Chemical Remanent Magnetization Research Articles

Dual-Polarity Early Carboniferous Remagnetization of the Fisset Brook Formation, Cape Breton Island, Nova Scotia

SUMMARY Red siltstones and volcanic flows of the Fisset Brook Formation of Cape Breton Island, Nova Scotia, were folded or tilted in two phases, one of Late Tournaisian and the other of Mid-Namurian age. Upon thermal demagnetization, both rock types yield three components of magnetization, herein denoted as L, I and H. The L component has low unblocking temperatures and a direction conforming to that of the present-day geomagnetic field. L is inferred to be of recent origin. The intermediate I component, carried by haematite, is of synfolding origin. Directions could be obtained through vector subtraction for the demagnetization interval of 300-550°C, and yield a mean of D/I = 160/+38 (k = 90.8, ag5 = S.lo), after 39 per cent of the tilt correction is applied. The H component has even higher unblocking temperatures, which overlap to a large degree with those of the I component, but analysis of intersecting great circles appears to yield a best-fit direction for H which is nearly antipodal to that of the I component. H is also synfolding, and yields a mean direction of D/I = 342/-38 (k = 120.9, ag5 = 6.9) after 60 per cent of tilt correction. Both components are interpreted as secondary chemical remanent magnetizations of Late Tournaisian to Early Namurian age. A comparison of all available Carboniferous results from the craton and the northern Appalachians indicates that palaeolatitudes for Nova Scotia changed from about 20s to about 10s in the interval between the Early and Late Namurian and that they changed again from about 10s to 0 between the Late Namurian and the Stephanian. Declinations show good agreement between Newfoundland, Nova Scotia, New Brunswick and the craton, with the exception of the Meguma terrain, which underwent a counterclockwise rotation with respect to the mainland in the Late Carboniferous, as noted previously by Spariosu, Kent & Keppie (1984).

Reply [to “Comment on ‘Magnetization of the oceanic crust: Thermoremanent magnetization or chemical remanent magnetization’ by C. A. Raymond and J. L. LaBrecque”

Reply [to “Comment on ‘Magnetization of the oceanic crust: Thermoremanent magnetization or chemical remanent magnetization’ by C. A. Raymond and J. L. LaBrecque”

Comment [on “Magnetization of the oceanic crust: Thermoremanent magnetization or chemical remanent magnetization” by C. A. Raymond and J. L. LaBrecque

Comment [on “Magnetization of the oceanic crust: Thermoremanent magnetization or chemical remanent magnetization” by C. A. Raymond and J. L. LaBrecque

Chemico-Viscous Remanent Magnetization in the Fe 3 O 4-y Fe 2 O 3 System

The chemical remanent magnetization (CRM) acquired when single-domain size magnetite (Fe(3)0(4)) oxidizes to maghemite (gammaFe(2)O(3)) in a 50-microtesla field at a series of 13 temperatures from 1000 to 6560C is of similar intensity to viscous remanent magnetization (VRM) acquired under the same field and temperature conditions by unoxidized magnetite. The remanences of the oxidized and unoxidized phases also have similar resistances to demagnetization. These similarities imply that the remanence of the oxidized material is a chemico-viscous remanent magnetization (CVRM) having some of the characteristics of both classic growth CRM and thermally activated VRM. At low temperatures in partially oxidized grains, VRM of the magnetite core and growth CRM of the maghemite surface layer contribute about equally to CVRM. Near the Curie point, intensity of CVRM increases to a Hopkinson-type peak. High-temperature CVRM is more resistant to demagnetization than the thermoremanent magnetization (TRM) produced from cooling through the Curie point.

