Physics
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.v31e2020k&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #V31E-2020
Physics
[1160] Geochronology / Planetary And Lunar Geochronology, [1194] Geochronology / Instruments And Techniques, [6240] Planetary Sciences: Solar System Objects / Meteorites And Tektites
Scientific paper
The decay of extinct nuclide 26Al to daughter nuclide 26Mg (half life of 7.3×105y) is considered to be a useful chronometer for the early solar system. The initial 26Al/26Al ratio of the solar system is estimated to be ~5×10-5 from the analyses of Ca, Al-rich refractory inclusions (CAIs) in primitive meteorites by the linear regression of multiple data on 26Al/26Mg vs. 26Mg/26Mg isochron diagram. Anorthite grains in CAIs show high 26Al/26Mg ratios (~400) with an excess of 26Mg/26Mg ratios as high as 100‰. By using the WiscSIMS IMS-1280, the Mg isotope ratio of anorthite can be obtained with a precision of better than 1‰, from which we can resolve time differences of only 10 ky among formation of individual CAIs. In order to obtain highly precise 26Al-26Mg isochrons, it is important to determine an accurate SIMS relative sensitivity factor (RSF) of 26Al/26Mg ratios from the analyses of plagioclase standard. However, uncertainties of Mg concentration in plagioclase standards are typically ~10% due to low concentrations of MgO (≤0.1wt.%) in naturally occurring plagioclase crystals. In order to obtain accurate initial 26Al/26Al ratios of CAIs, anorthite glass standards were prepared at the University of Chicago with MgO contents of 0.1%, 0.5% and 1.0 wt.% by melting Mg-, Ca-, Al-, and Si-oxides at 1620°C in N2 for 22 to 35 hours. Major and minor element concentrations (including MgO) of the glasses were analyzed precisely by EPMA (Cameca SX51 at University of Wisconsin). Operating conditions were 15 kV, 10 nA, 10 μm wide beam; for Mg, both peak and background were measured for 30 seconds each, and differential PHA was used. A synthetic akermanite standard was used, and a blank correction (0.01 wt% Mg) in Probe for EPMA software was applied. By obtaining ~30 EPMA point analyses for esch glass, MgO concentrations in these glasses were determined with precision of 1-3%. The homogeneity of Mg concentrations in the individual glass were examined by using SIMS and they show less than 1% variability on 26Al/26Mg SIMS raw measured ratios. By linear regression of EPMA-measured 26Mg/26Al ratios of anorthite glass standards against their SIMS raw 26Mg/26Al ratios (calibration line), we obtain an RSF for 26Al/26Mg ratios for SIMS anorthite measurements with the precision of 1%. By applying the RSF from the anorthite glass standards, we found that the initial 26Al/26Al ratio of one melted (igneous) CAI from Leoville meteorite was (5.053±0.040) ×10-5, which is possibly ~40 ky younger than those from unmelted and bulk CAIs. This small time difference may correspond to an active stage of protoplanetary disk evolution during the formation of young sun.
Davis Aileen M.
Fournelle John
Kita Noriko
Knight Keith
Mendybaev Ruslan
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