Thermochemical Remanent Magnetization Research Articles

Paleomagnetic investigation of the basal Maieberg Formation (Namibia) cap carbonate sequence (635 Ma): Implications for Snowball Earth postglacial dynamics

In this study, we investigate the paleomagnetism of the basal Maieberg Formation (Namibia) cap carbonate sequence to elucidate its magnetic properties and paleolatitude of deposition, establish global correlations, and contribute to the understanding of Snowball Earth postglacial dynamics. Two distinct magnetization components, C1 and C2, were identified. C1 is interpreted as a depositional or post-depositional remanent magnetization carried by detrital pseudo-single domain (PSD) magnetite, while the C2 component is a thermochemical remanent magnetization carried by fine authigenic single domain (SD)−PSD magnetite. The deposition paleolatitude provided by C1 is 33.3° ± 3.2°, which gives an initial quantitative approximation of the paleolatitude for the underlying Marinoan Ghaub diamictites. The thickness of the Keilberg Member cap dolostone is anomalously high for the paleolatitude calculated with C1, which suggests that other factors besides the influence of the paleolatitude on carbonate oversaturation may have influenced the sedimentary production of cap dolostones and the overall thickness of the flooding cap carbonate sequence. Possible explanations could include the influence of alkalinity input combined with local tectonic subsidence during a long glacial period with unusually low sedimentation rates, which appear to be in a favorable configuration for the substantial thickness of the Keilberg Member. Paleomagnetic field reversals at the Keilberg cap dolostone and analogous units globally suggest a longer duration of marine transgression than energy-balance deglaciation models and sedimentological-geochemical observations have constrained. Factors such as ocean warming, thermal expansion, and local glacio-isostatic adjustments imply extended marine transgressions beyond the deglaciation period. Still, magnetostratigraphic estimates for postglacial transgressive sequences require longer time scales by a factor of five or more. Thus, the conflict arising between estimates derived from paleomagnetic data and the constraints imposed by climate physics underscores uncertainties regarding an unconventional field state or a remanence acquisition mechanism within these cap carbonates that is not fully understood. Importantly, if such a phenomenon proves to be primary and global, the widespread occurrence of these stratigraphically compressed reversals would support the precise temporal correlation between Marinoan cap dolostones. The C2 pole correlates with Cambrian remagnetization poles observed in carbonates from West Gondwana, which now extend to the Congo craton. The remanence acquisition of C2 likely stems from diagenesis-related low-temperature authigenic magnetite formation after the conversion of iron-rich smectite to iron-poor illite. Cooling associated with the Kaoko orogen’s exhumation and tectonic uplift possibly locked the magnetic system at ca. 520 Ma, supported by the C2 pole position on the West Gondwana apparent polar wander path, although other explanations remain valid.

The last gasp of the Rogaland Igneous Complex, Norway: a palaeopole for the 920 Ma Tellnes intrusion

SUMMARY The Rogaland Igneous Complex (RIC) in southern Norway intruded into Sveconorwegian granulite crust beginning ∼930 Ma. Three massif anorthosite bodies, Egersund–Ogna, Helleren and Åna-Sira, were intruded some 10 Myr later by the Bjerkreim–Sokndal layered intrusion. The Garsaknatt leuconorite and the ilmenite-rich Tellnes norite, one of the youngest rock in the complex at ∼920 Ma, intrude the anorthosite or nearby country rock. Magnetic mineralogy and palaeomagnetic studies carried out on the Tellnes norite, the Garsaknatt leuconorite and the surrounding Åna-Sira anorthosite, indicate the magnetization of all three bodies are dominated by hemo-ilmenite carrying the remanence as a thermochemical remanent magnetization, although magnetite is present in some samples. The three bodies yield steep negative inclinations with northwesterly declinations (Tellnes, I = −71.9°, D = 305.0°, α95 = 10.6°; Garsaknatt, I = −73.1°, D = 312.7°, α95 = 4.7°; and Åna-Sira, I = −81.2°, D = 326.3°, α95 = 6.7°). When combined with data from other bodies in the RIC, the older anorthosites have steeper inclinations, and higher palaeolatitudes, while the younger units have less steep inclinations and shallower palaeolatitudes by nearly 10°, indicating northward plate motion during cooling of the intrusions. Age of the remanence is difficult to determine precisely, however, best estimates are ∼910 Ma for the older anorthosites and ∼900 Ma for the younger intrusions. Although these differences are significant, a unified pole position (35.6° N, 215.1° E), combining all the 111 sites from the RIC, strongly supports the assumed position of southern Baltica in Rodinia at ∼900 MA.

