Magnetic Domains Remanence Acquisition how rocks get magnetized

Magnetic Domains & Remanence Acquisition ……. . how rocks get magnetized

Magnetic Remanence When the magnetization of a body produces an external field (i. e. it possess a remanence), it has magnetostatic energy or an energy of self demagnetisation. [Atomic magnetic moments are dipoles and can most simply be modelled as pairs of magnetic charges. In a given magnetic particle the magnetic charges of adjacent atoms cancel internally within the particle but produce a magnetic charge distribution at the surface of the particle. For a uniformly magnetized spherical particle one hemisphere has a positive charge and the other a negative charge. This charge distribution gives rise to an energy known as the magnetostatic energy. ]

Magnetic Remanence (From Mc. Elhinny & Mc. Fadden, 2000)

Rock Magnetism: Magnetostatic Energy For a uniformly magnetized grain the magnetostatic energy is proportional to the square of the magnetization and the magnetostatic energy becomes extremely high for ferromagnetic materials with a high magnetization. To reduce this magnetostatic energy magnetic domains form within grains, which reduce the size of regions of uniform charge thereby reducing the surface magnetization.

Rock Magnetism: Domains Subdivision of a ferromagnetic grain into domains. (After Dunlop & Özdemir, 1997)

Rock Magnetism: Domains Grains with two or more magnetic domains are termed multi-domain. Internal to each domain the magnetization is equal to the saturation magnetization, but because charges of opposite sign are adjacent the net magnetization for the grain is less than the saturation magnetization. The region separating domains is known as the domain wall. Domain walls themselves possess a finite energy that is related to energy exchange between adjacent atoms and is proportional to the area of the wall. (After Butler, 1992)

Rock Magnetism: Domains With increasing grain size, and increased surface area, more magnetic domains are formed to counteract the increase in magnetostatic energy due to the increase in the surface area. Conversely, as grains get smaller the number of domains decreases until the energy involved in erecting a domain wall becomes larger than the decrease in magnetostatic energy resulting from dividing the grain into two domains. Grains with only one magnetic domain are termed single-domain. The grain diameter below which grains are single-domain is dependent on the grain shape and the magnetization. Grains with a low magnetization possess low magnetostatic energy and hence little incentive to form magnetic domains.

Rock Magnetism: Domains Single-domain, two-domain and multi-domain grain configurations (top) and the likely number of domains versus grain diameter for magnetite (bottom). The black bars represent the range of grain sizes for which that number of domains is the lowest energy domain state. (After Moon & Merrill, 1985; & Van der Voo, 1990)

Rock Magnetism: Domains arise due to a balance of energies within a grain. The major magnetic energies are; the exchange energy, Eex; the magnetostatic or demagnetising energy, Ed; and the anisotropy energy, Eanis. The total magnetic energy, Etot is given by: Etot = Eex + Ed + Eanis The Domain State of a magnetic grain is controlled by minimising these energies and thus is controlled by grain size, shape and mineralogy; magnetic field, temperature, stress and crystal defects as well as other more minor factors.

Rock Magnetism: Domains Exchange energy (Eex) is the energy associated with coupling through the interaction of the electrons of adjacent atoms. Eex is minimised by the parallel (ferromagnetic) or antiparallel (antiferromagnetic/ferrimagnetic) alignment of adjacent magnetic moments. Eex = -2 Je. Si • Sj Where Si • and Sj are the spin vectors for adjacent atoms and Je is the exchange integral. (Where Je > 0, energy is minimised by spins being parallel (ferromagnetism). Where Je < 0, energy is minimised by spins being antiparallel (antiferromagnetism, ferrimagnetism). )

Rock Magnetism: Domains Magnetostatic energy (Ed) also called the internal field energy arises from interaction of a crystals’ magnetization with itself. This energy is dependent on the geometry of the crystal. It is large for equant grains and small for elongate grains. An effect of this energy is shape anisotropy; it is easier for magnetisation to lie along the long axis of an elongate crystal. Shape anisotropy is thus uniaxial with two "easy" directions of magnetisation. Ed = ½ 0 NVM 2 Where N is the demagnetising factor, V is the volume and M is the magnetisation, and 0 is the permeability of free space.

