Lower-mantle water reservoir implied by the extreme stability of a hydrous aluminosilicate
Lower-mantle water reservoir implied by the extreme stability of a hydrous aluminosilicate
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DOI：
10.1038/ngeo2306
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发表时间：
2015-01-01
影响因子：
18.3
通讯作者：
Miyajima, Nobuyoshi
中科院分区：
文献类型：
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作者：
Pamato, Martha G.;Myhill, Robert;Miyajima, Nobuyoshi
形成海洋岛屿的玄武岩熔岩的源岩通常被推断为来自下地幔的热羽流的一部分。与一般的上地幔岩石相比，这些岩石富含水分。然而，实验表明，水在主要的下地幔相中的溶解度非常低，这提示了羽流可能来自深部地幔中尚未确定的富水原始物质储层。在这里，我们进行了高压实验，证明了Al2SiO4(OH)(2)-致密，含水硅酸镁d相的富铝端元在26gpa下超过2000摄氏度的温度下是稳定的。在这些条件下，富al相D在基性岩中是稳定的，这表明俯冲洋壳可能是对流下地幔中重要的长期储水层。我们认为，下地幔致密超基性岩石中含水矿物脱水形成的熔体将迁移到基性岩性中并结晶形成富al相d。当地幔岩石上升时，水将局部重新分配为名义上的无水矿物。这种上涌物质为洋岛玄武岩提供了一个潜在的来源，而不需要深地幔中富含水的原始物质的储层。
The source rocks for basaltic lavas that form ocean islands are often inferred to have risen as part of a thermal plume from the lower mantle. These rocks are water-rich compared with average upper-mantle rocks. However, experiments indicate that the solubility of water in the dominant lower-mantle phases is very low, prompting suggestions that plumes may be sourced from as-yet unidentified reservoirs of water-rich primordial material in the deep mantle. Here we perform high-pressure experiments to show that Al2SiO4(OH)(2)-the aluminium-rich endmember of dense, hydrous magnesium silicate phase D-is stable at temperatures extending to over 2,000 degrees C at 26 GPa. We find that under these conditions, Al-rich phase D is stable within mafic rocks, which implies that subducted oceanic crust could be a significant long-term water reservoir in the convecting lower mantle. We suggest that melts formed in the lower mantle by the dehydration of hydrous minerals in dense ultramafic rocks will migrate into mafic lithologies and crystallize to form Al-rich phase D. When mantle rocks upwell, water will be locally redistributed into nominally anhydrous minerals. This upwelling material provides a potential source for ocean-island basalts without requiring reservoirs of water-rich primordial material in the deep mantle.