The Electrical Conductivity of Liebermannite: Implications for Water Transport Into the Earth's Lower Mantle
The Electrical Conductivity of Liebermannite: Implications for Water Transport Into the Earth's Lower Mantle
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DOI：
10.1029/2020jb020094
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发表时间：
2020-08-01
影响因子：
3.9
通讯作者：
Jouffret, Laurent
中科院分区：
文献类型：
--
作者：
Manthilake, Geeth;Schiavi, Federica;Jouffret, Laurent
Liebermannite(KAlSi3O8)是深俯冲陆壳和洋壳中热力学稳定的主要矿物相。Liebermannite的晶体结构显示出在(Si，Al)O-6八面体单元边共享的双链组合之间形成的隧道，这些单元充当了大的不相容碱离子的储存库。在这项研究中，我们研究了12，15和24 GPaLibermannite在1500K温度下的电导率，以追踪大陆沉积物俯冲到地球下地幔的路径。此外，我们还观察了Liebermannite是否能够在深部地幔条件下隔离不相容的H2O。我们观察到由于K+离子的热激活跳跃而产生的锂离子的高电导率导致了大于1 S/m的高电导率。水合锂盐的红外光谱特征表明其晶体结构中同时存在分子H2O和羟基(OH-)。在中国东北部的地幔过渡带和菲律宾海的下地幔观察到的高电导率可以归因于大陆沉积物向地幔深处的俯冲路径。虽然在下地幔条件下，热解岩成分中的主要矿物相几乎不含H2O，但我们的研究表明，在这种条件下，锂贝壳甘露石可能是H2O的重要寄主。我们认为，深地幔岩柱形成的洋岛玄武岩中H2O含量相对较高，主要与深俯冲大陆沉积有关，其中Libermannite是H2O的主要载体。Liebermannite(KAlSi3O8，正式名称K-hollandite)是深俯冲大陆和洋壳中的重要矿物相。Liebermannite具有高的离子导电性，可以解释地球地幔过渡带和下地幔上部的低电阻率，暗示着水化地壳向下地幔俯冲。在此条件下合成的钙铝榴石样品表明，它的晶体结构中同时存在分子H2O和羟基(OH-)基团，这表明锂铝锰矿可能是地球深部地幔中水的重要宿主。与来自大洋中脊的玄武岩相比，洋岛玄武岩的水含量更高。据推测，大洋玄武岩是由深度俯冲的地壳成分形成的。我们还知道，下地幔相对干燥。因此，来自深部地幔的洋岛玄武岩中较高的水分含量很难调和。在这里，我们证明了洋岛玄武岩的源壳成分可能是水合的。芒硝石是深俯冲地壳中的关键矿物，能有效地容水。
Liebermannite (KAlSi3O8) is a principal mineral phase expected to be thermodynamically stable in deeply subducted continental and oceanic crusts. The crystal structure of liebermannite exhibits tunnels that are formed between the assemblies of double chains of edge-sharing (Si, Al) O-6 octahedral units, which act as a repository for large incompatible alkali ions. In this study, we investigate the electrical conductivity of liebermannite at 12, 15, and 24 GPa and temperature of 1500 K to track subduction pathways of continental sediments into the Earth's lower mantle. Further, we looked at whether liebermannite could sequester incompatible H2O at deep mantle conditions. We observe that the superionic conductivity of liebermannite due to the thermally activated hopping of K+ ions results in high electrical conductivity of more than 1 S/m. Infrared spectral features of hydrous liebermannite indicate the presence of both molecular H2O and hydroxyl (OH-) groups in its crystal structure. The observed high electrical conductivity in the mantle transition zone beneath Northeastern China and the lower mantle beneath the Philippine Sea can be attributed to the subduction pathways of continental sediments deep into the Earth's mantle. While major mineral phases in pyrolitic compositions are almost devoid of H2O under lower mantle conditions, our study demonstrates that liebermannite could be an important host of H2O in these conditions. We propose that the relatively high H2O contents of ocean island basalts derived from deep mantle plumes are primarily related to deeply subducted continental sediments, in which liebermannite is the principal H2O carrier.Plain Language Summary Liebermannite (KAlSi3O8, formally K-hollandite) is an important mineral phase in deeply subducted continental and oceanic crusts. Liebermannite exhibits high ionic conductivity and could explain low resistivity in the Earth's mantle transition zone and the upper part of the lower mantle, hinting toward the subduction of hydrated crusts into the lower mantle. The libermannite samples synthesized at these conditions indicate the presence of both molecular H2O and hydroxyl (OH-) groups in its crystal structure, suggesting that liebermannite could be an important host of H2O in the Earth's deep mantle. Compared to the basalts from mid-ocean ridges that exhibit trace quantities of water, ocean island basalts show higher water contents. It has been speculated that ocean basalts are derived from crustal components that were deeply subducted. It is also known that the lower mantle is relatively dry. Therefore, it is difficult to reconcile the higher water contents in oceanic island basalts that are derived from the deep mantle. Here we demonstrate that the source crustal components for oceanic island basalts were likely to be hydrated. Libermannite is a key mineral in deeply subducted crusts and can efficiently host water.