Results from a detailed aeromagnetic survey across the northeast Newfoundland margin, Part II: Early opening of the North Atlantic between the British Isles and Newfoundland

Poles of rotation for the North Atlantic have been derived from the results of a new aeromagnetic survey northeast of Newfoundland. Reconstruction of the North Atlantic at anomaly 34 time shows a band of large amplitude magnetic anomalies which parallels anomaly 34 on both sides of the Atlantic from Flemish Cap and Goban Spur to the Azores-Gibraltar Fracture Zone. A group of similar anomalies has also been identified in the Bay of Biscay. North of Goban Spur and Flemish Cap, these anomalies follow the ocean-continent boundary. Poles of rotation derived for this anomaly show that it forms an isochron (100–110 m.y.) during the long Cretaceous normal polarity interval. The cause of this anomaly is not definite, but it may represent an increase in the magnetization of the crust during a limited time within the Cretaceous Magnetic Quiet Zone by a process such as replacement of thermoremanent magnetization by chemical remanent magnetization as proposed by Raymond and LaBrecque. The North Atlantic has also been reconstructed at the time of the initial opening in the region between Flemish Cap and the Charlie-Gibbs Fracture Zone, using inferred ocean-continent boundaries on the west and east sides: it has been shown that the entire region could not have saparated at one time, but that spreading between the British Isles and Newfoundland had to progress from south to north. Consequently, when active sea-floor spreading was taking place between Goban Spur and Flemish Cap (about 110 m.y.) the region to the north was still being stretched. The calculated amount of stretching as derived from the reconstructions (about 25%) agrees well with the extension of the lithosphere obtained from modelling the subsidence history of this region, and with the results of deep seismic studies. Active spreading in the north started about 100 m.y. ago.

The geometry of archean & proterozoic lithospheric mantle domains beneath the western U.S.A.

The Betic Cordillera and the Moroccan Rif together form one of the smallest and tightest orogenic arcs on Earth and almost completely close the Mediterranean to the west. For the explanation of the geodynamic evolution of the mountain belt, palaeomagnetic data that generally found clockwise block rotations in the Iberian and anticlockwise rotations in the Moroccan part of the mountain belt, have played a key role in recent works. This palaeomagnetic study has found new constraints on the rotations and timing of the peridotitic bodies outcropping in the key position at the westernmost margin of the mountain belt, in Ceuta and Beni Bousera (Rif, northern Africa).Detailed thermal demagnetization of 115 individually oriented samples from 14 sites was combined with rock magnetic and scanning electron microscopic experiments to analyze the magnetic mineralogy responsible for the remanences and the mechanisms and relative times of their acquisition. In Ceuta, up to three magnetic components, and in Beni Bousera, up to two magnetic components have been found, that are all to be interpreted as chemical remanent magnetizations (CRM). The data suggests the following succession of geodynamic events affecting the peridotites until recent times: (1) after their exhumation and subsequent cooling about 20 Ma ago, they recorded a characteristic remanent magnetization of both normal and reversed polarities, carried by (pseudo-)single-domain magnetite grains; (2) after their dismembering, the Ceuta peridotites were tilted southward by 22–34° about a horizontal or tilted axis (up to plunge 50°) with an azimuth of 72–145° and the Beni Bousera peridotites were rotated anticlockwise by 72.3 ± 12.1° about a vertical axis and (3) both recorded another magnetic signal of normal polarity only, carried by multi-domain magnetite grains; and finally (4) the Ceuta peridotites rotated anticlockwise by 19.7 ± 5.9° about a vertical axis.This study provides the first palaeomagnetic data for the Ceuta peridotites that, with their tilt and recent small net rotation, had a distinct geodynamic evolution from the large net rotations about vertical axes in Beni Bousera and Ronda (Betic Cordillera). Moreover, earlier palaemagnetic data for Beni Bousera is improved, as mixed polarities have been found in the older of the remanences for the first time, and its interpretation as a CRM changes the rotation timing that was proposed previously. The sequence of events exposed in this work are important constraints that need to be incorporated in any geodynamic model of the evolution of the Betic–Rifean mountain belt.

Paleomagnetic Dating of Dedolomitization in Cambrian-Ordovician Arbuckle Group Limestones and Pennsylvanian Collings Ranch Conglomerate, Southern Oklahoma: ABSTRACT

Paleomagnetic and petrographic techniques have been used to date dedolomitization in stratigraphic and tectonic dolomites exposed in the Arbuckle Mountains, southern Oklahoma. The authors examined red dedolomites and their dolomite precursors from the Cambrian-Ordovician Arbuckle Group and dolomite clasts in the Pennsylvanian Collings Ranch Conglomerate. Authigenic hematite is associated with the dedolomite and precipitated as a result of the dedolomitization process. Dedolomite is associated with paleokarst and fractures, burrows, Liesegang bands, and red rims on conglomerate clasts. Magnetic directions from these dedolomitized rocks range from Dec = 145/sup 0/ to 154/sup 0/ and Inc = 2/sup 0/ to 9/sup 0/, with ks greater than 50 and /alpha/95s less than 5. The directions are constrained by fold tests to be post-structural (Late Pennsylvanian to Early Permian) and in the Collings Ranch to post-depositional. These directions correspond to a reversed Pennsylvanian pole position and the magnetizations are interpreted as chemical remanent magnetizations (CRM) acquired when hematite precipitated during dedolomitization.