Properties of Thermochemical Remanent Magnetization on Basalt Samples Containing Titanomagnetites with Increased Thermal Stability

Properties of Thermochemical Remanent Magnetization on Basalt Samples Containing Titanomagnetites with Increased Thermal Stability

The Properties of Thermochemical Remanent Magnetization Acquired by Slow Laboratory Cooling of Titanomagnetite-Bearing Basalt Samples from Different Temperatures and the Results of the Thellier Method

—The experiments have been carried out on the acquisition of thermochemical remanent magnetization (TCRM) in basalt samples containing titanomagnetite (TM) with the Curie temperature Тс ~200°C by their rapid heating to maximum temperatures Т* from 450 to 530°C followed by slow cooling in the laboratory magnetic field Blab. At different stages of the preliminary thermal treatment of the initial samples, a set of magnetomineralogical studies including electron microscopy, X-ray diffraction and thermomagnetic analyzes, and measurements of magnetic hysteresis parameters were performed. It is shown that as early as the very beginning of the cooling process, all samples demonstrate explosive growth of TCRM corresponding to the stage of rapid single-phase oxidation of the initial titanomagnetite fraction of basalt, and that TCRM is acquired by the increase of Тс and volume of single-phase oxidized parts of TM grains as well as by the growth of the volume of Ti-depleted (relative to the initial TM) cells of microstructure of the subsequent oxidative exsolution. The Arai–Nagata diagrams for the samples carrying TCRM have a form of a broken line consisting of two linear segments. The low-temperature interval T < Т* corresponds to a mixture of thermochemical and thermoremanent (TRM) magnetizations and gives a slightly overestimated Blab because of the effect of a low cooling rate during the acquisition of TCRM and TRM. The high-temperature interval corresponds to pure TCRM and the Blab strength determined from this interval is underestimated by 20–27%. It is recommended to reject samples whose Araii–Nagata diagram has two or more linear segments against the background single-component NRM.

Exact Analytical Solutions for Kinetic Equations Describing Thermochemical Remanence Acquisition for Single‐Domain Grains: Implications for Absolute Paleointensity Determinations

AbstractThe magnetic record, preserved by igneous rocks in the form of thermoremanent magnetization (TRM) or thermo‐chemical remanent magnetization (TCRM), is essential to reconstruct Earth's absolute paleointensity (API) but strongly depends on the kinetic conditions in which the remanence was acquired. In this paper, we present exact analytical solutions describing the time‐dependent processes of acquisition and thermal demagnetization of various kinds of thermally activated remanences for non‐interacting single‐domain grains with uniaxial shape anisotropy. Our solutions, derived in less‐restrictive conditions than previous studies, are also valid for TCRMs acquired either by growth of grain volume or by increase of the Curie point Tc. We first show that TCRMs by Tc increase and TRMs are of comparable intensity whereas TCRMs by volume growth are significantly less intense. We then model Arai‐Nagata diagrams for assemblies of coercivity‐variable grains and find that all Thellier‐type protocols yield reliable API determinations for TRMs and TCRMs by Tc increase, with the peculiarity of the IZZI protocol to produce small zigzags in the Arai‐Nagata diagram. In contrast, TCRMs by volume growth yield convex Arai‐Nagata diagrams. The most conspicuous kinetic effect is the influence of cooling rate on API determinations due to a ∼5% increase of the remanent magnetization for a 10‐fold increase in cooling time. We show that the situation is problematic when the cooling time of natural samples coincides with the geomagnetic secular‐variation time scales. Natural samples with cooling times sufficient to average out secular variation conversely yield reliable API determinations provided a cooling‐rate correction is applied.

Reliable Determination of Paleointensity in Igneous Rocks Bearing Thermochemical Remanent Magnetization

Oxidation decomposition of titanomagnetite grains to the magnetite–ilmenite lattice during the formation of igneous rock can lead to thermochemical remanent magnetization (TCRM). Computer simulation of TCRM in the system of single-domain interacting particles and numerical simulation of the Thellier experiments to determine the paleointensity have been carried out. Contrary to the concerns, the Thellier technique factually applied to the TCRM-bearing samples yields a result that is very close to the true value of the external magnetic field applied to the sample during formation of the TCRM. Together with the recently published laboratory experimental data, these calculations are indicative of a good perspective for obtaining reliable paleomagnetic data on TCRM-bearing samples and can expand the class of rocks suitable for determination of the paleointensity.

