Freshwater mussels of the genus Diplodon (Unionida) are common inhabitants of lakes and rivers in South America, and have slow growth and long life spans. We established the annual periodicity of incremental shell growth in Diplodon chilensis patagonicus (d'Orbigny, 1835) and calculated growth rates at different ages, using internal ring counting supported by dyeing methods and δ18O isotope analyses, in two Patagonian populations (Lago Steffen and its effluent Río Manso Inferior, Argentina). Longevities of ca. 90 years (Lago Steffen) significantly extend the life spans reported in the past. Growth rates for old individuals (>30 years) from both lake and river populations average 0.16 mm per year along the axis of minimal growth. We evaluated the seasonal periodicity of minor and trace elements (Mn, Mg, Sr, Ba, Na, S) in situ by Laser Ablation ICP-MS and Electron Probe Microanalyser analyses. Line-scans in a valve from Lago Steffen show that Mn, Sr and Ba are preferentially accumulated during the summer, while higher concentrations of Mg are found in the winter bands. Metal/Ca ratios may serve as long-term archives of environmental variables, e.g. metal concentrations in water, water temperature and primary productivity. Diplodon chilensis patagonicus valves exhibit excellent characteristics to construct an accurate chronological archive with time windows of up to around a century, resolving the environmental signal annually and even seasonally.