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采用增量方法,计算了地幔矿物(特别是MgsiO_3和Mg_2SiO_4同质多相变体)的氧同位素分馏.结果表明,地幔矿物相之间存在如下~(18)O富集顺序:辉石(Mg,Fe,Ca)_2Si_2O_6>橄榄石(Mg,Fe)_2SiO_4>尖晶石型(Mg,Fe)_2SiO_4>铁铁矿型(Mg,Fe,Ca)SiO_3>钙钛矿型(Mg,Fe,Ca)SiO_3.如果地幔内部处于氧同位素完全平衡状态,由于地幔不同深度矿物相化学组成与晶体结构的差异,预计过渡带中尖晶石结构的硅酸盐矿物比下地幔钙钛矿结构的硅酸盐矿物相对富集~(18)O,但相对于上地幔中的橄榄石和辉石亏损~(18)O,从而出现地幔的氧同位素分层.如果假定全球范围同位素平衡,地球内部的化学结构可用下述~(18)O富集顺序来描述:上地壳>下地壳>上地幔>过渡带>下地幔>地核.
Oxygen isotopic fractionation of mantle minerals (especially homogeneous heterogeneous transitions of MgsiO_3 and Mg_2SiO_4) was calculated using an incremental method. The results show that there is an order of ~ (18) O enrichment between the mantle mineral phases: pyroxene , Fe, Ca) _2Si_2O_6> olivine (Mg, Fe) 2SiO_4> spinel (Mg, Fe) 2SiO 4> iron ore type (Mg, Fe, Ca) SiO 3> perovskite type ) SiO_3. If the interior of the mantle is in complete equilibrium with oxygen isotopes, the silicate minerals of the spinel structure in the transitional zone are expected to be lower than the silicic acid of the lower mantle perovskite structure due to differences in the chemical composition of the mineral phases at different depths of the mantle Salt minerals are relatively enriched in ~ (18) O, but depleted in the upper mantle by ~ (18) O from the olivine and pyroxenes in the upper mantle, resulting in the stratification of oxygen isotopes in the mantle. Assuming a global isotopic balance, the chemical structure of the Earth’s interior The following sequence of ~ (18) O enrichment can be used to describe: upper crust> lower crust> upper mantle> transitional zone> lower mantle> nuclei.