Rapid Na, Cu exchange between synthetic fluid inclusions and external aqueous solutions: evidence from LA-ICP-MS analysis

Three sets of equilibration experiments (Set 1 to Set 3) were performed in cold-seal pressure vessels to investigate the compositional modification of quartz-hosted fluid inclusions after entrapment. Each set of experiment consisted of two stages. In a pre-run, fluid inclusions containing 5–10 wt% NaCl and selected trace elements were synthesized at 700°C/140–200 MPa. These samples were then loaded into new capsules together with Cu-bearing solutions and some mineral buffers, and re-equilibrated at 600–800°C and 70–140 MPa for 6–8 days. LA-ICP-MS analysis of individual fluid inclusions reveals that in re-equilibration experiments in which the outer fluid was composed of KCl (±FeCl₂) up to 83% of the original Na content of pre-existing fluid inclusions were lost, and up to 5660 ppm Cu were gained. Other elements with larger ionic radii (i.e. K, Fe, Ba, Sr) were not exchanged, demonstrating that the inclusions remained physically intact and that Na and Cu were transported through quartz by diffusion. The observed Na loss from pre-existing fluid inclusions correlates positively with Cu gain, with about 1 Cu atom being gained per 10 Na atoms lost. Thus, Na and Cu (plus probably H) were exchanged by interdiffusion. Remarkably, this processes resulted in up to 10 times higher Cu concentrations in re-equilibrated inclusions than were present in the outer fluid, i.e. Cu diffused 'uphill'. Large variations of Cu concentrations relative to the concentration of other elements are common also in natural fluid inclusion assemblages. However, no evidence for a correlation between Cu content and Na content was found so far, suggesting that Cu diffusion in natural samples may be dominated by processes other than Na–Cu interdiffusion.