Experimental evidence for silica-enriched Earth’s lower mantle with ferrous iron dominant bridgmanite
Experimental evidence for silica-enriched Earth’s lower mantle with ferrous iron dominant bridgmanite
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DOI：
10.1073/pnas.1917096117
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发表时间：
2020-10
期刊：
影响因子：
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通讯作者：
I. Mashino;M. Murakami;N. Miyajima;S. Petitgirard
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文献类型：
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作者：
I. Mashino;M. Murakami;N. Miyajima;S. Petitgirard
含铁硼镁石是地球内部最丰富的矿物。我们克服了测量含铁材料的实验挑战，使用布里渊散射技术优化的极端压力条件下结合金刚石压砧细胞。本文报道了用这种方法测量整个地幔压力范围内声波速度的结果。根据我们的结果，我们表明下地幔的大部分都富含Fe 2+，因为我们的模型的梯度与初步参考地球模型的梯度在几乎所有下地幔压力范围内（Fe 3 +/ΣFe = 0.2）都非常拟合。我们的下地幔模型也显示了一个独特的富硅的组成与Mg/Si的1.14相对于上地幔。确定地幔的化学成分对于了解地球的演化、动力学和起源具有重要意义。然而，由于缺乏全地幔高压条件下含铁硼镁石（Brd）声速的实验数据，制约了下地幔的矿物学模型。为了揭示这些问题，我们已经进行了声速测量含铁的Brd在金刚石压砧室（DAC）高达124 GPa使用布里渊散射光谱。结果表明，在整个下地幔压力范围内，含铁Brd的声速随含铁量呈明显的线性减小。我们的数据与整个富Fe ~（2+）Brd下地幔的地震结构有很好的吻合，表明富Fe ~（2+）Brd下地幔的大部分可能被占据。我们的下地幔模型显示了一个独特的富硅的组成与Mg/Si为1.14相对于上地幔（Mg/Si = 1.25），这意味着地幔对流已经足够低的化学均匀化地球的整个地幔。
Significance Iron-bearing bridgmanite is the most abundant mineral in the Earth’s interior. We overcome the experimental challenge of measuring iron-bearing materials by using Brillouin scattering techniques optimized for extreme pressure conditions in combination of diamond-anvil cell. By using such techniques, results of acoustic wave velocities in the pressure range of the entire mantle are reported here. Based on our results, we show that the greater part of the lower mantle is Fe2+-rich because the gradient of our model gives excellent fit to that of Preliminary Reference Earth Model for almost all the pressure range of the lower mantle with an Fe3+/ΣFe of 0.2. Our lower-mantle model also shows a distinctive Si-enriched composition with Mg/Si of 1.14 relative to the upper mantle. Determination of the chemical composition of the Earth’s mantle is of prime importance to understand the evolution, dynamics, and origin of the Earth. However, there is a lack of experimental data on sound velocity of iron-bearing Bridgmanite (Brd) under relevant high-pressure conditions of the whole mantle, which prevents constraints on the mineralogical model of the lower mantle. To uncover these issues, we have conducted sound-velocity measurement of iron-bearing Brd in a diamond-anvil cell (DAC) up to 124 GPa using Brillouin scattering spectroscopy. Here we show that the sound velocities of iron-bearing Brd throughout the whole pressure range of lower mantle exhibit an apparent linear reduction with the iron content. Our data fit remarkably with the seismic structure throughout the lower mantle with Fe2+-enriched Brd, indicating that the greater part of the lower mantle could be occupied by Fe2+-enriched Brd. Our lower-mantle model shows a distinctive Si-enriched composition with Mg/Si of 1.14 relative to the upper mantle (Mg/Si = 1.25), which implies that the mantle convection has been inefficient enough to chemically homogenize the Earth’s whole mantle.