Noble gas study of on- and off-axis alkali volcanism at the Hawaiian hotspot
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Hotspot (geology)
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Mantle plume
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The Hawaiian–Emperor Seamount Chain (ESC), in the northern Pacific Ocean, was produced during the passage of the Pacific Plate over the Hawaiian hotspot. Major and trace element concentrations and Sr–Nd–Pb isotopic compositions of shield and post-shield lavas from nine of the Emperor Seamounts provide a 43 Myr record of the chemistry of the oldest preserved Hawaiian magmatism during the Late Mesozoic and Early Cenozoic (from 85 to 42 Ma). These data demonstrate that there were large variations in the composition of Hawaiian magmatism over this period. Tholeiitic basalts from Meiji Seamount (85 Ma), at the northernmost end of the ESC, have low concentrations of incompatible trace elements, and unradiogenic Sr isotopic compositions, compared with younger lavas from the volcanoes of the Hawaiian Chain (<43 Ma). Lavas from Detroit Seamount (81 Ma) have highly depleted incompatible trace element and Sr–Nd isotopic compositions, which are similar to those of Pacific mid-ocean ridge basalts. Lavas from the younger Emperor Seamounts (62–42 Ma) have trace element compositions similar to those of lavas from the Hawaiian Islands, but initial 87Sr/86Sr ratios extend to lower values. From 81 to 42 Ma there was a systematic increase in 87Sr/86Sr of both tholeiitic and alkalic lavas. The age of the oceanic lithosphere at the time of seamount formation decreases northwards along the Emperor Seamount Chain, and the oldest Emperor Seamounts were built upon young, thin lithosphere close to a former spreading centre. However, the inferred distance of the Hawaiian plume from a former spreading centre, and the isotopic compositions of the oldest Emperor lavas appear to rule out plume–ridge interaction as an explanation for their depleted compositions. We suggest that the observed temporal chemical and isotopic variations may instead be due to variations in the degree of melting of a heterogeneous mantle, resulting from differences in the thickness of the oceanic lithosphere upon which the Emperor Seamounts were constructed. During the Cretaceous, when the Hawaiian plume was situated beneath young, thin lithosphere, the degree of melting within the plume was greater, and incompatible trace element depleted, refractory mantle components contributed more to melting.
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報告番号: 乙15021 ; 学位授与年月日: 2001-04-09 ; 学位の種別: 論文博士 ; 学位の種類: 博士(理学) ; 学位記番号: 第15021号 ; 研究科・専攻: 理学系研究科
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Helium isotopes are a robust geochemical tracer of a primordial mantle component in hot spot volcanism. The high 3 He/ 4 He (up to 35 R A , where R A is the atmospheric 3 He/ 4 He ratio of 1.39 × 10 −6 ) of some Hawaiian Island volcanism is perhaps the classic example. New results for picrites and basalts from the Hawaiian‐Emperor seamount chain indicate that the hot spot has produced high 3 He/ 4 He lavas for at least the last 76 million years. Picrites erupted at 76 Ma have 3 He/ 4 He (10–14 R A ), which is at the lower end of the range for the Hawaiian Islands but still above the range of modern mid‐ocean ridge basalt (MORB; 6–10 R A ). This was at a time when hot spot volcanism was occurring on thin lithosphere close to a spreading ridge and producing lava compositions otherwise nearly indistinguishable from MORB. After the hot spot and spreading center diverged during the Late Cretaceous, the hot spot produced lavas with significantly higher 3 He/ 4 He (up to 24 R A ). Although 3 He/ 4 He ratios stabilized at relatively high values by 65 Ma, other chemical characteristics such as La/Yb and 87 Sr/ 86 Sr did not reach and stabilize at Hawaiian‐Island‐like values until ∼45 Ma. Our limited 3 He/ 4 He record for the Hawaiian hot spot shows a poor correlation with plume flux estimates (calculated from bathymetry and residual gravity anomalies [ Van Ark and Lin , 2004 ]). If 3 He is a proxy for the quantity of primordial mantle material within the plume, then the lack of correlation between 3 He/ 4 He and calculated plume flux suggests that variation in primordial mantle flux is not the primary factor controlling total plume flux.
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Hotspot (geology)
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Submarine lavas erupted onto the Hawaiian arch 200–400 km north of Oahu show that the areal extent of Hawaiian volcanism is much larger than previously recognized. The North Arch volcanic field comprises 25 000 km2 of ∼0·5–1·15 Ma, volatile-rich, olivine-phyric alkalic lavas (alkalic basalt to nephelinite). These lavas are similar in composition to rejuvenated-stage lavas such as the Koloa Volcanics (Kauai) and Honolulu Volcanics (Oahu). North Arch lavas that encompass the compositional extremes have similar Sr, Nd and Pb isotopic ratios. Olivine accumulation and fractionation was the major post-melting process that affected the compositions of North Arch lavas. After correction for these processes, the inferred primary magma compositions show that they were derived by variable, factor of four, and relatively low extents of melting of garnet peridotite. Garnet and olivine were important residual phases during partial melting; in contrast to the Honolulu Volcanics, there is little evidence for residual hydrous phases, sulfides or Fe–Ti oxides. The mantle source for the North Arch lavas had Sr, Nd and Pb isotopic ratios intermediate between those of Pacific Ocean lithosphere and the inferred range for Hawaiian plume components. These data are consistent with a mixed lithosphere–plume source. Although the plume-derived component was probably from the Hawaiian plume, an alternative hypothesis is that during the middle Cretaceous, South Pacific lithosphere was contaminated by plumes that formed large oceanic plateaux (e.g. Ontong Java). This mixed source was subsequently partially melted as it passed near the Hawaiian plume.
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