کد مقاله | کد نشریه | سال انتشار | مقاله انگلیسی | نسخه تمام متن |
---|---|---|---|---|
6429138 | 1634753 | 2014 | 11 صفحه PDF | دانلود رایگان |
- Fine-grained CAI exhibit large and variable nucleosynthetic W isotope anomalies.
- Measured 183W/184W allow to correct 182W/184W of CAI for nucleosynthetic variability.
- Corrected Hf-W data define a bulk CAI isochron yielding solar system initial values.
- Hf-W and Al-Mg age intervals between angrites and CAI show good agreement.
- Inconsistent with a grossly heterogeneous distribution of 26Al in the solar protoplanetary disk.
Ca-Al-rich inclusions (CAI) are the oldest dated objects formed in the solar system and are pivotal reference points in early solar system chronology. Knowledge of their initial 182Hf/180Hf and 182W/184W is essential, not only for obtaining precise Hf-W ages relative to the start of the solar system, but also to assess the distribution of short-lived radionuclides in the early solar nebula. However, the interpretation of Hf-W data for CAI is complicated by nucleosynthetic W isotope variations. To explore their extent and nature, and to better quantify the initial Hf and W isotope compositions of the solar system, we obtained Hf-W data for several fine- and coarse-grained CAI from three CV3 chondrites. The fine-grained CAI exhibit large and variable anomalies in ε183W (εiW equals 0.01% deviation from terrestrial values), extending to much larger anomalies than previously observed in CAI, and reflecting variable abundances of s- and r-process W isotopes. Conversely, the coarse-grained (mostly type B) inclusions show only small (if any) nucleosynthetic W isotope anomalies. The investigated CAI define a precise correlation between initial ε182W and ε183W, providing a direct empirical means to correct the ε182W of any CAI for nucleosynthetic isotope anomalies using their measured ε183W. After correction for nucleosynthetic W isotope variations, the CAI data define an initial 182Hf/180Hf of (1.018±0.043)Ã10â4 and an initial ε182W of â3.49±0.07. The Hf-W formation intervals of the angrites D'Orbigny and Sahara 99555 relative to this CAI initial is 4.8±0.6 Ma, in good agreement with Al-Mg ages of these two angrites. This renders a grossly heterogeneous distribution of 26Al in the inner solar system unlikely, at least in the region were CAI and angrites formed.
Journal: Earth and Planetary Science Letters - Volume 403, 1 October 2014, Pages 317-327