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Abundance and geological significance of rhenium and osmium in pyrite samples from coals

LIU Gui-jian~1, PENG Zi-cheng~1,2 , YANG Gang~ 1 , CHOU Chen-Lin ~31.School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China 2.State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi'an 710075, China 3.Illinois State Geological Survey, Champaign, 3007 Valley Brook Drive, Champaign, IL 61822-6113, USA  
Two pyrite samples from coals in the Lower Shihezi Formation (the Early Permian), Huaibei Coalfield, Anhui Province, South China, have been handpicked and analyzed for abundance and isotopic compositions of rhenium and osmium using negative thermal ion mass spectrometry. These pyrite samples contain 1.22~1.29 ng/g Re and 0.004 6~ 0.005 4 ng/g Os. The model age of pyrite HBP6-4 is (233±9) Ma and that of pyrite HBP6-3 is (73.9±3.2) Ma. The model age of pyrite HBP6-4 (233 Ma) is significantly younger than the Early Permian age (258~286 Ma) of the coal seam. This may be caused by mixing late pyrite in the sample, since pyrite particles in coals may have formed at different stages of coal formation, as well as loss of radiogenic Os isotopes resulting from mineral/water interaction in the coal basin. The second pyrite, HBP6-3, has a model age of 73.9 Ma which is considerably younger than that of HBP6-4. A plausible explanation for such a young model age is that the Re-Os system in this pyrite sample has remained open until 73.9 Ma ago. The γ~TOs values of the two pyrite samples are +17 and +18, respectively, which are remarkably higher than those of mantle-derived rocks, and are consistent with its derivation from recycled crustal materials.
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