Magnetization, Chemical Remanent (CRM)
Chemical remanent magnetism (CRM) is imparted to ferro‐ and ferrimagnetic minerals by chemical processes, at temperatures below their Curie points, in the presence of an effective magnetic field. Here, chemical processes are considered broadly to include but not be limited to modifications in oxidation state, phase changes and crystal growth. The effective magnetic field is the resultant vector field acting on the chemically‐altered material, including the external and various interaction fields.
Nearly a century and half ago, Beetz (1860) discovered CRM during laboratory electrolytic depositions of iron; these observations supported Weber's hypothesis that some atoms possess intrinsic magnetization. These results were confirmed by Maurain (1901 and 1902) with electrolytic depositions of iron and nickel in the presence of external fields. Koenigsberger (1938, part 1, p. 122 & part 2, p. 319) noted the presence of coherent remanence in some sedimentary rocks, which he called crystallization remanence, and he advanced the hypothesis that it was “impressed by the Earth's field at temperatures between about 100°C and 500°C during the time of lattice changes in magnetite which result very probably from unmixing of Fe2O3.” With the rapid growth of paleomagnetism after World War II, it became apparent to many rock magnetists that the remanence of many sediments, especially red beds, was at least partly controlled by magnetic minerals chemically precipitated subsequent to deposition (e.g., Blackett, 1956).
In rocks, CRM is usually a secondary remanence; this is an important reason that in geophysics CRM studies lag behind investigations of TRM (Thermoremanent magnetization) and DRM (Depositional remanence), which are usually responsible for the primary remanence. In most paleomagnetic studies, it is advantageous to select samples that retain their primary TRM or DRM; secondary CRM is a nuisance to be avoided, if possible. However, in nature, CRM can rarely be entirely neglected. If a secondary CRM is superimposed on an extant primary TRM or DRM, one of the first objectives would be to remove (demagnetize) the CRM in order to expose the primary remanence. In many cases, especially for older, chemically‐altered sediments, CRM is the dominant characteristic remanence, the primary DRM having been obliterated by processes similar to those that are responsible for the CRM.
Theory for CRM
We now consider isothermal CRM production at temperature T A, caused by crystal growth of magnetic particles. The precipitating ferro‐ or ferrimagnetic particles grow from atomic/molecular paramagnetic nuclei to larger exchange‐coupled SP grains with spontaneous magnetization, M S. In an external field and thermal equilibrium, the particles are aligned according to Eq. (1). When the field is removed, and as long as the thermal fluctuations (kT) can easily overcome the anisotropy energy barriers (Kv), τ ≪ τ L, M decays quickly following Eq. (2), and there is no remanence. (τ L is a characteristic laboratory time on the order of minutes.) As the particle volumes increase, it becomes more difficult for thermal fluctuations to overcome the growing barriers to domain rotations. Because of the exponential dependence of τ on v in Eq. (3), the magnetization changes to a stable CRM over a very narrow range of volumes, Δv. At volume v AB, τ ≫ τ L, and CRM is said to be blocked. v AB is the critical blocking volume at T A. That is, at temperature T A, for v < v AB, the particle assemblage is super‐paramagnetic, while for v > v AB, the magnetization is blocked as stable CRM in single domain (SD) particles. This is analogous to TRM production, where v is considered constant, and τ increases as T cools below the Curie point, T C. Above the blocking temperature, T B, the SP magnetic moments are aligned according to Eq. (1) with τ ≪ τ L, so that when the external field is removed, the magnetization quickly decays following Eq. (2). At lower temperatures T < T B, τ ≫ τ L, and the magnetization is blocked as stable TRM.
CRM is usually difficult to distinguish from other remanences such as TRM and VRM (viscous remanence), because of overlapping stabilities and possible associations between CRM, partial‐TRM (PTRM) and high-temperature VRM. For example, grain growth CRM at T A > T R, where T R denotes room temperature, is in particles that have grown beyond the blocking volume, v AB; chemically precipitated particles with volumes v < v AB do not contribute to the CRM. On cooling from T A to T R, a PTRM will be produced in particles with blocking temperatures T A > T B > T R. Also, ubiquitous time effects might contribute significant VRM superimposed on the CRM. In addition, the resultant remanence is likely to grow on cooling from T A to T R, due to the increase in M S on cooling for most magnetic minerals. Such complexities make it difficult to uniquely isolate CRM from other remanences, hence it is probable that CRM occurrences in the paleomagnetic record are more numerous than is usually recognized.
