西藏日喀则地区蛇绿岩的氦同位素研究

STUDY ON THE HELIUM ISOTOPIC COMPOSITIONS OF XIGAZE OPHIOLITES, TIBET

测定了日喀则地区蛇绿岩组合岩石的稀有气体同位素组成。白朗的玄武岩具有比较均一的He同位素组成 ,3He/ 4He平均值为 5 35 9Ra;变橄岩的He同位素比值范围为 1 10 4～ 3 384Ra,平均为 2 383Ra;从东部的仁布和大竹到西部的下鲁和白朗 ,辉绿岩的3He/ 4He由低变高。采于吉定的新鲜辉绿岩平均高达 31 5 7Ra,该值接近于在夏威夷发现的高比值。分步加热法结果显示高R值的He是在低温步释放的 ,根据这种高R值的He可以推测雅鲁藏布江蛇绿岩的成因环境有地幔柱的作用 。

We have investigated the Xigaze ophiolites (Tibet) in the Yarlung Zangbo ophiolite belt (Tibet), SW China, by using noble gases isotopic compositions. The rocks investigated in this paper include serpentinized peridotites, basalts and dolerites. The 3He/ ? 4He ratios in these basalts are similar, having an average value of 5.36R a, and the He contents closely resemble each other. Two exceptions are basalt samples from Dagzhuka, with 3He/ ? 4He ratios that are lower than atmospheric value and He contents higher than those of the other basalts. The He concentrations in the serpentinized peridotites have a large variable, the highest one was found in sample WR-1 from Rinbung (2 18×10 -11mol/g), and the lowest one in sample WL-1 from Laze (0 071 3×10 -11mol/g). However, their 3He/ ? 4He values are comparable to each other, varying between 1 10R a and 3 38R a, with an average value of 2.38R a. The He contents in analyzed dolerites are higher than those in serpentinized peridotites and basalts,having an average of 3 43×10 -11mol/g. The 3He/ ? 4He ratios increase from Dazhuka and Rinbung in the east to Bailang and Xialu in the west. The He isotopic compositions are different among the samples with various alteration degrees. The 3He/ ? 4He ratio in sample with lower alteration degree is higher than that in sample of higher alteration degree. Furthermore, the He content in sample of higher alteration degree is higher than that in sample of lower alteration degree, because larger amount of radiogenic He were introduced into sample of higher alteration degree by metamorphism. The higher 3He/ ? 4He values of average 31.57R a were found in dolerites from Geding. Measurement of 3He/ ? 4He ratios by stepwise heating was carried out on one of the dolerite samples to investigate the micro-area preserving the He with high 3He/ ? 4He ratio. The result shows that the He with a 3He/ ? 4He value as high as 45.91R a was released at lower temperature. It is suggested that the He with high 3He/ ? 4He ratio was trapped in micro-inclusion from ascending magma during the rock formation,rather than the product of cosmogenic processes, because the dolerites are an intrusive rock that has not been strongly irradiated by cosmic rays. Furthermore, no evidence indicates that the high 3He/ ? 4He ratios in the dolerites can be attributed to the contribution of nucleogenic 3He, because both the lithium and potassium contents in dolerites are low. The higher 3He/ ? 4He values indicated in the sample is the representation of magma source. The 3He/ ? 4He ratios of the helium degassed from the dolerite at higher temperature are less than 8R a,and it can be explained by that the remnant primitive He with high 3He/ ? 4He ratio was diluted by radiogenic He in crystal lattice. This conclusion has been confirmed by reduplicate measurement on the same sample, in which the He content is 4 9×10 -12mol/g with 3He/ ? 4He ratio of 34.3R a. Hence, we suggest that the He occurring in Geding dolerites should be assigned to a hot-spot plume type. It is indicated that the Neo-Tethyan oceanic crust in Yarlung Zangbo was formed from ascended plume. This is supported also by the explosive structure of the ultra-high pressure minerals and the coexisting diamond in the Tibet ophiolites. It may imply that there was once a rift valley type ocean basin or an immature deep-large rift belt existed in the Yarlung Zangbo suture zone.