Palaeomagnetic dating of hydrothermal alteration in the Woodford Shale, Oklahoma, USA

AbstractAn oriented Woodford Shale core from the eastern Ardmore Basin was sampled to test if the shale was an open or closed system to hydrothermal fluids, and to determine the timing of alteration. Mineralized fractures are ubiquitous in the core, and the shale exhibits a complex paragenesis with multiple hydrothermal minerals, including biotite, magnesite, norsethite, gorceixite and potassium feldspar present in and around the fractures. These minerals suggest that the Woodford Shale was an open system during part of its diagenetic history. Vitrinite reflectance (Ro) measurements indicate values of ∼1.81 % (∼230 °C). Palaeomagnetic analysis reveals a characteristic remanent magnetization (ChRM) with south-southeasterly declinations and shallow inclinations that resides in magnetite. This ChRM is interpreted to be either a chemical remanent magnetization (CRM) or a thermochemical remanent magnetization (TCRM) that was acquired at 245 ± 10 Ma during Late Permian time based on the pole position (51.0° N, 115.6° E). Because the palaeomagnetic specimens show evidence of extensive hydrothermal alteration, the CRM/TCRM is interpreted to date the migration of hydrothermal fluids through the shale. The agreement in timing with other studies that report hydrothermal alteration in southern Oklahoma and the Ouachita Mountains in Late Permian time, suggest that there were post-collisional fluid-flow events which accessed reservoirs of warm fluids.

A Quantitative Model for the Thermochemical Remanent Magnetization of the Ocean Floor

A Quantitative Model for the Thermochemical Remanent Magnetization of the Ocean Floor

A whole rock absolute paleointensity determination of dacites from the Duffer Formation (ca. 3.467 Ga) of the Pilbara Craton, Australia: An impossible task?

We have conducted a whole-rock type magnetic and absolute paleointensity determination of the red dacite of the Duffer Formation from the Pilbara Craton, Australia. The age of the dated rock unit is 3467±5Ma (95% confidence). Vector analyses results of the step-wise alternating field demagnetization (NRM up to 100mT) and thermal demagnetization (from NRM up to 650°C) yield three components of magnetization. Curie point determinations indicate three characteristic temperatures, one at 150–200°C, a second one at ∼450°C and a third one at ∼580°C. Magnetic grain-size experiments were performed on small specimens with a variable field translation balance (VFTB). The coercivity of remanence (Hcr) suggests that the NRM is carried by low-coercivity grains that are associated with a magnetite fraction as is shown by the high-temperature component with blocking temperatures above 450°C and up to at least 580°C. The ratios of the hysteresis parameters plotted as a modified Day diagram show that most grain sizes are scattered within the Single Domain (SD) and the Superparamagnetic and Single Domain SP-SD domain ranges. In addition to the rock magnetic experiments we have performed absolute paleointensity experiments on the samples using the modified Thellier-Coe double heating method to determine the paleointensities. Partial-TRM (p-TRM) checks were performed systematically to document magnetomineralogical changes during heating. The temperature was incremented by steps of 50°C between room temperature and 590°C. The paleointensity determinations were obtained from the slope of Arai diagrams. Our paleointensity results indicate that the paleofield obtained was ∼6.4±0.68 (N=11) micro-Teslas with a Virtual Dipole Moment (VDM) of 1.51±0.81×1022Am2, from a medium-to high-temperature component ranging from 300 to 590°C that has been interpreted to be the oldest magnetization yet recorded in paleomagnetic studies of the Duffer Formation. The absolute paleointensity is relatively low and we interpret this low-paleofield bias a result of a thermochemical remanent magnetization (TCRM) process that indicates a possible underestimate of the paleofield by a factor of four for the red dacite of the Duffer Fm.

