Unmixed spinel in chromitite from the Iwanai-dake peridotite complex, Hokkaido, Japan: A reaction between peridotite and highly oxidized magma in the mantle wedge
Unmixed spinel in chromitite from the Iwanai-dake peridotite complex, Hokkaido, Japan: A reaction between peridotite and highly oxidized magma in the mantle wedge
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DOI：
10.2138/am.2005.1570
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发表时间：
2005-02
影响因子：
3.1
通讯作者：
A. Tamura;S. Arai
中科院分区：
文献类型：
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作者：
A. Tamura;S. Arai
摘要日本北海道岩内岳杂岩中的铬铁矿剖面中发现了结构不均匀的尖晶石[（Mg，Fe 2+）（Cr，Al，Fe 3+）2O4]。在反射光显微镜下发现的不均匀尖晶石在与纯橄榄岩相邻的铬铁矿层中具有斑点状和辛状结构。另一方面，周围纯橄榄岩和方辉橄榄岩中的尖晶石是均匀的。不均匀的尖晶石包括两个不同的相：富镁铝尖晶石和富铁尖晶石与铬含量的变化很大。相邻相的组成关系对应于600 °C下的固溶线，这表明由于富Al相和富Fe 3+相之间的可溶解性间隙，不均匀的尖晶石通过不混合过程形成。铬铁矿层中的原始尖晶石组合物，估计从散装尖晶石组合物，是镁铁（MgFe 23 +O4）和磁铁矿（Fe 2 + Fe 23 +O4）之间的中间。这种富Fe 3+的尖晶石组合物表明铬铁矿是在岩浆温度下高度氧化的条件下形成的。一个高度氧化的含水硅酸盐熔体，如矿物或absarokite，由熔融的含水地幔，可能与地幔楔中的橄榄岩反应，结晶的富Fe 3+的尖晶石。
Abstract Spinel [(Mg, Fe2+)(Cr, Al, Fe3+)2O4] with an inhomogeneous texture was observed in a chromitite section from the Iwanai-dake complex, Hokkaido, Japan, which mainly consists of highly depleted Alpine-type peridotite. The inhomogeneous spinel identified under reflected-light microscopy has speckled and symplectic textures in the chromitite layer adjacent to dunite. On the other hand, spinel in the surrounding dunite and harzburgite is homogeneous. The inhomogeneous spinel comprises two different phases: an MgAl-rich spinel and an Fe-rich spinel with wide variation of Cr content. The compositional relationships of contiguous phases correspond to the solvus at 600 °C, which suggest that the inhomogeneous spinel was formed by an unmixing process due to the miscibility gap between Al-rich and Fe3+-rich phases. The original spinel composition in the chromitite layer, estimated from bulk spinel composition, is intermediate between magnesioferrite (MgFe23+O4) and magnetite (Fe2+Fe23+O4). Such an Fe3+-rich spinel composition indicates that the chromitite was formed under highly oxidized conditions at magmatic temperatures. A highly oxidized hydrous silicate melt, such as minette or absarokite, generated by melting of hydrous mantle, possibly reacted with peridotite in the mantle wedge and crystallized the Fe3+-rich spinel.