Effects of Al content on water partitioning between orthopyroxene and olivine: Implications for lithosphere–asthenosphere boundary
Effects of Al content on water partitioning between orthopyroxene and olivine: Implications for lithosphere–asthenosphere boundary
复制标题
DOI：
10.1016/j.epsl.2014.05.041
复制
发表时间：
2014-08
影响因子：
5.3
通讯作者：
M. Sakurai;N. Tsujino;H. Sakuma;K. Kawamura;E. Takahashi
中科院分区：
文献类型：
--
作者：
M. Sakurai;N. Tsujino;H. Sakuma;K. Kawamura;E. Takahashi
研究了低水浓度（3 × 387 wt. %）下水在斜方辉石（Opx）和橄榄石（OI）之间的分配系数（D（Opx/OI））。ppm），在1.5-6 GPa的压力和1573 K的温度下进行了高压实验。实验是用Kawai型多砧和活塞缸装置，通过使用天然油和合成的Opx与不同的Al含量的起始材料进行的。水含量用真空型傅里叶变换红外光谱仪（FT-IR 6100，IRT 5000）获得。本研究中含Ol和Al的Opx的IR光谱与通过高压实验（Bai和Kohlstedt，1993）和天然岩石（Grant等人，2007年）。认为天然Opx红外光谱中的宽谱带是由于大量Al取代导致晶体畸变的影响。相反，不含Al的Opx的IR光谱与Rauch和Keppler（2002）报道的光谱不一致，可能是因为水逸度差异较大。D（无Al Opx/Ol）在所有压力条件下均为0.01或随压力增加而减小。但在相同条件下，含铝Opx的含水量明显大于Ol。此外，随着压力的增加，Opx中Al浓度对D（Opx/OI）的影响变得更大。Opx中的高Al含量显著增加D（Opx/Ol），并且趋势随着压力的增加而增加。D（Opx/Ol）在地幔条件下Opx的Al浓度接近于0的压力下急剧下降。这一结论表明，上地幔的粘度在深度大于不含Al的斜方辉石的深度时急剧下降，D（Opx/Ol）的急剧变化可以解释大洋和大陆岩石圈-软流圈边界的存在。
To investigate the partitioning coefficient of water between orthopyroxene (Opx) and olivine (Ol)(D (Opx/Ol)) under low-water concentrations (3∼ 387 wt. ppm) similar to the Earth's mantle conditions, high-pressure experiments have been conducted at pressures of 1.5–6 GPa and a temperature of 1573 K. The experiments were performed with Kawai-type multi-anvil and piston-cylinder apparatus by using starting materials of natural Ol and synthetic Opx with various Al contents. The water contents were obtained with a vacuum type Fourier transform infrared spectrometer (FT-IR6100, IRT5000). IR-spectra of Ol and Al-bearing Opx in this study are similar to those obtained by high-pressure experiments (Bai and Kohlstedt, 1993) and natural rocks (Grant et al., 2007), respectively. It is believed that broad bands in IR spectra of natural Opx are due to effect of crystal distortion by large Al substitution. On the contrary, IR-spectra of Al-free Opx are not consistent with those reported by Rauch and Keppler (2002) likely because of the large difference of water fugacity. D (Al-free Opx/Ol) is∼ 1 at all pressure conditions or decreases with increasing pressure. However, the water contents of Al-bearing Opx are significantly larger than those of Ol at the same conditions. In addition, the effect of Al concentration in Opx on D (Opx/Ol) becomes larger with increasing pressure. The high Al content in Opx significantly increases D (Opx/Ol) and the trend increases with increasing pressure. D (Opx/Ol) drops sharply at the pressure at which the Al concentration of Opx becomes nearly 0 in the Earth's mantle conditions. This conclusion indicates that viscosity of the upper mantle decreases sharply at depths larger than those in which orthopyroxene contains no Al. The dramatic change of D (Opx/Ol) may explain the lithosphere–asthenosphere boundary beneath oceans and continents.