The exposure of gabbroic sequences at Oceanic Core Complexes (OCC) along ultraslow- to slow-spreading ridges permits the study of the processes forming the lower oceanic crust. On top of the Atlantis Bank OCC along the ultraslow-spreading Southwest Indian Ridge, IODP Expedition 360 drilled Hole U1473A, mainly composed of primitive olivine gabbros interspersed with more evolved Ti-Fe oxide-bearing gabbros and minor felsic veins. These rocks record a complex history of protracted magmatism during continuous uplift and deformation of the gabbroic sequence. Extensive crystal-plastic deformation is dominantly recorded in the shallower sections of the drillhole, whereas the deeper sections better preserve primary magmatic features. We focus on microstructures, including intra-crystalline deformation of rock-forming minerals, and plagioclase crystallographic preferred orientations of olivine gabbros lacking evidence for exhumation-related crystal plastic deformation, to gain insights on the relationship between compaction, melt migration and melt accumulation during the early magmatic history of this section of lower oceanic crust. Olivine gabbros are characterized by ubiquitous grain-size variations, from coarse- to fine-grained intervals. Minerals in coarse-grained intervals show intra-crystalline deformation, while fine-grained crystals lack internal strain. Bent coarse-grained plagioclase associated with weak magmatic foliation and lack of lineation suggest that the coarse-grained intervals were deformed under weak compaction. On the other hand, crystallographic preferred orientations of undeformed fine-grained plagioclase show weak lineations, likely indicative of non-coaxial strain. We thereby infer that the coarse-grained intervals underwent ongoing weak compaction from the stage of olivine + plagioclase ± clinopyroxene crystal mush to the melt-poor stage, and that this process likely aided melt extraction and accumulation in discrete melt-rich zones where crystals orientated in the direction of magmatic flow. Crystallization of melts in the melt-rich zones ultimately formed the fine-grained intervals at different depths in Hole U1473A. This indicates that processes of compaction can lead to local chemical and grain-size heterogeneities in a lower crustal section, while had a minor role in the melt movement at larger scales (e.g., the whole crystal mush) within the oceanic crust.

大洋核复合体 (OCC) 沿超慢到慢扩张脊暴露辉长岩层序，可以研究下大洋地壳的形成过程。在沿超慢扩张的西南印度洋脊的亚特兰蒂斯银行 OCC 顶部，IODP Expedition 360 钻探了 U1473A 孔，主要由原始橄榄石辉长岩组成，散布着更进化的含钛铁氧化物的辉长岩和次要长英质脉。这些岩石记录了辉长岩层序连续隆升和变形过程中长期岩浆作用的复杂历史。广泛的晶体塑性变形主要记录在钻孔较浅的部分，而较深的部分更好地保留了原生岩浆特征。我们专注于微观结构，包括造岩矿物的晶内变形，和橄榄石辉长岩的斜长石晶体优选取向，缺乏与折返相关的晶体塑性变形的证据，以深入了解下大洋地壳该部分早期岩浆历史中压实、熔体迁移和熔体堆积之间的关系。橄榄辉长岩的特点是无处不在的粒度变化，从粗粒到细粒间隔。粗晶区间的矿物呈现晶内变形，而细晶晶则缺乏内应变。与弱岩浆叶理和缺乏线理相关的弯曲粗粒斜长石表明粗粒层段在弱压实作用下变形。另一方面，未变形细粒斜长石的晶体择优取向显示出弱线纹，可能表示非同轴应变。因此，我们推断粗粒层段从橄榄石 + 斜长石 ± 单斜辉石结晶糊阶段到贫熔阶段经历了持续的弱压实，并且这一过程可能有助于在晶体定向的离散富熔区中的熔体提取和积累在岩浆流动的方向。熔体在富熔区的结晶最终在 U1473A 孔中形成不同深度的细晶间隔。这表明压实过程会导致下地壳部分的局部化学和晶粒尺寸不均匀性，而在大洋壳内更大尺度（例如整个结晶糊）的熔体运动中起次要作用。因此，我们推断粗粒层段从橄榄石 + 斜长石 ± 单斜辉石结晶糊阶段到贫熔阶段经历了持续的弱压实，并且这一过程可能有助于在晶体定向的离散富熔区中的熔体提取和积累在岩浆流动的方向。熔体在富熔区的结晶最终在 U1473A 孔中形成了不同深度的细晶间隔。这表明压实过程会导致下地壳部分的局部化学和晶粒尺寸不均匀性，而在大洋壳内更大尺度（例如整个结晶糊）的熔体运动中起次要作用。因此，我们推断粗粒层段从橄榄石 + 斜长石 ± 单斜辉石结晶糊阶段到贫熔阶段经历了持续的弱压实，并且这一过程可能有助于在晶体定向的离散富熔区中的熔体提取和积累在岩浆流动的方向。熔体在富熔区的结晶最终在 U1473A 孔中形成不同深度的细晶间隔。这表明压实过程会导致下地壳部分的局部化学和晶粒尺寸不均匀性，而在大洋壳内更大尺度（例如整个结晶糊）的熔体运动中起次要作用。并且这个过程可能有助于在离散的富含熔体的区域中提取和积累熔体，在这些区域中晶体沿岩浆流动的方向取向。熔体在富熔区的结晶最终在 U1473A 孔中形成不同深度的细晶间隔。这表明压实过程会导致下地壳部分的局部化学和晶粒尺寸不均匀性，而在大洋壳内更大尺度（例如整个结晶糊）的熔体运动中起次要作用。并且这个过程可能有助于在离散的富含熔体的区域中提取和积累熔体，在这些区域中晶体沿岩浆流动的方向取向。熔体在富熔区的结晶最终在 U1473A 孔中形成不同深度的细晶间隔。这表明压实过程会导致下地壳部分的局部化学和晶粒尺寸不均匀性，而在大洋壳内更大尺度（例如整个结晶糊）的熔体运动中起次要作用。