Rock Magnetism: Domains The anisotropy energy (Eanis), the total anisotropy energy, is the sum of the magnetocrystalline, magnetostrictive and magnetoelastic anisotropies. The total anisotropic energy Eanis is thus given by: Eanis = Ek + Estric + Eme

Rock Magnetism: Domains The magnetocrystalline energy (Ek) arises from the interaction of the permanent magnetic moment with the anisotropic crystalline electric field. This simple means that atomic dipole moments align more easily along certain crystallographic axes than others. In magnetite, the [111] axes are the preferred orientation of magnetisation in the absence of an external field. This gives a cubic crystalline anisotropy and therefore 8 "easy" orientations of magnetisation. Shape anisotropy is a much stronger effect than crystalline anisotropy and dominates over it for elongations greater than a few percent.

Rock Magnetism: Domains Magnetization of a single crystal of magnetite along different crystallographic axes. [111] is the magnetocrystalline easy axis, while [100] is the magnetocrystalline hard axis. (After Nagata, 1961)

Rock Magnetism: Domains The magnetostrictive energy (Estric) arises from the fact that when a crystal is magnetised it changes shape. This introduces the possibility of magnetically induced mechanical stress in grains where the whole volume is not magnetised in the same direction.

Rock Magnetism: Domains The magnetoelastic energy (Eme) results from the effects of stress on a crystal, which alters the direction of spontaneous magnetisation. The origins of stress may be external (macrostress) or internal, (microstress), due to crystal imperfections e. g. dislocations, inclusions etc. Microstress is important when considering intra-crystal magnetic processes and remanence.

Rock Magnetism: Domains The total anisotropic energy Eanis is thus given by: Eanis = Ek + Estric + Eme For magnetically uniaxial grains Eanis = KVsin 2 Where K is the anisotropy constant, V is the grain volume and is the angle between the direction of magnetization and the easy axis.

Rock Magnetism: Domains Whether a grains will subdivide into two or more magnetic domains is strongly influenced by the size of the grain and at the critical size the energy of a single domain grain will be the same as the energy of the 2 domain state + the energy required to erect a domain wall between the two domains. ESD = E 2 D + EW ESD = ½ 0 NVM 2 For a sphere N is 4 /3 and the volume = d 3/6 where d is the diameter.

Rock Magnetism: Domains The energy of the wall (EW) = w. LW where w is the wall energy per unit area, L is the length of the wall and W is the width of the wall. Combining the above equations gives a critical diameter d such that: D = 4 w/ 0 NSDM 2 If we substitute values for magnetite the critical diameter comes out to be about 0. 04 m. This is actually less than the wall width for the domain walls in magnetite (about 0. 2 m).

Rock Magnetism: Single domain grains The theory of the magnetization of an assemblage of single -domain particles is essentially due to the work of Néel (1955). The grain-size below which particles are singledomain is termed the single-domain threshold grain size (d 0). This size is dependent on the grain shape and saturation magnetization. For haematite, which has a low saturation magnetization, this grain diameter is circa. 15 m, so a large proportion of haematite encountered in rocks is single-domain. Magnetite has a much higher saturation magnetization, and for cubic magnetite the critical diameter for single domain behaviour is approximately 0. 05 m. Elongate magnetite particles may still be single-domain up to 1 m. Single-domain particles can be very efficient carriers of remanent magnetization.

Rock Magnetism: Single domain grains Magnetite at 290°K (After Butler & Banerjee, 1975)

Rock Magnetism: Single domain grains During demagnetization the net magnetic moment of a single-domain grain cannot be reduced by internal cancellation of domain movements through domain wall movement. Instead, magnetic moments can only be made to change direction or rotated toward the applied field. However, there are resistances to the rotation of the magnetization, the dominant ones being shape anisotropy and magnetocrystalline anisotropy.

Rock Magnetism: Single domain grains Shape anisotropy is related, as the name suggests, to the shape of the grains. Highly elongate grains have a much lower magnetostatic energy when magnetized along their length rather than perpendicular to their length. This is because the percentage of surface area covered by magnetic charges is small when the magnetization lies along the long axis. Magnetization perpendicular to the long axis produces a substantial surface charge. The magnetic charge distribution produces a field internal to the grain, called the internal demagnetizing field, which opposes the magnetization of the grain. Therefore the internal demagnetizing field perpendicular to the long axis will be greater than that along the long axis.

Rock Magnetism: Single domain grains The difference in magnetization along and perpendicular to the long axis gives rise to a difference in magnetostatic energy, which represents a barrier to rotation of the magnetization through the perpendicular direction. To force the magnetization to rotate through this barrier an external magnetizing field is required, which is known as the microscopic coercive force. For needle-shaped singledomain magnetite, at room temperature, this field reaches a maximum of approximately 300 m. T, though in naturally occurring magnetite this extreme shape anisotropy is rarely encountered. More usually coercivities of magnetite lie within the range 30 -70 m. T. The coercivity of haematite is at least 0. 1 T and usually higher.

