江苏东海超高压榴辉岩的热导率及对大陆科学钻探研究的意义

Thermal Conductivity of Donghai UHP Eclogite and Its Significance for Studying Continental Scientific Drilling

对采自江苏东海毛北地区 (中国大陆科学钻探先导孔附近 )的新鲜榴辉岩样品进行了岩石热导率的测定 ,初步查明了该区榴辉岩热导率随矿物组成的变化关系 ,探讨了岩石结构特征和温度变化对热导率的影响 .本次所测东海超高压榴辉岩的热导率介于 3 .2 2 2～ 3.716Wm-1·K-1之间并随岩石中 2种主要矿物的相对含量比而变化 ,随着榴辉岩中石榴石对绿辉石体积比 (VGrt/VOmp)的增加而降低 ,近似的函数关系满足K =3 .76 7- 0 .18× (VGrt/VOmp) .岩石中矿物分布的不均匀性和面状构造的发育对榴辉岩热导率的影响较大 ,由此产生的热导率各向异性可达近 10 % .温度是影响热导率的另外一个重要因素 .结合本次的实测资料和相应的热导率 -温度关系 ,建立了东海地区榴辉岩热导率随温度的变化关系方程K(T)=1/ ( 7.85× 10 -2 +6 .95× 10 -4 ×T) ,根据这一方程并结合东海地区的地热梯度资料推算了榴辉岩热导率随 5 0 0 0m钻孔深度的变化关系 ,推测东海地区科学钻探施工至 5 0 0 0m深度时 ,榴辉岩的热导率将比地表平均降低 2 4% .该成果为钻探测井资料的解释以及该区地热结构模型的建立提供了重要依据和约束资料 .

UHP eclogite samples were collected from surface exposures around Chinese Continental Scientific Drilling (CCSD) drill-site in Donghai area have been measured on thermal conductivity to investigate the effect of mineral components and texture on thermal conductivity (TC) of eclogite. Measured thermal conductivities vary from 3.222 to 3.716 Wm -1·K -1 with average value 3.511 Wm -1·K -1, which depend on the volume ratio of garnet and omphacite (V Grt/V Omp) in the rocks. This correlation was fitted to the function K=3.767-0.18×(V Grt/V Omp), which shows the thermal conductivity of eclogite in this area decreased as increasing of V Grt/V Omp. Inhomogeneous distribution of minerals and the foliation texture in rocks also significantly affect the value of thermal conductivity and induce the anisotropy up to 10% in eclogite. For temperature dependence, according to calculations from the correlation between K-T, thermal conductivities under high temperature were also fitted to a function: K(T)=1/(7.85×10 -2+6.95×10 -4×T), T is absolute temperature. Based on this function and the published geothermal data of this area, a depth dependence of thermal conductivity can be concluded. The K of eclogite decreased as the increasing depth of CCSD drill hole. The K values of eclogite in surface and in bottom of hole are 3.511 Wm -1·K -1 and 2.687 Wm -1·K -1, respectively. The K of eclogite will be predicted to be decreased about 24% from surface to the end of 5 000 m depth of CCSD. These research results are helpful to establish the geothermal model of this area and to interpret well logging results from CCSD.