論文 / 著書情報
Article / Book Information
題目(和文)

ハワイ諸島下におけるプルーム-リソスフェア相互作用： ハワイ産マントル捕獲岩成因論に対する鉱物学的および地球化学的制約

Title(English)

Plume-lithosphere interaction beneath the Hawaiian Islands:
mineralogical and geochemical constraints on the petrogenesis of
Hawaiian mantle xenoliths

著者(和文)

藤田遼

Author(English)

Ryo Fujita

出典(和文)

学位:博士(理学),学位授与機関:東京工業大学,
報告番号:甲第12650号,授与年月日:2024年3月26日,
学位の種別:課程博士,
審査員:石川 晃,横山 哲也,上野 雄一郎,中島 淳一,太田 健二

Citation(English)

Degree:Doctor (Science),
Conferring organization: Tokyo Institute of Technology,
Report number:甲第12650号,
Conferred date:2024/3/26,
Degree Type:Course doctor,
Examiner:,,,,

学位種別(和文)

博士論文

Category(English)

Doctoral Thesis

種別(和文)

論文要旨

Type(English)

Summary

Powered by T2R2 (Tokyo Institute Research Repository)

（博士課程）Doctoral Program論 文 要 旨ＴＨＥＳＩＳ ＳＵＭＭＡＲＹ

系・コース：
Department of, Graduate major in

地球惑星科学
地球惑星科学

学生氏名：
Student’s Name系
コース

藤田 遼

申請学位（専攻分野）：

博士

Academic Degree Requested

Doctor of

審査員主査：
Chief Examiner（理学石川晃）

要旨（英文 800 語程度）
Thesis Summary （approx.800 English Words ）The Hawaiian Islands have been an attractive target for geological research because of the active
hotspot volcanism induced by the deep-rooted mantle plume. Studies of Hawaiian volcanoes have played acritical role in unraveling the mysterious nature and evolution of deep mantle structures. Peridotite xenoliths
from the Hawaiian Islands serve as mantle residues and provide a unique opportunity to investigate refractorycomponents in deep mantle plumes. However, the origin of these Hawaiian mantle xenoliths is controversial,with questions being raised as to whether they are derived from the plume, reflecting its chemical diversity, orare fragments of the Pacific plate unrelated to the Hawaiian plume. Given the complex interactions between theplume and the lithosphere revealed by geophysical and numerical studies, a comprehensive understanding of
the lithospheric evolution beneath the Hawaiian Islands is essential to reveal the origin of these xenoliths.
For this purpose, I studied the microstructure, crystallographic preferred orientations (CPOs), mineralchemistry, whole-rock chemical composition, and isotopic composition (He, Ne, Ar, and Os) of Hawaiian
mantle xenoliths from Kaʻula Island, Kauaʻi Island, Salt Lake Crater (SLC), and Pal- Ka‘au (PK) vents. Thisdissertation contains seven chapters.
Chapter 1 provides a general introduction and review of deep Earth structures, the geochemical
heterogeneity in the Hawaiian plume, the evolution of the Hawaiian lithosphere, and the geological insights into
the controversial origins of the Hawaiian mantle xenoliths.
Chapter 2 involved the reconstruction of the lithostratigraphy of the Hawaiian lithosphere based on
microstructure and mineral chemistry. Petrophysical and petrological analyses of the Hawaiian mantle xenoliths
underscore the influence of reactive transport of plume-related melts on the recrystallization and re-fertilizationof the Hawaiian lithosphere.
Equilibrium temperatures exhibit variability among sites, with the highest temperatures observed in
the following order: Kaʻula Island, Salt Lake Crater (SLC), PK vent, and Kauaʻi Island. Depth-related
temperature variations within the Hawaiian lithosphere are likely responsible for the temperature variations
observed in Hawaiian mantle xenoliths between these localities. The thermal gradient estimated from P-T
conditions in SLC garnet pyroxenites provides pressures corresponding to equilibrium temperatures in
Hawaiian peridotite xenoliths, yielding derived depths of ~1.5 GPa for Kauaʻi peridotites, ~1.7 GPa for PK
peridotites, ~1.9 GPa for SLC peridotites, and ~2.0 GPa for Kaʻula peridotites.
The depth-related chemical variation within the Hawaiian lithosphere is then constrained by a
combination of microstructure, mineral chemistry, and whole-rock geochemistry (Chapter 3). The Kauaʻi
peridotites, composed of refractory materials, correspond to the shallower layer of the Pacific lithosphere, while
the PK peridotites, which have a fertile composition, correspond to the deeper Pacific lithosphere. However, itis still unclear why the Kaʻula and SLC peridotites are both refractory and deep, which cannot be explained
through the typical ridge-melting model. This suggests that plume-lithosphere interaction modified the bottomof the pre-existing Pacific plate, creating a highly refractory layer at the base of the Hawaiian lithosphere.

