T18 The Anisotropy of Magnetic Susceptibility of Granitic Rocks: New Methodological Developments, Interpretations, and Challenges

Sunday, 27 October 2002
8:00 AM - 12:00 PM

NUMERICAL MODELLING OF ANISOTROPY OF MAGNETIC SUSCEPTIBILITY (AMS) IN PARAMAGNETIC ROCKS: EFFECTS OF MULTIPLE MINERAL PREFERRED ORIENTATIONS AND COMPOSITIONS

Paper number: T18-2
Program: Technical Sessions
Time: 8:15 AM – 8:30 AM

Authors

Abstract

Magnetic fabric (AMS) measurements quantify the bulk preferred orientationof all magnetic minerals that are present in a rock sample. In structuralgeology, however, the use of magnetic fabrics is complicated by (1) theeffects of multiple mineral preferred orientations, and (2) the proportionsof magnetic components that are present in the rock. In order to investigateseparately the role of the latter effects on magnetic fabrics, we havedeveloped a forward numerical model of AMS that concerns magnetocrystallinemineral species.

Numerical modelling provides an easy method to examine composite fabrics interms of angular dispersion or mixing. This approach requires generatingrepresentative and well-controlled synthetic samples. We shall discuss (i)the individual AMS tensor properties of chosen minerals (biotite,hornblende, clinopyroxene and tourmaline), (ii) the numerical generation ofangular dispersion, (iii) the computation of the total fabric, and (iv) therepresentativeness of the synthetic samples.

Then, for each mineral phase, we present results concerning the mixing oftwo distinct mineral preferred orientations expressed in terms of relativeabundance and angular separation. In particular, we shall discuss thechanges in anisotropy degree (P), lineation (L) and foliation (F) strengths,and changes in principal axes orientation. Finally, we investigate themixing of two and three mineral phases that are either lineated or foliated,in terms of their relative concentrations and their angular dispersions. Weshall focus on the effects on the total fabric of inverse (tourmaline) andoblique (hornblende, clinopyroxene) fabrics.

The above presented forward model allows the resultant magneticsusceptibility of a rock to be quantitatively related to: (i) the magneticproperties of the mineral species carrying the anisotropy, (ii) theirspecific preferred orientation and (iii) their relative abundance in the rock.