Investigation of phase changes of serpentine minerals: Guleman ophiolite example

Serpentinite rock is present in the Guleman ophiolite as an alteration of ultramafic rocks in almost all ophiolite sequences, bearing traces of both alteration and metamorphism. Serpentinites are typically formed along transform boundaries on the mantle or in fractured zones of oceanic ridges. Recognizable by their mesh-like texture and the presence of serpentinite minerals, serpentinites are the result of chemical reactions and mineralogical transformations occurring when water is expelled from hydrous minerals in oceanic crustal rocks, such as peridotite, due to high temperature and increased pressure. Therefore, serpentinite rocks form through the transformation of mantle rocks under the influence of water, resulting in the conversion of magnesium and iron-containing minerals into serpentinite minerals. Such rocks are particularly observed at transform boundaries of ophiolites and oceanic ridges, providing significant clues about the formation processes of the Earth's crust. In this study, phase transitions of serpentinite samples are aimed to be monitored from room temperature to 1100°C, using a constant heating rate (20°C/minute). For this purpose, measurements were taken using Differential Thermal Analysis (DTA) and Thermogravimetric Analysis (TGA) to determine accompanying mass changes, and the results were interpreted with XRD, SEM, and ICP-MS. Scanning Electron Microscope (SEM) examinations of ultramafics and serpentinites reveal serpentinite minerals in the form of fibrous-looking grains. Typically, the length of fine fiber bundles is 20 μm, and EDS images indicate that serpentinites exhibit a structural feature similar to asbestos. The phase thermal process of serpentinites (as observed in DTA analysis) shows an endothermic peak or decomposition of crystal structures at 660°C, associated with a 13% loss of water. Between 550-660°C, antigorite dehydroxylation reactions result in the development of forsterite. Antigorite and forsterite coexist in a phase composition up to approximately 660°C, after which enstatite minerals develop. Under constant pressure, it has been determined that antigorite remains stable between 650°C and 690°C, releasing water and silica.