Diagenetic processes and remagnetization in Permo-Triassic red beds.

Palaeomagnetic methods of analysis have largely been concentrated on distinguishing between the relatively hard and soft components which make up the natural remanent magnetization (NRM) of sedimentary rocks. Ihis has been because of the widespread beliefthat, in general, the hard cornponents are primary in origin whule the soft components are secondary, having been acquired a long time afler deposition. Also it is generally assumed that the primary components originate from depositional processes rathcr than diagenetie (post-depositional) alteration. Magnetic components acquired by in situ alteration are usually altributed to chemical remanent magnetization (CRM) with no more specific reference to tSe processes of magnetization. Furthermore the general practice has been Lo assume that tSe magnetic components with higher blocking temperatures or coercivities faithfully record the ambient geomagnetie field at or near tSe time of deposition despite any direct geological evidence of the age of tSe components. Ihis simplistie view of palaeomagnetism arises from tSe Neel equation which relates relaxation time to coercive force, particle volume and temperature for an assembly of non-interacting single-domain grajos:

A model of ocean basin crustal magnetization appropriate for satellite elevation anomalies

A model of ocean basin crustal magnetization measured at satellite altitudes is developed which will serve both as background to which anomalous magnetizations can be contrasted and as a beginning point for studies of tectonic modification of normal ocean crust. The model is based on published data concerned with the petrology and magnetization of the ocean crust and consists of viscous magnetization and induced magnetization estimated for individual crustal layers. Thermal remanent magnetization and chemical remanent magnetization are excluded from the model because seafloor spreading anomalies are too short in wavelength to be resolved at satellite altitudes. The exception to this generalization is found at the oceanic magnetic quiet zones where thermal remanent magnetization and chemical remanent magnetization must be considered along with viscous magnetization and induced magnetization.

Self-reversal of chemical remanent magnetization: a palaeomagnetic example

Summary An anomalous region with complicated thermal demagnetization characteristics has been observed in Siluro-Devonian andesites from the contact aureole of a 6.2 m wide Tertiary dyke in the Mull dyke swarm, Scotland. Outside this anomalous region, rocks from the contact aureole carry a simple two component remanence. The existence of a self-reversed CRM in samples from between 75 cm and 182 cm from the contact has been inferred from thermal demagnetization data. This magnetization has blocking temperatures greater than the peak temperatures reached in the aureole and causes a ‘beak’ shape in the vertical projection of vector plots of thermal demagnetization data. The self-reversed remanence is presumed to be carried by titanohaematite (x≅ 0.03) with a Curie point of 650–660°C (not isolated as a separate phase) and its blocking temperatures overlap with those of a primary Siluro-Devonian remanence and a normal Tertiary CRM overprint carried by hematite. The suggested mechanism for this self-reversed CRM is the transformation of maghaemite to haematite where super exchange across the phase boundary causes the self reversal. This is probably the best documented geological example of such a self-reversal.

Correction to “Magnetization of the oceanic crust: Thermoremanent magnetization or chemical remanent magnetization?” by C. A. Raymond and J. L. LaBrecque

Correction to “Magnetization of the oceanic crust: Thermoremanent magnetization or chemical remanent magnetization?” by C. A. Raymond and J. L. LaBrecque

Magnetization of the oceanic crust: Thermoremanent magnetization of chemical remanent magnetization?

