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

Смирнов, А. В.; Tarduno, J. A.

doi:10.1029/2004jb003445

Cited by 78 publications

(63 citation statements)

References 92 publications

(140 reference statements)

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“…The potential for biasing caused by the remanence carriers undergoing chemical changes below their Curie temperature during the original cooling of the unit (and therefore retaining a chemical or thermochemical remanence) is, as for many studies, not so well constrained. The nature of such effects and the extent to which they occur is presently an area of active research (Smirnov and Tarduno, 2005;Draeger et al, 2006;Yamamoto, 2006). Here, we merely point out that the results of the rock magnetic analysis are consistent with the units having acquired a purely thermal remanence.…”

Section: Discussionmentioning

confidence: 56%

The intensity of the geomagnetic field in the late-Archaean: new measurements and an analysis of the updated IAGA palaeointensity database

2009

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We firstly present the results of a detailed palaeointensity study performed on 54 samples from 9 volcanic units of late Archaean age (2724-2772 Ma) from the Pilbara Craton, Western Australia. These results were severely affected by magnetomineralogical alteration occurring during the laboratory heating process necessitating the application of a correction procedure. The correction allowed results from three lavas to pass strict selection criteria but we deem that only one of these exhibits sufficient internal consistency to be considered moderately reliable. It yields a virtual dipole moment of 47±6 ZAm 2 which is 60% of the present-day value. We combine this determination with a filtered dataset from the updated IAGA (International Association of Geomagnetism and Aeronomy) palaeointensity database, PINT08. Directional secular variation has recently been shown to have changed fundamentally since the Archaean, probably as a consequence of inner core growth since that time. However, here we argue that it is still unclear whether this evolution was accompanied by a single long timescale change in average poloidal field intensity. While the distribution of Precambrian palaeointensity determinations as a whole is significantly lower than that for the last 300 Myr, we show that this finding largely reflects data from the Proterozoic aeon. The distribution of more ancient measurements from the late Archaean-earliest Proterozoic is indistinguishable from that of the last 300 Myr which might suggest that a 'Proterozoic dipole low' period existed between two periods of higher field intensity. Were this pattern of long-term geomagnetic intensity variation to be supported by the addition of new data in the future, then it could indicate a related three-stage evolution in core dynamics, namely: vigorous thermal convection caused by high core-mantle heat flux early in the Earth's history, weaker thermal convection later as the heat flux fell, and finally, strong compositional convection since the inner core nucleated.

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Section: Discussionmentioning

confidence: 56%

The intensity of the geomagnetic field in the late-Archaean: new measurements and an analysis of the updated IAGA palaeointensity database

2009

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“…The correct identification of magnetic mineralogy is crucial for absolute paleointensity interpretation (Smirnov and Tarduno, 2005). The samples selected for Thellier paleointensity experiments should necessarily carry primary TRM (thermoremanent magnetization).…”

Section: Resultsmentioning

confidence: 99%

Early cretaceous absolute geomagnetic paleointensities from Córdoba Province (Argentina)

et al. 2006

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We present here new paleointensity and geochronology results from Early Cretaceous volcanic rocks of Sierra Chica de Cordoba (Argentina). The new K-Ar isotopic ages of 5 samples range from 136 to 122 Ma. Twenty five samples from 7 individual flows yielded acceptable paleointensity estimates. The mean paleointensity values per flow are ranging from 53.0 ± 1.9 to 25.4 ± 2.6 μT and the corresponding Virtual Dipole Moments (VDMs) are ranging from 9.3±1.3 to 4.6±0.5 (10 22 Am 2 ). This corresponds to the mean value of 7.3±1.7×10 22 Am 2 , which is compatible to the present geomagnetic axial dipole. Currently available selected paleointensity data from 80 to 130 Ma suggest that geomagnetic field strength frequently fluctuated before and during the Cretaceous Normal Superchron while the magnetic polarity maintained stable. The mean paleointensities derived from Córdoba lavas agree remarkably well with those obtained from the Paraná Magmatic Province (133-132 Ma). This reinforces the hypothesis about the unreliability of 'Mesozoic Dipole Low'.

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“…SEM and low-temperature magnetic analyses indicated that these phases are nearly stoichiometric magnetite and ilmenite produced by oxyexsolution (Smirnov and Tarduno, 2005). The sample manifests PSD hysteresis behavior ( Fig.…”

Section: Dmentioning

confidence: 99%

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

Смирнов

2009

Earth Planet Sp

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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 (R LT ) between a lowtemperature (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), pseudosingledomain (PSD), and multidomain (MD) magnetic states. The R LT 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 R LT 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 R LT 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.

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

Cited by 78 publications

References 92 publications

The intensity of the geomagnetic field in the late-Archaean: new measurements and an analysis of the updated IAGA palaeointensity database

The intensity of the geomagnetic field in the late-Archaean: new measurements and an analysis of the updated IAGA palaeointensity database

Early cretaceous absolute geomagnetic paleointensities from Córdoba Province (Argentina)

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

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