Is it possible to identify temporal differences among combustion features in Middle Palaeolithic palimpsests? The archaeomagnetic evidence: A case study from level O at the Abric Romaní rock-shelter (Capellades, Spain)

Archaeomagnetic dating is probably one of the most known applications of magnetic methods to archaeology but there are others still underutilized and of particular interest to Palaeolithic archaeology. Here, we report a novel application of archaeomagnetism as a technique to determine temporal diachronies among combustion features from the same surface within palaeolithic palimpsests. The approach is based on the subtle directional changes of the Earth's magnetic field through time (secular variation, SV) and on the ability of burned materials to record such variations under certain conditions. Three Middle Palaeolithic hearths from level O (ca. 55 ka BP) at the Abric Romaní rock-shelter (NE Spain), were archaeomagnetically investigated. The studied surface (black homogeneous carbonaceous facies), recorded the magnetic enhancement produced by fire with a tenfold increase in concentration-dependent magnetic parameters in the uppermost centimetre with respect to its unburned or deeper counterparts. Pseudo-single domain (PSD) Ti-low titanomagnetite was identified as the main remanence carrier. The irreversibility of thermomagnetic curves suggests that these samples did not undergo enough high temperatures as to record a full thermoremanence (TRM). Additionally, the occasional occurrence of maghaemitized magnetite is interpreted as an indication of a thermochemical remanent magnetization (TCRM), making these samples unsuitable for absolute palaeointensity determinations. Two well-defined (α95 < 5°) and statistically indistinguishable archaeomagnetic directions were obtained with their mean directions within their respective confidences circles at the 95% level. The lack of directional changes and the similarity in the magnetic properties suggest that these hearths recorded simultaneously or closely confined in time the Earth's magnetic field direction at the time of cooling. These results agree well with archaeological evidence which indicates a synchronic occupation of this activity area. The possibility of determining temporal differences among combustion features in prehistoric sites arises as a promising tool in palimpsest dissection studies and may help to reconstruct occupation patterns of prehistoric groups. The practical limits of the method are discussed as well as its potential to identify post-depositional mechanical alteration processes.

Holocene archeointensities from mid European ceramics, slags, burned sediments and cherts

The Earth’s geomagnetic field intensity in the past can be determined from archeological artifacts. These archeointensity data are important inputs for geomagnetic field models and local reference curves of Earth’s magnetic field. Although archeointensities have been measured on materials for more than half a century ago, data are still scarce before 1000 BC and for the Alpine area in general. This investigation presents new absolute archeointensity data from a time period of 5000–700 BC from Italy and Switzerland. The archeological materials that were studied are ceramics, copper slag, and burned sediments from fireplaces. In addition, we investigated archeointensities from burned cherts, in order to uncover if they are a suitable material for paleomagnetism. Rock magnetic properties of all samples indicate magnetite, and small amounts of maghemite and hematite in the pseudosingle domain range as the ferromagnetic carriers. The IZZI protocol was used for 96 specimens to obtain absolute intensities; 23 ceramics, slags and burned cherts passed the threshold criteria, which we applied. The choice of the threshold values allowed us to obtain the linear part in the Arai diagram, which corresponds to the characteristic remanent magnetization. Burned sediments did not pass the threshold criteria, most probably because they acquired a thermochemical remanent magnetization during their formation. We demonstrate that magnetic susceptibility and saturation isothermal remanent magnetization can be used to select cherts that are suitable for paleointensity determinations. After applying anisotropy and cooling rate corrections, the new archeointensity values are lower for some samples, but fit well with available models and other archeomagnetic data.

Thermochemical remanence acquisition in single‐domain particle ensembles: A case for possible overestimation of the geomagnetic paleointensity

Single‐domain (SD) particle ensembles are the best studied systems in rock magnetism. Yet even for this simple system, a complete theoretical treatment of the acquisition of thermochemical remanent magnetization (TCRM) is missing. On the basis of the SD theory of chemical remanent magnetization, it is generally taken for granted that TCRM acquisition must be less efficient than acquisition of a thermoremanent magnetization (TRM) and that consequently, the paleofield intensity is inevitably underestimated when a TCRM is interpreted as a TRM. Here it is shown that this conclusion holds only when a continuous chemical change (e.g., grain growth) takes place during remanence blocking. In cases where an initial TRM, at a later time and at lower temperature, is modified by chemical processes, the final TCRM can result in considerable paleofield overestimation when interpreted as a TRM. This is demonstrated for two different scenarios. First, partial grain dissolution after TRM acquisition and second, low‐temperature oxidation. These mechanisms are not unlikely to occur in nature, and numerical experiments indicate that both would lead to perfectly straight Arai plots, which makes such SD TCRMs indistinguishable from true TRMs in the Thellier experiment.

