科研之友
微信
新浪微博
Facebook
分享链接
摘要
We report on an investigation of the Al-26-Mg-26 isotope systematics in the D'Orbigny and Sahara 99555 angrites. High precision Mg isotope compositions and Al/Mg ratios were measured in mineral separates and whole rock samples from D'Orbigny and Sahara 99555 using multiple-collector inductively coupled plasma mass spectrometry (MC-ICPMS). Plagioclase separates from both angrites have resolvable excesses in Mg-26 (Delta Mg-26) that correlate with their respective Al/Mg ratios. Al-26-Mg-26 systematics in the mineral separates and whole rocks define precise isochrons that correspond to Al-26/Al-27 ratios of (5.06 +/- 0.92) x 10(-7) and (5.13 +/- 1.90) x 10(-7) and initial Delta Mg-26 values of -0.006 +/- 0.040 parts per thousand and -0.016 +/- 0.047%. for D'Orbigny and Sahara 99555, respectively. The slopes and initial Delta Mg-26 values are identical for these two meteorites within errors and the data for both angrites considered together define an isochron corresponding to a Al-26/Al-27 ratio of (5.10 +/- 0.55) x 10(-7) and initial Delta Mg-26 value of -0.012 +/- 0.019. Relative to the Efremovka E60 CAI, the Al-26/Al-27 values reported here for these angrites imply Al-26-Mg-26 ages of 4562.42 +/- 0.29 Ma and 4562.43 +/- 0.53 Ma for D'Orbigny and Sahara 99555, respectively. These Al-26-Mg-26 ages are concordant with model ages determined using other extinct radionuclide chronometers (e.g., Mn-53-Cr-53 and Hf-182-W-182), but are similar to 2 Myr younger than the absolute Pb-207-Pb-206 ages that have been reported recently for these angrites. The reason for this discrepancy is not presently known, but may imply disturbance of one or more of the isotope systems under consideration or a possible bias in the Pb-207-Pb-206 ages of the angrites resulting from natural or analytical causes.
- 出版日期2009-9-1
