鎂-鈣鋁榴石的高壓拉曼光譜與熱傳導性質研究
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2022
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石榴子石(Garnet)是地球地殼、地函和隱沒板塊中可發現的重要礦物，其晶體結構可以容納多種化學元素，如鎂、鈣與鐵(Mg/Ca/Fe)。以輝橄岩(Pyrolite)地函模型為例，其石榴子石內的鎂鋁榴石(Pyrope, 簡稱Py)占比約75%、鈣鋁榴石(Grossular, 簡稱Gr)約為10%以及鐵鋁榴石(Almandine, 簡稱Alm)約15%；相較而言，隱沒板塊中所發現的石榴子石的Gr的佔比則更高。因此，本研究利用鑽石高壓對頂砧模擬地球內部壓力環境，並使用合成單晶Py40Gr60和Gr樣品進行本次實驗。拉曼光譜是用於鑑定礦物內化學鍵與研究其振動模式。先前的研究指出，在室溫室壓環境條件下，石榴子石內化學成分對其中Si-O鍵結與拉曼振動模式存在著密切相關性但對高壓下對其Si-O鍵結與拉曼光譜振動模式的影響仍不清楚。故本研究的主要目的是藉由樣品中的Mg/Ca含量，進而比對高壓下Py-Gr固溶體之拉曼光譜的影響，並從高壓拉曼光譜探索它們在高壓下可能的穩定區間。結果隨著壓力的增加，發現特徵峰在特定壓力下拉曼位移有多個不連續區間，其發生在~6、~9、~17和~35 GPa，此觀察是前人沒有觀測到的。本研究還結合時間域熱反射技術，探討Gr以及中間相成分Py40Gr60樣品在受壓過程中熱傳導率隨壓力的變化，以了解石榴子石中的Mg/Ca含量比在地球深部熱傳導率的影響。實驗結果發現，室溫室壓下Gr的熱傳導率在(110)與(100)兩個晶面方向上出現差異，在Gr (100)上測得的熱導率比在Gr (110)上測得的高約1.3倍，而在Py40Gr60樣品上則無觀測到此現象。實驗測得Gr在(110)與(100)兩個晶面的熱傳導率隨壓力上升異向性增加，Py40Gr60的熱傳導率則介於Gr兩個晶面的熱傳導率之間，並且沒有出現異向性。結合本研究實驗結果、地函與隱沒板塊礦物學模型，我們計算了地函與隱沒板塊的整體熱傳導率，計算結果指出在模擬地球熱傳導率上可將不同Mg/Ca含量成分的石榴子石視為相同礦物。
This research aims to understand pyrope-grossular garnets' vibrational properties and thermal conductivity at high pressures. Garnets are common rock-forming minerals in the upper mantle, lower crust, and subducted slab. Therefore, knowledge of garnets’ physical properties in the pressures and temperatures relevant to the interior of Earth is important. Natural garnets have a wide chemical composition. Our study focuses on the physical properties of synthetic Pyrope-Grossular solid solutions, Gr and Py40Gr60. In this study, Raman spectroscopy and Time-domain thermoreflectance technique (TDTR) were used to observe the changes in the vibrational properties and thermal conductivity of samples with pressures.Our results found that there are several discontinuities at certain pressures in the Raman shift of garnets’ specific peaks along with the pressure increases; those discontinuities occur at ~6, ~9, ~17, and ~35 GPa. This observation has not been reported before. Furthermore, the thermal conductivity we measured on the Gr (100) is ~ 1.3 times higher than what we measured on the Gr (110) samples in ambient conditions. The values of the thermal conductivity of Py40Gr60 samples lie between the thermal conductivity of Gr (100) and Gr (110) at higher pressures. My experimental data with mineralogical models are able to constrain the thermodynamic properties of the Earth's upper mantle and subducted slabs.
This research aims to understand pyrope-grossular garnets' vibrational properties and thermal conductivity at high pressures. Garnets are common rock-forming minerals in the upper mantle, lower crust, and subducted slab. Therefore, knowledge of garnets’ physical properties in the pressures and temperatures relevant to the interior of Earth is important. Natural garnets have a wide chemical composition. Our study focuses on the physical properties of synthetic Pyrope-Grossular solid solutions, Gr and Py40Gr60. In this study, Raman spectroscopy and Time-domain thermoreflectance technique (TDTR) were used to observe the changes in the vibrational properties and thermal conductivity of samples with pressures.Our results found that there are several discontinuities at certain pressures in the Raman shift of garnets’ specific peaks along with the pressure increases; those discontinuities occur at ~6, ~9, ~17, and ~35 GPa. This observation has not been reported before. Furthermore, the thermal conductivity we measured on the Gr (100) is ~ 1.3 times higher than what we measured on the Gr (110) samples in ambient conditions. The values of the thermal conductivity of Py40Gr60 samples lie between the thermal conductivity of Gr (100) and Gr (110) at higher pressures. My experimental data with mineralogical models are able to constrain the thermodynamic properties of the Earth's upper mantle and subducted slabs.
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鎂鋁榴石, 鈣鋁榴石, 固溶體, 拉曼光譜學, 時域熱反射, 熱傳導率, 高壓, Pyrope, Grossular, Solid solution, Raman spectroscopy, Thermal conductivity, High pressures