Notices
Results 1 to 4 of 4

Thread: Radiometric Dating

  1. #1 Radiometric Dating 
    Forum Masters Degree Golkarian's Avatar
    Join Date
    Nov 2008
    Posts
    510
    I've heard that at Mt. St. Helens there were inaccurate dates given and apparently this was due to ineffective facilities being used (for that dates presumed), how would one tell if a facility was appropriate for a certain rock or fossil sample?


    Reply With Quote  
     

  2.  
     

  3. #2  
    Universal Mind John Galt's Avatar
    Join Date
    Jul 2005
    Posts
    14,169
    Dating on dacite from the lava dome was carried out by Steven Austin, a creationist. His research was published here. This is the abstract of the work

    The conventional K-Ar dating method was applied to the 1986 dacite flow from the new lava dome at Mount St. Helens, Washington. Porphyritic dacite which solidified on the surface of the lava dome in 1986 gives a whole rock K-Ar 'age ' of 0.35 ± 0.05 million years (Ma). Mineral concentrates from the dacite which formed in 1986 give K-Ar 'ages 'from 0.34 ± 0.06 Ma (feldspar-glass concentrate) to 2.8 ± 0.6 Ma (pyroxene concentrate). These 'ages 'are, of course, preposterous. The fundamental dating assumption ('no radiogenic argon was present when the rock formed ') is questioned by these data. Instead, data from this Mount St. Helens dacite argue that significant 'excess argon 'was present when the lava solidified in 1986. Phenocrysts of orthopyroxene, hornblende and plagioclase are interpreted to have occluded argon within their mineral structures deep in the magma chamber and to have retained this argon after emplacement and solidification of the dacite. The amount of argon occluded is probably a function of the argon pressure when mineral crystallization occurred at depth and/or the tightness of the mineral structure. Orthopyroxene retains the most argon, followed by hornblende, and finally, plagioclase. The lava dome at Mount St. Helens dates very much older than its true age because phenocryst minerals inherit argon from the magma. The study of this Mount St. Helens dacite causes the more fundamental question to be asked—how accurate are K-Ar 'ayes 'from the many other phenocryst-containing lava flows world-wide?

    The error on which this study is based is discussed here as part of a larger discussion debunking creationist attacks on dating methodology.. This is an extract.

    Xenoliths, zoned phenocrysts, and xenocrysts (like metamorphic and weathering features) are often easily identified under the microscope and sometimes even in the field. In some cases, a geologist may be interested in dating xenoliths, zoned phenocrysts, or the xenocrysts. However, obviously, if the geologist is interested in dating the younger matrix, he/she will look for and avoid any xenoliths, zoned phenocrysts or xenocrysts. While mainstream geologists know how to avoid xenoliths, zoned phenocrysts, and xenocrysts when dating igneous rocks, creationist Steve Austin apparently was not careful to avoid them when he conducted his "research" at Mt. St. Helens or the Grand Canyon, see " A Criticism of the ICR's Grand Canyon Dating Project by Chris Stassen and compare with Excess Argon within Mineral Concentrates from the New Dacite Lava Dome at Mt. St. Helens Volcano, by Steven Austin. [/url]


    Reply With Quote  
     

  4. #3  
    Forum Sophomore hokie's Avatar
    Join Date
    Jan 2009
    Posts
    175
    Radiocarbon dating for more recent materials can be crosschecked in some cases using thermo luminescent dating.

    As John points out the dating of materials is dependent on the correct collection of materials to be dated.
    Reply With Quote  
     

  5. #4  
    Universal Mind John Galt's Avatar
    Join Date
    Jul 2005
    Posts
    14,169
    And to build on what hokie has said, there are now many methods of dating, each appropriate for a particualr range of ages. In most cases more than one method can be used, so that ages can be confirmed by different techniques. Before the age of a particular horizon would be generally accepted mutltiple measurments would be required using more than one technique, producing results that were broadly consistent with established ages for adjacent and comparable formations.
    Reply With Quote  
     

Bookmarks
Bookmarks
Posting Permissions
  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •