Abstract Models for deep derivation of ophiolitic chromite from the mantle transition zone versus magmatic crystallization at lithospheric level are now highly controversial due to the discovery of ultrahigh-pressure (UHP) and super-reduced (SuR) minerals in ophiolitic chromitites and the presence of silicate mineral exsolution lamellae in chromite grains. According to experimental studies, chromite grains crystallized at low-pressures cannot exsolve silicate phases because of their low contents of CaO and SiO2. However, clinopyroxene needles in chromite grains from the Stillwater Complex, which is a typical layered mafic-ultramafic intrusion crystallized at crustal level, show morphological, compositional and crystallographic features very similar to exsolution lamellae in ophiolitic chromite grains. We propose that the Stillwater clinopyroxene ‘lamellae’ represent minute melt inclusions that were attached parallel to branches of skeletal chromite where they were trapped during continued growth of the grain. This interpretation is supported by the orientations of the lamella-like clinopyroxene and the coexistence of larger clinopyroxene inclusions and apatite crystals along with the clinopyroxene needles. Another possibility is that these lamella-like minerals precipitated from melt/fluid that filled cracks in the chromite grains. Trails of tiny inclusions preserved in some healed fractures are consistent with this mechanism, but are inconsistent with the regular orientations of most of the needle-like ‘inclusions’. The presence of such features in the Stillwater chromitites raises serious questions regarding the origin and significance of silicate lamellae in ophiolitic chromite grains and the use of such features to determine the depth of crystallization.

中文翻译：

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斯蒂尔沃特复合体铬铁矿中“硅酸盐出溶片层”的成因：蛇绿铬铁矿高压结晶的挑战

摘要 由于蛇绿铬铁矿中超高压 (UHP) 和超还原 (SuR) 矿物的发现以及硅酸盐的存在，从地幔过渡带深部衍生蛇绿岩与岩石圈水平的岩浆结晶的模型现在备受争议。铬铁矿颗粒中的矿物出溶片层。根据实验研究，在低压下结晶的铬铁矿晶粒不能溶出硅酸盐相，因为它们的 CaO 和 SiO2 含量低。然而，斯蒂尔沃特杂岩体中的单斜辉石针状体是典型的层状镁铁质-超镁铁质侵入体，在地壳水平结晶，显示出与蛇绿铬铁矿颗粒中的出溶片层非常相似的形态、成分和结晶特征。我们提出，斯蒂尔沃特单斜辉石“薄片”代表微小的熔体包裹体，这些包裹体平行于铬铁矿的骨架分支，在晶粒持续生长过程中被困在那里。这种解释得到了片状单斜辉石的取向以及较大的单斜辉石包裹体和磷灰石晶体与单斜辉石针的共存的支持。另一种可能性是这些薄片状矿物从填充铬铁矿晶粒裂缝的熔体/流体中沉淀出来。一些愈合裂缝中保留的微小包裹体轨迹与这种机制一致，但与大多数针状“包裹体”的规则取向不一致。