煤中黄铁矿的铼-锇同位素含量及其地质意义

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

采用Carius管溶样方法，通过热电离质谱对淮北煤田煤中黄铁矿样品的Re、Os含量及其同位素进行了测定，得出黄铁矿样品中Re和Os含量分别为l_22～1．29ng／g和0．0046～0．0054ng／g。对两个样品同位素定年测定得出，其年龄值分别为（73．9±3．2）Ma和（33±9）Ma，两个样品的年龄相差约258-286Ma。含量和同位素年龄差值表明两个样品是不同时代形成的黄铁矿。Re-Os同位素体系比值揭示黄铁矿所赋存的地质体受到来自地壳物质的显著混染。另外，γ^TOs参数也证实了这一点。该参数分别为＋17和＋18，表明了有富含Re母体的地壳物质的加入，这为煤中Re-Os含量及地质理论研究提供了参考资料。

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.