The mantle–crust boundary beneath oceanic spreading centres is a major chemical and thermal interface on Earth. Observations in ophiolites reveal that it is underlined by a dunitic transition zone (DTZ) that can reach a few hundred meters in thickness and host abundant chromitite ore bodies. The dunites have been deciphered as essentially mantle-derived in most ophiolitic massifs; that is, reactional residues of interactions between peridotite and percolating melt(s). Although both dunite and chromitite in ophiolites have been the focus of many studies, the reasons for their systematic association remain unclear. In this study we have explored the inclusion content of the chromite grains disseminated in the dunites from the DTZ exposed in the Maqsad area of the Oman ophiolite where a former asthenospheric diapir is exposed. Similarly to chromite in chromitite ore bodies, disseminated chromite grains in dunites contain a great diversity of silicate inclusions. Based on the major and minor element composition of 1794 single silicate inclusions in chromites from 285 samples of dunite and associated rocks in the DTZ, we infer that the disseminated chromites formed by a similar ‘metallogenic’ process to the chromitites, and that, as a whole, dunites from the DTZ actually represent the low-grade end-member of a single, giant ore body. The nature of the silicate inclusions (amphibole and mica among others) enclosed in chromite grains in dunites from the Maqsad DTZ precludes their crystallization from an anhydrous primitive basaltic melt, and rather calls for a crystallization from a melt hybrid between common mafic melts and more exotic Si-, Na- and volatile-rich fluids. The hybrid parent medium of both dunites and chromitites results from the interaction between an asthenospheric diapir (the mid-ocean ridge basalt source), and a colder, altered lithospheric lid and hydrothermal fluids responsible for this alteration. The excess silica in the hybrid melt is provided by the incongruent dissolution of enstatite from mantle harzburgite and/or from moderate degree of partial melting of the altered gabbroic crust. The chemical composition of the silicate inclusions is more variable when enclosed in the disseminated chromites than in the chromitites, suggesting a greater variability of melt and/or fluid fractions involved in the genesis of dunites than of chromite ores. Finally, the DTZ can be viewed as a metamorphic contact aureole between episodically rising asthenospheric diapirs and formerly accreted axial lithospheric lids. Our conclusion about the chicken and egg dilemma linking dunites and chromitites beneath oceanic spreading centres (i.e. do the chromitites form in response to the formation of dunites or conversely?) is that the mantle dunitization itself is a potential process for the release of Cr and its re-concentration as chromite ores, and that in turn the competition between orthopyroxene (± plagioclase) and chromite fractionation during this fluid–melt–peridotite reaction process is responsible for the great mineralogical and chemical variability of the DTZ dunites.

中文翻译：

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大洋扩张中心下方地幔-地壳过渡带中丹岩和铬铁矿的鸡与蛋两难：以地幔底辟（阿曼蛇绿岩）顶部形成的敦岩中铬铁矿型硅酸盐包裹体为例

海洋扩张中心下方的地幔-地壳边界是地球上主要的化学和热界面。对蛇绿岩的观察表明，它被一个单晶过渡带 (DTZ) 所强调，该过渡带可以达到几百米的厚度，并拥有丰富的铬铁矿矿体。在大多数蛇绿岩地块中，沙丘岩已被破译为基本上来源于地幔。也就是说，橄榄岩和渗透熔体之间相互作用的反应残余物。尽管蛇绿岩中的纯铁矿和铬铁矿一直是许多研究的重点，但它们系统关联的原因仍不清楚。在这项研究中，我们探讨了在阿曼蛇绿岩的 Maqsad 地区暴露的 DTZ 中散布在沙丘中的铬铁矿颗粒的夹杂物含量，那里暴露了一个前软流圈底辟。与铬铁矿矿体中的铬铁矿类似，纯砂岩中的浸染性铬铁矿颗粒也含有多种多样的硅酸盐包裹体。基于 DTZ 中 285 个纯岩和伴生岩石样品中铬铁矿中 1794 个单一硅酸盐包裹体的主要和微量元素组成，我们推断浸染型铬铁矿是通过与铬铁矿类似的“成矿”过程形成的，并且作为整体而言，来自 DTZ 的沙丘实际上代表了一个巨大矿体的低品位末端成员。硅酸盐包裹体（闪石和云母等）包裹在来自 Maqsad DTZ 的沙丘中的铬铁矿颗粒中的性质排除了它们从无水原始玄武岩熔体中结晶，而是要求从普通镁铁质熔体和更奇特的熔体之间的熔体混合物中结晶富含硅、钠和挥发性物质的流体。沙丘岩和铬铁矿的混合母介质是软流圈底辟（大洋中脊玄武岩源）与较冷、改变的岩石圈盖和导致这种改变的热液之间相互作用的结果。混合熔体中过量的二氧化硅是由顽火辉石从地幔方铅矿中的不一致溶解和/或由蚀变辉长岩地壳的中等程度的部分熔融提供的。当包裹在浸染铬铁矿中时，硅酸盐包裹体的化学成分比在铬铁矿中的变化更大，这表明与铬铁矿相比，熔体和/或流体部分与纯岩成因有关的可变性更大。最后，DTZ 可以看作是间断上升的软流圈底辟和以前增生的轴向岩石圈盖之间的变质接触光环。我们关于在海洋扩张中心下方连接沙岩和铬铁矿的鸡和蛋两难的结论（即铬铁矿的形成是响应沙岩的形成还是相反？）是，地幔单化本身是释放 Cr 及其释放的潜在过程。重新浓缩为铬铁矿，而在这种流体 - 熔体 - 橄榄岩反应过程中，斜方辉石（±斜长石）和铬铁矿分馏之间的竞争是导致 DTZ 砂岩的巨大矿物学和化学变异性的原因。