Aleksey V. Smirnov

Return to Rodinia? Moderate to high palaeolatitude of the São Francisco/Congo craton at 920 Ma

Abstract Moderate to high palaeolatitudes recorded in mafic dykes, exposed along the coast of Bahia, Brazil, are partly responsible for some interpretations that the São Francisco/Congo craton was separate from the low-latitude Rodinia supercontinent at about 1050 Ma. We report new palaeomagnetic data that replicate the previous results. However, we obtain substantially younger U–Pb baddeleyite ages from five dykes previously thought to be 1.02–1.01 Ga according to the 40Ar/39Ar method. Specifically, the so-called ‘A-normal’ remanence direction from Salvador is dated at 924.2±3.8 Ma, within error of the age for the ‘C’ remanence direction at 921.5±4.3 Ma. An ‘A-normal’ dyke at Ilhéus is dated at 926.1±4.6 Ma, and two ‘A-normal’ dykes at Olivença have indistinguishable ages with best estimate of emplacement at 918.2±6.7 Ma. We attribute the palaeomagnetic variance of the ‘A-normal’ and ‘C’ directions to lack of averaging of geomagnetic palaeosecular variation in some regions. Our results render previous 40Ar/39Ar ages from the dykes suspect, leaving late Mesoproterozoic palaeolatitudes of the São Francisco/Congo craton unconstrained. The combined ‘A-normal’ palaeomagnetic pole from coastal Bahia places the São Francisco/Congo craton in moderate to high palaeolatitudes at c. 920 Ma, allowing various possible positions of that block within Rodinia.

Grain size dependence of low-temperature remanent magnetization in natural and synthetic magnetite: Experimental study

Magnetic measurements at cryogenic temperatures (<300 K) proved to be useful in paleomagnetic and rock magnetic research, stimulating continuous interest to low-temperature properties of magnetite and other magnetic minerals. Here I report new experimental results on a grain size dependence of the ratio (RLT) between a low-temperature (20 K) saturation isothermal remanent magnetization (SIRM) imparted in magnetite after cooling in a 2.5 T field (field cooling, FC) and in a zero field environment (zero field cooling, ZFC). Synthetic magnetite samples ranged in mean grain size from 0.15 to 100 μm, representing nearly single-domain (SD), pseudosingle-domain (PSD), and multidomain (MD) magnetic states. The RLT ratio monotonically increases from 0.58 to 1.12 with the decreasing mean grain size, being close to unity for PSD grains (0.15-5 μm) and smaller than unity for MD magnetite (12-100 μm). The RLT ratio of 1.27 is observed for acicular magnetite characterized by nearly SD behavior. These observations indicate that within the range of ~0.15 to ~5 μm, the low-temperature SIRM may be higher than that expected from “normal” magnetic domain wall displacement. Such a behavior can be caused by the presence of a SD-like component in the magnetization of these grains, which origin, however, is uncertain. The natural rocks containing nearly stoichiometric magnetite manifest a dependence of the RLT ratio on magnetic domain state identical to that observed from synthetic magnetites. Therefore, the comparison of FC SIRM and ZFC SIRM at very low temperatures may allow a crude estimate of magnetic domain state in some magnetite-bearing rocks, such as shallow mafic intrusions or some marine sediments.

Absolute geomagnetic paleointensity as recorded by ∼1.09 Ga Lake Shore Traps (Keweenaw Peninsula, Michigan)

