chris@mimsy.UUCP (Chris Torek) (04/11/88)
>>In article <4006@cup.portal.com> roger_warren_tang@cup.portal.com writes: >>>"Radiation counts used to date things before a supposed catastrophe cannot [be trusted etc; I never saw the original, only mangled quotations] >In article <5300@uwmcsd1.UUCP> markh@csd4.milw.wisc.edu (Mark >William Hopkins) writes: >>Because the catastrophe itself may be a large source of radiation. In article <116@slu70.UUCP> guy@slu70.UUCP (Guy M. Smith) writes: >External radiation is irrelevant to radiometric dating. Right. The key to understanding this is that the dating is based on *ratios* of particular isotopes: >The technique involves measuring the amounts of specific radioactive >isotopes (e.g., Rb^87) and their stable daughter products (Sr^87 in >this case). Crudely, the Rb/Sr ratio is controlled by the age of the >rock as Rb converts to Sr at a known rate. External radiation has no >influence on the rate of decay. If I may say so, markh has the right idea but the wrong details, something he seems to do all too often. In this case the details are the important part. The point is not that the event (`catastrophe') may be a source of radiation, but rather that it may be a source of the isotopes used in dating. This changes the whole focus of the argument. If the absolute counts of either Rubidium-87 or Strontium-87 were influenced by the event, the ratio function [Rb/Sr](t) might have a sudden discontinuity. This can be checked, however, by choosing two more isotopes; as the number of isotopes checked goes up, the probability of error goes down. Moreover, there may be an independent means of verifying that the event did not affect the isotope ratio. Whether this is in fact the case I cannot say, although I doubt that anyone doing such dating would not anticipate this line of argument and have countered it before it even began. -- In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7163) Domain: chris@mimsy.umd.edu Path: uunet!mimsy!chris
hal@pur-phy (Hal Chambers) (04/11/88)
In article <11009@mimsy.UUCP> chris@mimsy.UUCP (Chris Torek) writes: >If the absolute counts of either Rubidium-87 or Strontium-87 were >influenced by the event, the ratio function [Rb/Sr](t) might have a >sudden discontinuity. This can be checked, however, by choosing two >more isotopes; as the number of isotopes checked goes up, the >probability of error goes down. As an example of the danger of using a single isotope ratio, several historic (19th centry) lava flows at Mauna Kea (sp?) have a Rb/Sr date of several hundred million years. Hal Chambers
guy@slu70.UUCP (Guy M. Smith) (04/13/88)
In article <11009@mimsy.UUCP>, chris@mimsy.UUCP (Chris Torek) writes: > If the absolute counts of either Rubidium-87 or Strontium-87 were > influenced by the event, the ratio function [Rb/Sr](t) might have a The problem with this idea is that you need to insert these isotopes into solid rock. Most geochronologists start with a large chunk of solid unaltered (hopefully) rock and take steps to eliminate contamination (like removing the weathering rind, etc). They then either grind up the whole rock or they extract certain minerals (generally a better technique). Uranium-lead dating is commonly done with zircons extracted from the original sample (you need 50-100 kg to start). It is difficult to see how an event of the sort under discussion could affect the elements buried centimeters or meters below the surface of a solid igneous rock. It would affect sediments but they are generally unsuitable for radiometric dating. As discussed by another poster, the exception is carbon-14 dating.