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

Abstract

Research Article| October 01, 2010 Volcanic biotite-sanidine 40Ar/39Ar age discordances reflect Ar partitioning and pre-eruption closure in biotite John M. Hora; John M. Hora 1Department of Geoscience, University of Wisconsin, 1215 W. Dayton Street, Madison, Wisconsin 53706, USA2Geowissenschaftliches Zentrum der Universität Göttingen, Abteilung Geochemie, Goldschmidtstraße 1, 37077 Göttingen, Germany Search for other works by this author on: GSW Google Scholar Brad S. Singer; Brad S. Singer 1Department of Geoscience, University of Wisconsin, 1215 W. Dayton Street, Madison, Wisconsin 53706, USA Search for other works by this author on: GSW Google Scholar Brian R. Jicha; Brian R. Jicha 1Department of Geoscience, University of Wisconsin, 1215 W. Dayton Street, Madison, Wisconsin 53706, USA Search for other works by this author on: GSW Google Scholar Brian L. Beard; Brian L. Beard 1Department of Geoscience, University of Wisconsin, 1215 W. Dayton Street, Madison, Wisconsin 53706, USA Search for other works by this author on: GSW Google Scholar Clark M. Johnson; Clark M. Johnson 1Department of Geoscience, University of Wisconsin, 1215 W. Dayton Street, Madison, Wisconsin 53706, USA Search for other works by this author on: GSW Google Scholar Shan de Silva; Shan de Silva 3Department of Geosciences, Oregon State University, Corvallis, Oregon 97331-5506, USA Search for other works by this author on: GSW Google Scholar Morgan Salisbury Morgan Salisbury 3Department of Geosciences, Oregon State University, Corvallis, Oregon 97331-5506, USA Search for other works by this author on: GSW Google Scholar Author and Article Information John M. Hora 1Department of Geoscience, University of Wisconsin, 1215 W. Dayton Street, Madison, Wisconsin 53706, USA2Geowissenschaftliches Zentrum der Universität Göttingen, Abteilung Geochemie, Goldschmidtstraße 1, 37077 Göttingen, Germany Brad S. Singer 1Department of Geoscience, University of Wisconsin, 1215 W. Dayton Street, Madison, Wisconsin 53706, USA Brian R. Jicha 1Department of Geoscience, University of Wisconsin, 1215 W. Dayton Street, Madison, Wisconsin 53706, USA Brian L. Beard 1Department of Geoscience, University of Wisconsin, 1215 W. Dayton Street, Madison, Wisconsin 53706, USA Clark M. Johnson 1Department of Geoscience, University of Wisconsin, 1215 W. Dayton Street, Madison, Wisconsin 53706, USA Shan de Silva 3Department of Geosciences, Oregon State University, Corvallis, Oregon 97331-5506, USA Morgan Salisbury 3Department of Geosciences, Oregon State University, Corvallis, Oregon 97331-5506, USA Publisher: Geological Society of America Received: 27 Jan 2010 Revision Received: 18 May 2010 Accepted: 21 May 2010 First Online: 09 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 © 2010 Geological Society of America Geology (2010) 38 (10): 923–926. https://doi.org/10.1130/G31064.1 Article history Received: 27 Jan 2010 Revision Received: 18 May 2010 Accepted: 21 May 2010 First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation John M. Hora, Brad S. Singer, Brian R. Jicha, Brian L. Beard, Clark M. Johnson, Shan de Silva, Morgan Salisbury; Volcanic biotite-sanidine 40Ar/39Ar age discordances reflect Ar partitioning and pre-eruption closure in biotite. Geology 2010;; 38 (10): 923–926. doi: https://doi.org/10.1130/G31064.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract 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. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.