Absolute geomagnetic paleointensity measurements were made on 255 samples from 38 lava flows of the ∼1.09 Ga Lake Shore Traps exposed on the Keweenaw Peninsula (Michigan, USA). Samples from the lava flows yield a well-defined characteristic remanent magnetization (ChRM) component within a ∼375°C–590°C unblocking temperature range. Detailed rock magnetic analyses indicate that the ChRM is carried by nearly stoichiometric pseudo-single-domain magnetite and/or low-Ti titanomagnetite. Scanning electron microscopy reveals that the (titano)magnetite is present in the form of fine intergrowths with ilmenite, formed by oxyexsolution during initial cooling. Paleointensity values were determined using the Thellier double-heating method supplemented by low-temperature demagnetization in order to reduce the effect of magnetic remanence carried by large pseudosingle-domain and multidomain grains. One hundred and two samples from twenty independent cooling units meet our paleointensity reliability criteria and yield consistent paleofield values with a mean value of 26.3 ± 4.7μT, which corresponds to a virtual dipole moment of 5.9 ± 1.1×1022 Am2. The mean and range of paleofield values are similar to those of the recent Earth’s magnetic field and incompatible with a “Proterozoic dipole low”. These results are consistent with a stable compositionally-driven geodynamo operating by the end of Mesoproterozoic.

Intrinsic paleointensity bias and the long-term history of the geodynamo

A key mechanism responsible for a systematic bias in the global geomagnetic paleointensity database. Many geodynamo models predict an inverse relationship between geomagnetic reversal frequency and field strength. However, most of the absolute paleointensity data, obtained predominantly by the Thellier method from bulk volcanic rocks, fail to confirm this relationship. Although low paleointensities are commonly observed during periods of high reversal rate (notably, in the late Jurassic), higher than present-day intensity values are rare during periods of no or few reversals (superchrons). We have identified a fundamental mechanism that results in a pervasive and previously unrecognized low-field bias that affects most paleointensity data in the global database. Our results provide an explanation for the discordance between the experimental data and numerical models, and lend additional support to an inverse relationship between the reversal rate and field strength as a fundamental property of the geodynamo. We demonstrate that the accuracy of future paleointensity analyses can be improved by integration of the Thellier protocol with low-temperature demagnetizations.

Paleomagnetism and U–Pb geochronology of the Black Range dykes, Pilbara Craton, Western Australia : a Neoarchean crossing of the polar circle

ABSTRACT We report a new paleomagnetic pole for the Black Range Dolerite Suite of dykes, Pilbara craton, Western Australia. We replicate previous paleomagnetic results from the Black Range Dyke itself, but find that its magnetic remanence direction lies at the margin of a distribution of nine dyke mean directions. We also report two new minimum ID-TIMS 207Pb/206Pb baddeleyite ages from the swarm, one from the Black Range Dyke itself (>2769 ± 1 Ma) and another from a parallel dyke whose remanence direction lies near the centre of the dataset (>2764 ± 3 Ma). Both ages are slightly younger than a previous combined SHRIMP 207Pb/206Pb baddeleyite weighted mean date from the same swarm, with slight discordance interpreted as being caused by thin metamorphic zircon overgrowths. The updated Black Range suite mean remanence direction (D = 031.5°, I = 78.7°, k = 40, α95 = 8.3°) corresponds to a paleomagnetic pole calculated from the mean of nine virtual geomagnetic poles at 03.8°S, 130.4°E, K = 13 and A95 = 15.0°. The poles reliability is bolstered by a positive inverse baked-contact test on a younger Round Hummock dyke, a tentatively positive phreatomagmatic conglomerate test, and dissimilarity to all younger paleomagnetic poles from the Pilbara region and contiguous portions of Australia. The Black Range pole is distinct from that of the Mt Roe Basalt (or so-called ‘Package 1’ of the Fortescue Group), which had previously been correlated with the Black Range dykes based on regional stratigraphy and imprecise SHRIMP U–Pb ages. We suggest that the Mt Roe Basalt is penecontemporaneous to the Black Range dykes, but with a slight age difference resolvable by paleomagnetic directions through a time of rapid drift of the Pilbara craton across the Neoarchean polar circle.

