This paper analyses the process and depth of formation of chromite crystals occasionally including ultrahigh pressure (UHP) minerals, recovered from some investigated ophiolites. We present direct microstructural evidence for crystallization of chromite ores from the Semail ophiolite, in place they are exposed. Similar crystal preferred orientation in olivine-rich layers and in septa from chromite layers indicate deformation of the olivine solid frame by dislocation creep at high temperature, synchronous with the crystallization of chromite. The preservation of the olivine framework throughout the development of joined chromite grains suggests that chromite replaced olivine. Chromite grains crystallized from a melt circulating in an open-system, stress-bearing network of dunitic aggregate, controlled by the olivine high-T slip system. The misorientation of joined crystals at grain boundaries define the ratio of twin vs. random boundaries that were closed to vs. open to fluid circulation, respectively, suggesting that chromite layers may represent planes of channelized melt circulation. The relationship of chromite deposits to ridge tectonics structures suggests that Cr-bearing melt was produced at the transition from an active spreading ridge to deep mantle shear zones acting as possible source of hydration. The occurrence of potential UHP inclusions in chromite crystals is assigned to continental contamination of melts ascending through the Arabian margin.

本文分析了铬铁矿晶体的形成过程和深度，偶尔包括从一些研究的蛇绿岩中回收的超高压 (UHP) 矿物。我们提供了从 Semail 蛇绿岩中结晶出铬铁矿矿石的直接显微结构证据，在它们暴露的地方。在富含橄榄石的层和铬铁矿层的隔膜中类似的晶体择优取向表明橄榄石固体框架在高温下通过位错蠕变发生变形，与铬铁矿的结晶同步。在连接铬铁矿晶粒的整个发展过程中橄榄石骨架的保存表明铬铁矿取代了橄榄石。铬铁矿晶粒从熔体中结晶，该熔体在由橄榄石高 T 滑移系统控制的开放系统、承受应力的双晶骨料网络中循环。晶界处连接晶体的错误取向分别定义了孪晶与随机边界的比率，它们分别对流体循环关闭与开放，这表明铬铁矿层可能代表通道化熔体循环的平面。铬铁矿沉积物与脊构造结构的关系表明，含铬熔体是在从活跃的扩张脊到深部地幔剪切带的过渡处产生的，可能是水合作用的来源。铬铁矿晶体中潜在 UHP 夹杂物的出现归因于通过阿拉伯边缘上升的熔体的大陆污染。这表明铬铁矿层可能代表通道化熔体循环的平面。铬铁矿沉积物与脊构造结构的关系表明，含铬熔体是在从活跃的扩张脊到深部地幔剪切带的过渡处产生的，可能是水合作用的来源。铬铁矿晶体中潜在 UHP 夹杂物的出现归因于通过阿拉伯边缘上升的熔体的大陆污染。这表明铬铁矿层可能代表通道化熔体循环的平面。铬铁矿沉积物与脊构造结构的关系表明，含铬熔体是在从活跃的扩张脊到深部地幔剪切带的过渡处产生的，可能是水合作用的来源。铬铁矿晶体中潜在 UHP 夹杂物的出现归因于通过阿拉伯边缘上升的熔体的大陆污染。