Synthesis, Elasticity, and Spin State of an Intermediate MgSiO 3 ‐FeAlO 3 Bridgmanite: Implications for Iron in Earth's Lower Mantle
Synthesis, Elasticity, and Spin State of an Intermediate MgSiO 3 ‐FeAlO 3 Bridgmanite: Implications for Iron in Earth's Lower Mantle
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中间 MgSiO 3 →FeAlO 3 Bridgmanite 的合成、弹性和自旋态：对地球下地幔中铁的影响
DOI：
10.1029/2020jb019964
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发表时间：
2020
期刊：
影响因子：
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通讯作者：
Li, Jie
中科院分区：
文献类型：
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作者：
Zhu, Feng;Liu, Jiachao;Lai, Xiaojing;Xiao, Yuming;Prakapenka, Vitali;Bi, Wenli;Alp, E. Ercan;Dera, Przemyslaw;Chen, Bin;Li, Jie
含铁铝的桥辉橄榄岩可能是地球下地幔中铁的主要寄主。本文报道了迄今最高Fe~(3+)-Al~(3+)偶联取代度的(Mg_(0.5)Fe~(3+))(Al_(0.5)Si_(0.5))O_3桥锰矿(FA50)的合成。X射线衍射测试表明，在常温下，FA50为LiNbO_3结构。在室温下压缩到18 GPa时，它又转变为水镁石结构，并保持稳定到102GPa和2600GPa.用Birch-Murnaghan状态方程拟合FA50水镁石的状态方程V0=172.1(4)ä3，K0=229(4)Gpa(固定)。计算得到的FA50桥锰矿的体积声速比镁SiO_3桥锰矿低约7.7%，这主要是因为铁的存在使晶胞质量增加了15.5%。这种差异可能代表了Fe~(3+)-Al~(3+)替代引起的声速异常的上限。X射线发射和同步穆斯堡尔谱测量表明，经激光处理后，约6%的Fe3+离子与Al3+发生交换，并在59 GPa时经历了高自旋到低自旋的转变。随着压力的增加，Fe3+的低自旋比例逐渐增加，在80 GPa时达到17-31%。由于富Fe~(3+)-Al~(3+)桥面辉石中的阳离子交换和自旋转变不会引起可分辨的晶胞体积缩小，而低自旋Fe~(3+)组分随压力逐渐增加，自旋转变不会在下地幔产生明显的地震特征。然而，它可能会影响铁的分配和同位素分馏，从而在下地幔引入化学不均匀性。
Fe‐Al‐bearing bridgmanite may be the dominant host for ferric iron in Earth's lower mantle. Here we report the synthesis of (Mg0.5Fe3+0.5)(Al0.5Si0.5)O3bridgmanite (FA50) with the highest Fe3+‐Al3+coupled substitution known to date. X‐ray diffraction measurements showed that at ambient conditions, the FA50 adopted the LiNbO3structure. Upon compression at room temperature to 18 GPa, it transformed back into the bridgmanite structure, which remained stable up to 102 GPa and 2,600 K. Fitting Birch‐Murnaghan equation of state of FA50 bridgmanite yieldsV0= 172.1(4) Å3,K0= 229(4) GPa withK0′ = 4(fixed). The calculated bulk sound velocity of the FA50 bridgmanite is ~7.7% lower than MgSiO3bridgmanite, mainly because the presence of ferric iron increases the unit‐cell mass by 15.5%. This difference likely represents the upper limit of sound velocity anomaly introduced by Fe3+‐Al3+substitution. X‐ray emission and synchrotron Mössbauer spectroscopy measurements showed that after laser annealing, ~6% of Fe3+cations exchanged with Al3+and underwent the high‐ to low‐spin transition at 59 GPa. The low‐spin proportion of Fe3+increased gradually with pressure and reached 17–31% at 80 GPa. Since the cation exchange and spin transition in this Fe3+‐Al3+‐enriched bridgmanite do not cause resolvable unit‐cell volume reduction, and the increase of low‐spin Fe3+fraction with pressure occurs gradually, the spin transition would not produce a distinct seismic signature in the lower mantle. However, it may influence iron partitioning and isotopic fractionation, thus introducing chemical heterogeneity in the lower mantle.