Stability of various hydrous phases in CMAS pyrolite-H2O system up to 25 GPa
Stability of various hydrous phases in CMAS pyrolite-H2O system up to 25 GPa
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DOI：
10.1007/s00269-003-0301-y
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发表时间：
2003-04
影响因子：
1.4
通讯作者：
K. Litasov;E. Ohtani
中科院分区：
文献类型：
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作者：
K. Litasov;E. Ohtani
我们对含水原始地幔成分进行了一系列熔融实验，以确定高压下致密水相的稳定性。在 10-25 GPa 的压力范围和 800 至 1400 °C 的温度范围内，确定了 CaO-MgO-Al2O3-SiO2 吡咯石与约 2 wt% 水的相关系。我们发现与橄榄石共存的 E 相在 10-12 GPa 和 1050 °C 以下是稳定的。与瓦兹利石共存的 E 相在 14-16 GPa 和 900 °C 以下稳定。超水相 B 在 18.5 GPa 下 1100 °C 以下和 25 GPa 下 1300 °C 以下的温度下在叶熔石中保持稳定。在 14-17 GPa 和 900-1100 °C 下，除瓦兹利石之外，叶熔石中没有其他水相稳定，这表明致密水合硅酸镁 (DHMS) 的稳定性存在差距。我们检测到瓦兹利石的稳定性场向较低压力（12 GPa 和 1000 °C）扩展。发现瓦兹利石的 H2O 含量不仅随着温度的升高而降低，而且随着压力的升高而降低。只有在俯冲板块下降到下地幔中的条件下，DHMS 相才能存在于热解成分中。在正常地幔和热羽流条件下，瓦斜锰矿和尖角橄榄石是主要的含水相。根据观察到的瓦兹利石水溶性随着压力的降低而增加，过渡区的顶部可能富含 H2O。由于俯冲板块和周围地幔之间的热平衡，最上面的下地幔可能是一个重要的脱水区，为上升的羽流提供流体。
We carried out a series of melting experiments with hydrous primitive mantle compositions to determine the stability of dense hydrous phases under high pressures. Phase relations in the CaO–MgO–Al2O3–SiO2pyrolite with ˜2 wt% of water have been determined in the pressure range of 10–25 GPa and in the temperature range between 800 and 1400 °C. We have found that phase E coexisting with olivine is stable at 10–12 GPa and below 1050 °C. Phase E coexisting with wadsleyite is stable at 14–16 GPa and below 900 °C. A superhydrous phase B is stable in pyrolite below 1100 °C at 18.5 GPa and below 1300 °C at 25 GPa. No hydrous phases other than wadsleyite are stable in pyrolite at 14–17 GPa and 900–1100 °C, suggesting a gap in the stability of dense hydrous magnesium silicates (DHMS). We detected an expansion in the stability field of wadsleyite to lower pressures (12 GPa and 1000 °C). The H2O content of wadsleyite was found to decrease not only with increasing temperature but also with increasing pressure. The DHMS phases could exist in a pyrolitic composition only under the conditions present in the subducting slabs descending into the lower mantle. Under the normal mantle and hot plume conditions, wadsleyite and ringwoodite are the major H2O-bearing phases. The top of the transition zone could be enriched in H2O in accordance with the observed increase in water solubility in wadsleyite with decreasing pressure. As a consequence of the thermal equilibration between the subducting slabs and the ambient mantle, the uppermost lower mantle could be an important zone of dehydration, providing fluid for the rising plumes.