Acquisition Of Chemical Remanent Magnetization Research Articles

Evaluating the Compatibility of Hematite (U‐Th)/He Data and Hematite‐Carried Secondary Magnetizations: An Example From the Colorado Front Range

AbstractAncient magnetization(s), often recorded by hematite (Fe2O3), provide key paleomagnetic constraints on plate interactions through time. Primary remanent magnetizations may be modified or overprinted by secondary processes that complicate interpretations of paleomagnetic data. Hematite (U‐Th)/He (hematite He) dating has the potential to resolve when secondary magnetizations were acquired. Here, we compare hematite He data and paleomagnetic results in Paleoproterozoic crystalline rocks, meters below a major nonconformity in the Colorado Front Range, USA. Prior work and new rock magnetic data indicate that pervasive hematite alteration records a secondary chemical remanent magnetization (CRM) during the Permo‐Carboniferous Reverse Superchron, coincident with the Ancestral Rocky Mountain orogeny. We target minor hematite‐coated faults cutting basement for (U‐Th)/He analyses because they are of sufficient hematite purity to yield geologically meaningful dates. Two samples yield overlapping and scattered individual hematite He dates ranging from ∼138 to 27 Ma (n = 33), significantly younger than the age of the late Paleozoic CRM. Scanning electron microscopy, electron probe microanalysis, and Raman spectroscopy indicate that aliquots have variable grain size distributions and fluorocarbonate impurities. Thermal history models support hematite on fault surfaces mineralized coeval with CRM acquisition during the late Paleozoic, and hematite He data scatter reflects variable He loss during Mesozoic burial owing to differences in grain size distribution from fault slip comminution and in chemistry among aliquots. Our results underscore the differences in temperature sensitivity and sampling requirements between paleomagnetic and hematite He investigations and illustrate that hematite He dates will usually be younger than preserved remanent magnetizations.

Estimating Paleointensities From Chemical Remanent Magnetizations of Magnetite Using Non‐Heating Methods

AbstractMany meteorites experienced aqueous alteration on their parent body. During this process, magnetite usually forms and acquires a chemical remanent magnetization (CRM) if growing in the presence of a magnetic field. The epoch of aqueous alteration on planetesimals encompasses the lifetime of the solar nebula. Therefore, magnetite‐bearing meteorites are potential sources of invaluable data regarding the intensity of the solar nebula magnetic field and its influence on planetary accretion. The major limitation encountered in meteorite paleomagnetic studies is the lack of an empirical law relating a CRM characterized using non‐heating methods and the magnetizing field intensity. This issue is usually bypassed using the empirical law for non‐heating methods calibrated for thermoremanent magnetizations (TRM), resulting in poorly constrained paleointensities. Here, we determine such an empirical law through a series of CRM acquisition experiments. Magnetite is grown in weakly magnetic sedimentary rocks and synthetic samples in a magnetic field while heated at 350°C for 5 hr in an argon atmosphere. All samples exhibit a high‐coercivity magnetization parallel and proportional to the field applied, identified as a CRM carried by magnetite. We determine the CRM empirical law by retrieving the applied field intensity using non‐heating methods. The empirical coefficients differ from the TRM ones by a factor of 1.9–3.2 depending on the method. We use these coefficients to revisit the paleointensities published for the CM chondrite Murchison and the C2 ungrouped Tagish Lake. This empirical law opens the door to the study of numerous magnetite‐bearing meteorites potentially carrying a CRM.

