Study on the Re-Os Isotope Composition of Graphite from the Lutang Graphite Deposit in Hunan Province
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摘要: 石墨具有较高的Re、Os含量，可望成为理想的Re-Os同位素测年对象，但迄今国内外研究较少，尤其在煤层经变质作用形成石墨过程中，其中Re-Os同位素体系的变化还有待研究。湖南鲁塘石墨矿是我国典型的隐晶质石墨矿床之一，矿体产于二叠系龙潭组煤系地层中。本文采用Carius管逆王水溶解样品，直接蒸馏、微蒸馏分离纯化Os，丙酮萃取法分离富集Re，热表面电离质谱法对鲁塘矿区石墨样品以及外围原煤进行了Re-Os同位素分析。结果表明：鲁塘石墨的Re含量为0.901~9.794 ng/g，Os含量为7.3~189.5 pg/g，Re-Os同位素等时线年龄为155.6±3.6 Ma，该年龄与鲁塘石墨矿东侧骑田岭岩体第二阶段中粒黑云母花岗岩锆石U-Pb年龄153~157 Ma一致，表明了龙潭组煤层受到骑田岭岩体"烘烤"作用，发生热接触变质作用，使得靠近骑田岭岩体原煤变质为石墨，形成石墨矿床。通过对比石墨、原煤和骑田岭岩体Re、Os含量及比值，发现石墨中的Re、Os主要来源于原煤，并根据石墨Re-Os等时线初始（187Os/188Os）i值（0.686±0.032），推测骑田岭岩体在侵入煤系地层过程中，有少量具有较低187Os/188Os值的Os被碳质吸附。Abstract: Graphite has relatively high Re and Os contents and is an ideal mineral for Re-Os isotope dating. However, there are few studies on Re-Os dating of graphite. Particularly for the graphite which was formed by deformation of coal beds, the variation in Re-Os isotope system needs further study. The Lutang graphite deposit in Hunan is one of the typical cryptocrystalline graphite deposits in China. The ore-bodies are hosted in coals of the Permian Longtan Formation. In this study, the Re-Os isotopic compositions of the graphite samples from Lutang deposit and the raw coal samples from the peripheral area were analyzed by digesting sample in the Carius tube, direct distilling Os from the Carius tube, extracting Re by acetone and determining Re and Os isotopes by Negtive Thermal Ionization Mass Spectrometry. The results indicate that the Re and Os contents are 0.901-9.794 ng/g and 7.3-189.5 pg/g, respectively. The Re-Os isotope isochron age is 155.6±3.6 Ma, which is consistent with the zircon U-Pb age (153-157 Ma) of StageⅡ medium-grained biotite granite of the Qitianling pluton on the east side of the deposit. It also indicates that the coal seam of the Longtan Formation is subjected to 'baking' by the Qitianling pluton, inducing the hot contact metamorphism and changing the raw coal near the Qitianling pluton to graphite to form a graphite deposit. The comparison of Re and Os content and Os isotopic composition between the graphite, coal and Qitianlign pluton demonstrates that the Re and Os in the graphite are mainly derived from raw coal. According to the initial Os isotope ratio of 0.686±0.032, it can be concluded that minor Os with low 187Os/188Os value was absorbed by carbonaceous material during the intrusion of coal seam by the Qitianling pluton.
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Key words:
- Lu tang /
- graphite deposit /
- genesis /
- Re-Os isotope /
- dating
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表 1 鲁塘石墨矿样品Re-Os同位素数据
Table 1. Re-Os isotope data of Lutang graphite deposit
样品种类 原样编号 样品质量(g) Re含量(ng/g) 普通Os含量(ng/g) 187Os含量(ng/g) 187Re/188Os 187Os/188Os 测定值 不确定度 测定值 不确定度 测定值 不确定度 测定值 不确定度 测定值 不确定度 石墨 LT2-1 0.2017 6.037 0.018 0.1650 0.001 0.0245 0.0004 176.1 0.8 1.137 0.018 LT2-2 0.20031 8.021 0.024 0.0891 0.001 0.0212 0.0003 433.6 3.0 1.824 0.029 LT3-1 0.20086 0.901 0.003 0.0073 0.000 0.0022 0.0000 597.3 6.0 2.275 0.031 LT3-2 0.20108 1.214 0.004 0.0051 0.000 0.0024 0.0000 1147 6 3.659 0.024 LT2-3 0.21409 5.447 0.016 0.1562 0.001 0.0230 0.0002 167.9 1.1 1.128 0.010 LT2-4 0.20301 9.794 0.029 0.0900 0.001 0.0238 0.0002 524.0 3.4 2.029 0.019 LT2-5 0.20114 8.156 0.026 0.1009 0.001 0.0222 0.0002 389.0 3.2 1.684 0.018 原煤 13LT-10-1 0.25023 12.39 0.13 0.0727 0.001 0.0123 0.0002 820.5 12.0 1.297 0.028 13LT-10-2 0.25063 15.73 0.22 0.1190 0.001 0.0400 0.0006 636.4 11.6 2.574 0.051 13LT-10-3 0.25052 10.55 0.04 0.0979 0.001 0.0262 0.0002 518.9 5.9 2.052 0.029 13LT-10-4 0.25081 6.56 0.081 0.1639 0.001 0.0271 0.0002 192.7 2.8 1.265 0.015 13LT06-1 0.20035 23.66 0.08 0.1509 0.001 0.0259 0.0002 754.9 4.7 1.316 0.011 13LT06-2 0.20029 15.20 0.06 0.0738 0.001 0.0177 0.0002 991.0 11.2 1.837 0.026 花岗岩 QTL-12 4.001 0.0217 0.0005 0.0066 0.0001 0.00043 0.00001 15.80 0.45 0.4985 0.0130 QTL-14 4.000 0.0629 0.0005 0.0034 0.0000 0.00040 0.00001 90.25 1.01 0.9210 0.0130 QTL-22 4.000 0.0177 0.0006 0.0023 0.0000 0.00026 0.00000 37.45 1.46 0.8882 0.0159 
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