Physics of Magnetism: Hysteresis When a ferromagnet is subjected to a cyclic change in the external field the magnetisation is not directly proportional to the applied field by there is a lag in the magnetisation, which is known as hysteresis. H is the applied field, J is the induced magnetization. Js is the saturation magnetization, Jr is the saturation remanence and Hc is the coercivity. The various hysteresis properties are not solely intrinsic properties but are dependent on grain size, domain state, stresses and temperature. Because hysteresis parameters are dependent on grain size, they are useful for magnetic grain sizing of natural samples.

Physics of Magnetism: Hysteresis (After Butler, 1992)

Natural Remanent Magnetization (NRM) The natural remanent magnetization (NRM) of a rock is the magnetization present in a rock prior to laboratory treatment and depends on the geomagnetic field and geological processes that have operated on the rock during and after its formation. NRM can typically contain a number of components of magnetization of differing ages. The NRM component acquired during the formation of a rock is usually referred to as primary NRM and those acquired subsequent to rock formation as secondary NRMs. Secondary NRMs can often mask or sometimes completely obscure the primary NRM. Therefore understanding the modes of acquisition of NRM by a rock is of critical importance in interpreting the significance of measured components of NRM.

Natural Remanent Magnetization (NRM) The principal forms of NRM are: 1) 2) 3) 4) Thermoremanent magnetization Chemical remanent magnetization Detrital remanent magnetization Viscous remanent magnetization.

NRM: Thermoremanent Magnetization (TRM) A thermoremanent magnetization (TRM) is the NRM produced in a rock when cooling it from above the Curie temperature in the presence of a magnetic field. However this magnetization will, in time, reach magnetic equilibrium with the surrounding field. A measure of this time is the relaxation time (t). Relaxation time is given in the equation: where C is a frequency factor, v is the volume of the grain, Bc is the grain's coercivity, Js is the spontaneous magnetization, k is Boltzmans constant and T is the absolute temperature.

NRM: Thermoremanent Magnetization (TRM) (After Van der Voo, 1990)

NRM: Thermoremanent Magnetization (TRM) (After Butler, 1992)

NRM: Thermoremanent Magnetization (TRM) One of the most important features of this equation is that the relaxation time is strongly dependent on the absolute temperature and directly related to the coercivity. At the Curie temperature the mineral will have a short relaxation time and will rapidly align with the applied field. The temperature at which a particular mineral acquires its magnetization is known as its blocking temperature (Tb). As a mineral phase can have a blocking temperature below the Curie point, albeit with a longer relaxation time, a TRM can be acquired over a range of blocking temperatures that are distributed from the Curie point down. As the temperature decreases through the Tb of an individual grain, the grain experiences a large increase in its relaxation time, effectively ‘freezing in’ the magnetization relative to geological or experimental time scales.

NRM: Thermoremanent Magnetization (TRM) In the case of igneous rocks the magnetization is acquired as the rock cools through the Curie temperature of the particular magnetic mineral (at temperatures above the Curie Temperature magnetic minerals lose their magnetic properties). As the mineral cools through the Curie Temperature it retains a record of the direction and strength of the Earth’s magnetic field.

NRM: Thermoremanent Magnetization (TRM) If the acquisition of a magnetization through a range of Tbs is regarded as a stepwise process, the TRM acquired over a particular temperature interval is termed Partial TRM or PTRM. The sum of all PTRMs should give the total TRM (Thellier, 1951). (After Van der Voo, 1990)

NRM: Thermoremanent Magnetization (TRM) A theoretical model for the acquisition of TRM in single domain ferromagnetic grains was given by Néel (1955). This model adequately explained the acquisition of TRM in cases where the assemblage of grains has a uniaxial anisotropy and grains are of equal size with a single Tb. In practice rocks would be expected to have a random distribution of isotropic axes and a variety of grain sizes with corresponding variations in Tb.

NRM: Chemical Remanent Magnetization (CRM) Chemical remanent magnetization (CRM) is produced by chemical reactions involving ferromagnetic minerals including precipitation of a new magnetic mineral phase and alteration of pre-existing minerals (both ferromagnetic and non-magnetic). The new ferromagnetic mineral ‘locks in’ a record of the earth's magnetic field direction at the time of its formation. In contrast with TRM, where grains acquire a magnetization at a constant volume and decreasing temperature, CRMs are acquired at a constant temperature with changes in volume.

