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位于地下670公里深处的地震不连续带是上、下地幔的分界面,其压强为24GPa,温度为2—3千K。在这样的条件下,上地幔的主要矿物——橄榄石〔(Mg、Fe)_2SiO_4〕、辉石〔(Mg、Fe)SiO_3〕和石榴石〔(Mg、Fe、Ca)_3Al_2Si_2O_2〕将转变为斜方畸变的具钙钛矿结构的〔(Mg、Fe)SiO_3〕矿物或以钙钛矿为主的矿物组合。由于钙钛矿型的硅酸盐至少在70GPa压力下还是稳定的,因而它被认为是下地幔(或许是整个地球)中最主要的矿物。尽管钙钛矿型的硅酸盐很重要,但直到1976年才被发现,而且由于合成这种矿物所需的压力和温度条件很难达到,所以对其物理性质还不甚了解。例如,钙钛矿热膨胀系数对下地幔可能存在的成分模式起着重要的制约作用。最近我们合成了足够量的钙钛矿型的(Mg_0.9,Fe_0.1)SiO_2以便用X射线衍射法测定其在温度达840K时的无压热膨胀。在高温下测得的平均热膨胀系数为4×10~(-6)K~(-1)。这么大的热膨胀系数表明:根据土地幔组分(如地幔岩或石榴石橄榄岩,Mg值≈0.89)的标准模式求得的无压密度,要比推断的下地幔在无压条件下的密度低大纣2%。这一结果显示:上地幔与下地幔的化学性质是不同的;这与地幔热对流的分层模式是一致的。
The seismic discontinuity belt located 670 kilometers underground is the interface between the upper and lower mantle. The pressure is 24 GPa and the temperature is 2-3,000 K. Under such conditions, the main minerals in the upper mantle, such as olivine [(Mg, Fe) 2 SiO 4], pyroxene (Mg, Fe) SiO 3 and garnet [Mg, Fe, Ca 3 Al 2 Si 2 O 2] An orthorhombic [(Mg, Fe) SiO 3] mineral with perovskite structure or a perovskite-based mineral assemblage. Since perovskite type silicate is stable at least under 70 GPa, it is considered the most dominant mineral in the lower mantle, perhaps the entire Earth. Although perovskite silicates are important, they were not discovered until 1976, and their physical properties were poorly understood due to the pressure and temperature conditions required to synthesize them. For example, the thermal expansion coefficient of perovskites plays an important role in controlling the possible compositional modes of the lower mantle. Recently, we synthesized a sufficient amount of perovskite type (Mg_0.9, Fe_0.1) SiO_2 in order to determine its unpressurized thermal expansion at 840K by X-ray diffraction. The average coefficient of thermal expansion measured at high temperature is 4 × 10 ~ (-6) K ~ (-1). Such a large coefficient of thermal expansion suggests that the unpressurized density obtained from the standard model of the mantle component (eg mantle or garnet peridotite, Mg value ≈ 0.89) is less than the density of the underlying mantle under pressureless conditions Low big bang 2%. This result shows that the chemical properties of the upper mantle and the lower mantle are different; this is consistent with the stratified pattern of thermal convection in the mantle.