Authigenic magnetite formation from goethite and hematite and chemical remanent magnetization acquisition

Authigenic magnetite formation from goethite and hematite and chemical remanent magnetization acquisition

Experimental and numerical simulation of the acquisition of chemical remanent magnetization and the Thellier procedure

The results of the Thellier–Coe experiments on paleointensity determination on the samples which contain chemical remanent magnetization (CRM) created by thermal annealing of titanomagnetites are reported. The results of the experiments are compared with the theoretical notions. For this purpose, Monte Carlo simulation of the process of CRM acquisition in the system of single-domain interacting particles was carried out; the paleointensity determination method based on the Thellier–Coe procedure was modeled; and the degree of paleointensity underestimation was quantitatively estimated based on the experimental data and on the numerical results. Both the experimental investigations and computer modeling suggest the following main conclusion: all the Arai–Nagata diagrams for CRM in the high-temperature area (in some cases up to the Curie temperature T c) contain a relatively long quasi-linear interval on which it is possible to estimate the slope coefficient k and, therefore, the paleointensity. Hence, if chemical magnetization (or remagnetization) took place in the course of the magnetomineralogical transformations of titanomagnetite- bearing igneous rocks during long-lasting cooling or during repeated heatings, it can lead to incorrect results in determining the intensity of the geomagnetic field in the geological past.

Temporal constraints on genesis of the Caravia-Berbes fluorite deposits of Asturias, Spain, from paleomagnetism

The Caravia-Berbes district is a major fluorite producing area in Europe. The fluorite occurs as mantos replacing breccio-conglomerates of the Middle Permian Caravia Formation and as veins and irregular replacive bodies in the unconformably underlying limestone of the Late Carboniferous Caliza de Montaña Formation. Paleomagnetic analyses were done using alternating field and thermal step demagnetization and saturation remanence methods. Ten sites (82 specimens) in massive and disseminated fluorite ore from the Emilio manto yielded a stable chemical remanent magnetization (CRM) in hematite inclusions with a direction of Decl. 20.1°, Incl. 67.3° (α95=6.1°). After correction for Neogene Pyrenean tilt, the manto's paleoinclination gives a CRM acquisition age of 206±8Ma that dates a major hydrothermal and ore emplacement event. The age is coeval with the onset of Pangea's break up as the Iberian microplate began to split from the Armorican terrane of Eurasia as part of the ocean-forming CAMP (Central Atlantic Magmatic Province) event. Another 11 sites (109 specimens) of silicified dolostone of the Caliza de Montaña Formation just below the Cueto L'Aspa manto yielded a stable CRM that resides in single and pseudosingle domain magnetite inclusions that has a direction of Decl. 328.7°, Incl. 76.6° (α95=4.9°). After Neogene tilt correction, the altered zone's paleopole gives a CRM acquisition age of 115±3Ma. This age shows that the western Cantabrian basin also underwent a major hydrothermal alteration and remagnetization event by fluid flow through steep re-activated faults when Iberia rotated 35±2° away from Eurasia during Aptian-Albian time.

Acquisition of chemical remanent magnetization during experimental ferrihydrite–hematite conversion in Earth-like magnetic field—implications for paleomagnetic studies of red beds

Hematite-bearing red beds are renowned for their chemical remanent magnetization (CRM). If the CRM was acquired substantially later than the sediment was formed, this severely compromises paleomagnetic records. To improve our interpretation of the natural remanent magnetization, the intricacies of the CRM acquisition process must be understood. Here, we contribute to this issue by synthesizing hematite under controlled ‘Earth-like’ field conditions (≲100 μT). CRM was imparted in 90 oriented samples with varying inclinations. The final synthesis product appeared to be dominated by hematite with traces of ferrimagnetic iron oxides. When the magnetic field intensity is ≳40 μT, the CRM records the field direction faithfully. However, for field intensities ≲40 μT, the CRM direction may deviate considerably from that of the applied field during synthesis. The CRM intensity normalized by the isothermal remanent magnetization (CRM/IRM@2.5 T) increases linearly with the intensity of growth field, implying that CRM could potentially be useful for relative paleointensity studies if hematite particles of chemical origins have consistent properties. CRM in hematite has a distributed unblocking temperature spectrum from ∼200 to ∼650 °C, while hematite with a depositional remanent magnetization (DRM) has a more confined spectrum from ∼600to680°C because it is usually coarser-grained and more stoichiometric. Therefore, the thermal decay curves of CRM with their concave shape are notably different from their DRM counterparts which are convex. These differences together are suggested to be a potential discriminator of CRM from DRM carried by hematite in natural red beds, and of significance for the interpretation of paleomagnetic studies on red beds.

