This is an Excel spreadsheet and graph that illustrates standard "batch" and Rayleigh decarbonation models and how they can be used to detect fluid infiltration during metamorphism of carbonates. It makes a good lecture demonstration, but with a few modifications can be turned into a laboratory exercise. Key variables are "hotlinked" directly into the batch and Rayleigh models so students can adjust them to get a feel for the influence of different model parameters. The included carbon isotope data are from marbles in the Jervois region, central Australia (Cartwright et al., 1997). Oxygen isotope data are also included in the spreadsheet.