We propose a model in which chemical remanent magnetization (CRM) acquired within the first 20 m.y. of crustal evolution may account for 80% of the bulk natural remanent magnetization of older basalts. The CRM of the crust is acquired as the original thermoremanent magnetization (TRM) is lost through low‐temperature alteration. The CRM intensity and direction are controlled by the posternplacement polarity history. This model explains several independent observations concerning the magnetization of the oceanic crust. The model accounts for amplitude and skewness discrepancies observed in both the intermediate‐wavelength satellite field and the short‐wavelength sea surface magnetic anomaly pattern. It also explains the decay of magnetization away from the spreading axis and the enhanced magnetization of the Cretaceous Quiet Zones, while predicting other systematic variations with age in the bulk magnetization of the oceanic crust. The model also explains discrepancies in the anomaly skewness parameter observed for anomalies of Cretaceous age. Further, our study indicates varying rates of TRM decay in very young crust which depicts the advance of low‐temperature alteration through the magnetized layer.

Two types of chemical remanent magnetization during the oxidation of magnetite

The chemical remanent magnetization (CRM), M CRM, resulting from oxidation of equidimensional magnetite to a mixture of 90% hematite and 10% cation-deficient magnetite is jointly controlled by the initial anhysteretic remanent magnetization, ( M ARM), of the parent magnetite and the field H CRM applied during oxidation. M CRM lies in the plane defined by H CRM and M ARM (which were perpendicular in our experiments) in an intermediate orientation determined by the relative strengths of H CRM (0.05, 0.1 and 0.2 mT) and M ARM (0.002–0.12 × saturation remanence). The CRMs are univectorial remanences that remain fixed in these stable intermediate directions throughout the course of stepwise alternating field and thermal demagnetization. Similar experiments in which acicular single-domain cation-deficient magnetite was oxidized to maghemite gave quite different results. M CRM was always parallel to M ARM. Its direction was unaffected by field strengths as high as H CRM = 1.5 mT. The essential factors controlling CRM direction seem to be the number and crystallographic structure of oxidation products. For single-phase oxidation without change in crystal lattice, CRM remains parallel to the initial remanence of the parent mineral. For multiphase oxidation to a mixture of rhombohedral and spinel phases, both initial remanence and the field acting during oxidation influence the CRM direction, which therefore has an intermediate orientation that parallels neither M ARM nor H CRM.

Acquisition of chemical remanent magnetization by synthetic iron oxide

A fundamental assumption of palaeomagnetism is that rocks can provide a reliable record of the Earth's magnetic field. It is known that as magnetic minerals undergo chemical change they acquire a form of chemical remanent magnetization (CRM)1. Indeed, the magnetic phases formed at different times in the history of the rock may add several components of CRM to its net magnetization2, so that a valid application of palaeomagnetism to the interpretation of geological history requires a knowledge of the different components of CRM and the time of their acquisition. There are two mechanisms responsible for CRM3: mineral alteration and crystal growth. To understand the properties and acquisition mechanisms of CRM associated with the growth of mineral crystals, which is known as grain-growth CRM4, we have developed a technique for the synthesis of haematite crystals under controlled conditions. We find that the acquired magnetization is parallel to the applied magnetic field.

The magnetic properties of an ironcrust in SE Belgium and synthetic Mn-substituted goethites

The magnetic properties of an ironband formed in Mesozoic sands and clay are given. X-ray, DTA, Mössbauer and magnetic analysis revealed that the main magnetic carrier is goethite, of which the crystallinity changes within the ironcrust due to the isomorphous substitution of Fe by Mn. Artificial Mn-substituted goethites, never previously reported, can be synthesized and their properties compared to the natural ones. The possibilities of using the chemical remanent magnetization of the ironcrust as a ‘marker’, to gain insight into its genesis, age and accretion velocity, are examined.

Origin of magnetization in the Phosphoria Formation at Sheep Mountain, Wyoming: A possible relationship with hydrocarbons

Hydrocarbon impregnated fenestral dolomites of the Phosphoria Formation were sampled on both flanks of Sheep Mountain Anticline, Wyoming. Demagnetization results indicate one dominant component that is either synfolding (Laramide in age) or predates folding (Cretaceous in age). Thermal demagnetization yields stable decay to temperatures between 500°C and 550°C and rock magnetic experiments suggest that the magnetization is dominated by a low coercivity component. Magnetic extracts contain spheres (<15 µ diameter) as well as other authigenic forms. Energy dispersive analysis demonstrates that Fe is the only detectable element present in the spheres and X‐ray diffraction analysis indicates that magnetite is present in the extracts; the spheres are interpreted to be authigenic magnetite. The magnetization is interpreted as a chemical remanent magnetization (CRM) residing in this authigenic magnetite. Chemical conditions created by the hydrocarbons may have caused precipitation of the authigenic magnetite and acquisition of the associated CRM.

