[meteorite-list] 26-AL isotope counting
From: Thetoprok_at_aol.com <Thetoprok_at_meteoritecentral.com>
Date: Thu Apr 22 10:22:35 2004 Message-ID: <180.1b53b63c.2c13575d_at_aol.com> --part1_180.1b53b63c.2c13575d_boundary Content-Type: text/plain; charset="US-ASCII" Content-Transfer-Encoding: 7bit Greetings List, I have a technical question / observation I'd like to discuss with some of you knowledgeabe in the area of isotopic counting, particularly 26-AL. It seems to me that there is some conflicting information out there concerning the true accuracy and / or validity of this test when determining ages and origins, (terr. or non terr.) through cosmic exposure. The recent issue with Shirokovsky brought this subject back to the surface for me. Don't get me wrong, this is not about Shirokovskys' pseudo-meteorite status, that rock is apparently not a meteorite for many reasons. It is about the comments made in response to this stones cosmic exposure test, that it has not experienced any time in space, based on 26-AL counting, asserting that this test is an absolute fool proof determinator. I have read in several publications, information that seems to contradict that reasoning. I have also spoken with several semi experts, (one author and a couple meteorcists) on the subject and opinions are not unanimous. Just recently a post from Ron Baalke discusses this, here is a snipet, pay particular a ttention to the last sentence; <Like all radioactive isotopes, short-lived ones decay to another <isotope. It is the distinctive nature of the daughter isotopes that record <the presence of the short-lived, extinct isotope. For example, 26Al decays <into magnesium-26, 26Mg. If present in a mineral grain that contains a small <amount of magnesium (most of which is in the form of non-radioactive 24Mg), <its decay leads to an anomalously high ratio of 26Mg to 24Mg. (In cases <where there is a lot of magnesium, the presence of 26Al cannot be <determined. And yet another reference; A quote from Harry Y. McSweens book, "Meteorites and Their Parent Planets", "The partitioning of 26 Al into eucrite magmas during melting caused the crust to become very hot, effectively insulating the mantle and preventing heat from escaping from the deep interior. Vesta lost heat so slowly that parts of its mantle remained molten long after the short lived 26 Al was exhausted, accounting for the general absence in eucrites of detectable 26 Mg, the decay product of 26 Al". What am I missing? I think I read in O. Richard Nortons first book that the Allende meteorites do not have any detectable 26 Al, is this true? If it is, and had Allende not been widely wittnessed, would it still be a meteorite? Is there any 'one' test that proves beyond a shadow of doubt if a stone is earthly or not? Thanks, Larry Atkins --part1_180.1b53b63c.2c13575d_boundary Content-Type: text/html; charset="US-ASCII" Content-Transfer-Encoding: quoted-printable <HTML><FONT FACE=3Darial,helvetica><FONT SIZE=3D2 FAMILY=3D"SANSSERIF" FACE= =3D"Arial" LANG=3D"0"><BR> Greetings List,<BR> <BR> I have a technical question / observation I'd like to discuss with some of y= ou knowledgeabe in the area of isotopic counting, particularly 26-AL.<BR> It seems to me that there is some conflicting information out there concerni= ng the true accuracy and / or validity of this test when determining a= ges and origins, (terr. or non terr.) through cosmic exposure. <BR> <BR> The recent issue with Shirokovsky brought this subject back to the surface f= or me. Don't get me wrong, this is not about Shirokovskys' pseudo-meteorite=20= status, that rock is apparently not a meteorite for many reasons. It is abou= t the comments made in response to this stones cosmic exposure test, that it= has not experienced any time in space, based on 26-AL counting, asserting t= hat this test is an absolute fool proof determinator.<BR> <BR> I have read in several publications, information that seems to contradict th= at reasoning. I have also spoken with several semi experts, (one author and=20= a couple meteorcists) on the subject and opinions are not unanimous. Just re= cently a post from Ron Baalke discusses this, here is a snipet, pay particul= ar attention to the last sentence;<BR> <BR> <BR> <BR> <Like all radioactive isotopes, short-lived ones decay to another<BR> <isotope. It is the distinctive nature of the daughter isotopes that reco= rd<BR> <the presence of the short-lived, extinct isotope. For example, 26Al deca= ys<BR> <into magnesium-26, 26Mg. If present in a mineral grain that contains a s= mall<BR> <amount of magnesium (most of which is in the form of non-radioactive 24M= g),<BR> <its decay leads to an anomalously high ratio of 26Mg to 24Mg. (In cases<= BR> <where there is a lot of magnesium, the presence of 26Al cannot be<BR> <determined.<BR> <BR> And yet another reference;<BR> <BR> A quote from Harry Y. McSweens book, "<I>Meteorites and Their Parent Planet= s"</I>, <BR> <BR> "The partitioning of 26 Al into eucrite magma= s during melting caused the crust to become very hot, effectively insulating= the mantle and preventing heat from escaping from the deep interior. Vesta=20= lost heat so slowly that parts of its mantle remained molten long after the=20= short lived 26 Al was exhausted, accounting for the general absence in eucri= tes of detectable 26 Mg, the decay product of 26 Al".<BR> <BR> What am I missing? <BR> <BR> I think I read in O. Richard Nortons first book that the Allende meteorites=20= do not have any detectable 26 Al, is this true? If it is, and had Allende no= t been widely wittnessed, would it still be a meteorite?<BR> <BR> Is there any 'one' test that proves beyond a shadow of doubt if a stone is e= arthly or not?<BR> <BR> Thanks,<BR> Larry Atkins<BR> <BR> </FONT></HTML> --part1_180.1b53b63c.2c13575d_boundary-- Received on Sat 07 Jun 2003 10:57:33 AM PDT |
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