Mg/Si ratios of aqueous fluids coexisting with forsterite and enstatite based on the phase relations in the Mg2SiO4-SiO2-H2O system
Mg/Si ratios of aqueous fluids coexisting with forsterite and enstatite based on the phase relations in the Mg2SiO4-SiO2-H2O system
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基于Mg2SiO4-SiO2-H2O体系中相关系的镁橄榄石和顽辉石共存的水相流体的Mg/Si比
DOI：
10.2138/am-2004-1010
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发表时间：
2004
影响因子：
3.1
通讯作者：
S. Ono
中科院分区：
文献类型：
--
作者：
T. Kawamoto;K. Matsukage;K. Mibe;M. Isshiki;K. Nishimura;N. Ishimatsu;S. Ono
采用同步x射线和外加热金刚石-铁锤池，在0.5-5.8 GPa和800-1000℃的温度下，直接观察了与MgSiO3 （enstatite）和/或Mg2SiO4 （forsterite）共存的水相流体。在MgSiO3 -H2O体系中，在1000°C时，forsterite在低于3gpa的压力下结晶，但不高于该压力。在Mg2SiO4 -H2O体系中，在1000°C时，橄榄石完全溶解在高达5 GPa的水溶液中。这些结果表明，与辉长辉石和橄榄石共存的水相流体在3gpa以下Mg/Si < 1，在3gpa以上Mg/Si < 2。与前人报道的与辉长辉石和复辉石共存的水流体Mg/Si比值对比表明，在3 GPa和1000℃左右，Mg/Si比值从富sio2到富mgo迅速变化。这种变化可能与这些条件下液态水可能发生的结构变化有关。与顽辉石和橄榄石共存的水流体的Mg/Si比值与饱和水橄榄岩部分熔体的Mg/Si比值相似。在上地幔的某个地方，这两种流体结合在一起形成一个单一的状态，彼此无法区分。
Abstract Direct observation of aqueous fluids coexisting with MgSiO3 (enstatite) and/or Mg2SiO4 (forsterite) was performed at 0.5-5.8 GPa and 800-1000 °C with an externally heated diamond-anvil cell and synchrotron X-rays. At 1000 °C in the MgSiO3 -H2O system, forsterite crystallizes below 3 GPa but not above that pressure. At 1000 °C in the Mg2SiO4 -H2O system, forsterite congruently dissolves into the aqueous fluids up to 5 GPa. These observations suggest that the aqueous fluids coexisting with enstatite and forsterite have Mg/Si < 1 below 3 GPa and 1 < Mg/Si < 2 above that pressure. Comparison with the previous studies reporting Mg/Si ratios of the aqueous fluid coexisting with enstatite and forsterite indicates that the Mg/Si ratios change rapidly from SiO2-rich to MgO-rich at around 3 GPa and 1000 °C. This change can be related to possible structural changes of liquid water under these conditions. The aqueous fluids coexisting with enstatite and forsterite do have Mg/Si ratios similar to those found in the partial melts of H2O-saturated peridotite. Somewhere within the upper mantle, these two fluids unite to form a single regime and cannot be distinguished from each other.