NRM: Chemical Remanent Magnetization (CRM) During chemical formation of a ferromagnetic mineral grains grow from a zero initial volume. Newly nucleated particles are small, have short relaxation times and are superparamagnetic. During the growth of the grains they become ferromagnetic and relaxation times increase dramatically. The diameter at which the grains change from being superparamagnetic to ferromagnetic is known as the blocking diameter (0. 02 m for haematite. As grains pass through the blocking diameter they record the applied magnetic field, and continued grain growth can produce a remanent magnetization that is stable over geological time.

NRM: Chemical Remanent Magnetization (CRM) (After Butler, 1992)

NRM: Chemical Remanent Magnetization (CRM) A number of factors govern the rate of CRM acquisition. Chemically immature sediments (those with an abundance of low-oxidation-state minerals) experience more rapid oxidation than chemically mature sediments and therefore acquire most of their CRM quickly. Therefore the chemical maturity of the sediment is a possible tool in recognising whether a sediment acquired its CRM rapidly or over a longer period of time. Secondly, the grain size of the sediment is of importance, given that fine-grained sediments have a larger surface to volume ratio and are likely to undergo more rapid chemical changes than coarser sediments. Finally the plaeoclimate and depositional environment also play a role. An oxygenating depositional environment is much more likely to result in rapid oxidation and warm moist paleo-climates tend to prolong the magnetization process in red beds.

NRM: Detrital Remanent Magnetization (DRM) Detrital remanent magnetization (DRM) is produced by the alignment of small magnetized particles during the deposition and lithification of sediments. The acquisition of DRM is a complicated process given the large number of processes involved during the formation of sedimentary rocks. There is a large variety of initial mineralogies, many minerals not being in equilibrium with each other or their depositional environment, and sediments are subject to large variety of post-depositional processes, such as bioturbation, prior to lithification.

NRM: Detrital Remanent Magnetization (DRM) (Modified after Cox & Hart, 1986) In sediments magnetic minerals make up a tiny proportion of the rock (<0. 1%). As the magnetic grains sink through the water column they align themselves with the ambient magnetic field (in this case the Earth’s magnetic field). When the sediment accumulates and solidifies into a rock the magnetic grains become ‘locked in’ and thus preserve a record of the magnetic field at the time of formation of the rock.

NRM: Detrital Remanent Magnetization (DRM) The classic model of DRM acquisition was proposed by Collinson (1965) which dealt only with the aligning effect of the applied magnetic field on a particle at the sedimentwater interface. This yielded a characteristic alignment time of 1 second, implying rapid and complete alignment of ferromagnetic particles with the geomagnetic field. However this model does not hold for natural cases or laboratory experiments as a number of other important considerations were not taken into account.

NRM: Detrital Remanent Magnetization (DRM) Firstly, magnetic grains and especially inequidimensional ones interact with each other, and the final alignment of the grains is therefore a compromise between alignment with the magnetic field and adjacent magnetic particles. Secondly, laboratory experiments indicate that the inclination of DRM tends to be consistently shallower than the applied field. A simple explanation for this is that grains tend to be magnetized along the long axis of the particle due to shape anisotropy. These long axes are subject to gravitational torque which tends to rotate them toward the horizontal.

NRM: Detrital Remanent Magnetization (DRM) There a number of problems with this explanation as measurements of the inclination error in natural sediments tend to be less than those in laboratory experiments, indicating that there are other factors to be taken into account, notably the possibility of further post-depositional DRM (p. DRM). PDRM has been attributed to Brownian motion, where magnetized particles are reoriented by the Brownian motion of the surrounding water. PDRM has been shown to be free of the inclination problem of DRM and it seems likely that DRMs in natural sediments are DRMs with some portion of later p. DRM.

NRM: Detrital Remanent Magnetization (DRM) Experimental production of PDRM in the laboratory plotted versus the inclination of the applied field. (After Kent, 1973) Most sediments, and particularly red beds, are thought to carry CRM, both DRM and p. DRM being subject to CRM overprinting early in the diagenetic history of the rock.

NRM: Viscous Remanent Magnetization (VRM) Viscous remanent magnetization (VRM) is the magnetization acquired during exposure to weak magnetic fields. It is proportional to the intensity of the ambient field and proportional to the logarithm of the time of exposure to the field. VRM at a given temperature is given by; where t is the time of exposure to the field and S is the viscosity coefficient. The viscosity coefficient (S) has been shown to be proportional to temperature. Because of the logarithmic growth of VRM with time, viscous magnetizations tend to be dominated by recent magnetic fields and generally rocks with a high proportion of VRM tend to have NRM aligned with the present geomagnetic field.