Paleomagnetism of the ~1.1 Ga Coldwell Complex (Ontario, Canada): Implications for Proterozoic geomagnetic field morphology and plate velocities

We report new paleomagnetic data from the ~1108 Ma intrusive Coldwell Complex (Ontario, Canada) to investigate the apparent reversal asymmetry observed in some Midcontinent Rift (MCR) rocks. The rocks of eastern and central part of the complex are reversely magnetized with a group mean direction of D = 114.8°, I = −63.7° (α95 = 3.6°, N = 30). The corresponding paleomagnetic pole at Plat = 47.2°N, Plong = 206.5°E (A95 = 4.8°) is located close to the paleomagnetic poles from nearly coeval reversely magnetized rocks of the MCR system, including the lower lava flows at Mamainse Point. The rocks of western part of the complex are normally magnetized with a group mean direction (D = 298.0°, I = 56.9°, α95 = 5.8°, N = 10) that passes the reversal test with respect to the reversed group mean direction. Our results do not support the previous model in which the complex was emplaced during two periods of reversed geomagnetic field polarity separated by a period of normal polarity and hence encompasses two geomagnetic reversals. Instead, our new data indicate that the Coldwell Complex records only two polarity intervals separated by a symmetrical reversal at ~1102–1105 Ma. This reversal is likely equivalent to the lowermost reversal recorded at Mamainse Point and provides further evidence that the apparent reversal asymmetry reflects a plate motion rather than a persistent nondipole field geometry. Together with a high-quality data from the ~1098 Ma North Shore Volcanics, our data indicate a rapid velocity of Laurentia at ~25 ± 4 cm/yr. The fast plate motion may reflect a decreased mantle drag due to vigorous mantle indicated by widespread intraplate magmatism at ~1.1 Ga.

Development of a low‐temperature insert for the measurement of remanent magnetization direction using superconducting quantum interference device rock magnetometers

Data on the directional changes of a full magnetization vector during cycling to cryogenic temperatures can provide important insights into the low-temperature magnetic properties of natural and synthetic materials. These data also provide an empirical basis for the application of low-temperature treatments in paleomagnetism, for example, the removal of viscous magnetization in magnetite-bearing rocks. However, existing instruments only allow continuous measurement of magnetization along a single axis, hampering experimental and theoretical advances in rock magnetism and the implementation of low-temperature techniques into regular paleomagnetic practices. Here we describe development of a novel low-temperature insert designed in collaboration with William S. Goree Inc., which allows measurement of directional behavior of a full magnetization vector during zero-field low-temperature cycling. Pilot experiments on well-controlled polycrystalline samples of pseudo-single-domain (PSD) and multidomain magnetite as well as on a natural sample containing PSD magnetite indicate that the orientation of a saturation isothermal remanent magnetization (SIRM) imparted at room temperature remains constant during low-temperature cycling to 20 K. This observation lends additional support to low-temperature cycling as a cleaning technique in paleomagnetism. The SIRM imparted in an individual crystal of magnetite showed systematic, albeit small changes upon both cooling and warming through the Verwey temperature, which may reflect switching between the easy magnetization directions. However, the switching effect may be significantly attenuated by crystallographic twinning in magnetite below the transition. Overall, our results demonstrate the potential of the directional low-temperature magnetometry for the advancement of our understanding of the properties of natural and synthetic materials.

Paleomagnetism and Geochemistry of ~1144‐Ma Lamprophyre Dikes, Northwestern Ontario: Implications for the North American Polar Wander and Plate Velocities

