菱鐵礦於高壓下電子自旋態轉變與熱傳導率之研究;Spin transition and thermal conductivity of (Fe0.78Mg0.22CO3) siderite under high pressure

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Title:	菱鐵礦於高壓下電子自旋態轉變與熱傳導率之研究;Spin transition and thermal conductivity of (Fe0.78Mg0.22CO3) siderite under high pressure
Authors:	趙耕賢;Chao, Keng-Hsien
Contributors:	應用地質研究所
Keywords:	菱鐵礦;電子自旋態轉變;熱傳導率;siderite;spin transition;thermal conductivity
Date:	2018-07-20
Issue Date:	2018-08-31 15:05:28 (UTC+8)
Publisher:	國立中央大學
Abstract:	菱鐵礦於高壓下電子自旋態轉變與熱傳導率之研究摘要　　　　　　深層碳循環控制地球表面與內部長期的碳含量收支。該循環係由海洋板塊將碳元素隱沒至地函，而碳元素會再被火山活動循環回地表。過去研究指出，含鐵碳酸鹽，例如:菱鐵礦，是深層碳循環中重要的地函含碳礦物。此外過去研究亦指出菱鐵礦會在大約40 GPa 至 55 GPa 發生電子自旋態轉變(由高自旋態轉變至低自旋態)並伴隨物理性質的劇烈變化，例如:彈性係數。熱傳導率為一種控制物質在兩端有溫差的狀況下，通過該物質的熱流多寡的重要物理性質，因此地球內部礦物的熱傳導率會影響其溫度構造與熱演化之過程。然而，由於過去實驗技術的困難，含鐵碳酸鹽在高溫高壓下的熱傳導率從未被研究過。本研究結合鑽石高壓砧，拉曼光譜與超快雷射技術(Time-domain thermoreflectance)，測量菱鐵礦從常壓至67 GPa 的熱傳導率變化。我們發現菱鐵礦的熱傳導率在電子自旋態轉變的壓力範圍內會發生劇烈的變化:當菱鐵礦所受之壓力介於40 GPa至55 GPa之間且低自旋態鐵的比例估計介於50%至85%之間時，熱傳導率會劇烈地增加三倍；在電子自旋態轉變幾乎結束時，熱傳導率會下降至最高值的1/9。這種在小壓力範圍內發生的劇烈熱傳導率變化意謂著如果菱鐵礦可以被板塊隱沒至1100至1500公里深，則可能會產生局部的熱流以及溫度異常，進一步影響局部礦物相的穩定度。 ;Spin transition and thermal conductivity of (Fe0.78Mg0.22CO3) siderite under high pressure ABSTRACT Deep carbon cycle is a cycle controlling the long-term budget of carbon on Earth’s surface and in Earth’s interior: the carbon is transported to the mantle by the subduction of slabs and recycled back to the Earth’s surface by the volcanic activities, respectively. Iron-bearing carbonate, for example, siderite, was proposed to be an important mantle carbon-hosting mineral in the deep carbon cycle. Previous literature showed that the siderite undergoes a pressure-induced iron spin transition (from high spin to low spin) around 40-55 GPa and the physical properties of siderite, such as elastic properties, change drastically across the spin transition. The thermal conductivity is a critical physical property that controls the heat flux flowing through a mineral when temperature gradient exists, and therefore, thermal conductivity controls the temperature profile and thermal structure evolution in Earth’s interior. However, the thermal conductivity of iron-bearing carbonate has never been investigated under relevant extreme temperature and pressure conditions due to the experimental difficulties. In this work, we combined the diamond anvil cell, Raman spectroscopy and time-domain thermoreflectance techniques to measure the thermal conductivity of siderite from ambient condition to 67 GPa, in particular across the spin transition. We found that the thermal conductivity varies drastically across the spin transition: when siderite is under 40-55 GPa, the thermal conductivity increases by three times as the fraction of low spin iron is estimated to be around 50-85% and suddenly drops to around 1/9 of its maximum value as the spin transition almost completes. These results imply that if the siderite could be transported to the depth of 1100-1500 km by the subduction of slabs, the thermal conductivity anomaly of siderite that varies drastically within a narrow pressure range might induce local heat flux and temperature anomalies, and therefore, influence the stability of local mineral phases.
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