In Chapter 4, the analytical protocol and method for HSE and VCE abundances, along with Os
isotopes, are developed for ultramafic samples. Following the developed method, the HSE geochemistry in theHawaiian mantle xenoliths was investigated to understand the nature and ancient origin of the Hawaiian mantle
xenoliths in Chapter 5. HSE abundances and Os isotopes in Kaʻula and SLC peridotites are dominated by thehighly depleted pattern (Pd/Ir <0.5) and unradiogenic Os isotope composition (187Os/188Os <0.126),
respectively. In contrast, PK and Kauaʻi peridotites display flat HSE patterns and near chondritic 187Os/188Os
ratios comparable to Pacific abyssal peridotites. Highly depleted HSE patterns and unradiogenic Os isotopes inKaʻula and SLC peridotites imply that these peridotites represent the ancient, recycled lithosphere within theHawaiian plume.
Chapter 6 examines the noble gas systematics of the Hawaiian mantle xenoliths and discusses thenature of chemical heterogeneity and the genetic relationship between primordial and recycled materialreservoirs. The study found that refractory and less-metasomatized peridotites have radiogenic 3He/4He ratios
compared to the MORB mantle. Model calculations indicate that the ancient (> 1.0 Ga) residual mantle could
host the radiogenic 3He/4He ratios observed in these coarse porphyroclastic peridotites. Consequently, theradiogenic 3He/4He ratios in the ancient residual mantle support the argument that recycling depleted oceanic
lithosphere cannot account for the high 3He/4He ratios but may contribute to an isotopic end-member component
characterized by depleted lithophile elements and radiogenic 3He/4He ratios.
In Chapter 7, a proposed model explains the evolution of the Hawaiian lithosphere, including theinteraction between the plume and the lithosphere. This model has implications for the distribution ofgeochemical heterogeneity in the Hawaiian plume and deep mantle. The result obtained in this study has
highlighted the petrophysical and chemical changes that occur in the Hawaiian lithosphere as a result of plumelithosphere interaction. During the main shield stage, there is a significant influx of magma, which causes staticrecrystallization and refertilization. This modification, in turn, results in rheological weakening and increased
intrinsic density. The plume residue, which is high-temperature, refractory, and buoyant, accumulates at thebasal lithosphere, causing thermal and density instability at the lithosphere-asthenosphere boundary. The
activated small-scale convection erodes and thins the lithosphere, which is then compensated for by the plumeremnant. This process may explain the dichotomous nature and origin of the shallower and deeper parts of theHawaiian lithosphere, as reflected in two distinct chemical composition groups of mantle xenoliths.
The presence of ancient and depleted signatures in the Kaʻula and SLC peridotites suggests that theHawaiian plume comprises refractory diapirs with intrinsic buoyancy. These diapirs consist of ancient, depleted,and recycled lithospheric mantle. The spatial correlation between two parallel volcanic chains and localities of
two mantle xenolith groups implies a cogenetic relationship between the Loa and ancient depleted components
within the Hawaiian plume. If the observed geochemical differences between Kea and Loa trends result frompreferential sampling of the deep mantle reservoir, the Pacific Large Low-Shear-Velocity Province (LLSVP)
may manifest as strong geochemical anomalies originating from ancient, subducted lithosphere composed of
isotopically enriched recycled crustal material and depleted refractory lithospheric mantle. The persistentdensity and longevity of LLSVPs may be due to subduction of cool and dense oceanic lithosphere, conductive
heating by the outer core, selective upwelling of buoyant refractory lithosphere as mantle plumes, andaccumulation of dense crustal material at the lowermost mantle that promotes LLSVPs growth.
備考 : 論文要旨は、和文 2000 字と英文 300 語を 1 部ずつ提出するか、もしくは英文 800 語を 1 部提出してください。
Note : Thesis Summary should be submitted in either a copy of 2000 Japanese Characters and 300 Words (Engｌish) or 1copy of 800
Words (English).
注意：論文要旨は、東工大リサーチリポジトリ（T2R2）にてインターネット公表されますので、公表可能な範囲の内容で作成してください。
Attention: Thesis Summary will be published on Tokyo Tech Research Repository Website (T2R2).