Abstract The new discovery of Fe-monticellite in serpentinized peridotites of the Happo ultramafic complex, central Japan, shows it replacing olivine and coexisting with antigorite. The monticellite occurs in several forms: as discrete grains; partially disconnected rings or circular ribbons; fringing mantles or in veins cutting olivine; or as aggregates of small equant grains. It shows a textural contrast to the intensely sheared matrix of antigorite. The coexistence with antigorite and diopside, and the Fe-rich compositions of monticellite indicates that it formed at 300–350 °C. This extremely low-temperature for monticellite formation is consistent with the observed textures indicative of its static formation at a stage of serpentinization that followed mylonitization during high-temperature (400–600 °C) serpentinization in the Happo complex. The monticellite contains minute inclusions of awaruite, which indicates formation under reducing conditions, as is characteristic of serpentinization of olivine. The monticellite is richer in Fe and Mn than relict olivine, being commensurate with deficiencies of these elements in coexisting antigorite. There is no evidence of Ca addition from an external source. However, where tremolite is replaced by serpentine, diopside grows at the opposite side of relict olivine that is partly replaced by monticellite. Thus, tremolite is the likely source of Ca for monticellite formation. These textural relationships and the thermodynamic calculations for paragenesis indicate that monticellite formed by reactions between olivine and aqueous fluids, carrying Ca released from tremolite, that were driven by a local gradient of SiO2 chemical potential during serpentinization.

中文翻译：

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八方超镁铁质复合体蛇纹岩中的铁蒙脱石

摘要 在日本中部八方超镁铁质杂岩体蛇纹石化橄榄岩中新发现铁蒙脱石，表明它取代橄榄石并与叶蛇纹石共存。蒙脱石以多种形式出现：作为离散颗粒；部分断开的环或圆形带；边缘地幔或在脉中切割橄榄石；或作为小颗粒的聚集体。它显示出与叶蛇纹石强烈剪切的基质的纹理对比。与叶蛇纹石和透辉石共存，以及蒙脱石的富铁成分表明它在 300-350 °C 形成。这种用于蒙脱石形成的极低温度与观察到的纹理一致，表明其在蛇纹石化阶段的静态形成，在八方复合体中高温（400-600°C）蛇纹石化过程中糜棱化之后。蒙脱石含有细小的铝镁石内含物，这表明在还原条件下形成，这是橄榄石蛇纹石化的特征。蒙脱石比残存橄榄石富含铁和锰，与共存叶蛇纹石中这些元素的缺乏相称。没有来自外部来源的 Ca 添加的证据。然而，在透闪石被蛇纹石取代的地方，透辉石生长在被蒙脱石部分取代的残余橄榄石的另一侧。因此，透闪石可能是形成蒙脱石的钙的来源。这些结构关系和共生作用的热力学计算表明，橄榄石和含水流体反应形成的蒙脱石携带着透闪石释放的钙，