We present new paleomagnetic and geochemical data from a suite of the ~1144-Ma ultramafic lamprophyre dikes that outcrop in the Canadian Shield northeast of Lake Superior (Ontario, Canada). Nineteen of 22 sampled dikes yielded consistent characteristic remanent magnetization directions of normal (n = 5) and reversed (n = 14) polarity. The primary origin of characteristic remanent magnetization is bolstered by positive baked contact tests and a reversal test. The group mean direction (D = 306.4°, I = 72.1°, α95 = 5.5°, N = 19) obtained from the lamprophyre dikes is statistically indistinguishable from the group mean direction (D = 297.4°, I = 65.5°, α95 = 8.3°, N = 8) previously reported for the nearly coeval ~1142-Ma Abitibi dikes. The geochemistry of the lamprophyre dikes suggests strong affinity with magmas derived from ocean island basalt-type mantle sources, consistent with the mantle plume hypothesis for the formation of the ~1.1-Ga North American Midcontinent Rift. The similarity in age, trend, paleomagnetism, and geochemistry indicates that the lamprophyre and Abitibi dike suites represent the earliest magmatic event associated with the commencement of rifting. The combined mean direction (D = 303.1°, I = 70.2°, α95 = 4.5°, N = 27) corresponds to a paleomagnetic pole at Plat = 55.8°N, Plong = 220.0°E (A95 = 7.3°). The new pole merits the highest classification on the Q-scale of paleomagnetic reliability and represents a key pole defining the North American apparent polar wander path during the late Mesoproterozoic. Combined with high-quality data from the ~1108-Ma Coldwell Complex, our data indicate an equatorward motion of Laurentia at 3.8 ± 1.4 cm/year, comparable with the present-day velocities of continental plates, before switching to extremely rapid motion between ~1108 and ~1099 Ma. Plain Language Summary Similar to a magnetic tape, rocks can retain the direction of ancient Earth’s magnetic field. Scientists use this record (known as paleomagnetism) to reconstruct past positions of continents and to decipher the geological history of our planet. We investigated paleomagnetism and chemical composition of the ~1.14 Ga-old intrusive rocks called lamprophyres exposed in Northwestern Ontario (Canada). We found that the paleomagnetic field directions recorded in lamprophyres are indistinguishable from those recorded by another similar age suite of basaltic intrusions called the Abitibi dikes, from the same area. The combined data from these rocks allowed us to constrain the position of an ancient supercontinent called Laurentia at ~1.14 billions of years ago more accurately than it was possible before. Our results convincingly show that, during that time, Laurentia moved with a velocity comparable to present-day plate velocities, before switching to an extremely rapid motion approximately 35 millions of years later. The lamprophyre and Abitibi rocks also share similar chemical signatures, close to those observed for ocean island basalts (e.g., Hawaii). These observations support the hypothesis that a failed ocean opening attempt called the North American Midcontinent Rift was instigated by the arrival of a hot mantle material upwelling to the Earth surface.

Nonheating methods for absolute paleointensity determination: Comparison and calibration using synthetic and natural magnetite-bearing samples

Nonheating paleointensity methods utilize an anhysteretic remanent magnetization (ARM) or a saturation isothermal remanent magnetization to model the natural thermal remanent magnetization (TRM) to avoid heating-induced alteration. We report the results of paleointensity experiments using the ARM, pseudo-Thellier, and ratio of equivalent magnetization (REM) methods conducted to investigate their relative efficiency in recovering the true paleofield strength and to provide additional estimates of their calibration factors. The experiments on synthetic magnetite-bearing samples representing single-domain (SD) and pseudo-single-domain (PSD) magnetic states indicated that the correction factors for the ARM-based methods depend on the magnetic grain size/domain state changing from ~6.3 (for SD grains) to ~4.1 (for ~1.5 µm PSD grains). The pseudo-Thellier method yielded correct absolute paleointensity values when normalization by the TRM/ARM demagnetization slope was used. When applied to samples of lava flows and dikes from the ~32 kyr Lemptegy volcano (France), both the ARM and pseudo-Thellier methods produced similar paleointensity estimates (28.0 ± 5.1 μT and 26.9 ± 4.7 μT, respectively) consistent with the available Thellier data for the 31–33 kyr time interval. The correction factors estimated from our synthetic samples for the REM (~3000) and for REMc (~1500) and REM′ (~1500) variants are consistent with the previously published estimates. However, all REM variants yielded unrealistically high estimates (>110 μT) of the paleofield strength from our natural samples. Our experimental results support ARM as a better proxy of TRM and suggest that the ARM-based methods currently represent the best alternative to heating-based absolute paleointensity method.