A Mid‐Triassic paleomagnetic age of the Kupferschiefer mineralization in Poland, based on a revised apparent polar wander path for Europe and Russia

Extensive mineralization occurs in southwest Poland as zoned blankets of Cu‐Ag‐(Pb‐Zn) sulphides in the Kupferschiefer and Zechstein Limestone across the base of the Late Permian Zechstein restricted marine sequence, and in the Weissliegendes sandstone at the top of the Early Permian Rotliegendes continental volcanic and siliciclastic sequence. The Rote Fäule (RF) zone, an oxidized, hematitic equivalent of the normally pyritic basal Zechstein, directly underlies the copper ore and is part of the metal zoning. Although the origin has generally been considered to be syngenetic, geologic controls of the RF‐ore systems in Poland suggest a late diagenetic origin by converting metalliferous brines migrating through the Rotliegendes clastic redbeds up along basement highs, oxidizing the pyritic Kupferschiefer to form the RF, and precipitating base and precious metals on the reduced side of this oxidation‐reduction front. A paleomagnetic study of the ore zone in five mines in Poland, the RF zone in three mines, and of the barren pyritic basal Zechstein from outcrops and quarries in Poland, Germany, and England was undertaken to determine the absolute age of the mineralization. In the barren pyritic rocks and the copper sulphide ore, thermal demagnetization and vector subtraction revealed an unstable low‐temperature first‐remanence component (76.9°N, 12.2°E; A63 = 1.6°), parallel to the ambient field, and a second, higher temperature component (82.6°N, 145.9°E; A63 = 1.9°), which could be Miocene (10–20 Ma) or, less likely, Cretaceous (110 Ma) or Jurassic (180 Ma). This second component is erased at 350°–400°C and is thought to be carried by a product of weathering after Alpine age uplift and erosion. No primary magnetization could be isolated in the copper or pyritic zones. The RF hematite, however, contained a very stable third, reversed component (49.0°N, 157.2°E; A63 = 2.2°) which was revealed typically between 500° and 620°C, and is considered to represent a secondary chemical remanent magnetization acquired during the mineralizing process. In order to date the isolated magnetic components, Carboniferous to Neogene apparent polar wander (APW) paths for Europe and Russia were constructed using paleopole catalogs. Mean paleopoles every 10 Ma from 350 to 0 Ma, with A63 circles of confidence, were calculated as running averages using overlapping 20‐ and 30‐Ma time intervals. The RF pole, corresponding to the third remanence component, coincides extremely well with a Middle Triassic age of 240 Ma on the 20‐Ma APW path and a Late Triassic age of 230 Ma on the 30‐Ma APW path. Statistical analysis yields upper and lower age limits of 250 and 220 Ma. This Triassic paleomagnetic age supports the proposed late diagenetic origin of the Kupferschiefer ore deposits, and suggests that the RF‐ore systems were formed immediately after Triassic continental rifting associated with the opening of the Tethys ocean to the south—a likely source of the thermal anomaly necessary to initiate adequate convective velocities within the Rotliegendes clastic basins.

Paleomagnetic Dating of Liesegang Bands

ABSTRACT Paleomagnetic analysis, in conjunction with petrographic studies, is used in this study to date the formation of hematite Liesegang bands in the Ordovician Upper Arbuckle Group in southern Oklahoma. The hematite bands form symmetrical patterns on both sides of calcite-filled fractures in dolomitic beds. The bands decrease in abundance and become more diffuse away from the fractures. Paleomagnetic specimens from distinctly banded dolomite near the fractures contain a relatively strong chemical remanent magnetization (CRM) with a southeasterly declination and shallow inclination. Samples farther from the fractures which are less distinctively banded or have no bands contain a weaker and less stable magnetization. Samples were collected from both flanks of the Arbuckle Anticline (late Pennsylvanian folding), and a fold test indicates that the CRM is postfolding. The pole position for the CRM corresponds to the Permian ( 280 Ma) part of the Apparent Polar Wander Path for stable North America. Petrographic evidence, stable demagnetization to above 600°C, and rock magnetic experiments indicate that the CRM resides i hematite. These results suggest that the Liesegang bands formed in the Early Permian, probably by precipitation of hematite from fluids that emanated from the fractures. The fluids also apparently caused dedolomitization and precipitation of calcite in intercrystalline pore spaces. Probable sources of the iron in the bands include these fluids, as well as iron released from dedolomitization and from oxidation of pyrite.

