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首页> 外文期刊>Earth and Planetary Science Letters: A Letter Journal Devoted to the Development in Time of the Earth and Planetary System >The effect of Al_2O_3 on Fe-Mg partitioning between magnesiowustite and magnesium silicate perovskite
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The effect of Al_2O_3 on Fe-Mg partitioning between magnesiowustite and magnesium silicate perovskite

机译：Al_2O_3对镁硅钙铁矿和硅酸钙钙钛矿中铁-镁的分配的影响

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摘要

We have measured the partitioning of Fe~(2+), Fe~(3+) and Mg~(2+) between magnesiowustite and magnesium silicate perovskite in both Al_2O_3-bearing and Al_2O_3-free systems. Starting compositions with varying Fe/(Fe + Mg) molar ratios were equilibrated in a multianvil apparatus between 24 and 25 GPa and 1650-1900 ℃. A time study indicated that run durations of at least 8 h were required for the partitioning to reach equilibrium at 1650 ℃. Multi-chamber rhenium metal capsules were employed that provided relatively oxidising conditions and allowed up to eight starting materials to be equilibrated in a single experiment. In the Al_2O_3-free system Fe partitions preferentially into magnesiowustite. In the Al_2O_3-bearing system, however, the proportion of Fe that partitions into perovskite increases with the perovskite Al_2O_3 content. The Fe-Mg distribution between the phases has been parameterised as a function of the perovskite Al_2O_3 concentration and we observe the Al_2O_3 dependence to be non-linear. The Fe~(3+)/ΣFe ratios of coexisting perovskite and magnesiowustite crystals were measured using electron energy-loss near-edge structure (ELNES) spectroscopy. In the Al_2O_3-bearing system perovskite Fe~(3+)/ΣFe ratios range between 60 and 80%, which is over three times greater than that measured for Al_2O_3-free perovskite. The results imply that increasing the Al_2O_3 content of perovskite only increases the Fe~(3+) solubility and has no measurable effect on the Fe~(2+) partitioning between magnesiowustite and perovskite. The variation in the Fe~(3+) solubility with the Al~(3+) content of perovskite is non-linear. We propose that this results from substitution of Fe~(3+) onto the perovskite six-fold coordinated site for low Al~(3+) concentrations but substitution of a FeAlO_3 component at higher Al~(3+) contents, where Fe~(3+) is on the eight-fold site charge balanced by Al on the six-fold coordinated site. A comparison with our experimental data suggests that the Fe partitioning between some (Mg, Fe) SiO_3 and (Mg, Fe) O inclusions found in diamonds is quite consistent with their origin as perovskite and magnesiowustite in the lower mantle. The partitioning between some (Mg, Fe)O and (Mg, Fe, Al) (Al, Si) O_3 diamond inclusions, however, is inconsistent with our results.
机译：我们已经测量了在含Al_2O_3-和不含Al_2O_3-的体系中菱镁矿和硅酸钙钙钛矿中Fe〜（2 +），Fe〜（3+）和Mg〜（2+）的分配。在多砧装置中，在24至25 GPa和1650-1900℃之间平衡具有不同的Fe /（Fe + Mg）摩尔比的起始成分。一项时间研究表明，在1650℃下，分配达到平衡需要至少8 h的运行时间。使用多室rh金属胶囊，该胶囊提供了相对氧化的条件，并允许在一次实验中平衡多达八种起始原料。在不含Al_2O_3的系统中，Fe优先分配为菱镁矿。然而，在含Al_2O_3的体系中，分配到钙钛矿中的Fe的比例随着钙钛矿Al_2O_3含量的增加而增加。已经将相之间的Fe-Mg分布参数化为钙钛矿Al_2O_3浓度的函数，并且我们观察到Al_2O_3的依赖性是非线性的。钙钛矿和菱镁矿共存钙钛矿的Fe〜（3 +）/ΣFe比值采用电子能损近缘结构（ELNES）光谱仪测量。在含Al_2O_3的钙钛矿中，Fe〜（3 +）/ΣFe的比例在60％到80％之间，是不含Al_2O_3的钙钛矿的三倍以上。结果表明，增加钙钛矿的Al_2O_3含量只会增加Fe〜（3+）的溶解度，而对菱镁矿和钙钛矿之间的Fe〜（2+）分配没有可测量的影响。 Fe_（3+）溶解度随钙钛矿中Al〜（3+）含量的变化是非线性的。我们认为，这是由于在低Al〜（3+）浓度下将Fe〜（3+）取代到钙钛矿六倍配位位上，而在较高Al〜（3+）含量下取代了FeAlO_3组分，其中Fe〜 （3+）在八重位点上，电荷由Al在六重位点上平衡。与我们的实验数据进行比较表明，钻石中发现的某些（Mg，Fe）SiO_3和（Mg，Fe）O夹杂物中的Fe分配与下地幔中钙钛矿和菱镁矿的起源非常一致。但是，某些（Mg，Fe）O和（Mg，Fe，Al）（Al，Si）O_3金刚石夹杂物之间的分配与我们的结果不一致。

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