Volcanic biotite-sanidine Ar-40/Ar-39 age discordances reflect Ar partitioning and pre-eruption closure in biotite

The Ar-40/Ar-39 radioisotope system is widely used to date eruption and cooling of volcanic tephra-marker horizons that commonly provide the only means of correlating and assigning numerical ages to stratigraphy in which they are contained. This chronometer bridges the gap between C-14 and longer-lived isotopic systems that are too imprecise for dating young samples. However, Ar-40/Ar-39 ages obtained from coevally erupted biotite and sanidine do not always match. Here, we use an independent chronometer, U-238-Th-230 disequilibrium, to demonstrate that Ar-40/Ar-39 age disparity is not caused by differences in pre-eruption crystallization times. Our findings indicate that the presence of extraneous Ar-40 in biotite, and its absence in sanidine, may result from violations of two assumptions implicit in Ar-40/Ar-39 geochronology on volcanic samples: (1) Prior to eruption, minerals are devoid of Ar-40 due to rapid loss to an "infinite reservoir" such as the atmosphere, and (2) closure to volume diffusion is geologically instantaneous and coincident with eruption. We propose a mechanism whereby the presence of extraneous Ar in certain minerals is explained by the relative sequence of four events in a magmatic system: (1) crystallization, (2) mineral closure with respect to Ar diffusion, (3) isotopic equilibration of magmatic and atmospheric Ar, and (4) quenching of the system by eruption. These data have potentially far-reaching implications for studies that depend on geochronological data, necessitating re-evaluation of interpretations based solely on biotite with no independent age control, particularly in young samples where the effects are most pronounced.