NRM: Isothermal Remanent Magnetization (IRM) Isothermal remanent magnetization (IRM) is acquired in the presence of a direct field at a constant temperature. IRM curves or hysteresis loops are often used in laboratory experiments to identify magnetic carriers in rocks. A demagnetized sample is subjected to an applied magnetic field (H) and the induced magnetization (Ji) is then measured. The induced magnetization per unit volume (J) is plotted against H, where H is increased in a series of steps up to maximum and then reversed and increased to a maximum in the reversed direction. The resultant hysteresis loop is characteristic of the remanence carrier(s) in the rock. The maximum induced magnetization is known as the saturation magnetization (Js) and depends linearly on the concentration of the ferromagnetic mineral involved.

Physics of Magnetism: Hysteresis When a ferromagnet is subjected to a cyclic change in the external field the magnetisation is not directly proportional to the applied field by there is a lag in the magnetisation, which is known as hysteresis. H is the applied field, J is the induced magnetization. Js is the saturation magnetization, Jr is the saturation remanence and Hc is the coercivity. The various hysteresis properties are not solely intrinsic properties but are dependent on grain size, domain state, stresses and temperature. Because hysteresis parameters are dependent on grain size, they are useful for magnetic grain sizing of natural samples.

NRM: Isothermal Remanent Magnetization (IRM) The applied field (H) required to achieve saturation (>700 m. T for haematite, 300 m. T for magnetite) can be used as an indication of the identity and domain states of the magnetic carriers. The reversed field required to reduce Js to zero is known as the coercivity of remanence (Hcr). Typical values of Hcr for magnetite are 20 -80 m. T and >300 m. T for haematite, though higher values are not unknown, indicating hard magnetic components.

NRM: Isothermal Remanent Magnetization (IRM) In nature occurrences of IRM tend to be restricted to outcrops that have been subjected to lightning strikes. Electrical currents of lightning can exceed 104 -105 amperes and induce magnetic fields of up 10 m. T within 1 m of the strike. These can generally be recognised by abnormally high NRM intensities and in some instances by a high scatter in the NRM directions. www. gsfc. nasa. gov/

NRM: Isothermal Remanent Magnetization (IRM) The current travels radially from the point of impact, and the distance it travels depends on the conductivity of the rock, and whether it is wet. The resulting magnetic directions are usually highly scattered.

NRM: AF Demagnetization (After Van der Voo, 1990) Alternating field (AF) demagnetization is achieved by the cycling of a magnetized rock sample through hysteresis loops with decreasing amplitude in a zero Dc field. The magnetic moment of grains with a coercivity less than the peak field applied is thereby nullified. The process is repeated for successively higher fields until the NRM is effectively demagnetized or the maximum peak field is attained.

NRM: Thermal Demagnetization Progressive thermal demagnetization is achieved by stepwise heating to the maximum unblocking temperature. Samples are then cooled in a field-free chamber which allows for random orientation of particles or domain moments. The magnetization of the rock sample is measured at room temperature between each heating cycle. The lower blocking temperature components are progressively removed leaving those of high thermal stability. Treatments are typically in 20 -100°C steps though this depends on the nature of the NRM. Treatments are usually concentrated over temperature intervals where a large proportion of magnetic grains un-block, (i. e. a thermally discrete spectrum or where detailed analysis is required.

NRM: Thermal Demagnetization (After Van der Voo, 1990)

NRM: Thermal Demagnetization (After Butler, 1992)

NRM: Thermal Demagnetization (After Butler, 1992)

NRM: Thermal Demagnetization Normalized Intensity 1. 0 0° 200° 400° Temperature (°C) 600°

NRM: Thermal Demagnetization One drawback with the technique is that the heating cycle can produce mineralogical alteration within the rock and for the magnetic minerals these alteration products include; production of Maghemite from Magnetite at 150250°C, Hematite from Maghemite at 350 -450°C, Hematite from Magnetite at >500°C and the reduction of Hematite to Magnetite at >550°C. Alterations such as these will change the magnetic characteristics of the sample and it becomes particularly important that field-cancellation in the furnace be as complete as possible to avoid the acquisition of spurious TRM during cooling.

Magnetism in Oxides Ilmeno. Haematite series Titano. Magnetite series

NRM: Thermal Demagnetization Up, W NRM 1. 0 520 o. C 530 o. C 540 o. C 550 o. C 500 m. A/m N 2 GP 35 (Hornblendite) 4 6 x 100 o. C 8