直至核幔边界的Mg_2SiO_4成分熔体剪切粘滞度的平衡分子动力学研究  

The Shear Viscosity of Mg 2SiO 4 Melt Under the Core Mantle Boundary Conditions Using Equilibrium Molecular Dynamics

作　　者：张毅刚[1] 聂高众[1] 

机构地区：[1]中国科学院地质研究所

出　　处：《岩石学报》1998年第4期409-418,共10页Acta Petrologica Sinica

基　　金：国家杰出青年基金;国家科技部攀登项目

摘　　要：熔体的粘滞度极大地影响熔体的各种动力学行为。虽然有一些有关酸碱性熔体粘滞度的数据，但涉及地球更深部过程的基性和超基性熔体的粘滞度的数据很少。本项研究利用平衡分子动力学方法研究Ｍｇ２ＳｉＯ４成分熔体温度从２１６９Ｋ到５５９５Ｋ，压力从１０５Ｐａ到１３５ＧＰａ的粘滞度，所获得的粘滞度数据介于Ｕｒｂａｉｎｅｔａｌ．（１９８２）和ＩｖａｎｏｖａｎｄＳｔｅｎｇｅｌｍｅｙｅｒ（１９８２）的实验数据之间，比Ｓｈａｗ（１９７２）和Ｐｅｒｓｉｋｏｖ（１９９０）的经验方程高大约高０．６个对数单位。研究结果表明该熔体的粘滞度随温度的变化比酸碱性岩浆要小得多，在大约１０５Ｐａ条件下随温度有如下关系式：ｌｏｇη（Ｐａｓ×１０－１）＝４．２１５×１０３Ｔ（Ｋ）－２．６９９Ｍｇ２ＳｉＯ４熔体粘滞度随压力的增加而线性增加。这与前人关于聚合程度低的液体的行为一致。在５５００Ｋ左右，从１０５Ｐａ到１３５ＧＰａ，它的粘滞度大约变化一个数量级。这与酸碱性熔体相比也是较小的。本项研究获得的粘滞度与压力的关系如下：η（Ｐａｓ×１０－１）＝７．２６７×１０－５Ｐ（ＧＰａ×１０－１）＋４．１５８×１０－３总之，Ｍｇ２ＳｉＯ４熔体粘滞度的温度和压力效应均大?Shear viscosity controls the dynamic behaviors of melts. Although many viscosity data exist for acidic alkaline melts, there are few viscosity data for basic and ultra basic melts at very high temperatures and pressures. These data are especially important for our understanding of dynamic processes in the very deep Earth. In the present study the equilibrium molecular dynamics method is used to obtain the viscosity of Mg 2SiO 4 melt in the temperature range from 2169K to 5595K and pressure from 10 5 Pa to 135 GPa. The potential model used is from Belonoshko and Dubrovinsky (1996). Direct integration of Green Kubo formula is used for obtaining shear viscosity (Allen and Tildesley, 1987). The control parameters for molecular dynamics simulation are as follows: single time step = 0 0002ps, total time step = 24ps, equilibration period = 4ps. The cutoffs were chosen according to Fincham (1993). To have an idea about the influence of these control parameters on the calculated viscosity, we have made 7 molecular dynamics runs all at 3000K and 2 700 g/cm 3. These 7 runs give us an averaged viscosity of 0 0051 Pa·s with a standard deviation of 0 0017. The shear viscosity data obtained in the present study are in between the experimental data of Urbain et al. (1982) and Ivanov and Stengelmeyer (1982), and are higher than the empirical formula of Shaw (1972) and Persikov (1990) by about 0 6 log unit. It is shown that the viscosity of Mg 2SiO 4 melt has a much smaller variance with temperature compared with acidic alkaline type melt. At about 10 5Pa, the relationship between viscosity and temperature for the Mg 2SiO 4 melt is as follows: log η( Pa·s ×10 -1 )=4 215× 10 3T( K )-2 699 with the correlation coefficient R 2=0 837.The viscosity of Mg 2SiO 4 melt is positvely and linearl

关 键 词：核幔边界 Mg2SiO4 熔体 粘滞度 平衡分子 动力学 

分 类 号：P183.2[天文地球—天文学]