Palaeointensity and Brunhes palaeomagnetic field models

Statistical models of geomagnetic secular variations in terms of a Giant Gaussian Process (GGP) are compared with palaeontensity databases (Brunhes epoch, effusive rocks). We tested two GGP models TK and QC (suggested by Tauxe-Kent and Quidelleur-Courtillot) against 392 volcanic palaeointensity data, and found both models to be incompatible with the data. Even though at present there is no accepted GGP model of the Brunhes chron, we found interesting to clarify the details of this negative test. While such an incompatibility is clear enough for the TK model, because of its too small dipole coefficient, the incompatibility of the QC model was due the 12 per cent of the data with very small intensities; remaining 88 per cent of data is fully compatible with QC model QC. The troublesome, low-palaeointensity records belong to different sites and ages so their excess presence is not an effect of particular location or time. We found no evidence that the marked discrepancy between the empirical data and theoretical calculations on the QC model can be explained by the possibility that the low-intensity values belong to known excursions of the geomagnetic field. As an alternative, we conjecture that selected low-intensity values can possibly stem from acquisition of chemical remanent magnetization instead of the thermoremanence.

The Timing of Diagenesis and Thermal Maturation of the Cretaceous Marias River Shale, Disturbed Belt, Montana

AbstractThe hypothesis that chemical remanent magnetization (CRM) in argillaceous rocks may be due to release of Fe during smectite illitization has been tested by study of spatial and temporal relationships of CRM acquisition, smectite illitization, and organic-matter maturation to deformation in the Montana Disturbed Belt. New K-Ar ages and stacking order and percentages of illite layers in illite-smectite (I-S) are consistent with conclusions from previous studies that smectite illitization of bentonites in Subbelts I and II of the Disturbed Belt was produced by thrust-sheet burial resulting from the Laramide Orogeny. Internally concordant, early Paleogene, K-Ar age values (55–57 Ma) were obtained from clay subfractions of thick bentonites which were significantly different in terms of their ages (i.e. Jurassic Ellis Formation and late Cretaceous Marias River Shale), further supporting a model of smectite illitization as a result of the Laramide Orogeny. Internally concordant K-Ar ages were found also for clay sub-fractions from a thick bentonite at Pishkun Canal (54 Ma) and from an undeformed bentonite near Vaughn on the Sweetgrass Arch (48 Ma). In Subbelts I and II, a greater degree of smectite illitization corresponds to increased thermal maturation, increased natural remanent magnetization intensity, and increased deformation (dip of beds). A dissolution-precipitation model over a short duration is proposed for the formation of illite layers in Subbelts I and II. A characteristic remanent magnetization was developed before or just after folding began in the early Paleogene. More smectite-rich I-S, low thermal maturity, and the absence of a CRM were noted in one outcrop of an undeformed rock on the Sweetgrass Arch. Strontium isotope data allow for the possibility that internal or externally derived fluids may have influenced illitization, but the K-Ar age values suggest that illitization was probably in response to conductive heating after the overthrusting had occurred. The differences in K-Ar dates among the bentonites studied herein may be due to differences in the timing of peak temperature related to differences in distance below the overthrust slab, in rates of burial and exhumation, and in initial temperature.

