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Shoji Arai – One of the best experts on this subject based on the ideXlab platform.

  • Alkali Basalt from the seifu seamount in the sea of japan post spreading magmatism in a back arc setting
    Solid Earth, 2020
    Co-Authors: Shoji Arai, Tomoaki Morishita, Naoto Hirano, Hirochika Sumino, Hiroshi Sato, Tomoyuki Shibata, Masako Yoshikawa, Rie Nauchi, Akihiro Tamura

    Abstract:

    Abstract. We present geochemical and 40Ar∕39Ar age data
    for a peridotite xenolith-bearing Basalt dredged from the Seifu Seamount (SSM
    Basalt) in the northeast Tsushima Basin, southwest Sea of Japan. An
    40Ar∕39Ar plateau age of 8.33±0.15  Ma (2 σ ) was
    obtained for the SSM Basalt, indicating that it erupted shortly after the
    termination of back-arc spreading in the Sea of Japan. The SSM Basalt is a
    high-K to shoshonitic Alkali Basalt that is characterized by light rare
    earth element enrichment. The trace element features of the Basalt are
    similar to those of ocean island Basalt, although the Yb content is much
    higher, indicating formation by the low-degree partial melting of spinel
    peridotite. The Nd, Sr, and Pb isotopic compositions of the SSM Basalt
    differ from those of back-arc basin Basalts in the Sea of Japan. The Sr–Nd
    isotopic composition of the SSM Basalt suggests its source was depleted
    mid-ocean ridge mantle containing an enriched mantle (EM1) component. The SSM Basalt was
    formed in a post-back-arc extension setting by the low-degree partial melting of
    an upwelling asthenosphere that had previously been associated with the main
    phase of back-arc magmatism.

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  • silica and lree enriched spinel peridotite xenoliths from the quaternary intraplate Alkali Basalt jeju island south korea old subarc fragments
    Lithos, 2014
    Co-Authors: Yonghoon Woo, Kyounghee Yang, Youngwoo Kil, Sunghyo Yun, Shoji Arai

    Abstract:

    Abstract Spinel harzburgite to lherzolite xenoliths are entrapped in Quaternary intraplate Alkali Basalts on Jeju Island, South Korea. These xenoliths are unusual in containing late-stage secondary orthopyroxene, free of deformation and exsolution that is replacing olivine as the main pervasive metasomatic mineral. These xenoliths are characterized by high Mg# in olivine, orthopyroxene, and clinopyroxene (89–93) and variable Cr# of spinel (9–53), representing residues left after variable degrees of melt extraction (~ 25%). In contrast to their depleted major-element compositions, clinopyroxenes in the xenoliths are enriched in most incompatible trace elements. Clinopyroxenes display enrichment in light rare earth elements (LREE) or spoon-shaped REE with a general enrichment in La over Ce, and depletion in high field strength elements (HFSE; e.g., Nb-Ta, Zr-Hf, Ti). Orthopyroxenes (either primary or secondary) are characterized by low TiO2, high Al2O3, and moderate CaO contents, and resemble those of sub-continental arc peridotites from the eastern Pacific. The geochemical evidence, in addition to the formation of secondary orthopyroxene, indicates that Jeju peridotite xenoliths have been subjected to different degrees of metasomatism by subduction-related silica- and LREE-enriched fluids (or melts). However, chemical equilibrium is evident between the primary and secondary orthopyroxene, implying that the duration of post-metasomatic high temperatures enabled complete resetting/reequilibration of the mineral compositions. The metasomatic enrichment pre-dates the host Jeju Quaternary magmatism, and a genetic relationship with the host magmas is considered unlikely. We therefore propose that the Jeju peridotite xenoliths went through a two-stage evolution, with their composition primarily controlled by early fractional melt extraction, which was subsequently modified by residual slab-derived fluids (or melts). Following enrichment in the peridotite protolith in the mantle wedge, the upper mantle beneath proto-Jeju Island was transformed from a subarc environment to an intraplate environment. The Jeju peridotites, representing old subarc fragments, were subsequently transported to the surface, incorporated into ascending Quaternary intraplate Alkali Basalt. The result of this study implies that long term material transfer in the transformation of geotectonic setting from a subarc to intraplate may have played a significant role in the evolution of the subcontinental lithospheric mantle, resulting in the enriched mantle domains, such as EMI or EMII.

