The Nature of Secondary Natural Magnetizations in Some Igneous and Baked Rocks

Wilson, R. L.; Smith, Peter J.

doi:10.5636/jgg.20.367

Cited by 31 publications

(9 citation statements)

References 15 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…Indeed, AF treatment fails in recovering the direction of the PRM of samples GS4 and Gs9 (Prevot 1975) in spite of the very low coercivity of the VRM. Because of the low blocking temperatures involved in the viscous magnetization process (Wilson & Smith 1968;Dunlop 1969;Biquand 1974;F'revot 1975;Dunlop & Hale 1977) heating in a zero field is the most powerful cleaning method for highly viscous volcanic rocks, whatever the coercivity of VRM.…”

Section: Discussionmentioning

confidence: 99%

Some aspects of magnetic viscosity in subaerial and submarine volcanic rocks

Prévot

1981

Geophysical Journal International

View full text Add to dashboard Cite

The magnetic viscosity of 334 Upper Tertiary and pre-Bruhnes Quaternary volcanic rocks from the Massif Central (France) and the Steens Mountain (Oregon), and of 40 basaltic cores from DSDP leg 37 has been investigated. Thellier's viscosity index follows a log normal distribution with mean values equal to 6 and 3.5 per cent for subaerial and submarine rocks respectively. For subaerial rocks, the average intensity of the viscous remanent magnetization (VRM) acquired in the Earth's field since the beginning of the Bruhnes polarity epoch (t = 0.7 Myr) is estimated to be equal to one-quarter of the average intensity of the primary remanence. Alternating field demagnetization of VRMs acquired in low fields for acquisition times t ranging from 2 day to 32 month indicates the resistance to alternating fields is quite different from sample to sample and increases linearly with log t.NBel's diagnostic parameter of domain structure of the grains involved in magnetic viscosity shows that hard VRM is carried by single-domain grains and soft VRM carried by multidomain particles. Single domain particles carrying hard VRM in subaerial volcanic rocks are almost equant magnetite intergrowths with size near the superparamagnetism threshold, resulting from high temperature oxidation of titanomagnetite. Soft VRM is carried by low Curie point homogeneous titanomagnetite. Unlike his single domain theory, Nkel's multidomain theory of magnetic viscosity does not account quantitatively for the resistance of VRM to alternating fields.

show abstract

Section: Discussionmentioning

confidence: 99%

Some aspects of magnetic viscosity in subaerial and submarine volcanic rocks

Prévot

1981

Geophysical Journal International

View full text Add to dashboard Cite

show abstract

“…Avchyan and Faustov [1966] noted that the VRM of certain red clays, which was extremely resistant to af demagnetization (see section 7h) was easily erased by heating to 100 øC for 1.5 hours. Briden [ 1965] An equation very similar to (23) has been used to predict that the maximum reheating temperature necessary to erase secondary NRM acquired viscously (at room temperature) by terrestrial rocks during the Brunhes epoch (•* 10 6 years) is about 300øC [Wilson and Smith, 1968] or, taking account of F(T), about 200øC . Thermal demagnetization is thus an extremely efficient means of erasing VRM.…”

Section: The Observed Field Dependence Of Magnetic Viscosity Is Not Pmentioning

confidence: 99%

Theory of the magnetic viscosity of lunar and terrestrial rocks

Dunlop¹

1973

Reviews of Geophysics

127

View full text Add to dashboard Cite

Lunar materials exhibit two distinct types of viscous or time-dependent magnetic behavior. Igneous rocks and largely recrystallized breccias, whose magnetic properties are due to multidomain iron, typically have weak magnetic viscosity, but decay persists for very long times following even a brief exposure to a field. Lunar soils and low metamorphic grade breccias, which contain an impor[ant fraction of metallic iron of singledomain and superparamagnetic size, generally acqtlire an anomalously strong viscous remanence, whose decay time is about equal to the 'time of exposure to the field. Four theories of magnetic viscosity are reviewed in this paper in an attempt to interpret the very different viscous properties of these two types of rocks. The Richter (1937) and N6el (1949) theories are appropriate to soils and low-grade breccias, whereas igneous and recrystal-liZed rocks are better described by the multidomain theories of N6el (1950) and Stacey (1963). Both the Stacey and N•el theories correctly predict logarithmic'magnetic viscosity, in spite of the fact that the central role played by the internal demagnetizing field in multidomain grains is ignored in N6el's formulation. This apparent paradox has been resolved. Analysis of the particularly simple Case of a two-domain particle shows that .the distribution of asymmetrical nonidentical potential barriers required by N•e! is automatically generated from the simpler distribution of symmetrical identical barriers proposed by Stacey through the action of the demagnetizing field. Experimental evidence on many facets of viscous magnetization, from terrestrial as well as lunar materials, is reviewed in detail before a final evaluation of the various theories is made. One interesting conclusion is tha.t the magnetic viscosity of multidomai n particles, although relatively weak, is still too strong to be explained by • displacements of entire domain walls. Either displacements .of small wall segments or rotation of pseudo-single-domain moments could account for the enhancement of magnetic viscosity in these particles.

show abstract

“…With its soft magnetic properties, the Neroly sandstone would be particularly vulnerable to acquiring VRM. In the laboratory, thermal demagnetization from 200° to 300°C will usually remove VRM [ Wilson and Smith , 1968; Dunlop , 1969] acquired by magnetite during the most recent polarity epoch. The natural remanent magnetization of the Neroly sandstone, however, is 90% removed after heating to 330°C, and an underlying component is not found.…”

Section: Discussionmentioning

confidence: 99%

Highly magnetic Upper Miocene sandstones of the San Francisco Bay area, California

Hillhouse

Jachens

2005

Geochem Geophys Geosyst

View full text Add to dashboard Cite

[1] A high-resolution aeromagnetic survey of the San Francisco Bay area shows prominent positive anomalies over distinctive blue sandstones of Late Miocene age. The total-field survey was measured at a nominal height of 300 m above the land surface along flight lines spaced 0.5 km apart. Anomalies with amplitudes up to 200 nT correlate with sandstones of the San Pablo Group, and these anomalies are similar in strength to the magnetic signatures of serpentinites and basalts in the surveyed region. Andesitic sandstone of the Neroly Formation, the upper part of the San Pablo Group, has high magnetic susceptibility (0.013 SI units, volume) and relatively strong natural remanent magnetization (0.29 A/m). Total magnetization of the sandstone is two thirds induced and one third remanent magnetization. The presence of coarse-grained magnetite detritus, low coercivity of remanence, low thermal stability of remanence, and multidomain properties is consistent with the NRM being a viscous remanent magnetization that grew during the Brunhes normal-polarity chron. The strong magnetic signature of the Upper Miocene sandstones allows their delineation over distances as great as 100 km, through areas where they are concealed by landslides and younger deposits. The sandstones are important structural markers for understanding the complex folding and faulting associated with active fault systems in the San Francisco Bay area.

show abstract

The Nature of Secondary Natural Magnetizations in Some Igneous and Baked Rocks

Cited by 31 publications

References 15 publications

Some aspects of magnetic viscosity in subaerial and submarine volcanic rocks

Some aspects of magnetic viscosity in subaerial and submarine volcanic rocks

Theory of the magnetic viscosity of lunar and terrestrial rocks

Highly magnetic Upper Miocene sandstones of the San Francisco Bay area, California

Contact Info

Product

Resources

About