Types of Magnetism Acquired by Rocks

As a rock is formed, it acquires a magnetisation which is, in general, parallel to the Earth’s magnetic field at that time and place. In the case of the igneous units studied in this the- sis, this occurs as the magnetic moments within the rocks align themselves with the ambi-

ent geomagnetic field as the lavas/intrusives cool after extrusion/emplacement. A mechanism by which magnetic minerials will equilibrate in an applied field, results from forcing the magnetic moments to rotate through the anisotropy energy barriers (see sec- tion 3.1.2) as a result of thermal activiation (Butler., 1992; Tauxe, 2010). The simplest ef- fect of thermal activation is the natural decay of remanent magnetisation over time and the acquisition of a new “viscous” magnetisation aligned with the ambient field. At a given temperature some of the grains will have enough energy to overcome the anisot- ropy energy and js will be rotated to the ‘easy’ axis.In this situation, the temperature re- quired to achieve this on laboratory timescales is known as the (un)blocking temperature. Given enough time, in a zero-field environment, all the moments will become randomised and magnetisation in the sample will decay to zero. The time for a remanent magnetisa- tion to decay to 1/e of its original magnitude is referred to as the relaxation time, (Butler.R.F, 1992; Tauxe, 2010).

Prior to any laboratory treatment the measured intensity and direction of the field in a sample is known as the NRM. The NRM is often constituted of more than one component. The primary NRM is that acquired by a rock as it is first formed.

As well as the primary NRM, the NRM of a rock may also include secondary NRM compo- nents which overprint and obscure the primary NRM signature. The causes of acquisition of secondary NRMs are varied and can included chemical changes in the rock, such as hydrothermal alteration or burial metamorphism, which might also affect the ferromag- netic minerals; proximity to areas affected by lightning strikes; reheating of the rocks whether it be through burial or proximity to the emplacement of intrusive units (e.g. dykes, sills and/or plutons/batholiths) and/or subsequent emplacement of further vol- canic units; and long-term exposure the magnetic field.

Primary NRMs can be acquired in three main ways:

a) as a thermoremanent magnetization (TRM), which is acquired during cooling of the rocks below their Curie Temperature. This is the form of primary NRM acquired by the majority of igneous rocks.

b) chemical remanent magnetization (CRM), formed by growth of ferromagnetic grains below the Curie Temperature (Tc, in the case of a ferromagnetic minerals, the tempera-

ple, metamorphic or hydrothermal alteration. In addition, if ferromagnetic minerals are exposed to elevated temperatures, they may acquire a thermochemical remanent magen- tisation (TCRM) as result of chemical alteration. For example, titanomagnetite exposed to elevated temperatures can continue to exsolve in a solid solution series below its Curie Temperature as a consequence of oxidation. A CRM can be acquired one of two ways: it may result from the alteration of minerals already existing in the rock; alternatively it may be acquired when a ferromagnetic mineral is precipitated out of a solution. In both cases, this occurs at temperatures below the mineral’s blocking temperature. This is particularly relevant to the rocks of the Onverwacht Group given that they are extensively serpen- tinised. Therefore, the source of any magnetisation the BGB rocks may record should be considered in light of a potential CRM or TCRM.

c) a detrital remanent magnetization (DRM), resulting from the alignment of detrital fer- romagnetic minerals with the ambient field as they form sediments. In an ideal scenario, the TRM is acquired by a rock which is dominated by single domain grains (SD), but this is seldom the case. In reality, truly SD grains are relatively rare within most igneous rocks, which tend to be dominated by PSD and MD grain sizes (Butler., 1992; Tauxe, 2010). Grains larger than 10µm tend to acquire the TRM inefficiently and are prone to acquiring secondary magnetisations. This means that SD and PSD grains are likely to reliably record a primary NRM whilst MD sized grains are more liable to later overprints as a result of subsequent heating and/or burial events. It is known extrusive volcanic rocks which cooled quickly have relatively small grain size distributions in the SD and PSD range, whilst intrusive rocks tend to have a larger percentage of MD sized grains (Butler.R.F, 1992). However, given the age of the samples studied in this work, despite them being predomi- nantly extrusive with the exception of the Neelshoogte Pluton samples, it would be im- prudent to assume that a solely primary magnetisation will be recorded, even if the mag- netic carriers fall within the SD-PSD grain size range.

Other types of magnetisation that should be considered when interpreting the results of this study include: viscous remanent magnetisation (VRM) which results from exposure to a weak magnetic field over a period of time (applicable to the rocks of the BGB given their age) and a thermoviscous remanent magnetisation (TVRM) also of relevance to the BGB samples as it is acquired during prolongued heating events subsequent to the emplace- ment of the rocks. Isothermal remanent magnetisation (IRM) may also be of importance when understanding the potential magnetic signal in the studied samples. An IRM is im-

parted during exposure to a strong magnetic field over short periods of time. A natural IRM is acquired as secondary component as a result of lightning strikes (the magnetic field within 1 m of a lightning bolt can be between 10 to 100 mT, Butler, 1992). Given the moun- tainous nature of the BGB, in an otherwise mostly flat landscape, thunder storms are com- mon. Care was taken when selecting sampling sites to avoid those which were overly ex- posed or at the top of ridges. In addition, where lightning strikes were of concern, samples from a site were collected over a larger area to minimise the effect of any potential im- parted IRM. Nevertheless, when considering the results of the BGB the effect of a lightning induced IRM must be assessed. IRMs can also be imparted during laboratory by placing samples in a strong magnetic field.