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  • silica enrichment of group ii xenoliths by evolved Alkali Basalt from jeju island south korea implication for modification of intraplate deep seated rocks
    Mineralogy and Petrology, 2012
    Co-Authors: Kyounghee Yang, Shoji Arai, Csaba Szabo, Hoonyeong Jung

    Abstract:

    Group II xenoliths, corresponding to the lithology of dunite, wehrlite to olivine clinopyroxenite and olivine websterite to websterite, occur in Pleisto-Holocene Alkali Basalts from Jeju Island, South Korea. The large grain size (up to 5 mm), moderate mg# [=100 × Mg/(Mg + Fetotal) atomic ratio] of olivine (79–82) and pyroxenes (77–83), and absence of metamorphic textural features indicate that they are cumulates of igneous origin. Based on textural features, mineral equilibria and major and trace element variations, it can be inferred that the studied xenoliths were crystallized from Basaltic melts enriched in incompatible trace elements and belong to the Jeju Pleisto-Holocene magma system. They appear to have been emplaced near the present Moho, an estimated 5–8 kbars beneath Jeju Island. Consolidation of cumulates was followed by infiltration of silica-enriched metasomatic melt, producing secondary orthopyroxenes at the expense of olivine. The metasomatic agent appears to have been a silica-enriched residual melt evolved from an initially slightly silica-undersaturated Alkali Basalt to silica-saturated compositions by fractional crystallization under relatively high pressure conditions. The result of this study indicates that relatively young olivine-bearing cumulates could have been metasomatized by a silica-enriched melt within underplates, suggesting that silica enrichment can occur in intraplate Moho-related rocks as well as in the upper mantle of the subarc area.

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G. Rivalenti – One of the best experts on this subject based on the ideXlab platform.

  • co2 fluid and silicate glass as monitors of Alkali Basalt peridotite interaction in the mantle wedge beneath gobernador gregores southern patagonia
    Lithos, 2009
    Co-Authors: Marco Scambelluri, Riccardo Vannucci, A. De Stefano, M Preitemartinez, G. Rivalenti

    Abstract:

    Abstract A suite of mantle-wedge amphibole + phlogopite-bearing spinel peridotite xenoliths in Plio-Pleistocene Alkali Basalts from Southern Patagonia (Gobernador Gregores, Santa Cruz Province, Argentina) contains carbonic fluid inclusions, glass and carbonate in several textural domains. Here we present a microstructural and fluid inclusion study showing that fluid (corresponding to pure CO 2 ) and glass post-date the hydrous mantle assemblage and formed soon before and/or during xenolith entrainement in the host Alkali Basalt. The high densities preserved by a number of CO 2 inclusions indicate that fluid infiltration took place at mantle depths. The low densities pertaining to the majority of analyzed fluid inclusions derive from inclusion re-equilibration during xenolith ascent. Glass occurs in reaction haloes around clinopyroxene, amphibole and phlogopite, where it hosts microlites of new pyroxene, olivine and locally carbonate. Glass veins cut the mantle minerals and locally contain primary carbonate. Glasses vary widely in composition depending on the textural domains and attain Si- and Alkali-rich compositions (SiO 2  = 47.0–68.3 wt.%; Na 2 O + K 2 O = 5.8–12.2 wt.%). Incompatible trace element patterns of glasses in anhydrous xenoliths are closely similar to those of the host Alkali Basalts, whereas the compositions of interstitial and vein glasses in the hydrous xenoliths indicate that a compositional control has been exerted by the local mineral assemblage (mainly amphibole). The δ 18 O values of carbonate from the glass pockets and veins in the xenoliths, as well as of carbonate globules and amygdales in the host Basalts are in the range 19.62 to 21.04‰. Corresponding δ 13 C values are − 9.25 to − 10.12‰ and − 7.59 to − 9.32‰, respectively. These values are very different from those of primary carbonatites and the δ 18 O values clearly exceed those expected for minerals and glasses from mantle assemblages. The similarity of isotopic ratios of carbonates from both xenoliths and host lavas and their shift towards low δ 13 C and high δ 18 O values may be the result of Basalt-peridotite interaction during ascent of the mantle xenoliths. Our study points to a close relationship between the infiltration of carbonic fluid together with fractions of the host Alkali Basalt, and melting of hydrous peridotite-forming minerals. Assuming an initial average content of 1.5–2 wt.% CO 2 in the primary Alkaline melt and considering that the dissolved amounts of CO 2 and H 2 O in such a melt at 400 MPa can be in the order of 0.3 wt.% and 3.5% respectively, approximately 75 to 50% of the total carbon dioxide load was released by the uprising host magma. This process led to infiltration and entrapment of high-density CO 2 inclusions in the GG mantle rocks, to hydrous phase breakdown and to carbonate precipitation in veins and at some reaction sites after the primary mantle minerals. We propose that formation of CO 2 inclusions, glass and carbonate in hydrated mantle xenoliths is not unique of carbonatite metasomatism: comparable effects can be produced by degassing Alkaline magmas.