Paleomagnetism of Devonian red beds in the Appalachian Plateau and Valley and Ridge provinces

Samples of red and green fluvial to marine sandstones in the Hampshire Formation/Catskill Group from regional‐scale folds across the structural trend of the Valley and Ridge (VR) and Appalachian Plateau (AP) provinces of West Virginia were analyzed to test models for remagnetization of red sandstones. The red sandstones contain a dominant secondary Pennsylvanian‐Permian magnetization with south declinations (154.8°–166.9°) and shallow inclinations (0.2°–19.3°) that resides in hematite and is interpreted as a chemical remanent magnetization (CRM) acquired during the Kiaman reversed superchron. Incremental fold tests for this CRM yield generally prefolding results from the AP rocks and synfolding results from the VR rocks. This suggests that the remanence acquisition in the AP may have occurred within the time span of deformation. A second, high unblocking temperature apparent synfolding CRM is also found in a few samples of the red sandstones and is distinguished by slightly steeper inclinations than the dominant component on northwest dipping beds. Specimens from green sandstone have weak intensities and are dominated by a modern viscous remanence. Thin section analysis shows the presence of authigenic specular hematite cement and submicron particle size red hematite pigment. Geochemical/fluid inclusion studies indicate that the rocks were exposed to mixed methane‐saturated formational and meteoric fluids with no evidence that external warm orogenic fluids altered the rocks. A working model for CRM acquisition involves (1) methane reduction of previously formed iron phases and mobilization of iron and (2) a return to oxidizing conditions and precipitation of new authigenic hematite as a result of the introduction of meteoric fluids just prior to and during uplift. The green sandstones probably formed as a result of gleying which occurs in paleosols although they could be a relic of or preserve evidence of the reduction phase.

The lepidocrocite-maghemite-haematite reaction chain-I. Acquisition of chemical remanent magnetization by maghemite, its magnetic properties and thermal stability

SUMMARY We report on the magnetic properties and the acquisition of a chemical remanent magnetization (CRM) in a field of 100 μT as a function of temperature and time during the lepidocrocite–maghemite–haematite reaction chain. The development of CRM was monitored at a series of 13 temperatures ranging from 175 to 550 °C; data acquisition was done at the specific formation temperatures for durations of up to 500 hr. Up to acquisition temperatures of 200 °C it takes a considerable time (up to 7 hr) before the CRM is measurable. This time decreases with increasing temperature, reflecting the activation energy of the reaction to form the first maghemite. During the lepidocrocite conversion, formation of two types of maghemite is suggested by two peaks in the CRM versus time curves. Magnetic properties were analysed after various stages in the reaction. They indicate a mixture of superparamagnetic and single-domain maghemite. The first reaction product (obtained after annealing at 200 °C) is a fine-grained yet crystalline maghemite (labelled type A). Before massive maghemite formation occurs, the coercive and remanent coercive forces go through a minimum at intermediate temperatures of 250–300 °C (annealing for 2.5 hr). This minimum lowers to 200–250 °C with increasing annealing time (500 hr). This is probably the result of two processes acting simultaneously—formation of superparamagnetic maghemite particles of a second less crystalline maghemite type (labelled type B) and removal of stacking faults in type A maghemite. The second process is suggested by analogy to the behaviour of natural magnetite/maghemite systems on annealing. Removal of stacking faults is reported to result in a magnetic softening of the grain assemblage. Annealing at 300–350 °C removes most of the lepidocrocite and the second maghemite type, type B, becomes prominent. Haematite formation sets in at slightly higher temperatures, yet the type B maghemite is in part thermally stable up to 600 °C enabling Thellier–Thellier experiments. This stability is also inferred from Arrhenius fitting that shows a comparatively high activation energy for the maghemite to haematite reaction. In Thellier–Thellier experiments the CRM showed a markedly downward convex Arai–Nagata plot while a second thermoremanent magnetization (TRM) showed perfect linear behaviour as expected. This feature may be used to recognize CRM in natural rocks.

