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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上地幔的变形和熔岩相互作用：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岩-岩石相互作用，对来自日本北海道 Horoman 橄榄岩杂岩体的变形地幔橄榄岩进行了微观结构和岩石学分析。Horoman 橄榄岩杂岩体在岩性上是非均质的，包含各种超镁铁质和镁铁质岩石。我们研究了一个 3 × 70 m 大小的露头，其中包含层状尖晶石哈茨堡石、斜长石针铁矿和镁铁质岩石。结果表明，活性熔体优先沿着已经变形的橄榄岩中的叶状物迁移，并且这些富含熔体的区域变得特别容易进一步变形。这一推论得到了以下因素的支持：（1） 变形橄榄岩中岩层和叶状边界的平行性，以及橄榄岩中橄榄石的形状和晶体学择优取向（SPO 和 CPO）;（2） 橄榄岩和镁铁质层边界附近镁的扩散趋势和钙镁矿晶粒的模态组成;（3） 橄榄石晶体中 NiO 含量的变化;（4） 橄榄石 CPO 的变化与斜方 （010）[100] 滑移系统模式和弱纤维 - [010] 模式;（5） 强的钙镁石 SPO、镁铝石的尖头形状以及无晶间变形。结果，结合先前报道的 Horoman 橄榄岩杂岩体的 P-T 条件，表明变形的橄榄岩和具有层状结构的镁铁质岩石代表 1050-1150 °C 的温度和 0.7-1.5 GPa 的压力。 我们的结果表明，压力的降低导致从无熔体系统过渡到含熔体系统，从而改变变形机制，从无熔体系统中的位错蠕变到含熔体系统中的扩散蠕变，应变局限在细晶粒富熔体层中。变形机制的变化可能发生在洋中脊下方的最上层地幔中，那里强烈的流变对比受熔融分数的空间变化控制。