HR: 11:05h
AN: V32B-04    [Abstracts]
TI: NeAr Dating: New Dimensions for Ar-Ar Dating Using Nucleogenic Neon Isotopes
AU: * Hall, C M
EM: cmhall@umich.edu
AF: Dept. of Geological Sciences, U. of Michigan, 1100 N University Ave, Ann Arbor, MI 48109-1005, United States
AB: The neutron reactions that produce ³⁷Ar from Ca, ³⁸Ar from Cl and ³⁹Ar from K form the very heart of the ⁴⁰Ar/³⁹Ar dating system. Not only can ages be derived, but much can be deduced from the effective mineral separation performed by step-heating analysis. However, the normal suite of elements detected using Ar isotopes cannot determine the presence of some minerals. Specifically, the absence of Na means that it is not possible in principle to uniquely determine the compositions of degassing feldpsars and the inability to measure Mg limits the discrimination of some mafic phases. Mineral and glass samples of known composition have been irradiated to determine the important nucleogenic Ne isotopes produced from F, Na and Mg. Mg produces two isotopes from the reactions ²⁴Mg(n,α)²¹Ne and ²⁵Mg(n,α)²²Ne, with a production ratio for (²²Ne/²¹Ne)_Mg of about 0.25. For Na, the important reactions are ²³Na(n,α)²⁰F(β-)²⁰Ne with a production ratio for (²⁰Ne/²²Ne)_Na of about 5.3. The thermal neutron reaction for F is ¹⁹F(n,γ)²⁰F(β-)²⁰Ne with (²⁰Ne)_F/(³⁹Ar)_K equal to about 1.1. Because there are only 3 isotopes and 4 end member isotopic compositions, it is not possible to uniquely deconvolve the above nucleogenic sources along with atmospheric Ne. Fortunately, most unirradiated minerals analyzed have had extremely low levels of atmospheric Ne. A maximal correction for atmospheric Ne can be done assuming an atmospheric ²⁰Ne/³⁶Ar ratio. Measuring Ne isotopes along with Ar isotopes is challenging, requiring extra time and cryo-separation of the two species. In addition, there are unresolved issues dealing with the relative rates of Ne and Ar diffusion and Ne recoil effects. However, there is promise for the method for all whole-rock samples, amphiboles, feldspars and any mineral with expected complex exsolution textures. Examples of a variety of Ne-enhanced argon age spectra will be shown.
DE: 1040 Radiogenic isotope geochemistry
DE: 1100 GEOCHRONOLOGY
DE: 1115 Radioisotope geochronology
DE: 1194 Instruments and techniques
DE: 3600 MINERALOGY AND PETROLOGY
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting