Acquisition of viscous remanent magnetization

Viscous remanent magnetization (VRM) has been frequently suggested as a possible source of magnetic anomalies in ocean crust and as an important contributor of natural remanent magnetization in some volcanic and sedimentary rocks. Most references to VRM assume a linear dependence with log(t), yet long-term experimental data on VRM, while scarce often disagree with this simple notion. Here, we investigate the angular-dependence, grain-size dependence, initial-state dependence, and time (t) dependence of VRM acquisition. We observe a non-linear log(t) dependence of VRM acquisition over all time intervals (>104 s) for all the samples regardless of various experimental conditions. Echoing earlier studies, we also find that VRM acquisition is strongly dependent on the initial magnetic state of the samples. Samples in a thermally demagnetized initial-state were most susceptible to VRM acquisition. When VRM is produced parallel to the initial thermal remanence, VRM acquisition is negligible. VRM acquisition is more prevalent for magnetites in single-domain and multidomain (MD) grains than in the pseudo-single-domain range. In MD magnetites, frequent removal and restoration of samples (for the purpose of zero-field remanence measurement) apparently increases the efficiency of VRM acquisition.