Possible TCRM acquisition of the Kilauea 1960 lava, Hawaii: failure of the Thellier paleointensity determination inferred from equilibrium temperature of the Fe−Ti oxide

Abstract Natural rock samples may not always be the ideal material for the Thellier-type method as they occasionally result in high paleointensities. The Kilauea 1960 lava, Hawaii, is one such example. Several previous studies have suggested that one of the possible causes for this undesirable behavior is an acquisition of thermochemical remanent magnetization (TCRM) during lava formation. In order to examine this possibility quantitatively, equilibrium temperatures of titanomagnetite grains, which are associated with samples previously subjected to Thellier experiments, are estimated by a Fe−Ti oxide geothermometer. The results show that two specimens from the rock magnetic group giving relatively ideal Thellier paleointensities have clustered equilibrium temperatures of about 800–900 and 700–800°C. In contrast, two swarmed temperatures around 300 and 700°C were observed for the specimen from a group yielding high paleointensities. Although these are semi-quantitative estimates, when the time scales of Fe−Ti interdiffusion and lava cooling are taken into consideration, the last specimen could have acquired the TCRM during its formation. For such specimens, simple calculation predicts that TCRM/TRM (thermoremanent magnetization) ratios could be 1.19–1.72 for the blocking temperature range of 400–480°C, assuming a grain-growth model. The extent of this overestimation (20–70%) is comparable to the magnitude of the observations. It is therefore suggested that attention be paid to titanomagnetite grains with well-developed ilmenite lamellae, as these could be potential sources of overestimations of Thellier paleointensities of up to a few tenths of percentage points.

Thermochemical remanent magnetization in Precambrian rocks: Are we sure the geomagnetic field was weak?

Thellier paleointensity determinations from two dikes of the Early Proterozoic (∼2.46 Ga) Matachewan dike swarm (Canada) yield field values of 2.14 ± 0.18 and 9.81 ± 1.65 μT. Corresponding values of virtual axial dipole moment are very low (0.54 ± 0.05 × 1022 and 2.49 ± 0.42 × 1022 Am2, respectively) when compared with the modern field. The characteristic remanent magnetization (ChRM) was isolated over a narrow range of high unblocking temperatures (∼520–580°C). Detailed rock magnetic analyses indicate that the ChRM is carried by nearly stoichiometric pseudo single domain magnetite. Scanning electron microscopy (SEM) reveals that the magnetite is in the form of fine intergrowths with ilmenite, formed by oxyexsolution during cooling. The high‐temperature oxidation defined in the SEM images could have continued at temperatures below the Curie point of magnetite. In this case, the ChRM would be a thermochemical remanent magnetization (TCRM) rather than a thermal remanent magnetization (TRM). Estimates of the TCRM/TRM ratio show that the Thellier data could underestimate the true field value by a factor of 4 without violating experimental selection criteria. This uncertainty in TRM fidelity translates into a potential range of field values that spans that defined by the modern field (∼8 × 1022 Am2) and proposed low Precambrian levels (∼2 × 1022 Am2). Therefore understanding further how TCRM is acquired and records the field represents a major challenge if these and many other similar rocks are to be used in Thellier studies aimed at defining the strength of the Precambrian field.

New paleomagnetic, rock magnetic, and petrographic results from the Valle Group, Baja California, Mexico: Exploring the causes of anomalously shallow paleomagnetic inclinations

A detailed paleomagnetic, rock magnetic, and sedimentologic study was conducted on Cenomanian‐age strata of the Valle Group at El Campito, Baja California, to examine the burial compaction effects on the paleomagnetic inclination of marine sedimentary rocks. Alternating field and thermal demagnetization isolated a characteristic remanence at 16 of 18 sites with a mean of Dec = 342.6°, Inc = 19.1°, and α95 = 3.4°. The inclination is 38.7° shallower than that predicted by the reference paleopole for North America. Laboratory experiments indicate that compaction could account for only 7.5° of the inclination shallowing. Fold tests at different scales yield both positive and negative results, making a tight constraint on the remanence's age impossible. A primary postdepositional remanence is supported by bedding‐parallel foliations in the anisotropy of anhysteretic remanence fabric. Although primary anomalously shallow inclinations could indicate significant southerly, then northerly, paleolatitudinal offset, a more likely scenario is a late Cenozoic low‐temperature remagnetization, which is suggested by alteration along the edges of some detrital silicate grains and a bimodal magnetic grain size distribution revealed by isothermal remanence data. The synthesis of the data indicates that the Vizcaino terrane was contiguous with the Cordillera margin during remagnetization and underwent a subsequent 13.5 ± 5.4° clockwise rotation. We tentatively interpret that the remagnetization resulted from burial and/or Miocene volcanism, which caused an acquisition of thermochemical remanent magnetization (TCRM). This study reveals a complicated remanence that resists easy interpretation and suggests caution in studies of similar rocks.