Paleomagnetic constraints on the Archean geomagnetic field intensity obtained from komatiites of the Barberton and Belingwe greenstone belts, South Africa and Zimbabwe

The intensity of the geomagnetic field during Archean is an important source of information about the evolution of the Earth’s core. Hale [Earth Planet. Sci. Lett. 86 (1987b) 354] reported a very low equatorial paleointensity of 5 μT at ca. 3.5 Ga obtained from komatiites of the Barberton greenstone belt, South Africa, and regarded their remanences as thermal overprints due to a metamorphic event. However, this result has a major drawback since remanences of the Barberton komatiites are carried by magnetites formed during serpentinization that typically results in acquisition of chemical remanent magnetization (CRM). We report the results of detailed paleomagnetic, rock magnetic, and paleointensity investigations using komatiite samples from the Barberton (ca. 3.5 Ga) and Belingwe (ca. 2.7 Ga) greenstone belts in South Africa and Zimbabwe, respectively. Paleodirectional data for Barberton are consistent with results of previous work, and the characteristic remanent magnetization (ChRM) of the Belingwe samples reveals a positive fold test, suggesting that samples from both areas record the Archean geomagnetic field. The main carriers of ChRM are Ti-free magnetites presumed to be akin to single domain grains, and their rock magnetic properties are very stable when subjected to laboratory heating up to 600–700 °C. Our microscopic observations provide support that these are secondary minerals formed during serpentinization. Thellier–Thellier paleointensity experiments for the Barberton and Belingwe samples yielded very low mean virtual dipole moment (VDM) estimates of (1.8±1.3)×1022 A m2 and (1.1±0.9)×1022 A m2, respectively. These correspond to about 24 and 15% of the present day value. Considering the low metamorphic grade for the sampling areas, however, it seems to be somewhat difficult to regard the entire ranges of komatiite ChRM as thermal overprints acquired by reheating during metamorphic events. We hence propose as the most likely scenario the possibility that most fractions of ChRM are survivors of primary grain growth CRM. Taking the theoretical and experimental estimates of the ratio of CRM to thermoremanent magnetization (TRM) into account, this suggests that these low mean VDM values can provide constraints on the lower limits of the geomagnetic field intensities in both greenstone belts at ca. 3.5 and 2.7 Ga.

Rock-magnetic changes with reduction diagenesis in Japan Sea sediments and preservation of geomagnetic secular variation in inclination during the last 30,000 years

A rock-magnetic and paleomagnetic study was conducted on a sediment core of about 4.4 m long taken from the northeastern part of the Japan Sea. The core covers the last about 30 kyrs, which was dated by nineteen radiocarbon (14C) ages. Remanent magnetization is carried dominantly by magnetite. Reductive dissolution of magnetic minerals occurs between 1.2 and 1.6 m in depth (about 5–8 ka in age). A rapid downcore decrease of anhysteretic remanent magnetization (ARM) begins at the shallowest depth. Saturation isothermal remanent magnetization (SIRM) follows, and a decrease of magnetic susceptibility (k) takes place at the deepest. Within this zone, coercivity of natural remanent magnetization (NRM) and the ratios of ARM to k and SIRM to k also decreases with depth. These observations indicate that finer magnetic grains were lost earlier than larger grains. A decrease of S ratios, wasp-waisted hysteresis curves, and a deviation from a mixing trend of single-domain and multi-domain grains in a Day plot occur as the dissolution proceeds, which suggests that high coercivity minerals like hematite are more resistive to dissolution than low coercivity minerals like magnetite. The start of the dissolution at 1.2 m in depth is synchronous with increases in organic-carbon and total-sulfur contents, but the horizon does not coincide with the present Fe-redox boundary at about 0.02 m below the sediment-water interface. From low-temperature magnetometry, it is estimated that magnetites with maghemite skin are reduced to pure magnetites prior to dissolution. There is no evidence for precipitation of secondary magnetic phases and acquisition of chemical remanent magnetization (CRM). Neither pyrrhotite nor greigite was detected. Information of paleomagnetic directions have survived the reductive dissolution. Inclination variations of this core resembles closely to the secular variation records available around Japan. Well-dated records older than 10 ka are still very rare, and hence our new record could be useful for establishing regional secular variations.