CRM in Igneous Rocks
In igneous rocks, CRM can be produced by phase transformations or nucleation and growth of new magnetic minerals at temperatures below the Curie point of the new magnetic species. The effective magnetic field at the site of the new magnetic particles determines the direction and intensity of the CRM. In igneous rocks, magnetic interactions among multiple phases, such as during exsolution of the iron‐titanium solid solution series, might significantly modify the external field through magnetostatic or exchange interactions with phase(s) having higher blocking temperatures. Occasionally, such interactions might produce CRM with oblique directions or opposite polarity to the external magnetic field and the existing magnetic phase (negative magnetic interactions), and in rare cases a self‐reversal may result (Néel, 1955).
CRM Origin of Marine Magnetic Anomalies
The first remanence of freshly extruded submarine basalts is TRM in stoichiometric titanomagnetites, xFe2TiO4(1‐x)Fe3O4, with x ∼0.6 ± 0.1, that is, about 60% molar ulvöspinel, with Curie points between 100°–150°C (Readman and O'Reilly, 1972). The initial magnetic phase is transformed at the sea floor by topotactic low temperature oxidation to cation deficient titanomaghemites. The oxidation is thought to proceed by a net cation migration out of the crystal lattice to accommodate a higher Fe3+/Fe2+ ratio and accompanying changing proportions of Fe:Ti. The resulting cation deficient phases retain the cubic crystal structure with smaller lattice dimensions, higher Curie points, approaching 500°C, and lower saturation magnetizations, responsible for the rapid diminution of the amplitudes of the marine magnetic anomalies away from spreading centers (e.g., Klitgord, 1976). Therefore, marine magnetic anomalies over the world's oceans can be considered to be preserved predominantly as CRM in oxidized Fe‐Ti oxides.
The secondary CRM in submarine basalts usually retains the same polarity as the initial TRM recorded upon extrusion. This is indicated by the agreement, for overlapping time intervals, between the polarity time scales from marine magnetic anomalies, continental lavas and marine sediments, as well as the magnetic polarity of oriented dredged and drilled submarine basalts. The agreement of CRM and TRM directions is further supported by laboratory low temperature oxidation experiments of predominantly SD titanomagnetites (e.g., Marshall and Cox, 1971; Johnson and Merrill, 1974; Özdemir and Dunlop, 1985). The superexchange interactions vary with changes in cation distribution and lattice dimensions, as indicated by the reduced saturation magnetization and higher Curie points for the more oxidized titanomaghemites. However, it is possible that the orientations of the sub‐lattice magnetic moments remain intact during low temperature oxidation of the titanomagnetite minerals, so that the ensuing CRM retains or inherits the original TRM direction.
Evidence from magnetic anomalies and magnetic properties of drilled oceanic basalts suggests an increase of the magnetization of extrusive submarine basalts of oceanic crust older than about 40 Ma (e.g., Johnson and Pariso, 1993). At present, the data are too sparse for one to be confident of the generality of this phenomenon or to select from several mechanisms that might be responsible.
CRM Influence on Paleointensity Studies
At present, only TRM can be used for obtaining absolute paleointensities of the Earth's magnetic field (see Paleointensity from TRM ). Hence, all paleointensity methods require that the NRM be essentially pure TRM, or that the TRM can be readily isolated from the NRM. The paleointensity methods compare the NRM of each specimen to a new laboratory TRM produced in a known laboratory field, HL. Because chemical and mineralogical alterations of specimens during laboratory heatings are common and often preclude reliable paleointensity determinations, the different paleointensity methods apply various pre‐ and post‐heating tests to assess the extent of chemical changes on the remanence and paleointensity experiments. Several paleointensity methods use a single heating to above the specimen's highest Curie temperature to produce a total laboratory TRM. However, because reaction rates increase with temperature, such a procedure tends to maximize chemical alterations.
