Abstrakt: |
The mobility of HO and DO by diffusion through quartz is illustrated with HO-rich fluid inclusions synthesized at 600 °C and 337 MPa, within the α-quartz stability field. Inclusions are re-equilibrated at the same experimental conditions within a pure DO fluid environment. Consequently, a gradient in volatile fugacities is the only driving force for diffusion, in the absence of pressure gradients and deformation processes. Up to 100 individual inclusions are analyzed in each experiment before and after re-equilibration by microscopic investigation, microthermometry, and Raman spectroscopy. Changes in fluid inclusion composition are obtained from the ice-melting temperatures, and density changes are obtained from total homogenization temperatures. After 1-day re-equilibration, inclusions already contain up to 11 mol % DO. A maximum concentration of 63 mol % DO is obtained after 40-day re-equilibration. DO concentration profiles in quartz are determined from the concentration in inclusions as a function of their distance to the quartz surface. These profiles illustrate that deep inclusions contain less DO than shallow inclusions. At equal depths, a variety of DO concentration is observed as a function of fluid inclusion size: Small inclusions are stronger effected compared with large inclusions. A series of 19-day re-equilibration experiments are performed at 300, 400, 500, and 600 °C (at 337 MPa), at the same conditions as the original synthesis. The threshold temperature of diffusion is estimated around 450 °C at 337 MPa, because DO is not detected in inclusions from re-equilibration experiments at 300 and 400 °C, whereas maximally 26 mol % DO is detected at 500 °C. Our study indicates that the isotopic composition of natural fluid inclusions may be easily modified by re-equilibration processes, according to the experimental conditions at 600 °C and 337 MPa. [ABSTRACT FROM AUTHOR] |