Thermoremanent magnetization carried by synthetic analogues of the altered olivines of igneous rocks

Abstract

The deuteric alteration of olivines in nature has been simulated in the laboratory by heating members of the forsterite-fayalite solid solution at temperatures between 850°C and 1280°C and under oxygen fugacities between 10 −13.1 and 10 −5.7 atmospheres. Magnetite was produced analogous to the “dendritic” and “symplectite” intergrowths found in nature. The thermoremanent magnetization (TRM) acquired by the magnetite was intense and stable, and was found to correlate with the temperature ( T ox ) at which the deuteric alteration had been carried out. In the lower T ox range (850–950°C) TRM intensity increased with increasing T ox and this may be a rare example of TRM intensity increasing with increasing grain size as expected from the monodomain model. The highest intensities are higher than that of any magnetite TRM recorded in the literature. Applying the monodomain model to the higher T ox range (1000–1200°C) yields implausible values for particle volume, whereas the application of multidomain models yields plausible demagnetizing factors. Discrepancies between the TRM acquisition curves and the temperature dependence of coercive force point to the limitations of the simple multidomain model.