Hydrothermal systems develop via interspersed thermal events over tens to hundreds of thousands of years. The timing of how these systems evolve is commonly established via application of geochronology to a variety of phases and/or the indirect correlation of dated stratigraphy. Here we report 40Ar/39Ar results from adularia extracted from a single mineralized fracture in the late Quaternary Tauhara geothermal system of New Zealand. By utilizing both the age and Ar diffusion properties, we demonstrate how adularia can provide reliable temporal and thermal constraints on the evolution of geologically youthful and active geothermal systems. Our results indicate that adularia formation occurred after 30 ka (mean age 15 ± 10 ka), possibly resulting from subsurface fracturing induced by a 25.4 ka hydrothermal eruption. Simulation of transient heat effects upon Ar retention in adularia with respect to the thermal history of the Tauhara geothermal system confirm that this age is consistent with the time of adularia crystallization. Overall, 40Ar/39Ar dating on geologically young hydrothermal adularia with respect to its thermal history may be used to assess the timing and potential events (e.g., eruptions and fracturing) related to hydrothermal system evolution.