|
|
Hokkaido University Collection of Scholarly and Academic Papers >
Theses >
博士 （理学） >
Identification of multiple components of noble gas isotopes in backarc lithospheric mantle
|
Files in This Item:
|
|
|
Related Items in HUSCAP:
|
| Title: | Identification of multiple components of noble gas isotopes in backarc lithospheric mantle |
| Other Titles: | 背弧域マントルの希ガス同位体比に基づいた不均質同定 |
| Authors: | 横倉, 伶奈 Browse this author |
| Issue Date: | 23-Mar-2023 |
| Publisher: | Hokkaido University |
| Abstract: | I investigated isotope ratios of noble gases (3He/4He and 40Ar/36Ar) extracted by crushing and laser microprobe methods for five upper mantle-derived xenoliths collected from northeast China. The original depth of five mantle xenoliths is estimated to be approx. 25.9–36.6 km by combining equilibrium temperatures with CO2 densities of fluid inclusions in the mantle xenoliths. The estimated depth corresponds to the uppermost part of the lithospheric mantle beneath back-arc region. The ranges of 3He/4He, 40Ar/36Ar, and 3He/36Ar obtained by crushing method are 6.45–0.14 Ra, 1509–284, and 2.3 × 10-1–5.7 × 10-5, respectively. In addition to sub-continental lithospheric mantle (SCLM)-like components, it indicates contribution of components having radiogenic 3He/4He and atmospheric 40Ar/36Ar. Considering a location property, where a subducted slab may be stagnated below, the component showing atmospheric 40Ar/36Ar and radiogenic 3He/4He could be associated with the subducted slab. That is, the noble gas characteristics could be explained by two-component mixing between SCLM and slab-related components having atmospheric 40Ar/36Ar and extremely low 3He/4He. Although, based on the calculation of radiogenic addition of 40Ar within the slab-related components, they could keep atmospheric 40Ar/36Ar for several million years, 40Ar/36Ar of the hydrous minerals in the slab would increase to over 350 in 4 Ma even if the initial 40Ar/36Ar is assumed as 295.5. In contrast, pore-fluid (ancient seawater in a subducted slab) could keep atmospheric 40Ar/36Ar in 4 Ma because it has K content lower than that in the hydrous minerals. Hence, I proposed that the present noble gas isotope characteristics is responsible for a component related to the pore-fluid in the subducted slab. The heterogeneous noble gas isotope ratios (ex. 6.45–0.14 Ra) were obtained from the mantle xenoliths originating from narrow depth range (25.9–36.6 km). It may reflect local metasomatism induced by multi-components with various origins. Observation of inclusions would be effective in investigating the cause of the heterogeneity of noble gas isotope ratios extracted from the xenoliths by crushing method. On the basis of shape and chemical composition, the inclusions were classified into three types. I performed quantitative verification of the correlation between 3He/4He and area ratio of three types of inclusions on the polished surface of ten olivine grains in each sample. As a result, it is calculated that type (3) inclusions with negative crystal shape have a relatively high 3He/4He, and types (1) and (2) inclusions may have a relatively low 3He/4He. In order to verify the speculation and to determine the 40Ar/36Ar for each type of the inclusions, I applied laser spot analysis of noble gas isotopes for type (2) and type (3) inclusions. The type (2) inclusions showed relatively low 3He/4He and atmospheric 40Ar/36Ar. In contrast, type (3) inclusions showed relatively high 3He/4He and 40Ar/36Ar, which were almost typical of SCLM values. The atmospheric 40Ar/36Ar and relatively low 3He/4He. The type (2) inclusions are composed of melt inclusions. Considering the growth stage of inclusion form, type (2) inclusions would be vestiges of melt infiltrated to the upper mantle beneath northeast China later than type (3) inclusions, whose shape represents a matured stage.
Taking the tectonic setting of the studied area into account, the coexistence of the inclusions with different noble gas isotopic compositions indicates that the upper mantle with SCLM-like noble gas isotopic compositions was infiltrated by the melt associated with pore-fluid. The occurrence of the melt with pore-fluid-like noble gas isotope characteristics in the mantle beneath back-arc region implies that the pore-fluid plays an important role in the process of transporting surface water back to the mantle. |
| Conffering University: | 北海道大学 |
| Degree Report Number: | 甲第15288号 |
| Degree Level: | 博士 |
| Degree Discipline: | 理学 |
| Examination Committee Members: | (主査) 教授 圦本 尚義, 教授 栗谷 豪, 助教 馬上 謙一, 教授 山本 順司 （九州大学大学院理学研究院） |
| Degree Affiliation: | 理学院（自然史科学専攻） |
| Type: | theses (doctoral) |
| URI: | http://hdl.handle.net/2115/89641 |
| Appears in Collections: | 課程博士 (Doctorate by way of Advanced Course) > 理学院(Graduate School of Science)
学位論文 (Theses) > 博士 （理学）
|
|
