Rock Magnetic Characterization of the Ultramafic-Mafic Complex of San Juan de Otates, Central Mexico.

Magnetite formation in serpentinites is a very common process, however variations in the magnetic properties of serpentinized ultramafic rocks are scarcely studied. We analyzed 60 cores of ultramafic rocks with several degrees of serpentinization. These rocks belong to the San Juan de Otates ultramafic-mafic complex, central-Mexico. Magnetic field and temperature-dependent measurements as hysteresis parameters, IRM, FORC and magnetic susceptibility vs. temperature were used to determine volume, size and type of magnetic minerals, as well as the magnetic domain of the minerals carriers of magnetization. Our results suggest that: 1) magnetite is the main magnetic carrier; 2) serpentinized peridotites have the highest remanent magnetization and susceptibility values, while pyroxenites have the lowest values, and mafic rocks have significant magnetic signal; 3) serpentinization degree in the ultramafic rocks varies from 6 to 100%, being dunites those of highest values. This approach allowed us to constrain to what extent the magnetite grain size depends on the serpentinization process. Results of this analysis let us better understand how does magnetite grows up, distributes and its relation to serpentinization degree, but also opens the possibility to a magnetic anomaly global approach.