超含水相B在高溫高壓下的熱傳導率及其對地球深部水循環的影響

Abstract

地球表面覆蓋了大量的水，這些水可透過板塊隱沒進入地球深部，影響地球內部的物理與化學性質，是我們了解地球動力學及熱演化的重要關鍵。緻密含水鎂矽酸鹽（dense hydrous magnesium silicates, DHMSs）為隱沒板塊中隨著溫度與壓力變化相變而成的一系列含水礦物，在其晶體結構中可以含有大量水（OH-），對地球深部水循環有重要的影響。其中，高溫高壓礦物實驗顯示，超含水相 B（superhydrous phase B, ShyB）是一種可穩定存在於地函過渡帶至下部地函最上部的重要含水礦物，並具有攜帶大量水的能力。然而，超含水相 B的熱性質及其對隱沒板塊熱演化與隱沒動力學的影響尚未被充分理解。 本研究將高溫高壓實驗合成的超含水相 B，置於高壓鑽石砧及電阻式加熱系統中模擬地球內部的高壓及高溫環境，並利用時域熱反射技術精確量測其熱傳導性質，記錄隨著溫度與壓力上升的熱導率變化。結果顯示，超含水相 B的熱導率隨著壓力會單調上升，隨溫度的升高則會下降，與先前已被研究的其他含水相的低熱導率表現一致。 本研究進一步結合前人研究的其他含水礦物熱導率數據，建立隱沒板塊的熱演化數值模型。模型中顯示，由於含水礦物的體積占比低，對隱沒板塊整體的熱結構影響較小。但一系列含水相的熱導率變化會阻擋由高溫地函所帶來的熱流入板塊內部，或在部分深度區間反而促進熱流入，進而在板塊表層部分形成局部的溫度差異；一旦造成隱沒板塊內部溫度較低，則會促使含水礦物能攜帶更多水至更深的地球內部。此溫度差異也會導致周圍地函物質的密度與相變深度產生差異。同時，相變過程中伴隨的脫水現象可能因此在不同深度發生，形成地震波低速帶，並與深層地震活動相關聯。

Water covers a significant portion of the Earth's surface, which could be brought into the mantle through plate tectonics and subduction, influencing the physical and chemical properties in the Earth's interior. Mineral physics experiments have indicated that dense hydrous magnesium silicates (DHMS), a series of hydrous minerals formed in subducting slabs under the increasing pressure and temperature conditions, can contain water in their crystal structures and play a crucial role in the deep Earth water cycle. Among them, superhydrous phase B (ShyB) is expected to carry substantial amounts of water and stably exist at depths ranging from the mantle transition zone to the uppermost lower mantle. The thermal conductivity of ShyB could crucially affect the thermal state of a subducting slab, while it remains poorly understood. In this study, synthesized ShyB was placed in a high-pressure diamond anvil cell coupled with a resistive heater, which is used to simulate the deep Earth's high-pressure and high-temperature conditions. The thermal conductivity of ShyB was precisely measured using time-domain thermoreflectance (TDTR) techniques, with changes recorded as both pressure and temperature increased. The results show that ShyB's thermal conductivity increases monotonically with pressure, but decreases with temperature, consistent with the thermal conductivity behaviors of other hydrous phases observed in previous studies. Combined with previous thermal conductivity data, a numerical model was developed to examine the thermal evolution of subducting slabs. The model indicates that while the volume fraction of hydrous minerals is relatively small, their thermal properties influence the slab's internal thermal structure. At certain depth ranges, depending on the relative thermal conductivity of hydrous phases and lithosphere, the changes in thermal conductivity of hydrous phases either inhibit or enhance the heat transfer into the slab, creating local temperature differences at the slab's surface. These differences can maintain a lower internal temperature in the subducting slab, which facilitates the transport of water deeper into the Earth. Additionally, these temperature variations slightly affect the density and the phase transition depths of surrounding mantle materials. The dehydration accompanying phase transitions at different depths may lead to the formation of seismic low-velocity zones, which are associated with deep seismic activities.