Paleomagnetism of Archean rocks from northwestern Ontario: III. Rock magnetism of the Shelley Lake granite, Quetico Subprovince

1984 ◽  
Vol 21 (8) ◽  
pp. 879-886 ◽  
Author(s):  
David J. Dunlop ◽  
Larry D. Schutts ◽  
Christopher J. Hale
Keyword(s):  
Remanent Magnetization ◽  
Rock Magnetism ◽  
Natural Remanent Magnetization ◽  
Thermal Decay ◽  
Magnetic Phases ◽  
Northwestern Ontario ◽  
Component Type ◽  
Thermal Overprint

The Shelley Lake granite of northwestern Ontario contains five magnetic phases: deuteric and post-crystallization hematites, which are relatively abundant but carry only 1–4% of the natural remanent magnetization (NRM); primary magnetite in coarse (50–500 μm) grains, both optically homogeneous and subdivided by hematite lamellae; micrometre-size secondary magnetite in chloritized biotites; and submicrometre-size magnetite, whose presence is inferred from low blocking temperatures in thermal decay curves of the NRM. The NRM is a composite of type 1 and type 2 remanences, which differ in direction by about 90° (see companion paleomagnetic paper). Both NRM components occur in normal (N) and reverse (R) polarities. Type 1 remanences (1N/1R) have the hallmarks of multidomain (MD) behaviour: high blocking temperatures but low coercivities, exponential alternating field (AF) decay curves, generally MD results of the Lowrie–Fuller test, and MD to transitional values (0.3–10) of the Koenigsberger Qn ratio. Furthermore, intensities of 0.6 Oe (0.06 mT) laboratory thermoremanent magnetizations (TRM's) match those of 1R and some 1N NRM's. We argue on this evidence that 1R and at least part of 1N NRM's are TRM's residing in coarse MD-size primary magnetite. This primary TRM dates from initial cooling of the Shelley Lake pluton around 2580 Ma. Thermal decay spectra of single-component type 2 NRM's (2N/2R) resemble those of 1R. However, the considerable overlap of 2N/2R and 1R blocking temperatures in multivectorial NRM's demonstrates that type 2 remanence must be a chemical or thermochemical rather than a thermal overprint.

Paleomagnetism of Archean rocks from northwestern Ontario: IV. Burchell Lake granite, Wawa–Shebandowan Subprovince

1984 ◽  
Vol 21 (10) ◽  
pp. 1098-1104 ◽  
Author(s):  
David J. Dunlop
Keyword(s):  
Natural Remanent Magnetization ◽  
Reverse Polarity ◽  
Polar Wander ◽  
Apparent Polar Wander Path ◽  
Northwestern Ontario ◽  
Normal Polarity ◽  
The Mean ◽  
Low K

The late Archean Burchell Lake granite of the Shebandowan greenstone belt in northwestern Ontario has a characteristic natural remanent magnetization (NRM) resembling the type 1 NRM of the nearby Shelley Lake granite of the Quetico gneiss belt. Of 36 stably magnetized samples, 21 had predominantly normal polarity (1N) NRM and 15 had reverse polarity (1R). The mean direction based on stable end-point and vector subtracted directions is D = 2.3 °I = 48.9 °(k = 23.9, α95 = 10.0°, N = 10 sites). Intersecting remagnetization circles gave a similar direction. The corresponding paleopole BL1 lies at 83.2°E, 71.1°N, near track 6 of the Laurentian apparent polar wander path around 2600 Ma. Although the Burchell Lake pluton is not dated radiometrically, neighbouring granites give K/Ar biotite ages of 2550–2600 Ma, and it is reasonable to suppose that the NRM dates from the time of intrusion or shortly thereafter. A type 2 NRM like that of the Shelley Lake granite was isolated in 12 samples. Its mean direction is D = 80.9 °I = −11.1°, but the precision is unacceptably low (k = 7.7).