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  • CO2 fluid and silicate glass as monitors of Alkali Basalt/peridotite interaction in the mantle wedge beneath Gobernador Gregores, Southern Patagonia
    Lithos, 2008
    Co-Authors: Marco Scambelluri, Riccardo Vannucci, A. De Stefano, M. Preite-martinez, G. Rivalenti

    Abstract:

    Abstract A suite of mantle-wedge amphibole + phlogopite-bearing spinel peridotite xenoliths in Plio-Pleistocene Alkali Basalts from Southern Patagonia (Gobernador Gregores, Santa Cruz Province, Argentina) contains carbonic fluid inclusions, glass and carbonate in several textural domains. Here we present a microstructural and fluid inclusion study showing that fluid (corresponding to pure CO 2 ) and glass post-date the hydrous mantle assemblage and formed soon before and/or during xenolith entrainement in the host Alkali Basalt. The high densities preserved by a number of CO 2 inclusions indicate that fluid infiltration took place at mantle depths. The low densities pertaining to the majority of analyzed fluid inclusions derive from inclusion re-equilibration during xenolith ascent. Glass occurs in reaction haloes around clinopyroxene, amphibole and phlogopite, where it hosts microlites of new pyroxene, olivine and locally carbonate. Glass veins cut the mantle minerals and locally contain primary carbonate. Glasses vary widely in composition depending on the textural domains and attain Si- and Alkali-rich compositions (SiO 2  = 47.0–68.3 wt.%; Na 2 O + K 2 O = 5.8–12.2 wt.%). Incompatible trace element patterns of glasses in anhydrous xenoliths are closely similar to those of the host Alkali Basalts, whereas the compositions of interstitial and vein glasses in the hydrous xenoliths indicate that a compositional control has been exerted by the local mineral assemblage (mainly amphibole). The δ 18 O values of carbonate from the glass pockets and veins in the xenoliths, as well as of carbonate globules and amygdales in the host Basalts are in the range 19.62 to 21.04‰. Corresponding δ 13 C values are − 9.25 to − 10.12‰ and − 7.59 to − 9.32‰, respectively. These values are very different from those of primary carbonatites and the δ 18 O values clearly exceed those expected for minerals and glasses from mantle assemblages. The similarity of isotopic ratios of carbonates from both xenoliths and host lavas and their shift towards low δ 13 C and high δ 18 O values may be the result of Basalt-peridotite interaction during ascent of the mantle xenoliths. Our study points to a close relationship between the infiltration of carbonic fluid together with fractions of the host Alkali Basalt, and melting of hydrous peridotite-forming minerals. Assuming an initial average content of 1.5–2 wt.% CO 2 in the primary Alkaline melt and considering that the dissolved amounts of CO 2 and H 2 O in such a melt at 400 MPa can be in the order of 0.3 wt.% and 3.5% respectively, approximately 75 to 50% of the total carbon dioxide load was released by the uprising host magma. This process led to infiltration and entrapment of high-density CO 2 inclusions in the GG mantle rocks, to hydrous phase breakdown and to carbonate precipitation in veins and at some reaction sites after the primary mantle minerals. We propose that formation of CO 2 inclusions, glass and carbonate in hydrated mantle xenoliths is not unique of carbonatite metasomatism: comparable effects can be produced by degassing Alkaline magmas.

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Brian J Fryer – One of the best experts on this subject based on the ideXlab platform.