Erroneous fold tests as an artifact of alteration chemical remanent magnetization

[1] Several studies have shown that an alteration chemical remanent magnetization (CRM) may have a direction intermediate between that of the remanence of the parent phase and the field during remagnetization. In a fold a surviving natural remanent magnetization (NRM) is exposed at varying angles to the remagnetizing field, giving rise to varying degree of remanence deflection. A laboratory experiment was carried out to investigate how the CRM deflection varies with angle between parent phase NRM and remagnetizing field, HCRM, and we discuss how this variation would affect the results of a fold test. The experiment was performed on a Silurian lava, with a well-defined single component NRM carried by deuterically oxidized titanomagnetite (TM). Specimens of the lava were arranged in a fold configuration with NRM at various angles to a 50 μT laboratory field. Heating to 525°C resulted in oxidation of part of the primary TM phase to hematite, leading to the formation of stable remanence components of intermediate direction, as revealed by thermal demagnetization. Stepwise unfolding of the CRM gave a syntectonic fold test result. The variation of remanence deflection with angle between HCRM and initial NRM can be understood in terms of vector addition of fields or remanence components. A model for CRM acquisition derived from the experimental results was employed to explore CRM in other fold configurations. It appears from analysis of synthetic data that it is possible to falsely conclude both synfold and prefold magnetization from fold tests on postfold or synfold CRM.

The origin of magnetization and geochemical alteration in a fault zone, Kilve, England

The origin of an apparently syndeformational chemical remanent magnetization (CRM) and geochemical alteration in a fault zone in the Bristol Channel Basin, southwest England, was investigated. Deformation in the fault zone occurs in Jurassic aged, organic-rich limestones and consists of numerous normal and oblique-slip faults and associated folds. Migration of basinal, radiogenic fluids is indicated by elevated 87Sr/86Sr values for calcite veins that occur throughout the fault zone. Some of the calcite veins contain hydrocarbons sourced from deeper strata. Elevated 87Sr/86Sr values in the host Jurassic limestones indicate that they were also extensively altered by radiogenic fluids that migrated through microfractures in addition to major fault and fracture planes. Folded and tilted host limestones contain a pervasive secondary CRM residing in magnetite that was acquired during deformation in the Tertiary. The association between this pervasive CRM and the pervasive geochemical alteration is consistent with a genetic connection between the orogenic fluids and the CRM although the timing of CRM acquisition (Tertiary) is not consistent with structural interpretations for the timing of most veining. An alternative remagnetization mechanism which is not triggered by externally derived fluids, such as diagenesis of hydrocarbons, might account for the CRM. Hydrocarbon-bearing veins also contain a CRM that resides in magnetite, although the time for remanence acquisition is not well constrained by field tests. © 1998 John Wiley & Sons, Ltd.

Chemical remagnetization and burial diagenesis: Testing the hypothesis in the Pennsylvanian Belden Formation, Colorado

Lower Pennsylvanian Belden Formation carbonate rocks from Colorado were subjected to paleomagnetic, rock magnetic and geochemical studies to test whether there is a connection between a widespread chemical remanent magnetization (CRM), carried by authigenic magnetite, and burial diagenesis. Thermal demagnetization results indicate the presence of two components of natural remanent magnetization (NRM) after removal of a low unblocking temperature (NRM‐250°C) remanence that is interpreted to be a modern, viscous magnetization. An intermediate unblocking temperature (250–400°C) remanence component with normal and reversed polarity Tertiary directions is interpreted to be a thermoviscous remanent magnetization. Many limestones also contain a high unblocking temperature (400–570°C) remanence component which is interpreted to be a CRM. Fold tests from different parts of the basin indicate that the CRM was acquired either before or during Laramide folding. This CRM is interpreted to be carried by authigenic magnetite that formed by replacement of pyrite. Hysteresis ratios are consistent with those reported for other remagnetized carbonates and indicate that the CRM is carried by single‐domain/pseudo single‐domain magnetite. Although elevated 87Sr/86Sr values indicate passage of radiogenic fluids through the limestones, the results of contact vein tests do not support the hypothesis that these fluids were responsible for the CRM. The time of CRM acquisition, which varies from late Paleozoic to Cretaceous, coincides with the modeled time of organic matter maturation in different parts of the basin. This suggests that diagenetic reactions, that were triggered by low to moderate burial temperatures, may have caused the magnetite authigenesis and probably gave rise to the CRM.

