As the largest and best exposed example of paleo fast‐spreading oceanic crust on land, the Samail ophiolite in the Sultanate of Oman represents an ideal natural laboratory for investigating deep crustal processes at fast‐spreading mid‐ocean ridges. We studied two layered gabbro sequences from different stratigraphic depths: one from the middle of the plutonic crust showing decimeter‐scale modal layering (i.e., varying phase proportions) with olivine abundances gradually decreasing from layer bases to tops (Wadi Somerah, Sumail block) and one located near the crust‐mantle boundary showing millimeter‐scale olivine‐rich layers (Wadi Wariyah, Wadi Tayin block). Our multimethod approach of field, petrographic, geochemical, and microstructural observations focuses on documenting layered textures that are widely observed within the lower oceanic crust as well as understanding their formation mechanisms within the context of small scale crustal accretion processes beneath fast‐spreading mid‐ocean ridges. Results from the mid‐crustal sequence indicate moderate cooling rates (Ca‐in‐olivine: log[dT/dt; °C yr⁻¹] = −2.21 ± 0.7) and correlated variations in mineral compositions and microstructures. We infer that decimeter‐scale layers in Wadi Somerah were deposited by density currents of crystal‐laden magma within a sill environment that potentially experienced occasional magma replenishment. The millimeter layering in Wadi Wariyah is best explained by Ostwald ripening emphasizing initial heterogeneities possibly being provoked by cyclical nucleation of olivine through the competing effects of element diffusion and rapid cooling. Fast cooling is recorded for the crustal base (Ca‐in‐olivine: log[dT/dt; °C yr⁻¹] = −1.19 ± 0.5, Mg‐in‐plagioclase: log[dT/dt; °C yr⁻¹] = −1.35°C ± 0.6) demonstrating that heat locally can be lost very efficiently from the lowermost crust.

中文翻译：

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来自阿曼苏丹国萨迈尔蛇绿岩的下部大洋地壳火成岩层的形成

作为陆地上快速传播的古地壳的最大，暴露最好的例子，阿曼苏丹国的Samail蛇绿岩是研究快速传播的中洋海脊深层地壳过程的理想天然实验室。我们研究了来自不同地层深度的两个层状辉长岩层序：一个从深部地壳中部开始，显示出十米尺度的模态分层（即，不同的相比例），橄榄石的丰度从层底到顶逐渐减小（Wadi Somerah，Sumail区块），以及一个位于地幔边界附近，显示了毫米级富橄榄石层（Wadi Wariyah，Wadi Tayin地块）。我们在野外，岩石学，地球化学，微观结构观察的重点是记录下层大洋壳中广泛观察到的层状纹理，以及在快速扩张的中海脊下方小规模地壳增生过程的背景下理解其形成机制。地壳中部序列的结果表明冷却速度适中（钙橄榄石：log [dT / d t ; °C yr ^-1 ] = −2.21±0.7）以及矿物成分和微结构的相关变化。我们推断，瓦迪萨默拉的十亿分之一尺度的层是由处于潜在岩浆补充作用的坎s环境中充满晶体的岩浆的密度流所沉积的。Wadi Wariyah中的毫米分层可以通过奥斯特瓦尔德（Ostwald）熟化得到最好的解释，强调初始异质性可能是由于元素扩散和快速冷却的竞争效应引起的橄榄石的周期性成核引起的。记录了地壳基底的快速冷却（钙橄榄石：log [d T / d t ;°C yr ^-1 ] = −1.19±0.5，镁斜长石：log [d T / dt ; °C yr ^-1 ] = -1.35°C±0.6）表明，局部热量可以从最下面的地壳中非常有效地散失。