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세미나

세미나 상세보기 입니다.
발표자 Prof. Tomoo Katsura
소속 Bayerisches Geoinstitut, Germany
제목 [초청세미나]Sharp 660-km discontinuity controlled by extremely narrow binary post-spinel transition
문의처 이성근 교수님 (880-6729)
자료 Katsura CV 08.03.19.pdf [20785 byte] 자료 다운로드
시간 2019-08-23 14:32~15:30
장소 25-1동 304호
내용요약

Sharp 660-km discontinuity controlled by extremely narrow binary post-spinel transition

 

The Earth's mantle is characterized by a sharp seismic discontinuity at a depth of 660-km that can provide insights into deep mantle processes. The discontinuity occurs over less than 2km - or a pressure interval less than 0.1 GPa - and is thought to result from the post-spinel transition, that is, the decomposition of the mineral ringwoodite to bridgmanite plus ferropericlase. Existing high-pressure-temperature experiments have lacked the pressure control required to test whether such sharpness is the result of isochemical phase relations or chemically distinct upper and lower mantle domains. Here, we obtain the isothermal pressure interval of the Mg-Fe binary post-spinel transition by applying advanced multi-anvil techniques with in situ X-ray diffraction with help of Mg-Fe partition experiments. It is demonstrated that the interval at mantle compositions and temperatures is only 0.01 GPa, corresponding to 250 m. This interval is indistinguishable from zero at seismic frequencies. These results can explain the discontinuity sharpness and provide new support for whole mantle convection in a chemically homogeneous mantle. The present work suggests that distribution of adiabatic vertical flows between the upper and lower mantles can be mapped based on discontinuity sharpness.


 

H2O-enhanced ionic conductivity in olivine

 

Electrical conductivity at the top of the asthenosphere near ridges shows significantly higher conductivity than dry peridotite. It also shows conductivity anisotropy. Although this high and anisotropic conductivity was interpreted either by incipient melt or proton conduction, whose electric charge carriers are interstitial protons, trace amounts of volatile in the DMM do not allow such interpretation. In this study, we propose a new hypothesis that ionic conductivity, whose charge carriers are Mg vacancies, is largely enhanced by trace amounts of H2O to produce high and anisotropic conductivity in the asthenosphere. Based on this hypothesis, we first measured Mg lattice diffusivity in iron-free olivine aggregates as a function of pressure, temperature and H2O content using a multi-anvil apparatus and secondary ion mass spectrometer, which gave the activation energy, activation volume and H2O-content exponent of 250(30) kJ/mol., 4.3(3) cm3/mol. and 1.2(2), respectively. Next, we measured high-temperature ionic conductivity in olivine single crystals along three crystallographic orientations also as a function of pressure, temperature and H2O content using a multi-anvil presses with in situ impedance analyses. The obtained activation energy, activation volume, and H2O-content exponent are 250-405 kJ/mol., 4(1) cm3/mol. and 1.3(2), respectively, which are identical to those of Mg diffusivity within the errors. Meanwhile, the ionic conductivity along [001] crystallographic axis is more conductive than [100] and [010] axes. These results, especially the large H2O-content exponent support the H2O-enhanced ionic conductivity hypothesis. The H2O-enhanced ionic conductivity may contribute to the bulk conductivity of olivine under asthenosphere conditions.

 

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