Precise determination of phase relations in pyrolite across the 660 km seismic discontinuity by in situ X-ray diffraction and quench experiments
Precise determination of phase relations in pyrolite across the 660 km seismic discontinuity by in situ X-ray diffraction and quench experiments
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DOI：
10.1016/j.pepi.2003.08.010
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发表时间：
2004-06-15
影响因子：
2.3
通讯作者：
Funakoshi, K
中科院分区：
文献类型：
--
作者：
Nishiyama, N;Irifune, T;Funakoshi, K
通过原位 X 射线衍射和淬火实验，在接近 660 km 地震间断面的压力和 1600 ℃ 的固定温度下，观察了叶绿石成分随压力增加的矿物组合变化。根据原位X射线衍射实验获得的结果，在约20-22GPa的压力下观察到了尖橄榄石(Rw)与多数石榴石和富含CaSiO3的钙钛矿。根据 Matsui 等人的方镁石压力标度，尖菱铁矿分解为富含 MgSiO3 的钙钛矿和镁方铁矿 (Mw)，在 22.0 +/- 0.2 GPa 下完成，根据 Shim 等人的旧压力标度，在 21.7 +/- 0.1 GPa 下完成。多数石榴石持续到约 24 GPa，此时叶绿石转变为下地幔矿物组合，即 MgSiO3-钙钛矿、富含 CaSiO3 的钙钛矿和镁方铁矿。因此，多数石榴石与下地幔组合在约 22-24 GPa 的压力下共存。在淬火实验中，在22.5 GPa和1600℃下合成了Mg2SiO3-钙钛矿、菱镁矿、富含CaSiO3的钙钛矿和多数石榴石的组合体，其中Mg-钙钛矿含有2.8wt.%的Al2O3，并且Fe含量明显低于共存的镁方铁矿。镁钙钛矿和含三价铁的镁方铁矿之间的 Fe-Mg 分配系数 (K-app = 0.27 +/- 0.06) 与无铝体系中的 Fe-Mg 分配系数非常接近，这表明这些 P-T 条件与尖角铁矿分解的条件接近。本研究中的原位X射线衍射和淬火实验的结果都得出了一个收敛结果，即在硫铁矿中，在4600℃配子到钙钛矿转变之前，菱铁矿分解为镁钙钛矿和镁方铁矿。镁钙钛矿中的 Al 含量与热熔石中的 K-app 之间的关系是非线性的，这与之前报道的更简单的 MgO-FeO-Al2O3-SiO2 体系中镁钙钛矿和镁方铁矿之间的 Fe-Mg 分配一致。 (C) 2004 年由 Elsevier B.V. 出版
Mineral assemblage changes in a pyrolite composition with increasing pressure were observed by in situ X-ray diffraction and quench experiments at pressures near that of the 660 km seismic discontinuity and at a fixed temperature of 1600 degreesC. According to results obtained by in situ X-ray diffraction experiments, ringwoodite (Rw) was observed with majorite garnet and CaSiO3-rich perovskite at pressures of about 20-22GPa. Dissociation of ringwoodite to MgSiO3-rich perovskite and magnesiowustite (Mw) was completed at 22.0 +/- 0.2 GPa according to Matsui et al.'s periclase pressure scale, and at 21.7 +/- 0.1 GPa according to Shim et al.'s old pressure scale. Majorite garnet persisted to about 24 GPa where pyrolite transformed to a lower mantle mineral assemblage, i.e. MgSiO3-perovskite, CaSiO3-rich perovskite, and magnesiowustite. Thus, majorite garnet coexists with the lower mantle assemblage at pressures of about 22-24 GPa. In the quench experiments, an assemblage of Mg2SiO3-perovskite, niagnesiowustite, CaSiO3-rich perovskite, and majorite garnet was synthesized at 22.5 GPa and 1600degreesC, in which Mg-perovskite contained 2.8 wt.% Al2O3, and was significantly poorer in Fe than coexisting magnesiowustite. The Fe-Mg partition coefficient between Mg-perovskite and magnesiowustite including ferric iron (K-app = 0.27 +/- 0.06) is very close to that in the Al-free system, which suggests that these P-T conditions are in the vicinity of those of ringwoodite decomposition. Both the results of in situ X-ray diffraction and quench experiments in the present study yield a convergent result that ringwoodite decomposes into Mg-perovskite and magnesiowustite before the gamet-to-perovskite transition at 4600 degreesC in pyrolite. The relation between the Al content in Mg-perovskite and K-app in pyrolite is non-linear, which is consistent with the Fe-Mg partitioning between Mg-perovskite and magnesiowustite previously reported for a simpler MgO-FeO-Al2O3-SiO2 system. (C) 2004 Published by Elsevier B.V.