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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)

要約

Type(English)

OutlinePowered by T2R2 (Tokyo Institute Research Repository)論文要約

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

Ryo Fujita
Ishikawa laboratory
Department of Earth and Planetary Sciences
Tokyo Institute of Technology

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 a critical role in unraveling the mysterious nature and evolution of deep
mantle structures. Peridotite xenoliths from the Hawaiian Islands serve as mantle residues andprovide a unique opportunity to investigate refractory components 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, or arefragments of the Pacific plate unrelated to the Hawaiian plume. Given the complex interactions
between the plume and the lithosphere revealed by geophysical and numerical studies, acomprehensive understanding of the lithospheric evolution beneath the Hawaiian Islands isessential to reveal the origin of these xenoliths.
For this purpose, I studied the microstructure, crystallographic preferred orientations(CPOs), mineral chemistry, 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. This dissertation contains seven chapters.
Chapter 1 provides a general introduction and review of deep Earth structures, thegeochemical heterogeneity in the Hawaiian plume, the evolution of the Hawaiian lithosphere, andthe geological insights into the controversial origins of the Hawaiian mantle xenoliths.
Chapter 2 involved the reconstruction of the lithostratigraphy of the Hawaiian lithospherebased on microstructure and mineral chemistry. Petrophysical and petrological analyses of theHawaiian mantle xenoliths underscore the influence of reactive transport of plume-related meltson the recrystallization and re-fertilization of 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 KauaʻiIsland. 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 1.0-1.2 GPa for Kauaʻi peridotites, 1.1-1.3 GPa for PK peridotites, 1.2-1.4 GPafor SLC peridotites, and 1.5-1.7 GPa for Kaʻula peridotites.
The depth-related chemical variation within the Hawaiian lithosphere is then constrainedby a combination of microstructure, mineral chemistry, and whole-rock geochemistry (Chapter3). The Kauaʻi peridotites, composed of refractory materials, correspond to the shallower layer ofthe Pacific lithosphere, while the PK peridotites, which have a fertile composition, correspond tothe deeper Pacific lithosphere. However, it is still unclear why the Kaʻula and SLC peridotites areboth refractory and deep, which cannot be explained through the typical ridge-melting model.
This suggests that plume-lithosphere interaction modified the bottom of the pre-existing Pacificplate, 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, alongwith Os isotopes, are developed for ultramafic samples. Following the developed method, theHSE geochemistry in the Hawaiian mantle xenoliths was investigated to understand the natureand ancient origin of the Hawaiian mantle xenoliths in Chapter 5. HSE abundances and Osisotopes in Kaʻula and SLC peridotites are dominated by the highly depleted pattern (Pd/Ir <0.5)
and unradiogenic Os isotope composition (187Os/188Os <0.126), respectively. In contrast, PK andKauaʻi peridotites display flat HSE patterns and near chondritic 187Os/188Os ratios comparable toPacific abyssal peridotites. Highly depleted HSE patterns and unradiogenic Os isotopes in Kaʻulaand SLC peridotites imply that these peridotites represent the ancient, recycled lithosphere within
the Hawaiian plume.
Chapter 6 examines the noble gas systematics of the Hawaiian mantle xenoliths anddiscusses the nature of chemical heterogeneity and the genetic relationship between primordial
and recycled material reservoirs. 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, the radiogenic 3He/4He ratios
in the ancient residual mantle support the argument that recycling depleted oceanic lithospherecannot 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 the interaction between the plume and the lithosphere. This model has implications forthe distribution of geochemical heterogeneity in the Hawaiian plume and deep mantle. The result
obtained in this study has highlighted the petrophysical and chemical changes that occur in theHawaiian lithosphere as a result of plume-lithosphere interaction. During the main shield stage,
there is a significant influx of magma, which causes static recrystallization 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 the basal
lithosphere, causing thermal and density instability at the lithosphere-asthenosphere boundary.
The activated small-scale convection erodes and thins the lithosphere, which is then compensatedfor by the plume remnant. This process may explain the dichotomous nature and origin of theshallower and deeper parts of the Hawaiian lithosphere, as reflected in two distinct chemicalcomposition groups of mantle xenoliths.
The presence of ancient and depleted signatures in the Kaʻula and SLC peridotites
suggests that the Hawaiian 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 acogenetic 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
persistent density 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, and accumulation of dense crustal material at thelowermost mantle that promotes LLSVPs growth.