地幔中的水与熔融

Water and partial melting in the Earth s mantle.

自二十世纪九十年代起,通过对天然样品的观察、矿物中水的溶解度的实验研究、以及地球物理的观测,人们逐步认识到地球内部以矿物晶格缺陷形式蕴含着至少与现今海洋水量相当的水。地球内部的水除了显著影响矿物的热导率、电导率、流变学、扩散等性质之外,还对不同构造背景下的地幔熔融产生了重要影响。水作为强不相容元素,在地幔发生熔融时会更倾向于进入到熔体相中,从而改变熔体的结构,影响与残留固相之间的平衡。总体上讲,在等温等压条件下,地幔中水的存在可以显著降低其固相线并增大熔融程度;而在绝热减压过程中,水的存在则可以加深初始熔融的深度,增大地幔整体熔融的区间,从而降低平均熔融程度。近年来,越来越多的证据表明,水对于从岩石圈-软流圈边界的低程度熔融到地幔超大规模熔融产物(大火成岩省)的形成都具有重要作用。本文综述了不同构造背景下(大洋中脊、岛弧、地幔过渡带、大火成岩省、大陆板内、洋岛等)水对地幔熔融影响的新进展,并提出现有研究的不足和未来的挑战。

With the investigation of water in natural samples,experiments on the water solubility in mantle minerals,and geophysical surveys on the deep mantle structure since the 1990s,it has been gradually known that the mass of water preserved in the deep mantle in the form of point defects in crystal lattice could be comparable with that of the present ocean.Besides its remarkable impacts on the physical properties of minerals such as thermal and electrical conductivity,viscosity,and elemental diffusivity,water could also affect the production of mafic magmatism from different geological backgrounds.Water plays a role as a strong incompatible element during mantle melting,and its preferential partitioning into the melts than the residue solid can largely affect the structure of the melts and consequently impact the melt-solid equilibrium.In principle,for a mantle under specific pressure and temperature,water can lower the solidus temperature and increase the partial melting degree.In contrast,for an adiabatically decompressing mantle,water can deepen the initial melting depth and decrease the average partial melting degree.During the past several years,more and more evidence supports that water accounts for not only the low degrees of partial melting at the lithosphere-asthenosphere boundary,but also the high degrees of partial melting required for the formation of large igneous provinces.In this work,the new progresses on the water contents of mantle-derived magmas and their effects on partial melting in different tectonic settings(e.g.mid-ocean ridge,island arc,mantle transition zone,large igneous provinces,continental intraplate,oceanic island)were briefly introduced.We also point out the limitations of the present works and the challenges in the future.