High-temperature chemical alterations during the Thelliers’ paleointensity procedure is one of the more common causes for failed or abbreviated paleointensity experiments. If the CRM is expressed as a greater PTRM capacity, which increases with temperature, the data will form a concave‐up PNRM‐PTRM plot (Figure M107, curve B). Provided the lower temperature data are linear and the PTRM checks show no evidence of alteration, then the lower temperature data can be used to calculate a paleointensity (Figure M107 curve B, points T1‐T3). Recent studies suggest that at least 50% of the NRM should be used to obtain reliable results (e.g., Chauvin et al., 2005).
CRM production that increases the PTRM capacity may result from precipitation of new magnetic particles or from unmixing of titanomagnetite grains to a more Fe‐rich phase with higher saturation magnetization. Chemical modifications, which lead to higher PTRM, usually cause the PNRM‐PTRM points to lie above the ideal line, and the calculated paleointensity will be lower than the actual paleo‐field. Alternatively, if the chemical alterations decrease the PTRM potential by destroying magnetic particles or by transforming them to a phase with lower intrinsic magnetic moments, then the PNRM‐PTRM points will plot below the ideal line, with higher apparent paleointensities than the actual values (e.g., Figure M107, curve D and PTRM check 3). For a special case, where CRM acquisition grows linearly with temperature, the PNRM‐PTRM plot might be linear (Figure M107, line C); however, these data plot above the ideal line (Figure M107, line A), and the calculated paleointensity would be lower than its actual value. This result emphasizes that linear PNRM‐PTRM data are a necessary but not sufficient condition for obtaining reliable paleointensities.
The adverse effects of chemical alterations and CRM on paleointensity studies do not always arise from heatings in the laboratory. It is also possible that the NRM is not a pure TRM but contains a significant CRM component. Yamamoto et al. (2003) suggested that high temperature CRM contributes to the NRM of the Hawaiian 1960 lava, which results in higher than expected paleointensities. Alternatively, low‐temperature hydrothermal alteration might produce CRM in new magnetic particles that contribute to the NRM. If these particles have not grown significantly beyond their blocking volumes, vAB, they would be demagnetized at T ≥ T A, leading to rapid decrease of the NRM. This scenario might explain the precipitous diminution of the NRM observed for some basalts, with decreases on the order of 20% to more than 50% in the first few temperature steps of the Thellier experiment. When this decrease in NRM cannot be attributed to viscous remanence, it is possible that the NRM is augmented by CRM. It is no longer pure TRM.
CRM in Sedimentary Rocks
Oxidized Red Sediments
Red beds are a broad and loosely defined category of highly oxidized sediments with colors ranging from brown to purple, usually resulting from secondary fine particles of hematite, maghemite and/or ferric oxyhydroxide. The color is a complex function of the mineralogy, chemical composition and particle sizes of the iron oxides, as well as the impurity cations and their concentrations; however, for paleomagnetism, color is unimportant. Red sediments have been used extensively for paleomagnetism since the late 1940s, because they are widely distributed geographically and with respect to geologic time. In addition, the remanence of red sediments is often stable and sufficiently intense for paleomagnetic measurements, even with early‐generation magnetometers. Already in the 1950s, it was deduced that low temperature oxidation was responsible for transforming the original magnetite to fine particles of hematite, maghemite, and/or goethite, which provide the pigment and CRM of red beds (Blackett, 1956).
Larson and Walker (1975) studied CRM development during early stages of red bed formation in late Cenozoic sediments; they showed that in their samples CRM occurred in several authigenic phases including hematite and goethite. The CRM, which obscured the original DRM, had formed over multiple polarity intervals, as indicated by different polarities in several generations of authigenic minerals. Complex multi‐generation patterns of CRM, with several polarities within single specimens, have also been observed in Paleozoic and Mesozoic red beds. The influence of the secondary CRM on the primary remanence depends on the relative stability and intensity of the CRM carriers as compared with the primary DRM. In many cases the DRM may have been entirely obliterated by diagenetic processes, and the CRM is the dominant characteristic remanence. However, there are examples of red beds, where the primary DRM in specularite hematite remains the characteristic remanence with respect to CRM (e.g., Collinson, 1974). Sometimes distinct CRM components can be isolated by thermal demagnetization, selective leaching in acids (e.g., Collinson, 1967) and removal of altered phases of sediment by selective destructive demagnetization (Larson, 1981).
