Geol 319: Important Dates

Wednesday, Oct 3rd – Last magnetics lecture

Wednesday afternoon – Midterm review (4:30 pm, M 210)

Friday, Oct 5th – Midterm exam

Week of Oct 8th – 14th: - No lectures for the week

Rock magnetismAll rock magnetism is related to dipole moments at atomic scales

Contributions to magnetization arise from

1. Dipole moments of electron “spin”

2. Dipole moments of the electron orbital shells

If these dipole moments are organized, the macroscopic crystal will be magnetically susceptible.

There are several varieties of macroscopic magnetization:

• Diamagnetism• Paramagnetism• Ferromagnetism• Anti-ferromagnetism• Ferrimagnetism

Rock magnetism

Diamagnetism

• All material is diamagnetic, but this is only evident for matter that has no other magnetism

• Prevails if net electron spin dipole is zero (no unpaired electrons)

• Orbital dipole moments self-organize – oppose the external magnetic field

• Result is a small, net negative magnetic susceptibility

• Naturally occurring materials with detectable diamagnetism are gypsum, salt, quartz and graphite

Rock magnetismDiamagnetism

• Weak effect, but nearly universal

• Water, organic material are diamagnetic

• In a strong magnetic field the magnetic repulsion caused by diamagnetism will “levitate” inanimate objects

• University of Nijmegan has many examples

http://www.hfml.sci.kun.nl/froglev.html

(note 16 Tesla field strength!)

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Rock magnetism

Para-magnetism

• Caused by effect of unpaired electrons

• Spin moments do not cancel

• Presence of external field aligns dipoles

• Net positive, weak susceptibility

• Magnetic rocks that are heated above the “Curie” temperature are paramagnetic, change to ferrimagnetic or antiferromagnetic as they cool

• As in diamagnetism, the dipoles do not interact with each other – the effect remains weak

Rock magnetism

Ferro-magnetism

• Like paramagnetism, caused by effect of unpaired electrons

• Dipole moments are now coupled through lattice structures

• Coupling creates aligned magnetic “domains”

• External field induces domains to become aligned with each other

• Alignment can be retained even after removal of external field (“hard magnetism”, or remenant magnetism)

• Extremely large, positive susceptibility 106 times as strong as diamagnetism and paramagnetism

• Occurs in pure iron, nickel, cobalt – these are not naturally occurring compounds

Rock magnetism

Anti-ferromagnetism

• Coupling between adjacent domains is anti-parallel

• Net magnetization is theoretical zero

• Lattice defects break perfect anti-symmetry, causing “parasitic anti-ferromagnetism”

• Intermediate, positive susceptibility

• Most notable example in nature is hematite (Fe2O3 )

• Susceptibility of iron oxides is variable, and much stronger if magnetite (Fe2O3 ) is present

Rock magnetism

Ferrimagnetism

• Like anti-ferro magnetism, coupling between adjacent domains is anti-parallel

• Here one set of domains dominates, with either:

1. Unequal numbers of domains (magnetite, Fe2O3 )

2. Or, one set of domains is stronger (pyrrohotite, Fe2S1+x)

Causes very strong, positive susceptibility

Rock magnetism

The three types of magnetism above are only possible when magnetic domains are present.

Ferromagnetism

Antiferromagnetism

Ferrimagnetism

Rock magnetismRemanent magnetism

• In ferromagnetism, anti-ferromagnetism and ferrimagnetism, the material can acquire a permanent magnetization

• Recall, we assumed:

• This is a straight line relationship

• In reality shape of the M vs H curve is not straight, and it depends on the history of the magnetization

• The effect is known as “hysterisis”

• The magnetization left at zero external field is the remanent magnetism

Rock magnetismRemanent magnetization

Remanent magnetization is first acquired when the rock formed. Several mechanisms for this are recognized:

Thermoremenant magnetization: Arises when magnetic material cools below the “Curie” point (at which paramagnetism becomes ferrimagnetism)

Detrital magnetization: Occurs during the slow settling of fine grained particles (clays and silts)

Chemical magnetization: Occurs when grains of magnetic minerals grow, or recrystallze

Viscous magnetization: Produced by long exposure to an external field.

Isothermal magnetization: Residual magnetization left behind followign the removal of an unusually strong magnetic field. Occurs locally following lightning strikes, may prove very confusing.

Magnetics case studies

Sugarbush geological mapping

Aves ridge, eastern Caribbean

Top: magnetic anomalies. Bottom: Bathymetry and basement-sediment interface (from seismics). Horizontal bars are derived from the apparent wavelength of magnetic data.

Western North America: Total field anomaly, with structural provinces

Southwestern USA: Reduced-to-pole aeromagnetic anomaly map. Inset shows variable flight line spacing.Symbols indicate major mineralization zones.

Southwestern USA: Residual RTP magnetic anomaly near the Butte mining

camp, Montana

Schematic model of batholith emplacement

Barbados accretionary wedge

Magnetic field profile over the Barbados accretionary wedge. Two alternative models for the central magnetic anomaly show a) a sliver of oceanic crust within the accretionary wedge, and b) the rise of metamorphosed sediments. Arrows and figures show the magnetization vectors used in the modelling.

St. Lawrence lowlands

Top: Mount St. Gregoire, and a nearby magnetic anomaly.

Bottom: A large magnetic anomaly 11 km from Mt. Bruno

St. Lawrence lowlands

(profiles and prism models)

Ayre Peninsula, South Australia: High level aeromagnetic survey. Note positions of ground profiles 1, 2, 3

Ayre Peninsula, South Australia: Magnetic profiles and gravity profiles

Northern Middleback Range, South Australia:

Iron ores will only have a significant magnetic field signature only if they have a significant magnetite to hematite ratio.

Leslie Kimberlite

Archeological mapping, Mexico

Pyroclastic rocks, Teotihuacan, Mexico: Anomalies 2 and 3 correspond to known tunnels, anomaly 1 revealed an undiscovered tunnel.

Landfill magnetic survey