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arXiv physics/0010083physics.geo-ph
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MgSiO3钙钛矿分子动力学压力-体积-温度状态方程及对lower mantle组成的约束

Pressure-volume-temperature equation of state of MgSiO3 perovskite from molecular dynamics and constraints on lower mantle composition

Frederic C. Marton, Joel Ita, Ronald E. Cohen

摘要:

下地幔的组成可以通过检查组成模型的密度和地震速度作为深度的函数来研究。为了做到这一点,有必要知道的体积和热弹性性质的组成成分下地幔条件。采用非经验变分诱导呼吸(VIB)原子间作用势,结合分子动力学模拟,在下地幔温度和压力条件下,确定了MgSiO 3钙钛矿的热状态方程。我们将我们的压力-体积-温度结果拟合为形式为P(V,T)= P0(V,T0)+ Delta Pth(T)的热状态方程,其中T0 = 300 K,P0是等温通用状态方程。热压力Δ Pth可以由线性关系Δ Pth = a + b T表示。在压力为0-140 GPa、温度为300-3000 K的情况下,我们发现V0 = 165.40 A^3,KT 0 = 273 GPa,K 'T0 = 3.86,a = -1.99 GPa,b = 0.00664 GPa K^-1。通过将V0固定为实验确定的值162.49 A^3,并计算沿下地幔地热的密度和体积声速剖面,我们发现下地幔不能仅由XMg在0.9-1.0范围内的(Mg,Fe)SiO 3钙钛矿组成。然而,使用67体积%钙钛矿(XMg = 0.93-0.96)和33体积%镁橄榄石(XMg = 0.82-0.86)的热解组合物,我们得到的密度和速度分布与合理的地热地震模型非常吻合。

英文摘要:

The composition of the lower mantle can be investigated by examining densities and seismic velocities of compositional models as functions of depth. In order to do this it is necessary to know the volumes and thermoelastic properties of the compositional constituents under lower mantle conditions. We determined the thermal equation of state (EOS) of MgSiO3 perovskite using the nonempirical variational induced breathing (VIB) interatomic potential with molecular dynamics simulations at pressures and temperatures of the lower mantle. We fit our pressure-volume-temperature results to a thermal EOS of the form P(V,T) = P0(V,T0) + Delta Pth(T), where T0 = 300 K and P0 is the isothermal Universal EOS. The thermal pressure Delta Pth can be represented by a linear relationship Delta Pth = a + b T. We find V0 = 165.40 A^3, KT0 = 273 GPa, K'T0 = 3.86, a = -1.99 GPa, and b = 0.00664 GPa K^-1 for pressures of 0-140 GPa and temperatures of 300-3000 K. By fixing V0 to the experimentally determined value of 162.49 A^3 and calculating density and bulk sound velocity profiles along a lower mantle geotherm we find that the lower mantle cannot consist solely of (Mg,Fe)SiO3 perovskite with XMg ranging from 0.9-1.0. Using pyrolitic compositions of 67 vol % perovskite (XMg = 0.93-0.96) and 33 vol % magnesiowustite (XMg = 0.82-0.86), however, we obtained density and velocity profiles that are in excellent agreement with seismological models for a reasonable geotherm.

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