The effect of shape anisotropy on TRM direction Robert s. COe Earth Sciences Board

Summary. A more exact treatment than has previously been given of the effect of magnetic shape anisotropy on TRM direction is presented. This takes into account the variation with temperature of the spontaneous magnet-ization, weak-field susceptibility and distribution of blocking temperatures. Detailed numerical computations for three examples of basalts with very different magnetic properties show that the effects of shape anisotropy can be estimated very accurately from a simple equation derived for a limiting case if one uses in place of the real TRM susceptibility an equivalent TRM susceptibility 0.68-0.85 times as large. Application of these equations suggests that shape anisotropy has a very small average effect in subaerial basalts, but may in some cases substantially shallow the direction of TRM in submarine basalts. In particular,about 10 degrees of the inclination anomaly observed in DSDP leg 37 basalts could be caused by shape anisotropy. These results assume the absence of significantly large and systematic internal fields due to short-range magnetic interactions. Further work is needed to determine how much the TRM direction in strongly magnetic rocks actually deviates from that of the external field