• journal_title：Geology
• Contributor：John M. Hora ; Brad S. Singer ; Brian R. Jicha ; Brian L. Beard ; Clark M. Johnson ; Shan de Silva ; Morgan Salisbury
• Publisher：Geological Society of America
• Date：2010-
• Format：text/html
• Language：en
• Identifier：10.1130/G31064.1
• journal_abbrev：Geology
• issn：0091-7613
• volume：38
• issue：10
• firstpage：923
• section：Articles

The ⁴⁰Ar/³⁹Ar 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 ¹⁴C and longer-lived isotopic systems that are too imprecise for dating young samples. However, ⁴⁰Ar/³⁹Ar ages obtained from coevally erupted biotite and sanidine do not always match. Here, we use an independent chronometer, ²³⁸U-²³⁰Th disequilibrium, to demonstrate that ⁴⁰Ar/³⁹Ar age disparity is not caused by differences in pre-eruption crystallization times. Our findings indicate that the presence of extraneous ⁴⁰Ar in biotite, and its absence in sanidine, may result from violations of two assumptions implicit in ⁴⁰Ar/³⁹Ar geochronology on volcanic samples: (1) Prior to eruption, minerals are devoid of ⁴⁰Ar 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.