Measurement of Thermal Diffusivity for Eclogite and Basalt underHigh Temperature and High Pressure Conditions
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摘要: 榴辉岩和玄武岩是大洋消减带最重要的组成部分，研究其在高温高压条件下的导热性质有助于了解消减带的岩石圈热结构和消减带动力学。以瞬态平面热源法为基础开展高温高压实验，测量了榴辉岩和玄武岩在1.0和2.0GPa、278~973K的热扩散系数。结果表明，二者均随温度的升高而减小；在278K、1.0GPa下，榴辉岩的热扩散系数比玄武岩高约30%，热导率比玄武岩高约50%。在消减板块的深度剖面上，上部玄武岩的导热能力弱，阻碍俯冲板块与周围热的地幔之间的能量交流，使俯冲板块保持冷的、坚硬和脆性，这种特性使应变积累并最终导致破裂和地震的发生；俯冲板块的下半部分，玄武岩-榴辉岩的相变使俯冲板块与周围环境的温差减小，有利于当地的动力学稳定。Abstract: Knowledge of the heat-transport properties of eclogite and basalt, which are the most important parts of the oceanic subduction zone, is essential for understanding the thermal regimes and the geodynamics subduction zone.Based on the pulse heating method, we conducted high pressure experiment, and measured thermal diffusivities of eclogite and basalt at 1.0 and 2.0GPa and 373-973K.The results show that the thermal diffusivity of both eclogite and basalt decreases with the increase of temperature, and that, at 1.0 GPa and room temperature (278K), the thermal diffusivity and thermal conductivity of eclogite are about 30% and 50% higher than that of basalt, respectively.In the depth profiles of the subduction plate, the low thermal conductivity of upper basalt hinders the heat exchange between the subduction plate and the surrounding mantle, so that the subduction plate remains cold, hard and brittle, which results in strain accumulation and eventually leads to rupture and earthquake.In the lower half of the subduction plate, the basalt-eclogite phase transition reduces the temperature difference between the subduction plate and surrounding environment, which is favorable for the local dynamic stability.
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图 1 高温高压下热扩散系数测量样品组装图
Figure 1. Sample assembly for thermal diffusivity measurement at high pressure
图 2 高压下榴辉岩和玄武岩热扩散系数的测量结果
Figure 2. Thermal diffusivity of eclogite andbasalt under high pressure
表 1 样品描述
Table 1. Description of the samples
Sample Color Structure Particlesize Mineral composition Locality andsource Density/(g/cm3) Porosity/(%) Basalt Black Porphyriticisotropy Fine-grained 60% labradorite，20% peridot，15% augite，5% magnetite particles Damapingof HebeiProvince 2.88 2.3 Eclogite Blackgray Crystalloblastic anisotropic Coarsegrained 40% garnet，35% omphacite，20% symplectitic，a smallamount of hornblende Zhucheng ofShandongProvince 3.53 3.3 
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表 2 样品的化学成分及其质量分数
Table 2. Chemical compositions and their mass fraction of the samples
(%) Sample SiO2 Al2O3 FeO* MgO CaO Na2O K2O MnO P2O5 TiO2 L.O.I Total Basalt 44.88 13.78 13.57 8.99 8.93 3.70 0.97 0.17 0.76 2.47 1.78 99.99 Eclogite 45.96 16.16 14.66 7.87 9.07 3.21 0.60 0.22 0.58 1.93 0.78 100.82 Note：(1)FeO* and L.O.I refer to the total iron content and the burning loss, respectively;
(2)The analysis method is XRF (X-Ray Fluorescence).
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表 3 热扩散系数与温度关系以函数D=a+b/T拟合的拟合系数
Table 3. Fitting parameters of thermal diffusivity with D=a+b/T
Sample Pressure/(GPa) a b Coefficient of determination R2 Basalt 1.0 0.64357 157.921 0.9892 Eclogite 1.0 0.66193 242.209 0.9620 Eclogite 2.0 0.70923 259.355 0.9954
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