苏鲁超高压变质岩区深部流体He-Ar的系统关系:中国大陆科学钻探工程在线流体监测的解析

He-Ar systematics in the crustal fluids from the Sulu ultrahigh pressure metamorphic terrene:Inferences from the on-line monitoring of the CCSD drilling mud gas geochemistry.

中国大陆科学钻探工程在线监测从泥浆中分离出的气体地球化学组成揭示了一段重要的气体异常,从2004年12月24日夜里开始到12月29日晚上结束。从12月10日到24日晚上11点30分的气体的Ar、He和N2基本上沿着趋势A分布,而12月26日早上7点半到29日晚上7点半的数据沿趋势C分布。相对于趋势A,趋势C中的气体含有相对升高的Ar,Ar/He和Ar/N2的平均值分别为3653和0.0142,明显高于空气的比值1800和0.0119。趋势A中的气体的Ar/He和Ar/N2比值分别围绕1851和0.0118变化,其中的Ar/He比值稍微高于空气的比值,但Ar/N2比值近似于空气比值,表明背景地下流体含非常低的Ar,而He和N2主要是大气组分。在趋势A和趋势C之间的数据(时间段B)具有和空气接近的Ar/N2比值,但平均Ar/He比值为3265,明显高于空气比值,反映了该段气体具有相对亏损的He。苏鲁-大别山地区的热年代学研究已经表明云母和角闪石的Ar/Ar的冷却年龄大大地高于磷灰石或锆石的He的冷却年龄,说明Ar在超高压变质地体折返早期就已封闭,而He一直保持开放状态,直到超高压变质岩接近地表。这种Ar和He对温度变化的不同反应,导致大部分的He在超高压岩石折返过程中脱气并释放到空气中,Ar则相对圈闭在固体岩石或封闭的断裂带中。在He-Ar的系统关系上,表现为来自于超高压岩石或断裂带中的流体具有富集Ar、亏损He及升高的Ar/He比值。气体组分从趋势A向趋势C的骤然跳跃,反映了地下流体组分的强烈变化,即具有相对富集Ar的深部流体的贡献大大增强。

In the final stage of Chinese Continental Scientific Drilling Project, the on-line monitoring of the mud gas geochemistry has recorded an anomalous session started at 11 :30 pm on Dec. 24, 2004, and ended abruptly at 7:30 pro on Dec. 29, 2004. The corresponding drilling depths were approximately from 4930 to 5030 in. These gases fall into three groups based on their Ar, He, and N-2 geochemistry. Group A, started from Dec. 10 to 11 :30 pm, Dec. 24, 2004, has Ar, He, and N-2 concentrations, and Ar/He and Ar/N-2 ratios very close to their atmospheric values. Such characteristics demonstrate that Group A gases are dominated by the atmospheric component that was dissolved at equilibrium into the drilling mud. Subtle chemical variations in Group A gases, for example slightly greater than atmospheric Ar/He ratios, suggest that the drilling mud also has minor contributions from crustal fluids with very low load of Ar. Group B, started from 11 :30 pm, Dec. 24 to 7:30 am, Dec. 26, 2004, is characterized by Ar and N2 concentrations similar to their atmospheric values, but lower contents of He ranging from 0.00019% to 0.00056%, averaged at 0. 0031%. As a consequence, Ar/He ratios in most of the Group B gases have elevated values as high as 4683, while they have average Ar/N-2, ratio of 0. 0118, similar to the atmospheric value 0.0119. These characteristics suggest that Group B gases have a contribution from a fluid component with very low or no load of He. Group C which started at 7:30 am, Dec. 26 and ended abruptly at 7 :30 pin, Dec. 29, 2004 is extraordinary in that gases in this group have: (1) highly increased Ar concentrations ranging from 1.0230% up to 1.2108%, and averaged at 1.0883%; (2) lower average concentrations of He (0.00031%) and N-2 (76.504%) which are substantially lower than their atmospheric values; and (3) highly elevated Ar/He and Ar/N, ratios which range from 2374 to 5922, and from 0. 0133 to 0. 0154, respectively. These features in Group C gas geochemistry strongly suggest that another fluid component highly enriched in Ar relative to either He or was incorporated into the drilling mud. This component may be resulted from the differences in Ar and He or N-2 diffusion properties in crustal rocks. Higher blocking temperatures of Ar in K-rich mineral phases A than those of He in U and/or Th rich mineral phases led to most of Ar started to be retained in minerals or grain boundaries in early stages of exhumation of Sulu ultrahigh pressure ( UHP) rocks, in contrast, He kept on opening until UHP rocks reached higher crustal levels close to the Earth surface. This process potentially resulted in highly fractionation of Ar from He, and thus high Ar contents and Ar/Hc ratios in UHP rocks or fluids derived from such rocks. The rapid shift from Group A to Group C gases is an indicative of input A of such extraordinary fluids from UHP rocks to the drilling mud.