2020 Fiscal Year Annual Research Report
高圧実験と地震波観測による統合的アプローチを用いた沈み込み帯下の構造と進化の解明
| Project/Area Number |
19H02002
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| Research Institution | Ehime University |
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Principal Investigator |
GREAUX Steeve 愛媛大学, 地球深部ダイナミクス研究センター, 助教 (90543166)
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| Co-Investigator(Kenkyū-buntansha) |
肥後 祐司 公益財団法人高輝度光科学研究センター, 回折・散乱推進室, 主幹研究員 (10423435)
河合 研志 東京大学, 大学院理学系研究科(理学部), 准教授 (20432007)
佐藤 友彦 東京工業大学, 理学院, 研究員 (80714831)
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| Project Period (FY) |
2019-04-01 – 2023-03-31
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| Keywords | mantle composition / high pressure / ultrasonic measurement / seismological model / triplication |
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| Outline of Annual Research Achievements |
P- and S-wave velocities (VP and VS) are the unique tool we have to explore the inaccessible deep Earth via comparisons with the observed seismic
velocities. In this FY we measured VP and VS of Harzburgite (Hz), which constitute the main body of the subducted lithosphere. The Hz samples were synthesized at high Pressure and Temperature at GRC (Ehime University), starting from a glass of Hz composition fabricated in a laser levitation furnace at IPGP (Paris University)
The sound velocities of the Hz aggregates were determined by in
situ ultrasonic interferometry, X-ray radiography and diffraction techniques, at high P and T in the multianvil press at the beamline Bl04B1 located at the synchrotron facility SPring-8. The analyzed samples consisted of majorite, akimotoite and olivine at P ~20 GPa and bridgmanite and periclase at P> 25 GPa, accordingto the synthesis Pressure. An additional experiment on Mid-Ocean Ridge Basalts (MORB) at P ~25 GPa was also conducted to support the results of the previous FY. Preliminary results indicated the VS/VP of Hz are higher than those of seismic models across the 660-km depth discontinuity, suggesting harzburgitic fragments could explain the high shear seismic velocities observed above the 660, while slower materials, such as MORB or partial melt are necessary to explain the low velocities beneath the 660.
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| Current Status of Research Progress |
Current Status of Research Progress
2: Research has progressed on the whole more than it was originally planned.
Reason
In this FY, we completed a first set of data on the harzburgite composition, which will permit precise estimates of the seismic velocities of the subducted litosphere across the 660-km discontinuity. These data add to the those on Mid-Ocean Ridge Basalts (MORBs) completed in the previous FY. We are now able to estimate with accuracy the velocities of the subducted crust up to ~25 GPa, which corresponds to the depths of the uppermost lower mantle. The results are to be compared with local seismic models established by triplication methods.
This plan is however delayed due to the ongoing pandemic situation leading to difficulties in effectively collaborate with the seismologists of our team who reside in Tokyo and Taiwan, although some efforts were made to achieve this work remotely, which will be continued in the next FY.
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| Strategy for Future Research Activity |
In the next FY, we will carry out ultrasonic measurements on pyrolite and (x)pyrolite + (1-x)MORB compositions, at pressures higher than 25 GPa. These experiments were designed on the basis of our current data, which suggest a mixture of pyrolite and MORB with x ~30-40% is likely to reproduce the low velocities of the uppermost lower mantle. We also plan to investigate the effect of partial melting on the sound velocities of the mantle materials by adding molten FeS or molten basalts the pyrolitic aggregates. This work will be supported by a JSPS post-doc, which has been already approved, and which is planned to start from September 2021.
The obtained velocity data will be compared to new local seismic models derived from the triplication methods developed by our team. The combined velocity data and local seismological models will provide improved knowledge on the structure of the mantle in various subducted slab regions across the globe.
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Research Products
(8 results)
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[Presentation] Low Velocity Zones in the Martian Upper Mantle Highlighted by Sound Velocity Measurements2020
Author(s)
Siersch N., Xu F., Greaux S., Badro J., Kuwahara H., Kondo N., Kono Y., Higo Y., Plesa A.-C., Rivoldini A. and Antonangeli D.
Organizer
American Geophysical Union Fall meeting (San Francisco, USA)
Int'l Joint Research
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