results and implications for the Columbia supercontinent

The configuration and the timing of assembly and break-up of Columbia are still matter of debate. In order to improve our knowledge about the Mesoproterozoic evolution of Columbia, a paleomagnetic study was carried out on the 1420Ma Indiavaí mafic intrusive rocks that crosscut the polycyclic Proterozoic basement of the SW Amazonian Craton, in southwestern Mato Grosso State (Brazil). Alternating field and thermal demagnetization revealed south/southwest ChRM directions with downward inclinations for sixteen analyzed sites. These directions are probably carried by SD/PSD magnetite with high coercivities and high unblocking temperatures as indicated by additional rock magnetic tests, including thermomagnetic data, hysteresis data and the progressive acquisition of isothermal remanent magnetization. Different stable magnetization components isolated in host rocks from the basement 10km NW away to the Indiavaí intrusion, further support the primary origin of the ChRM. A mean of the site mean directions was calculated at Dm=209.8°, Im=50.7° (α95=8.0°, K=22.1), which yielded a paleomagnetic pole located at 249.7°E, 57.0°S (A95=8.6°). The similarity of this pole with the recently published 1420Ma pole from the Nova Guarita dykes in northern Mato Grosso State suggests a similar tectonic framework for these two sites located 600km apart, implying the bulk rigidity of the Rondonian-San Ignacio crust at that time. Furthermore these data provide new insights on the tectonic significance of the 1100–1000Ma Nova Brasilândia belt—a major EW feature that cuts across the basement rocks of this province, which can now be interpreted as intracratonic, in contrast to previous interpretation. From a global perspective, a new Mesoproterozoic paleogeography of Columbia has been proposed based on comparison of these 1420Ma poles and a 1780Ma pole from Amazonia with other paleomagnetic poles of similar age from Baltica and Laurentia, a reconstruction in agreement with geological correlations.

Remanent magnetization of maghemitized basalts from Krafla drill cores, NE-Iceland

We investigated the natural remanent magnetization (Jr) of hydrothermally altered basalts from two drill cores KH1 (200 m) and KH3 (400 m) situated at the rim of the Krafla caldera in NE Iceland, where a geothermal field (>150°C) is still active. Low temperature oxidation along with mineral reactions in the chlorite zone (<350°C) is the prevailing cause for the maghemitization and a strong decrease of Jr to occur in our study. Despite a significant decrease of Jr with respect to fresh basalts in surface outcrops of the same area, the stepwise demagnetization analyses of Jr show the presence of a stable magnetic component with the expected inclination of 77° in Iceland. Because the alteration temperature (<350°C) is above the Curie temperatures of most of the original titanomagnetite (40°–350°C), we suggest that a normal direction of remanence is chemically acquired during the low temperature alteration. We observed only one reliable negative inclination at 293.2 m in the KH3 core, which we rather interpret to be acquired during a geomagnetic excursion with reverse polarity than caused by a self-reversal mechanism.

Paleomagnetism of the Upper Vendian Basu formation of the Bashkirian Meganticlinorium revisited

We have carried out paleomagnetic studies of the Upper Vendian sedimentary rocks from the Bashkirian Meganticlinorium (Southern Ural). The rocks were sampled at three localities spread over more than 100 km. Totally, more than 300 samples were collected from about 40 sampling sites. Stepwise thermal demagnetization up to 700°C revealed a stable component of magnetization of either polarity in 25 sites. The fold test and the reversal test for this component are positive, which is usually regarded as a sound argument in favor of the primary origin of magnetization. However, the Basu paleomagnetic pole (longitude 187.3°E, latitude 1.1°N) is located near the Late Ordovician-Early Silurian segment of the apparent polar wander path for Baltica, which might indicate a Paleozoic remagnetization of Vendian rocks. In this work we analyze different interpretations of the obtained results and evaluate the reliability of the Late Riphean and Vendian paleomagnetic data for Baltica.

A comprehensive rock­magnetic, paleomagnetic, paleointensity and geochronologic study along the western Trans­Mexican Volcanic Belt: geodynamic and geomagnetic implications

Presentamos un estudio paleomagnético, de magnetismo de rocas geocronológico y de paleointensidad conjunto, en rocas volcánicas Miocénicas del sector de la Faja Volcánica Transmexicana (FVTM). En el cual se emplearon 58 sitios compuestos por 7 a 21 unidades volcánicas de enfriamiento consecutivas de 4 secciones estratigráficas separadas. Nuestros nuevos fechamientos radiométricos sugieren que los flujos de lava de la secuencia Jesús Maria fueron emitidos durante un corto lapso de 11.1±0.8 a 10±0.8 Ma durante el cron de polaridad normal C5n.2n. La cercana sección de Atotonilco presenta características litológicas similares a la secuencia de Jesús Maria, con los 3 flujos de lava de la cima probablemente perteneciendo al cron C5n.1r. Nuestros resultados geocronológicos de la sección Funicular indican que estas lavas fueron emitidas durante un lapso más amplio, de 5.2±0.7 a 2.8±0.5 Ma. Con base en las observaciones de mineralogía magnética y de magnetismo de rocas de todas las muestras se determina que la mineralogía magnética es primaria e inalterada. Las titanomagnetitas de bajo-Ti, son resultado de la oxi-exosolución de las titanomagnetitas originales durante el enfriamiento inicial de los flujos. Los componente estables de la magnetización se determinaron después de experimentos detallados de desmagnetización térmica y de campos alternos. Nuestros resultados combinados con los datos paleomagnéticos regionales existentes indican que las áreas de muestreo no han sufrido desplazamientos N-S, ni rotaciones de bloque significativos desde hace al menos 11 Ma. Las determinaciones de paleointensidad aceptadas son de buena calidad técnica con chequeos estándares positivos de magnetización parcial termoremanente (pTRM). Los datos de paleointensidad obtenidos van de 17.5 a 34.3 mT, sugiriendo la existencia tanto de una alta intensidad del campo geomagnético como de fluctuaciones bajas durante el Mioceno. Nuestros resultados sugieren que las rocas preservan una memoria magnética del campo geomagnético del Mioceno tardío y delimitan las reconstrucciones paleogeográficas. Los nuevos resultados paleomagnéticos contribuyen potencialmente a la reconstrucción de los modelos de evolución tectónica de la FVMT.