蛇绿岩地幔岩中自由SiO2的发现及其地质意义

自由SiO_2系指石英及其同质多型物(polymorphs)柯石英、斯石英等。石英广泛分布于地壳中的各种岩石中,柯石英和斯石英只存在于超高压岩石和陨石坑中。由于石英和非饱和SiO_2的橄榄石不能共生,因此在地幔橄榄岩和超镁铁岩中不存在原生石英。最近笔者在西藏罗布莎蛇绿岩的地幔岩(方辉橄榄岩)的豆荚状铬铁矿中发现了自由SiO_2和柯石英相。根据高温高压相平衡实验资料,橄榄石、辉石这样的硅酸盐矿物在地幔深部的压力条件下可以分解成简单氧化物,如FeO(方铁矿)、MgO(方镁石)以及SiO_2(斯石英)等。由此推测,西藏蛇绿岩地幔岩中自由SiO_2可能是来自于下地幔的矿物,是地幔柱作用将其搬运到上地幔浅部。

The discovery of free SiO2 in mantle peridotite of ophiolite, Tibet

Free SiO-2 is represented by quartz, coesite and stishovite polymorphs. Quartz widely occurs in different rocks of the earth's crust, whereas coesite and stishovite are only formed under high pressures and temperatures. Since olivine is silica-unsaturated, quartz and olivine never coexist in the same rock. Therefore, in mantle peridotite and other ultramafic rocks, primary quartz can never be found. Recently, free SiO-2 (quartz and coesite) was discovered from podiform chromitites in mantle peridotites of Luobusa ophiolite, Tibet. High-pressure and high-temperature phase equilibrium experiments show that silicates such as olivine and pyroxene will be decomposed into simple oxides like FeO (wustite), MgO (periclase) and SiO-2 (stishovite) beneath the lower mantle (>670 km). It is thus thought that the free silica (quartz and coesite) associated with mantle minerals was possibly derived from the lower mantle and that mantle plume might have been the mechanism for transporting these minerals to the shallow part of the ophiolitic mantle.