Palaeomagnetic study of the Ronda peridotites (Betic Cordillera, southern Spain)

A palaeomagnetic study of the Ronda peridotites (southern Spain) has been carried out on 301 samples from 20 sites, spread along the three main outcrops of the ultrabasic complex: Ronda, Ojén and Carratraca massifs. Different lithologies and outcrops with different degrees of serpentinization have been sampled and analysed. Rock magnetic experiments have been carried out on a representative set of samples. These measurements include: Curie curves, hysteresis cycles, isothermal remanent magnetization (IRM) acquisition curves, thermal demagnetization of IRM imparted along three orthogonal axes and magnetic bulk susceptibility. Results indicate that magnetite is the main magnetic mineral present in the samples. Stepwise thermal and alternating field (AF) demagnetization of the natural remanent magnetization (NRM) reveals the presence of a characteristic remanent magnetization (ChRM) carried by magnetite, and in some sepentinized samples, a northward component with variable unblocking temperatures up to 250–575 °C. The appearance and the relative intensity of this northward component are strongly related to serpentinization degree. Taking into account the geological history of the peridotites, the ChRM has been considered as a thermo-chemical remanent magnetization acquired during the first serpentinization phase associated to the post-metamorphic cooling of this unit. On the basis of radiometric and fission track analysis, the ChRM is proposed to have been acquired between 20 and 17–18 Ma. The inclination of the mean direction of the ChRM statistically coincides with the expected inclination for stable Iberia during the Oligocene–Miocene. The declination of the ChRM differs from the expected declination, indicating clockwise block rotations of 41±12° about vertical axes since the cooling of the peridotites. When applying a compositional layering correction, the ChRM directions fail to pass this kind of fold test, thus, the compositional layering was not a palaeohorizontal during ChRM acquisition time. Normal and reversed polarities of the ChRM are reported, showing that at least one reversal of the Earth's magnetic field took place during ChRM acquisition process. A tentative polarity zonation within the peridotitic outcrops is also suggested. No evidence is found from these data for the previously proposed simultaneity between post-metamorphic cooling and rotation of the peridotites.

Palaeointensity study of the Hawaiian 1960 lava: implications for possible causes of erroneously high intensities

It is known that the Hawaiian 1960 lava sometimes yields higher palaeointensities than the IGRF (36.2 μT). In order to clarify the causes, we have performed a comprehensive investigation of palaeointensity measurements on 19 cores from the lava. According to various rock magnetic analyses, they are classified into three groups with degrees of the deuteric oxidation. In Coe's version of the Thellier method, a significantly larger mean palaeointensity of 49.0 ± 9.6 μT (N= 17) was observed similar to previous studies. These palaeointensities showed a dependence on the oxidation indices, which can be explained by thermochemical remanent magnetization (TCRM) acquisition during the natural cooling stage of the lava. Although it is generally difficult to distinguish the TCRM from thermoremanent magnetization (TRM), a low-temperature demagnetization (LTD) treatment on NRM and TRM in the Thellier experiment seems to allow us to detect the influence of the TCRM on NRM. Another possible tool for TCRM detection is the double-heating technique (DHT) of the Shaw method combined with LTD (LTD–DHT Shaw method). It gave a better average of 39.4 ± 7.9 μT (N= 9), though the success rate was much lower. Most of the inappropriate results were screened by a combination of the double-heating test, the ARM correction and the LTD treatment. If two outliers are further excluded, the average is improved to be 35.7 ± 3.3 μT (N= 7), which agrees well with the expected intensity. The results of both methods emphasize the importance of taking and measuring samples from the parts of various oxidation degrees in a lava flow.