Magnetostratigraphy of the Koobi Fora Formation, Lake Turkana, Kenya

The Koobi Fora Formation, a Pliocene and Pleistocene sequence of sedimentary deposits northeast of Lake Turkana, has yielded numerous fossils and stone artifacts of early hominids. Stratigraphic correlation of the hominid‐bearing deposits throughout the Turkana region was established primarily by the chemistry and isotopic ages of volcanic tuffs and complemented by magnetostratigraphic studies. We have reinterpreted previously published magnetostratigraphy from the upper part of the Koobi Fora Formation because the original stratigraphy and dating of tuffs have been revised. In our reinterpretation we include previously unpublished data from the uppermost part of the formation. The upper magnetozones correlate with parts of the Brunhes Normal‐Polarity Chron and Matuyama Reversed‐Polarity Chron (about 0.6–0.85 Ma) and are separated from the magnetozones of the upper part of the Matuyama (2.0–1.25 Ma) by a disconformity. The Olduvai Normal‐Polarity Subchron is represented within the Matuyama, but the lower part of the Matuyama (2.4–2.0 Ma) is missing due to an erosional disconformity. We have also determined magnetozones in the lower part of the Koobi Fora Formation, which had not been sampled for paleomagnetism during the earlier studies. Our time calibration of the magnetozones is made possible by isotopic dating of several tuffs and by chemical correlation of Koobi Fora tuffs with dated tuffs in the Shungura Formation of southern Ethiopia. The tephra correlations are corroborated by the excellent concordance between the magnetostratigraphies of the Koobi Fora and Shungura formations. The lower part of the Koobi Fora spans the interval from about 4 Ma to 2.4 Ma, within the Gilbert Reversed‐Polarity and Gauss Normal‐Polarity chrons. Rock magnetic studies indicate that detrital magnetite carries most of the stable remanence, although hematite contributes to the remanence as indicated by thermal demagnetization. The hematite, which presumably formed by postdepositional oxidation of original iron oxide grains, carries chemical remanent magnetization (CRM) that diminishes the sharpness of polarity zone boundaries. The CRM accumulated continuously within the uppermost 10 m of sediment below the land surface, so that the CRM reinforces the depositional remanent magnetization within thick magnetozones but obscures magnetozones having durations of roughly less than 70,000 years in sections where the sedimentation rate was approximately 15 cm/1000 years.

Absolute dating of dedolomitization and the origin of magnetization in the Cambrian Morgan Creek Limestone, central Texas

Goethite associated with dedolomite in the Morgan Creek Limestone in central Texas carries a chemical remanent magnetization (CRM). The pole position (88°N, 96°W; k =67, α 95=2.7), which is close to that of the modern pole, and the fact that only normal directions are present suggest that the component is a post-Brunhes (<700,000 yr) CRM. Dedolomite is common in the Morgan Creek and consists of calcite rhombohedra with relict dolomite. Goethite occurs surrounding the rhombs and in rhombic zones within the crystals. The goethite precipitated when iron was released from ferroan dolomite during the dedolomitization process. The paleomagnetic data, therefore, date the dedolomitization process as modern. Dedolomitization is probably related to surface weathering of the Morgan Creek. In addition to goethite associated with dedolomite, the Morgan Creek contains hematite replacing fossils and glauconite, and detrital hematite. The Morgan Creek also contains three other magnetic components. One of these components is a viscous remanent magnetization aligned with the present magnetic field. Another component is a Triassic/Jurassic CRM residing in hematite. A fourth component in the Morgan Creek is an early Paleozoic magnetization residing in hematite and minor magnetite and represents the vector addition of a detrital remanent magnetization and one or more CRMs. The results of this study demonstrate that paleomagnetic and petrographic techniques can be used to date a diagenetic event and also illustrate the complex nature of magnetization in a sedimentary unit.