Culler, T. S. (2000) Lunar Impact History from 40Ar/39Ar Dating of Glass Spherules. Science, 287 (5459). 1785-1788 doi:10.1126/science.287.5459.1785
Reference Type | Journal (article/letter/editorial) | ||
---|---|---|---|
Title | Lunar Impact History from 40Ar/39Ar Dating of Glass Spherules | ||
Journal | Science | ||
Authors | Culler, T. S. | Author | |
Year | 2000 (March 10) | Volume | 287 |
Page(s) | 1785-1788 | Issue | 5459 |
Publisher | American Association for the Advancement of Science (AAAS) | ||
DOI | doi:10.1126/science.287.5459.1785Search in ResearchGate | ||
Mindat Ref. ID | 2514918 | Long-form Identifier | mindat:1:5:2514918:4 |
GUID | c810f0a2-75b4-4636-aafc-548251d3ecb3 | ||
Full Reference | Culler, T. S. (2000) Lunar Impact History from 40Ar/39Ar Dating of Glass Spherules. Science, 287 (5459). 1785-1788 doi:10.1126/science.287.5459.1785 | ||
Plain Text | Culler, T. S. (2000) Lunar Impact History from 40Ar/39Ar Dating of Glass Spherules. Science, 287 (5459). 1785-1788 doi:10.1126/science.287.5459.1785 | ||
In | (2000, March) Science Vol. 287 (5459) American Association for the Advancement of Science (AAAS) |
References Listed
These are the references the publisher has listed as being connected to the article. Please check the article itself for the full list of references which may differ. Not all references are currently linkable within the Digital Library.
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Not Yet Imported: Geological Society of America Special Papers - book-chapter : 10.1130/SPE247-p155 If you would like this item imported into the Digital Library, please contact us quoting Journal ID 25243 | |
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Each of the 155 Apollo 14 spherules was hand-picked from the 14163 soil sample cleaned and sorted according to color into one of six categories: black/opaque (85 total spherules) yellow (36) gray (13) orange/red (10) green (9) and white (2). These variations in color are thought to relate to the concentration of titanium and to a lesser extent iron in the glass of the spherules [ | |
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]. Each color group was placed in a separate pit in an aluminum irradiation disk along with MMhb-1 [ | |
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] and FCs (31) fast neutron fluence monitors (standards). The samples underwent ∼100 hours of irradiation at the Oregon State University Reactor (fast neutron fluence parameter J = 0.0261 ± 0.001). We used the cadmium-lined in-core instrument transfer facility which virtually eliminates thermal neutron reactions such as 37 Cl(n γ) 38 Ar and 40 K(n p) 40 Ar. Correction factors for interfering isotopes (31) are ( 36 Ar/ 37 Ar) Ca = (2.64 ± 0.02) × 10 −4 ( 39 Ar/ 37 Ar) Ca = (7.04 ± 0.06) × 10 −4 and ( 40 Ar/ 39 Ar) K = (8 ± 3) × 10 −4 . Mass discrimination is monitored by periodic analyses of air pipettes. Each spherule was degassed stepwise by an 8-W Ar-ion laser in 5 to 30 steps depending on the size and potassium concentration of the spherule. The extraction line we used has a cryo-trap operated at −122°C to remove condensable gases and two SAES C-50 getters (one was turned off to lower instrument blanks). Isotopic analyses were done with a MAP 215 noble gas mass spectrometer. The laser extraction line and mass spectrometer are all fully automated. Because the initial isotopic composition of Ar in the spherules cannot be assumed our analytical strategy focused on step-heating to generate data facilitating the use of 40 Ar/ 36 Ar versus 39 Ar/ 36 Ar isochrons. This approach relies on releasing gas comprising binary mixtures of initial Ar (of uniform isotopic composition) with radiogenic Ar ( 40 Ar*). In most cases low laser power steps are enriched in a low 40 Ar/ 36 Ar component whereas heating at higher power produced higher 40 Ar/ 36 Ar because of the release of proportionately more 40 Ar*. In some cases additional components of Ar were clearly associated with anomalous Ca/K (determined from 37 Ar/ 39 Ar) which we construe to reflect Ca-rich clasts that were incompletely degassed during impact melting; we excluded such analyses from isochron regressions. In some other cases more than two components of Ar are indicated by nonlinear scatter on isochron diagrams. In such cases the age error is multiplied by the square root of the mean squared weighted deviates such that isochron ages with excess scatter are deweighted and have little impact on our conclusions. Terrestrial atmospheric contamination is efficiently removed in sample cleaning and preanalysis bakeout as indicated by consistently low 40 Ar/ 36 Ar in even the lowest temperature extraction steps and by the linearity of the isochron data. | |
Summary data for all isochron ages are available at Science Online (www.sciencemag.org/feature/data/1044416.shl). | |
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Supported by the Ann and Gordon Getty Foundation. We thank W. Alvarez A. Deino J. Donovan D. Karner K. Ludwig and T. Teague for discussions and various other invaluable contributions to this research K. Nishiizumi for curating the samples and NASA for providing them. |
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