Paleomagntism of Archean rocks from northwestern Ontario: II. Shelley Lake granite, Quetico Subprovince

1984 ◽  
Vol 21 (8) ◽  
pp. 869-878 ◽  
Author(s):  
David J. Dunlop
Keyword(s):  
Dipole Field ◽  
Polar Wander ◽  
Orthogonal Vector ◽  
Site Type ◽  
Apparent Polar Wander Path ◽  
Northwestern Ontario ◽  
Heating Event ◽  
The Mean

The 2580 ± 20 Ma Shelley Lake granite of the Quetico gneiss belt in northwestern Ontario preserves two distinct natural remanent magnetizations (NRM's) of different ages. Type 1 NRM of either normal (1N) or reverse (1R) polarity was isolated in 43 samples, using three methods: stable end-point directions, orthogonal vector plots, and converging remagnetization circles. The mean direction based on both 1N and 1R results is D = 5.9 °I = +56.4 °(k = 95.4, α95 = 4.4°, N = 12 sites), which is significantly different from both the present Earth's field (PEF) and axial dipole field directions at the site. Type 2 NRM, either normal (2N) or reverse (2R), was successfully separated from 1N/1R in 13 samples. Its mean direction, combining both polarities, is D = 70.3 °I = −27.8 °(k = 20.1, α95 = 9.4°, N = 13 samples).The paleopole SL1 corresponding to 1N/1R falls at 65.8°E, 77.6°N, near track 6 of the Laurentian apparent polar wander path (APWP) around 2600 Ma. This remanence is carried principally by primary multidomain magnetite with blocking temperatures up to 580 °C. If it is a primary thermoremanence, as is argued here, SL1 provides a well dated tie point for the APWP at 2580 ± 20 Ma.Paleopole SL2 corresponding to 2N/2R falls at 157.1°W, 1.3°S, around 1250 Ma on the APWP. The remanence is probably a chemical or thermochemical overprint acquired during a mild heating event (T < 300 °C, t > 1100 Ma) that reset 40Ar/39Ar feldspar ages. An earlier tentative assignment of SL2 to track 6 around 2800 Ma is incorrect.

Paleomagnetism of Archean rocks from northwestern Ontario: Wabigoon gabbro, Wabigoon Subprovince

1983 ◽  
Vol 20 (12) ◽  
pp. 1805-1817 ◽  
Author(s):  
David J. Dunlop
Keyword(s):  
Remanent Magnetization ◽  
Natural Remanent Magnetization ◽  
Blocking Temperature ◽  
Virtual Geomagnetic Pole ◽  
Polar Wander ◽  
Apparent Polar Wander Path ◽  
Northwestern Ontario ◽  
Paleomagnetic Pole ◽  
Geomagnetic Pole ◽  
Chemical Remanent Magnetization

The Wabigoon gabbro of the Archean Wabigoon greenstone belt in northwestern Ontario preserves a univectorial natural remanent magnetization (NRM) with D = 246°, I = 12° (k = 19.5, α95 = 10.5°, N = 11 sites). The precision is reduced if sample means are averaged, however (k = 9.3, α95 = 9.2°, N = 29 samples). The paleomagnetic pole falls either at 160°W, 11°S (δp = 5.3°, δm = 10.6°), corresponding to an age of ~1300 Ma on the Laurentian apparent polar wander path, or the reverse of this, 20°E, 11° N, corresponding to a late Archean age (~2800 Ma). No ~1300 Ma igneous or metamorphic event is known in the area; a major west-northwest-trending dike about 9 km south of the gabbro yields a virtual geomagnetic pole at 122°W, 45°N and seems to be of Abitibi age (~2150 Ma) rather than Mackenzie age (~1250 Ma). A few gabbro samples and some greenstones from the intrusive baked zone have hybrid remanences in which a higher blocking temperature Kenoran-age (~2600 Ma) NRM is superimposed on the gabbro characteristic NRM. However, the Kenoran component may be a younger chemical remanent magnetization (CRM) residing in hematite. The hypothesis that the gabbro characteristic remanence is itself a hybrid of Kenoran and Keweenawan (~1100 Ma) NRM's, which would explain both the high between-sample scatter and the lack of a ~1300 Ma remagnetizing event, is considered but rejected because fewer than 10% of the gabbro samples exhibit multivectorial swings during alternating field or thermal cleaning. Two geomagnetic field reversals are recorded at interior sites, but only one or none is recorded near the margin of the intrusion. The different cooling histories of margin and interior, as well as the bulk of the other evidence, favour magnetization during initial cooling in late Archean time.

scholarly journals On the Primordial Condensation and Accretion Environment and the Remanent Magnetization of Meteorites

1971 ◽  
Vol 13 ◽  
pp. 311-329 ◽  
Author(s):  
Aviva Brecher
Keyword(s):  
Remanent Magnetization ◽  
Rock Magnetism ◽  
Laboratory Data ◽  
Natural Remanent Magnetization ◽  
Parent Body ◽  
Origin And Evolution ◽  
Consistent Model ◽  
Magnetic Remanence ◽  
Condensation Growth ◽  
Theoretical Results

Attention is drawn to the fact that neither astronomical observations, nor laboratory data can, as yet, sufficiently constrain models of the origin and evolution of the solar system. But, if correctly approached and interpreted, the magnetic remanence of meteorites could help in constructing a self-consistent model.In the context of various models for the early evolution of a solar nebula, the possible roles assigned to ambient magnetic fields and the paleointensities required to establish the stable natural remanent magnetization (NRM in range 10-4 to 10-1 cgsm) observed in meteorites, are discussed. It is suggested that the record of paleofields present during condensation, growth, and accumulation of grains is likely to have been preserved as chemical (CRM) or thermochemical (TCRM) remanence in unaltered meteoritic material. This interpretation of the meteoritic NRM is made plausible by experimental and theoretical results from the contiguous fields of rock magnetism, magnetic materials, interstellar grains, etc. Several arguments (such as the anisotropy of susceptibility in chondrites, suggesting alignment of elongated ferromagnetic grains, or the characteristic sizes and morphology of carrier phases of remanence, etc.) as well as general evidence from meteoritics (cooling rates, chemical and mineralogical data) can be used to challenge the interpretation of NRM as thermo-remanence (TRM) acquired on a “planetary” parent body during cooling of magnetic mineral phases through the Curie point in fields of 0.2 to 0.9 Oe.Fine-particle theories appear adequate for treating meteoritic remanence, if models based on corresponding types of permanent magnet materials, e.g., powder-ferrites for chondrites; diffusion hardened alloys for iron meteorites, are adopted, as suggested here.Finally, a potentially fruitful sequence of experiments is suggested for separating the useful component of NRM in determining the paleofield intensity and its time evolution.

Rock magnetism as a tool for investigating the use of archaeological artefacts from baked clay

2021 ◽  
Author(s):  
Evdokia Tema ◽  
Enzo Ferrara ◽  
Lorenzo Zamboni ◽  
Marica Venturino ◽  
Margherita Reboldi ◽  
...  
Keyword(s):  
Iron Age ◽  
Remanent Magnetization ◽  
Rock Magnetism ◽  
Magnetic Behavior ◽  
Archaeological Site ◽  
Natural Remanent Magnetization ◽  
Rock Magnetic Properties ◽  
Ring Shape ◽  
Magnetic Analysis

