REVIEW OF AARM STUDIES IN GRANITIC ROCKS

Anhysteretic remanent magnetization (ARM) is imparted by the combination of a strong alternating field (decaying from an initial value of 100-200 millitesla) and a weak steady field (50-100microtesla), and ARM intensity is generally proportional to the strength of the steady field. This proportionality is sometimes termed "anhysteretic susceptibility," and like the more familiar weak-field AC susceptibility, ARM anisotropy can be mathematized as a second-rank tensor. Because AARM is controlled entirely by remanence-carrying ferromagnetic (s.l., e.g., magnetite, pyrrhotite) grains, it may differ in geologically meaningful ways from AMS, which reflects a weighted average of contributions from all mineral phases present. This paper reviews some of the important properties and characteristics of ARM and its anisotropy, as well as previous and potential applications in studies of granitic rocks. In general, separation of ferromagnetic and paramagnetic anisotropies is useful: (a) when the two types of minerals crystallize at different points in the flow/deformation/metamorphic history; (b) when quantitative measures of petrofabric intensity are sought; (c) when minerals bearing an "inverse" anisotropy (e.g., single-domain magnetite) are important; and (d) when the relationship between natural remanence and paleomagnetic field properties are of interest. AARM is helpful in all of these cases, and is especially well-suited for the latter two, in contrast to various other methods of isolating paramagnetic and ferromagnetic anisotropies.