2005 Fiscal Year Final Research Report Summary
Study on the chemical state of the Earth's deep interior based on chemical and isotopic compositions of kimberlites and mantle materials
| Project/Area Number |
15340183
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| Research Category |
Grant-in-Aid for Scientific Research (B)
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| Allocation Type | Single-year Grants |
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| Section | 一般 |
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| Research Field |
Petrology/Mineralogy/Science of ore deposit
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| Research Institution | The University of Tokyo |
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Principal Investigator |
KANEOKA Ichiro The University of Tokyo, Earthquake Research Institute, Professor Emeritus, 地震研究所, 名誉教授 (30011745)
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| Co-Investigator(Kenkyū-buntansha) |
NAKAI Shun-ichi The University of Tokyo, Earthquake Research Institute, Associate Professor, 地震研究所, 助教授 (50188869)
SANO Yuji The University of Tokyo, Ocean Research Institute, Professor, 海洋研究所, 教授 (50162524)
TAKIGAMI Yutaka Kanto Gakuen University, Faculty of Law, Professor, 法学部, 教授 (40206909)
MIURA Yayoi The University of Tokyo, Earthquake Research Institute, Research Associate, 地震研究所, 助手 (90282730)
HONDA Satoru The University of Tokyo, Earthquake Research Institute, Professor, 地震研究所, 教授 (00219239)
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| Project Period (FY) |
2003 – 2005
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| Keywords | Kimberlite / Helium isotope ratio / West Greenland / Lower mantle / Volatile elements / Tungsten isotope ratio / Mantle xenolith / Ocean-island basalt |
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| Research Abstract |
In this study, we aimed to reveal the chemical state of the Earth's deep interior by analyzing kimberlites and various mantle-related materials and examine some models on the chemical structure of the Earth's interior. To collect in-situ kimberlites, we performed sampling in China and Brazil, but found them to have been much altered. We could get a knowledge that fresh kimberlites might have been only found in a limited place where kimberlite magmas were intruded at a depth like sills. By analyzing noble gases in fresh kimberlites from West Greenland, which we had got from foreign investigators, we have revealed that their magma sources are similar to those of ocean-island basalts and probably derived from the lower mantle based on their high ^3He/^4He ratios. Since kimberlites are enriched in volatile elements and show unfractionated platinum-group element patterns relative to those of chondrites, it can be inferred that the lower mantle might have been less degassed and more reduced
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state compared to the upper mantle. However, we could not identify the difference in tungsten isotope ratios between kimberlites and mantle-related materials of the upper mantle origin.
On the other hand, we observed ^3He/^4He ratios lower than that of the air in mantle xenoliths from Far Eastern Russia, which suggests the effect of subducted slabs to the mantle in that area. For xenoliths of lower crust and upper mantle origin from the Indian Ocean area, we observed ^3He/^4He ratios similar to those of MORBs and ^<40>Ar/^<36>Ar ratios affected by the air. Furthermore, we have revealed that basalts of less than 1 Ma from Mauritius Island, which belongs to the Reunion hotspot, show Sr, Nd and Pb isotope ratios which are closer to MORB values than those of older basalts. This can be explained by increasing of the effect of plume magmas by the upper mantle materials of MORB types with time. Such tendency can also be observed in noble gas isotopes in alkaline basalts taken from the ocean floor around the Hawaiian Islands and it suggests a possibility that upper mantle materials might have been affected by the ocean-island basalts. Less
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