40Ar-39Ar geochronometry of pseudotachylytes by vacuum encapsulation: North Cascade Mountains, Washington, USA

Research Article| January 01, 2001 40Ar-39Ar geochronometry of pseudotachylytes by vacuum encapsulation: North Cascade Mountains, Washington, USA Jerry F. Magloughlin; Jerry F. Magloughlin 1Department of Earth Resources, Colorado State University, Fort Collins, Colorado 80523-1482, USA Search for other works by this author on: GSW Google Scholar Chris M. Hall; Chris M. Hall 2Department of Geological Sciences, University of Michigan, 2534 C.C. Little Building, Ann Arbor, Michigan 48109-1063, USA Search for other works by this author on: GSW Google Scholar Ben A. van der Pluijm Ben A. van der Pluijm 2Department of Geological Sciences, University of Michigan, 2534 C.C. Little Building, Ann Arbor, Michigan 48109-1063, USA Search for other works by this author on: GSW Google Scholar Author and Article Information Jerry F. Magloughlin 1Department of Earth Resources, Colorado State University, Fort Collins, Colorado 80523-1482, USA Chris M. Hall 2Department of Geological Sciences, University of Michigan, 2534 C.C. Little Building, Ann Arbor, Michigan 48109-1063, USA Ben A. van der Pluijm 2Department of Geological Sciences, University of Michigan, 2534 C.C. Little Building, Ann Arbor, Michigan 48109-1063, USA Publisher: Geological Society of America Received: 17 May 2000 Revision Received: 06 Oct 2000 Accepted: 06 Oct 2000 First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (2001) 29 (1): 51–54. https://doi.org/10.1130/0091-7613(2001)029<0051:AAGOPB>2.0.CO;2 Article history Received: 17 May 2000 Revision Received: 06 Oct 2000 Accepted: 06 Oct 2000 First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation Jerry F. Magloughlin, Chris M. Hall, Ben A. van der Pluijm; 40Ar-39Ar geochronometry of pseudotachylytes by vacuum encapsulation: North Cascade Mountains, Washington, USA. Geology 2001;; 29 (1): 51–54. doi: https://doi.org/10.1130/0091-7613(2001)029<0051:AAGOPB>2.0.CO;2 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 To determine a reliable means of dating pseudotachylytes, we obtained total gas ages, plateau ages and argon-retention ages on samples from the North Cascade Mountains, Washington. Quartz ampoule vacuum-encapsulation analysis of small grains (a few milligrams) from six samples allowed evaluation of 39Ar loss through recoil during irradiation, and laser ablation on four samples allowed textural control. In microlitic samples, recoil-loss 39Ar ranges from 0.2% to 8.4%. The "total gas" ages incorporating this 39Ar are inconsistent, but retention ages are much more internally consistent, and within analytical error they match or nearly match plateau ages from most of the microlitic samples at 54–55 Ma. Samples analyzed by laser ablation suggest one episode of faulting and pseudotachylyte formation at 55–59 Ma, and an earlier episode at ca. 80 Ma. The partly glassy pseudotachylyte yielded laser-ablation, total gas, and retention ages of ca. 65 Ma. The microlitic pseudotachylytes indicate formation close to 55 Ma, with possible older faulting and pseudotachylyte formation at ca. 81–84 Ma. The good consistency among the retention ages, the laser ages with highest precision, and especially the plateau dates, combined with the ages falling within the anticipated time span, provides confidence in the geologic reality of the ages. Nevertheless, it is clear that large clasts and crystal fragments within these particular veins did not completely degas or may have an anomalous inherited radiogenic 40Ar component. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.