To obtain a better understanding of melt–rock interactions in the upper mantle, microstructural and petrological analyses were conducted on deformed mantle peridotites from the Horoman peridotite complex, Hokkaido, Japan. The Horoman peridotite complex is lithologically heterogeneous and contains various kinds of ultramafic and mafic rocks. We studied an outcrop of 3 × 70 m in size that contains layered spinel harzburgite, plagioclase lherzolite, and mafic rocks. The results indicate that reactive melts migrated preferentially along the foliation in the already deformed peridotite, and that these melt-rich zones became especially prone to further deformation. This inference is supported by (1) the parallelism of the boundaries of rock layers and foliation in the deformed peridotite, and the shape and crystallographic preferred orientations (SPOs and CPOs) of olivine in the peridotites; (2) the diffusive trends of magnesium and modal compositions of pargasite grains near the boundaries between peridotite and mafic layers; (3) variations in the NiO content of olivine crystals; (4) variations in olivine CPOs with orthorhombic (010)[100] slip system patterns and weak fiber-[010] patterns; and (5) the strong pargasite SPOs, the cuspate shapes of the pargasites, and the absence of intercrystallite deformation. The results, combined with previously reported P–T conditions for the Horoman peridotite complex, indicate that the deformed peridotites and mafic rocks with a layered structure represent temperatures of 1050–1150 °C and pressures of 0.7–1.5 GPa. Our results suggest that a decrease in pressure led to the transition from a melt-free to a melt-bearing system with a consequent change in the deformation mechanism, from dislocation creep in the melt-free system to diffusion creep in the melt-bearing system, with strain localization in the fine-grained melt-rich layers. The change in deformation mechanism is likely to have occurred in the uppermost mantle beneath a mid-ocean ridge, where strong rheological contrasts are controlled by spatial variations in the melt fraction.

中文翻译：

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上地幔的变形和熔体-岩石相互作用：来自日本 Horoman 橄榄岩层状结构的见解


为了更好地了解上地幔中的熔体-岩石相互作用，对日本北海道 Horoman 橄榄岩杂岩中的变形地幔橄榄岩进行了微观结构和岩石学分析。霍罗曼橄榄岩杂岩岩性不均匀，含有多种超镁铁质和镁铁质岩石。我们研究了一块大小为 3 × 70 m 的露头，其中含有层状尖晶石方辉橄榄岩、斜长石二辉橄榄岩和镁铁质岩石。结果表明，活性熔体优先沿着已经变形的橄榄岩中的叶理迁移，并且这些熔体丰富的区域特别容易进一步变形。这一推论得到以下支持：（1）变形橄榄岩中岩层和叶理边界的平行性，以及橄榄岩中橄榄石的形状和晶体择优取向（SPO和CPO）； (2)镁的扩散趋势以及橄榄岩和镁铁质层边界附近的菱镁矿晶粒的模态组成； (3)橄榄石晶体中NiO含量的变化； (4) 具有斜方晶系(010)[100]滑移系模式和弱纤维-[010]模式的橄榄石CPO的变化； (5) 强的斜长石 SPO、尖长石的形状以及不存在晶间变形。结果结合之前报道的霍罗曼橄榄岩杂岩的 P-T 条件，表明变形橄榄岩和具有层状结构的镁铁质岩石的温度为 1050-1150 °C，压力为 0.7-1.5 GPa。 我们的结果表明，压力的降低导致从无熔体系统向熔体承载系统的转变，从而导致变形机制发生变化，从无熔体系统中的位错蠕变到熔体承载系统中的扩散蠕变，应变集中在细晶粒富熔层中。变形机制的变化很可能发生在洋中脊下方的最上地幔中，那里强烈的流变对比是由熔体分数的空间变化控制的。