2017 Fiscal Year Annual Research Report
Hydrogen grain boundary diffusion in olivine and its contribution to conductivity anomaly
| Project/Area Number |
17J11576
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| Research Institution | Okayama University |
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Principal Investigator |
Zhao Chengcheng 岡山大学, 自然科学研究科, 特別研究員(DC2)
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| Project Period (FY) |
2017-04-26 – 2019-03-31
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| Keywords | hydrous forsterite / hydrous olivine / synthesis methods / quality of sintering |
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| Outline of Annual Research Achievements |
Forsterite powder was synthesized using SiO2 and MgO oxide mixture. D-bearing forsterite was synthesized, whose final D concentration was expected to be D2O = 50% H2O. To ensure high water content in forsterite, pressure was chosen to be 8 GPa using 18/11 cell assembly. However, the forsterite aggregate was loosely sintered when kept at 1100 oC for 4 hours. To increase the quality of sintering, following methods was tried. (1) Slow cooling with cooling rate of 1 oC/min and 3 oC/min from 1500 oC to 1100 oC; (2) Two-step synthesis, previous synthesized D-bearing Fo sintered again; (3) Phase back transformation (wadsleyite to forsterite) from 16 GPa to 10 GPa.
Slow cooling cannot ensure well sinter D-bearing forsterite. Two-step synthesis can get well sintered forsterite, but it costs too much. The third method also can get well sintered sample, but under SEM its grain boundary was not closed and there is swirl texture at center, which precluded it to be used as starting material for diffusion source.
So olivine was used instead. D-bearing olivine aggregate was synthesized at 8 GPa, 1100 oC and was well sintered, with D2O = 50% H2O (Fig. 1). Double capsule technique was adopted to control oxygen fugacity around NNO buffer. However, during synthesis of diffusion path, H-bearing olivine aggregate seems to be well sintered. But when ultrasonic machine was used to core sample out, sample usually broke into powder. When the inner Pt capsule was changed to Ni foil, sintering did not become better. It still needs further trials and errors
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| Current Status of Research Progress |
Current Status of Research Progress
3: Progress in research has been slightly delayed.
Reason
This project was proceeded smoothly at beginning. I succeeded in sintering well sintered D-doped olivine instead of D-doped forsterite as diffusion path. During this process, many sintering methods including slow cooling, two-step synthesis and phase back transformation from wadsleyite to forsterite were tried. None of them works well, either due to their high cost or bad sintering quality. Olivine turns out to be good candidate to continue such process. However, D-doped well sintered olivine could be obtained, H-doped olivine using the same method did not work, which still need further trials and errors.
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| Strategy for Future Research Activity |
On the other hand, the electrical conductivity of forsterite buffered by SiO2 or MgO gave a significant result, which seems to be able to explain the high electrical conductivity near the depth of LAB. At 4 GPa, MgO buffered Fo can incorporated more water, while its conductivity with the same water content is smaller than the latter. At 8 GPa, the IR spectra of both buffered Fo become similar. Their conductivity is expected to be similar. However, due to their low dehydration temperature, all the data were obtained at low temperature, whose extrapolation might be inaccurate. In order to get high temperature data, next we plan to measure the conductivity of dry SiO2 or MgO buffered Fo to high temperature. During heating and cooling cycles, Fo will absorbed several tens or even hundreds ppm of water. Therefore, we can know the true conductivity at mantle related pressure and temperature condition.
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