Phase relations of a serpentine composition between 5 and 14 GPa: significance of clinohumite and phase E as water carriers into the transition zone
Phase relations of a serpentine composition between 5 and 14 GPa: significance of clinohumite and phase E as water carriers into the transition zone
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DOI：
10.1007/s004100000208
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发表时间：
2001-01
影响因子：
3.5
通讯作者：
R. Stalder;P. Ulmer
中科院分区：
文献类型：
--
作者：
R. Stalder;P. Ulmer
用光学显微镜、显微拉曼光谱和电子探针分析了在800-1,200℃和5-14 Gpa(相当于深度150-420公里)下对蛇纹石块体成分[接近于Mg_3Si2O5(OH)4]进行的高压实验产物。所有电荷都表现出强烈的化学分区。液体、熔体和含水固体主要集中在胶囊的顶部、底部和沿壁。装药的中心部分没有水。流体相和水合镁相的镁硅比均高于镁橄榄岩。相比之下，胶囊的中心富含二氧化硅。观测到的分带既不能用重力沉降来解释，也不能仅仅用温度梯度来解释。最有可能的情况是，流体通过表面力从固体中分离出来，从而通过优先溶解氧化镁来建立化学梯度。如果考虑到强烈的化学分带，可以通过将块体分成几个具有不同块体成分的区域来解释出现的比相规则所允许的更多的相。结果表明，与文献报道的纯MSH中的OH-斜锰矿相比，添加少量的F可以提高单斜辉石的稳定场。单斜辉石与顽辉石共存，在5 GPa时高达975℃，在12 GPa时高达1100℃。在14 GPa时(接近α/β-Mg_2SiO_4转变)，E相成为最重要的水载体。新的结果表明，与蛇纹石和A相等其他重要的水载体相比，斜绿泥石具有较高的热稳定性，因此可能是向地球过渡带输送水的重要地幔矿物。
Run products from high pressure experiments at 800–1,200 °C and 5–14 GPa (corresponding to depths of 150 to 420 km) on a serpentine bulk composition [close to Mg3Si2O5(OH)4] were analysed by optical microscopy, micro-Raman spectroscopy and electron microprobe. All charges exhibit strong chemical zoning. Fluid, melt and hydrous solids were mostly concentrated at the top, bottom and along the wall of the capsules. The central part of the charge was devoid of H2O. Both fluid and hydrous magnesian phases exhibit a Mg/Si ratio higher than forsterite. In contrast, the centre of the capsule was enriched in SiO2. The observed zoning can neither be explained by gravitational settling nor by a thermal gradient alone. Most likely the fluid was separated from the solids by surface forces and thereby established the chemical gradient by preferentially dissolving MgO. If strong chemical zoning is taken into account, the occurrence of more phases than allowed by the phase rule can be explained by separating the bulk into several domains of different bulk compositions. Results indicate that small amounts of F increase the stability field of clinohumite, Mg9Si4O16(OH,F)2, compared to OH-clinohumite in pure MSH previously reported. Clinohumite coexists with enstatite up to 975 °C at 5 GPa, and up to 1,100 °C at 12 GPa. At 14 GPa (close to the α/β-Mg2SiO4transition) phase E becomes the most important water carrier. The new results indicate that clinohumite could be an important mantle mineral for transporting water into the Earth's transition zone due to its high thermal stability compared to other important water carriers such as serpentine and phase A.