Serpentinization of oceanic peridotites: 1. A high-sensitivity method to monitor magnetite production in hydrothermal experiments

A new method using the magnetic properties of magnetite, Fe3O4, was developed to monitor experimental serpentinization. The saturation remanent magnetization signal (Jrs) was measured during the course of experiments designed to react San Carlos olivine, (Mg-0.91, Fe-0.09)(2)SiO4, with water at 250 to 350 degrees C and 500 bars. At the end of the experiments, the ratio with saturation magnetization (Jrs/Js ratio) allowed to convert each successive Jrs measurement into an in situ amount of magnetite produced by the serpentinization reaction. Water weight loss was also measured on the end product to determine the final degree of serpentinization. The application of this procedure to a series of experiments performed at 300 degrees C/500 bars for various run duration (9 to 514 days) and starting olivine grain size (1 to 150 mu m) shows a linear relationship between magnetite production and reaction progress. This relationship can be safely transposed to other experimental conditions using thermochemical modeling and/or the Fe content of the product phases. We show that this high-sensitivity magnetic method is a powerful tool to precisely monitor serpentinization kinetics in Fe-bearing systems. It represents, in addition, a new indirect mean for monitoring the production of hydrogen which is bound to magnetite production rate.