Effects of composition and pressure on electronic states of iron in bridgmanite
Effects of composition and pressure on electronic states of iron in bridgmanite
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DOI：
10.2138/am-2020-7309
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发表时间：
2020-07-01
影响因子：
3.1
通讯作者：
Gillet, Philippe
中科院分区：
文献类型：
--
作者：
Dorfman, Susannah M.;Potapkin, Vasily;Gillet, Philippe
下地幔的主要矿物(Mg，Fe，Al)(Fe，Al，Si)O-3中的铁的电子状态控制着地幔的物理性质，包括密度、弹性以及导电性和导热性。然而，铁的电子态的确定一直存在争议，部分原因是对穆斯堡尔谱结果的不同解释，这些结果用于确定铁的自旋态、价态和位置占有率。我们用能域穆斯堡尔谱对一组四个桥锰矿样品进行了研究，这些样品的组成范围很广：10-50%Fe/总阳离子，0-25%Al/总阳离子，12-100%Fe3+/总Fe。在金刚石顶压室中，在Fe3+的高自旋到低自旋跃迁以下和以上的压力下进行的测量提供了一个桥锰矿的穆斯堡尔参考库，并证明了压力和成分对铁的电子态的影响。结果表明，虽然桥锰矿B位中Fe~(3+)的自旋跃迁如预测的那样发生，但它对观测到的1.4 mm/S的四极分裂影响不大，仅使该位的中心位移减少到0 mm/S。因此，在高压下，Fe~(3+)和Fe~(2+)的中心位移可以很容易地区分开来，在高压下，Fe~(3+)和Fe~(2+)呈现出两个明显的穆斯堡尔位，中心位移接近1 mm/S，四极分裂分别为2.4~3.1 mm/S和3.9 mm/S。在桥锰矿地震性质的实验测量中，为了限制成分和氧化还原状态的影响，需要对桥锰矿中的Fe~(3+)/总铁进行正确的定量。在与深部地幔有关的压力下，富铁、混价的桥辉石中，高达20%的铁可能是Fe2.5+的电荷转移组分，这将增强地幔底部富铁不均质的电性和热导率。
Electronic states of iron in the lower mantle's dominant mineral, (Mg,Fe,Al)(Fe,Al,Si)O-3 bridgmanite, control physical properties of the mantle including density, elasticity, and electrical and thermal conductivity. However, the determination of electronic states of iron has been controversial, in part due to different interpretations of Mossbauer spectroscopy results used to identify spin state, valence state, and site occupancy of iron. We applied energy-domain Mossbauer spectroscopy to a set of four bridgmanite samples spanning a wide range of compositions: 10-50% Fe/total cations, 0-25% Al/total cations, 12-100% Fe3+/total Fe. Measurements performed in the diamond-anvil cell at pressures up to 76 GPa below and above the high to low spin transition in Fe3+ provide a Mossbauer reference library for bridgmanite and demonstrate the effects of pressure and composition on electronic states of iron. Results indicate that although the spin transition in Fe3+ in the bridgmanite B-site occurs as predicted, it does not strongly affect the observed quadrupole splitting of 1.4 mm/s, and only decreases center shift for this site to 0 mm/s at similar to 70 GPa. Thus center shift can easily distinguish Fe3+ from Fe2+ at high pressure, which exhibits two distinct Mossbauer sites with center shift similar to 1 mm/s and quadrupole splitting 2.4-3.1 and 3.9 mm/s at similar to 70 GPa. Correct quantification of Fe3+/total Fe in bridgmanite is required to constrain the effects of composition and redox states in experimental measurements of seismic properties of bridgmanite. In Fe-rich, mixed-valence bridgmanite at deep-mantle-relevant pressures, up to similar to 20% of the Fe may be a Fe2.5+ charge transfer component, which should enhance electrical and thermal conductivity in Fe-rich heterogeneities at the base of Earth's mantle.