&lt;p&gt;Even though multidisciplinary approaches applied to the investigation of archaeological findings are widely used, the use of rock magnetic properties is still poorly exploited in the determination of the use of ancient artefacts. In this study, we present the results of a combined archaeological, morphological and magnetic analyses applied on the ring-shape clay artefacts found at the archaeological site of Villa del Foro, in Northern Italy. The materials studied are dated between the sixth and the first half of fifth century BC and are found in large quantities in different trenches of the archaeological excavation. To investigate their thermal history and to exploit their possible use as kiln supports, cooking stands, or loom weights, we have investigated their natural remanent magnetization (NRM) and the magnetic mineralogy changes occurred during laboratory heating. Magnetic analysis used for the determination of the firing temperatures show thermal stability up to 500-600 &lt;sup&gt;o &lt;/sup&gt;C, while further laboratory heating at 700 &lt;sup&gt;o &lt;/sup&gt;C introduces magnetic alteration. Thermal demagnetization of the samples generally shows a strong and stable thermal remanent magnetization. In few cases, a clear secondary component is present, suggesting partial re-heating or displacement at temperatures ranging from 200 &lt;sup&gt;o&lt;/sup&gt;C to 450 &amp;#176;C. Such secondary magnetic component can be indicative of a secondary heating or of a displacement of the rings from their initial firing position while still hot. Even though the studied rings belong to casually different morphological typologies, no connection among type and magnetic behavior was observed, suggesting that the ring&amp;#8217;s morphology does not define neither their production conditions nor the final use of the artefacts. The estimated firing temperatures of around 600-700 &lt;sup&gt;o&lt;/sup&gt;C are compatible with the heating of the rings during their manufacture rather than related to cooking activities. In combination with the archaeological evidence and the morphological analysis it is thus suggested that the rings were used as weight looms and baked only during their production procedures. Such a pilot study can be used as reference for the identification of similar objects found in Italy and Europe during the Iron Age and confirms the great potential of rock magnetic analysis in the investigation of the technology and use of ancient baked clays.&lt;/p&gt;

Viscosité magnétique potentielle, aimantation détritique et viscosité réelle d'un sédiment lacustre quaternaire

1987 ◽  
Vol 24 (9) ◽  
pp. 1903-1912 ◽  
Author(s):  
Daniel Biquand ◽  
François Sémah
Keyword(s):  
Remanent Magnetization ◽  
Rock Magnetism ◽  
Viscosity Index ◽  
Natural Remanent Magnetization ◽  
Blocking Temperature ◽  
Magnetic Viscosity ◽  
Paleomagnetic Study ◽  
Temperature Spectra

The magnetic viscosity of sediments, as indicated by thermal demagnetization of natural remanent magnetization (NRM), depends on two main parameters: (i) the specific magnetic viscosity of the material and (ii) the efficiency of the primary magnetization process. In an attempt to determine the relative importance of these two variables, we studied a Lower Pleistocene lacustrine sequence bearing a primary reversed detrital remanent magnetization (DRM).Using natural samples and small cores made of crushed sediment, our study is based on the thermodynamic theory of rock magnetism developed by L. Néel, who established an equivalence between time and temperature, that is, between viscous remanent (VRM) and thermoremanent (TRM) magnetization processes. The determination of the blocking temperature spectra from 20 to 152 °C allows us to calculate the maximum theoretical VRM acquired in situ at each horizon, while the detailed thermal study of the NRM permits an appraisal of the DRM quality. This leads us to define a geological viscosity index, which accounts, in a rather convincing manner, for the behaviour of the samples observed during the classical paleomagnetic study. For the section studied, it appears that the variations of this index are closely correlated with the efficiency of the DRM acquisition process.

THERMOMAGNETIC ANALYSIS AND ORE GENESIS

1967 ◽  
Vol 4 (6) ◽  
pp. 1119-1125
Author(s):  
E. J. Schwarz
Keyword(s):  
Remanent Magnetization ◽  
Rock Magnetism ◽  
Thermal Conditions ◽  
Natural Remanent Magnetization ◽  
Ore Deposits ◽  
Ore Genesis ◽  
Thermomagnetic Analysis ◽  
Stable Part ◽  
Physical Changes ◽  
Genesis Of Ore Deposits

The potential of rock magnetism in studies on the genesis of ore deposits is examined. It is suggested that techniques based on magnetic properties other than the direction of the stable part of the natural remanent magnetization might prove usefully applicable. More specifically the analysis of the type of remanent magnetization in ores and their wall rocks is suggested as a worthwhile approach in the study of ore genesis. Other methods suggested are based on the occurrence of chemical or physical changes affecting ferromagnetic minerals in ores during heating. It may be possible to relate the results of such experiments to the thermal conditions prevailing at the time of either formation of minerals in ore deposits or acquisition of stable remanent magnetization.

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