Paleomagnetic dating of the smectite‐to‐illite conversion: Testing the hypothesis in Jurassic sedimentary rocks, Skye, Scotland

A hypothesized connection between clay diagenesis and magnetite authigenesis is supported by the results of paleomagnetic, rock magnetic, geochemical, and petrographic studies on Jurassic sedimentary rock of Skye, Scotland. The results have implications for understanding remagnetization mechanisms and for the development of a paleomagnetic method to date clay diagenesis. Previous diagenetic studies indicate that rock in north Skye contains abundant detrital smectite, whereas the clays in the same age rock in south Skye have altered to illite because of Tertiary igneous activity. Geochemical (87Sr/86Sr, δ13C, δ18O) studies confirm that sedimentary rocks in south Skye are altered. The magnetization in the rocks in north Skye is weak and unstable. In contrast, the rocks in south Skye contain a multicomponent magnetization. At intermediate temperatures (225°–450°C) a magnetization with southerly declinations and negative inclinations or its antipodal equivalent is present. At higher temperatures (450°–580°C) a magnetization with northerly declinations and positive inclinations or its antipodal equivalent is removed. Both magnetizations have directions similar to Tertiary igneous rocks on Skye. Rock magnetic studies indicate the presence of pyrrhotite and magnetite. The intermediate‐temperature component resides in pyrrhotite and/or magnetite and is interpreted as a thermoviscous remanent magnetization or a thermochemical remanent magnetization related to the heat and/or hydrothermal activity associated with the igneous activity. The high‐temperature component resides in magnetite and is a chemical remanent magnetization (CRM). The presence‐absence test and the timing of acquisition for this CRM suggest that magnetite authigenesis is related to the smectite‐to‐illite conversion and that clay diagenesis is a viable remagnetization mechanism.

A Palaeomagnetic Study of 143 Ma Kimberlite Dikes In Central New York State

SUMMARY A palaeomagnetic study of 143 Ma serpentinized kimberlite dikes near Ithaca, New York, yields a dual-polarity, high unblocking temperature, and high coercivity magnetization which passes the reversal test and two baked contact tests. The mean pole position (58N, 203E; A95 = 3.8, N = 7) differs from published late Jurassicearly Cretaceous North American poles currently used to define the apparent polar wander path. The angular dispersion in mean directions (&=3.5) is low but the presence of reversals argues that the Ithaca kimberlites magnetization should represent sufficient time for averaging of palaeosecular variation. Similar findings apparently typify palaeomagnetic studies of other serpentinized kimberlite, supporting the suggestion that thermo-chemical remanent magnetization in this lithology prolongs the duration of magnetization acquisition sufficiently to average secular variation per dike. A consistent but weak foliation in anisotropy of ARM parallels the N-S and vertical orientation of the Ithaca dikes, but is apparently unrelated to the northwest-down or southeast-up remanence. The Ithaca kimberlites pole may therefore record a previously undocumented sharp bend or 'cusp' at -143Ma, and the initiation of a Cretaceous and Cenozoic interval of apparent polar wander that generally follows a great circle along the 200E meridian to geographic north. A coeval (-145 Ma) kimberlite pole from southern Africa transferred to North America agrees with the Ithaca kimberlites pole position whereas reported poles from the Berriasian stratotype (southern France) and 144Ma Svalbard dolerites provide less diagnostic tests of the Ithaca kimberlites pole due in part to uncertainties in Europe-North America reconstructions.

A paleomagnetic study of the layered mafic intrusion at Sept-Îles, Quebec

A paleomagnetic study of gabbroic and anorthositic members of the Sept-Îles layered mafic intrusion has isolated a remanence with direction D = 333°, I = −29 °(remanence A) and a corresponding pole of 141 °E, 20°N. The rocks are cut by numerous diabase dykes of unknown age. The remanence carried by the dykes has direction D = 188°, I = −85 °(remanence B), with a corresponding pole of 116°E, 59°S. Another remanence, statistically identical to B (D = 186°, I = −85°), is found at dyke contacts and in the local host rock; its associated pole, 115°E, 61°S, is identical to the one derived from the dykes. This overprint is significantly different from remanence A, and is obviously related to dyke emplacement. The location of the pole derived from remanence A is in excellent agreement with many other Cambrian poles from a variety of locations throughout cratonic North America. The rocks at Sept-Îles have been previously dated radiometrically at 540 Ma. Thus, all evidence indicates that remanence A is an original thermochemical remanent magnetization, acquired during initial cooling of the intrusion in the Cambrian and prior to the emplacement of the dykes.