REAL-TIME MONITORING OF REACTION RATE AND CHEMISTRY OF INCIPIENT SERPENTINIZATION OF OCEANIC LITHOSPHERE

Serpentinization of mafic-to-ultramafic rocks by aqueous fluids is a widespread process in the Solar system that has important implications concerning the presence and role of water in planetary systems and the origin of life. However, many questions remain concerning details of the serpentinization reaction, especially as related to fluid chemistry and the type and composition of solid phases that form as the reaction proceeds. The synthetic fluid inclusion (SFI) technique was used to trap fluids in olivine at elevated temperature and pressure in order to follow in-situ serpentinization of olivine at 280°C in a closed system at low water/rock ratios. Serpentinization experiments were designed to test the effects of thermal cycling and fluid composition on reaction progress. Fluid compositions included H₂O-NaCl, H₂O-MgCl₂ and H₂O-NaCl-MgCl₂ (with an NaCl/MgCl₂ ratio similar to that of seawater) with salinities ranging from 1-10 wt.% salt. Reaction progress was monitored using microthermometry, and Raman and Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS) to analyze the fluid inclusions over a period of several months. Results show that alteration of olivine starts after a few days in most of the experiments (except in experiments using H₂O-MgCl₂). Thermal cycling between 280°C and room temperature did not affect reaction progress. In the seawater experiments (3.5 wt% H₂O-NaCl-MgCl₂) the salinity of the FI increases uniformly and consistently (~0.5 wt% every 5 days) starting when serpentine and brucite first nucleate. Salinity increases were observed until the newly formed minerals completely filled the cavity (with salinities of ~7.1 to 7.5 wt%). Due to the small size (micron scale) of the inclusions and their contained reaction products, TOF-SIMS analyses were performed to characterize the chemical composition of the new formed phases and the host at different stages of reaction to better constrain the reaction progress. Preliminary results show small variations in Mg/Fe ratio from the newly formed serpentine crystals inside the FI (more Mg rich) and the olivine host (less Mg rich).