Siderite and its role in the magnetic mineralogy of Mesozoic swarm dikes of Southeast of Brazil

The emplacement of dikes and alkaline complexes found in the Santos-Rio de Janeiro swarm is claimed to be related to processes of the separation of South America and Africa. The referred dikes are diabases and lamprophyres, and they crop out in a pattern where they are parallel to each other. Their dimensions range from a few centimeters up to 2 m wide for the lamprophyres dikes, and up to > 10 m for the diabase dikes. Extensive paleomagnetic and anisotropy of magnetic susceptibility (AMS) work has been done in these dikes. In order to study the magnetic mineralogy of these samples different techniques were used, such as: hysteresis loops, field-dependence of magnetic susceptibility (K), K(T) (low and high temperature), low temperature remanent magnetization curves (ZFC/FC), room temperature saturation isothermal magnetization (RTSIRM), AC susceptibility as a function of temperature, and Mossbauer spectroscopy at room temperature and 4.2 K. The ZFC/FC curves of diabase dikes have the typical Verwey transition at around 120 K suggesting magnetite as the magnetic carrier of them. On the other hand, the ZFC/FC remanence curves of the lamprophyres dikes have as the main feature a transition near 38 K. This is diagnostic of the presence of siderite and it is supported also by Mossbauer spectroscopy. The Mossbauer spectra at room temperature are represented by two small sextets associated with magnetite and paramagnetic doublets of Fe3+ and Fe2+. At 4.2 K siderite is below its Tn and has magnetic order, thus the Mossbauer spectra at low temperature show, besides the magnetite sextet and doublets of Fe3+ and Fe2+, an additional sextet that is associated with siderite. The contribution of siderite for the AMS study is being investigated.