  • trace element evidence for volatile influenced differentiation in a flow of Alkali Basalt peng hu island taiwan
    Canadian Mineralogist, 2008
    Co-Authors: John D Greenough, Brian J Fryer

    Abstract:

    A 20-m-thick Alkali Basalt flow on the Peng Hu Island, Taiwan, shows three well-developed segregation veins in its upper 8.5 meters. Each pegmatitic and “vesicular” (ocelli-bearing?) segregation displays distinct whole-sample chemical traits that are shared with the enclosing Basalt. Augite, plagioclase and olivine exhibit major-element (electron-microprobe data) and trace element compositions (laser ablation microprobe – inductively coupled plasma – mass spectrometry) that reflect this whole-rock chemical stratification. Thus the chemical stratification is a product of igneous processes and is not the result of secondary alteration. Elements defining the stratification (K, Rb, Li, Na, Zn in minerals and whole-rock data, but Cl, S, As, Pb and Sr are also important based on whole-rock data) tend to be complexed and moved by volatiles in various geological environments. Conventional crystal-fractionation models do not reproduce the observed variations in these elements up through the flow. The patterns of data suggest that rising plumes of vesicles carried volatile-complexed (scavenged) elements to high levels in the flow at the time the segregation veins were forming and the interior of the flow was largely molten.

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  • trace element evidence for volatile influenced differentiation in a flow of Alkali Basalt peng hu island taiwan
    Canadian Mineralogist, 2008
    Co-Authors: John D Greenough, Brian J Fryer

    Abstract:

    A 20-m-thick Alkali Basalt flow on the Peng Hu Island, Taiwan, shows three well-developed segregation veins in its upper 8.5 meters. Each pegmatitic and “vesicular” (ocelli-bearing?) segregation displays distinct whole-sample chemical traits that are shared with the enclosing Basalt. Augite, plagioclase and olivine exhibit major-element (electron-microprobe data) and trace element compositions (laser ablation microprobe – inductively coupled plasma – mass spectrometry) that reflect this whole-rock chemical stratification. Thus the chemical stratification is a product of igneous processes and is not the result of secondary alteration. Elements defining the stratification (K, Rb, Li, Na, Zn in minerals and whole-rock data, but Cl, S, As, Pb and Sr are also important based on whole-rock data) tend to be complexed and moved by volatiles in various geological environments. Conventional crystal-fractionation models do not reproduce the observed variations in these elements up through the flow. The patterns of data suggest that rising plumes of vesicles carried volatile-complexed (scavenged) elements to high levels in the flow at the time the segregation veins were forming and the interior of the flow was largely molten.

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  • evidence for volatile influenced differentiation in a layered Alkali Basalt flow penghu islands taiwan
    Bulletin of Volcanology, 1999
    Co-Authors: John D Greenough, Brian J Fryer

    Abstract:

    An approximately 20-m-thick Alkali Basalt flow on the Penghu Islands contains ∼20 cm thick, horizontally continuous (>50 m), vesicular layers separated by ∼1.5 m of massive Basalt in its upper 8.5 m. The three layers contain ocelli-like “vesicles” filled with nepheline and igneous carbonate. They are coarse grained and enriched in incompatible elements relative to the massive Basalt with which they form sharp contacts. These vesicular layers (segregation veins) formed when residual liquid in the underlying crystal mush was forced (gas filter pressing) or siphoned into three thermally induced horizontal cracks that opened successively in the advancing crystal mush of the flow’s upper crust. Most vesicular layer trace elements can be modelled by residual melt extraction after 25–40% fractional crystallization of massive Basalt underlying each layer. Sulphur, Cl, As, Zn, Pb, K, Na, Rb, and Sr show large concentration changes between the top, middle, and bottom layers, with each vesicular and underlying massive Basalt forming a chemically distinct “pair.” The large changes between layers are difficult to account for by crystal fractionation alone, because other incompatible elements (e.g., La, Sm, Yb, Zr, Nb) and the major elements change little. The association of these elements (S, Cl, etc.) with “fluids” in various geologic environments suggests that volatiles influenced differentiation, perhaps by moving Alkali, Alkaline earth, and chalcophile elements as magma-dissolved volatile complexes. Volatiles may have also led to large grain sizes in the segregation veins by lowering melt viscosities and raising diffusion rates. The chemical variability between layers indicates that a convection and concentration mechanism acted within the flow. The specific process cannot be determined, but different rates of vesicle plume rise (through the flow) and/or accumulation in the upper crust’s crystal mush might account for the chemical pairing and extreme variations in Cl, S, As, and C. This study emphasizes the importance of sampling vesicular rocks in flows. It also suggests that volatiles play important physical and chemical roles in rapidly differentiating mafic magmas in processes decoupled from crystal fractionation.

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