Chemical remanent magnetization in oceanic sheeted dikes

Marine lineated magnetic anomalies often exhibit anomalous skewness that indicates non‐vertical boundaries between normally and reversely magnetized crust. The acquisition of secondary magnetization components well after formation of the crust has been suspected as one possible cause for anomalous skewness. A petrographic and rock magnetic study on sheeted dikes recovered from the lower part of the deepest drill hole in the oceanic crust, ODP (Ocean Drilling Program) Hole 504B, demonstrates that these rocks carry a secondary chemical remanent magnetization (CRM) and not a thermoremanent magnetization (TRM). Primary titanomagnetites have mostly been altered to non‐magnetic phases while the remanent magnetization resides in secondary magnetite that formed below its Curie temperature (∼600°C at ambient pressure) mainly by the alteration of olivine. The natural remanent magnetization (NRM) has been compared with laboratory anhysteretic remanent magnetization (ARM) and TRM. The intensities relate NRM < ARM < TRM, each by a factor of ≈ two. Because previous studies have shown that CRMs are much lower in intensity than TRMs and because of the common presence of secondary magnetite we conclude that the NRMs of the lower Hole 504B dikes are CRMs. The dikes exhibit the same reversed magnetization as the overlying extrusive basalts, and, thus, the timing of CRM acquisition is most likely confined to the time span between the crustal age of Hole 504B and the next younger field reversal, i.e. 0.25 m.y. after emplacement.

Chemical remanent magnetism related to the Dead Sea Rift: Evidence from Precambrian igneous rocks of Mount Timna, southern Israel

A paleomagnetic and mineralogical study of shallow intrusive basement rocks on Mount Timna (Arabo Nubian Massif in Sinai) shows that although all the igneous rocks are of late Precambrian age, a remanent magnetic direction similar to the subrecent field (Miocene to present) is identified in samples of quartz‐monzodiorite, monzodiorite, dikes of various composition, and altered gabbro. The average direction of these rock units is (D/I) 359°/41°, α95=4°, and the pole is at 83.6°N,223.2°E. The “subrecent” direction appears both as an overprint direction and as the sole stable vector in dolerite, rhyolite, and andesite dikes. The magnetic mineral assemblage of these rocks includes secondary minerals, such as hematite and goethite, which formed by oxidation and hydration of the original magnetite and Ti‐magnetite. The subrecent direction is interpreted to have been acquired as a chemical remanent magnetization (CRM) by hydrothermal activity and circulation of thermal brines through fractures related to the adjacent Dead Sea transform. The hydrothermal activity occurred before uplift and erosion exposed the basement rocks, i.e., in the middle Miocene, during the early stages of activity of the Dead Sea rift. The north trending declinations indicate that Mount Timna has not rotated about a vertical axis after acquisition of the CRM, a conclusion that confirms a previous structural analysis. An absence of reversals implies that the duration of CRM acquisition was probably less than ∼1 m.y., the maximum length of normal polarities since the Oligocene. Other field directions were found in an alkali granite (one site, 343°/14°, α95=22°), in a dolerite dike (one site, 318°/0°, α95=2°) and in a gabbro (seven sites, 303°/56°, α95=6°). The field direction in the granite is similar to that for Early Cretaceous, a time of magmatic activity in Timna. Northwest trending declinations and shallow inclinations found in several samples of the dikes are carried by unaltered parts of the rocks. These directions are interpreted as late Precambrian‐Early Cambrian in age, indicating a near‐equatorial location of the region at that time.