During the past more than five decades, paleomagnetic studies of red sediments have contributed significantly to magnetostratigraphy, plate tectonics and rock magnetism. Many data of apparent polar‐wander paths are from red beds, where it is assumed that the CRM was produced soon after deposition, so that the paleomagnetic pole accurately represents the depositional age of the sediments. Moreover, CRM is not subject to inclination shallowing, which often affects the primary DRM. The utility of red sediments for high resolution studies of the geomagnetic field and paleosecular variation is limited and depends on how pervasive the CRM is as compared to the primary DRM, the time lag between the CRM and initial DRM, and the duration of CRM production.
The presence of magnetite (and siderite, FeCO3) in oil impregnated sediments was discovered by Bagin and Malumyan (1976), and Donovan et al. (1979) reported correspondence of near surface magnetic anomalies over an oil field with a higher concentration of magnetic minerals in the sediments. Paleomagnetic studies of remagnetized hydrocarbon‐impregnated Paleozoic sediments (McCabe et al., 1987; Benthien and Elmore, 1987) indicate a relationship between hydrocarbon migration and the precipitation of authigenic magnetite particles, carrying the secondary CRM. This scenario is supported by extracted magnetite spherules up to several tens of microns in diameter and the presence of other authigenic textures in the sediments. All these studies suggest a net reduction of ferric ions in oxides, hydroxides and silicates, caused by the biodegradation (oxidation) of the hydrocarbons and the precipitation of more reduced iron oxide phases such as magnetite (Fe3O4), siderite (FeCO3) and wustite (FeO). Of these, only magnetite is ferrimagnetic; hence it is responsible for the CRM and for being preferentially extracted during magnetic separations.
Rapidly deposited marine and lacustrine sediments are increasingly being used to study high‐resolution behavior of the Earth's magnetic field, including secular variation and relative paleointensities. However, to accurately interpret the sedimentary record, geochemical processes that influence the magnetic signal must be understood. In anoxic and suboxic environments, bacterial sulfate reduction produces H2S, which reacts with the detrital iron oxides to precipitate sulfide minerals (Berner, 1970, 1984). An abundance of sulfate favors reactions that produce relatively more stable pyrite (FeS2), which does not carry remanence. When the sulfate supply is more limited, the formation of ferrimagnetic pyrrhotite (Fe7S8) and/or greigite (Fe3S4) is preferred. Pyrrhotite formation is less common in sediments because it is thought to require pH > 11 (Garrels and Christ, 1965), which is outside the range of values measured in sedimentary pore waters. However, for extremely low sulfur activity, it is possible for pyrrhotite to form in sediments.
This report on CRM is not exhaustive and reflects the interests, biases and limitations of the author. It is an update of a similar article written over fifteen years ago (Levi, 1989). In the future, as paleomagnetists address more difficult tectonic and geomagnetic questions, requiring data from structurally more complex, metamorphosed, and older formations, it will be increasingly likely that CRM will contribute to the NRM. Paleomagnetists have become more adept at isolating different remanence components, using detailed and varied demagnetization procedures. It is usually assumed that the most resistant remanence, whether with respect to increasing temperatures, alternating fields, or a particular leaching agent is also the primary component. However, CRM stabilities are highly variable, and this assumption is unlikely to be satisfied universally. During the past fifteen years, there has been progress in understanding several aspects of CRM, including (a) the recognition that even for some very young subaerial lavas the NRM may comprise a low‐temperature CRM component, and (b) that in some active sedimentary environments, diagenesis leads to CRM in ferrimagnetic iron sulfides. A more comprehensive understanding of CRM is needed to assist paleomagnetists to interpret complex, often multicomponent, NRMs with probable CRM overprints. This goal would be advanced by conducting controlled field and laboratory CRM experiments to (1) recognize the varied geochemical environments that produce different magnetic minerals and their associated CRMs; (2) determine the ranges of magnetic and mineralogical stabilities with respect to different demagnetization procedures and for isolating different CRM components; and (3) develop procedures for identifying the timing and sequencing of multi‐component CRMs.
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