ニュージーランド, オケテ・ベイサナイト中の多起源性かんらん石斑晶 Polygenetic olivine phenocrysts in Okete basanite, New Zealand

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ニュージーランド,オケテのベイサナイトおよびそのかんらん岩捕獲岩とメルトの反応物中のかんらん石,スピネルを記載した。かんらん石斑晶は二種類に大別できる。斑晶 (I) (コアでFo<sub>85</sub>-Fo<sub>76</sub>, リムでFo<sub>76</sub>) は小型で自形性が高く,非変形で,細粒スピネルを包有する。斑晶 (II) (コアはFo<sub>90</sub> 以下で均質,リムはFo<sub>85</sub>-Fo<sub>76</sub> の正累帯) は大型で,自形∼他形である。コアは変形組織を有し,リムのみスピネルを包有する。斑晶 (I) のうち,低Fe<sup>3+</sup> グループのスピネル (高Cr#) を包有するものは,かんらん岩中の斜方輝石とメルトの反応物起源,高Fe<sup>3+</sup> グループのスピネル (低Cr#) を有するものは,メルトから結晶化したものであろう。斑晶 (II) は外来結晶 (かんらん岩起源) 起源で,化学的,形態的に様々な改変を受けている。ピクライト玄武岩やアルカリかんらん石玄武岩のような,かんらん石斑晶に富む火山岩中のかんらん石斑晶は,メルトとかんらん岩との反応により,多起源性たり得る。

Generation of polygenetic olivine phenocrysts is discussed for a basanite from Okete Quarry, New Zealand, based on descriptions of its olivine and spinel and the reaction product between the basanite and its peridotite xenoliths. The olivine phenocrysts can be divided into two types, I and II. The phenocrysts (I) are relatively small, subhedral to euhedral and has minute spinel inclusions sparsely throughout the grain. They are free of the textures indicating deformation and fracture. The phenocrysts (II) are relatively large and have spinel inclusions only in the rim. Their core has deformation textures and/or fluid inclusion trails. The olivine phenocrysts (I) are normally zoned: the core ranges from Fo<sub>85</sub> to Fo<sub>76</sub> and the rim has Fo<sub>76</sub>. The spinel inclusions are highly variable in Cr/(Cr+Al+Fe<sup>3+</sup>) atomic ratio (0.02-0.5), Cr# (=Cr/[Cr+Al] atomic ratio) (0.1-0.6), TiO<sub>2</sub> content (17-1 wt%) and Fe<sup>3+</sup>/(Cr+Al+Fe<sup>3+</sup>) atomic ratio (=Fe<sup>3+</sup> ratio) (0.7-0.2), and can be grouped into two, low-Fe<sup>3+</sup> (Fe<sup>3+</sup> ratio <0.5) and high-Fe<sup>3+</sup> (Fe<sup>3+</sup> ratio >0.5) spinels. The Cr ratio is correlated positively with the Cr# and negatively with the TiO<sub>2</sub> content and Fe<sup>3+</sup> ratio. The phenocrysts (II) frequently have compositionally flat cores, which are relatively Mg-rich (up to Fo<sub>90</sub>), and zoned rims (Fo<sub>85−76</sub>). Their spinel inclusions mostly belong to the high-Fe<sup>3+</sup> group. The generation of olivine crystals and their spinel inclusions of various origins can be observed at the boundaries between the basanite and its peridotite xenoliths. Orthopyroxene in the peridotite xenoliths was decomposed into olivine, clinopyroxene, spinel and melt (glass) by the reaction with the basanite melt. The spinel from the reaction product is relatively high-Cr#. Subhedral olivine which includes chromian spinels with a wide compositional range has overgrown on the mantle olivine, which has been gradually disintegrated into the basanite matrix.<br>     The olivine phenocrysts (I) with low-Fe<sup>3+</sup> spinel inclusions were derived from the orthopyroxene/melt reraction product, and those with high-Fe<sup>3+</sup> spinels were precipitated from the basanite melt. The olivine phenocrysts (II) are of xenocrystal origin and have been modified by the melt to various degrees. The olivine phenocrysts in olivine-rich magmas such as picrite basalts and alkali olivine basalts can be polygenetic due to the reaction between the mantle peridotite and more primitive magmas.

収録刊行物

  • 岩鉱 : 岩石鉱物鉱床学会誌 : journal of mineralogy, petrology and economic geology  

    岩鉱 : 岩石鉱物鉱床学会誌 : journal of mineralogy, petrology and economic geology 93(7), 235-249, 1998-07-05 

    Japan Association of Mineralogical Sciences

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各種コード

  • NII論文ID(NAID)
    10002090255
  • NII書誌ID(NCID)
    AN10047992
  • 本文言語コード
    ENG
  • 資料種別
    ART
  • ISSN
    09149783
  • NDL 記事登録ID
    4579231
  • NDL 雑誌分類
    ZM46(科学技術--地球科学--岩石・鉱物・鉱床)
  • NDL 請求記号
    Z15-71
  • データ提供元
    CJP書誌  CJP引用  NDL  J-STAGE 
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