The effect of low‐temperature hydrothermal alteration on the remanent magnetization of synthetic titanomagnetites: A case for acquisition of chemical remanent magnetization

The effect of hydrothermal alteration on the thermoremanent magnetization (TRM) of synthetic titanomagnetite (TM40: Fe2.6Ti0.4O4) has been studied, to simulate the alteration that occurs in the oceanic crust. Pseudo‐single‐domain titanomagnetite grains, similar in size to those often found in oceanic basalts, were dispersed in a permeable but rigid glass matrix. This resulted in a TRM in the sample which was subsequently oxidized in acidic solutions while a magnetic field (0.1 mT) was applied perpendicular to the TRM direction. The experiments were conducted in a non‐magnetic stainless steel pressure vessel at 150°C in solutions of acidity varying from pH=2 to pH=7. In addition to being time and temperature dependent, the acquired chemical remanent magnetization (CRM) was also found to be very pH dependent. The degree of maghemitization increased drastically as the acidity of the hydrothermal solution was increased in accordance with a process controlled by the loss of iron ions in aqueous solutions. Long‐term storage experiments at carefully chosen temperatures demonstrated that no significant viscous remanent magnetization was acquired during heating. It was found that during the alteration of TM40 to titanomaghemite the CRM is along the ambient field direction from the onset, and not partly or wholly along the TRM direction as has been found in previous air oxidation experiments. This has important implications for the possible cause of anomalous skewness of marine magnetic anomalies and for the anomalous directions of natural remanent magnetization found in some oceanic basalt samples.

A direct comparison of the properties of CRM and VRM in the low-temperature oxidation of magnetite

SUMMARY We report results of a collaborative investigation of the experimental properties of chemical remanent magnetization (CRM) acquired when nearly single-domain size magnetite undergoes low-temperature oxidation to maghemite and of viscous remanent magnetization (VRM) acquired by the daughter phase maghemite. Magnetite grains 100-200 nm in diameter were oxidized for varying lengths of time at a temperature of 220 C. The acquisition of CRM in fields of 0.2,0.5 and 1 Oe was monitored at frequent intervals during heating runs lasting 30 hr. Zero-field decay of this remanence was then monitored for periods of 160-430 hr. Subsequently VRM acquisition and decay were measured for similar time periods for the daughter phase maghemite (z = 0.96). The principal results are as follows.

Synfolding magnetization in the Jurassic Preuss Sandstone, Wyoming‐Idaho‐Utah Thrust Belt

The Jurassic Preuss Sandstone, exposed in five thrust plates of the Wyoming‐Idaho‐Utah thrust belt, carries directions of remanent magnetization that group most tightly after only partial unfolding. Field, petrographic, and rock magnetic evidence indicates that the carrier of this magnetization is detrital, low‐Ti titanomagnetite. The detrital titanomagnetite was remagnetized at low temperatures (75°–150°C) probably completely during folding. Anisotropy of magnetic susceptibility and petrographic observations indicate that the detrital titanomagnetite has been affected by tectonic strain. We suggest that low‐temperature remagnetization of the detrital titanomagnetite was either a viscous partial thermoremanent magnetization, the acquisition of which was enhanced by stress, or a piezoremanent magnetization that involved stress‐induced movement of domain walls during intracrystalline strain, or was a combination of the two mechanisms. Stress may promote remagnetization at temperatures much lower than predicted by current theoretical models. Other mechanisms, such as acquisition of chemical remanent magnetization during folding, deflection of a prefolding magnetization by internal strain, or combination of components of magnetization with different direction cannot account for the geometry of magnetization in the Preuss. The locus of acquisition of synfolding magnetization in the Preuss migrated in conjunction with deformation in the thrust belt. A model is presented in which synfolding magnetization was acquired during cooling and folding as strata moved up thrust ramps. A lack of reverse‐polarity directions remains a puzzling feature of the remanence. The remanent direction is tentatively interpreted to reflect the predominant polarity state during its acquisition over an extended rather than a discrete time period during folding in Late Cretaceous and early Tertiary (?) periods of predominantly normal polarity.