Several ODP-IODP expeditions drilled oceanic core complexes interpreted as exhumed portions of lower crust close to the ridge axis, and provide the community with invaluable sampling opportunity for further constraining magmatic processes involved in the formation of the slow-spreading lower oceanic crust. ODP Hole 735B presents the most primitive lithologies sampled at Atlantis Bank oceanic core complex (Southwest Indian Ridge) in a ∼250 m thick section that was previously interpreted as a single crustal intrusion. We combined detailed structural and petrographic constraints with whole rock and in situ mineral analyses of this section in order to precisely determine the processes of emplacement, crystallization, and melt migration within the lower crust. The lower half of the unit is comprised of alternating olivine gabbros and troctolites showing intrusive contacts, magmatic fabrics, and crystal-plastic fabrics. Such structures and primitive lithologies are lacking in the upper half, rather uniform, gabbroic sequence. Whole rock compositions highlight the cumulative character of both lower and upper units and a great compositional variability in the lower sequence, whereas the upper sequence is homogeneous and differentiates up-section. In situ analyses of mineral phases document magma emplacement processes and provide evidence for ubiquitous reactive porous flow (RPF) during differentiation. We show that the whole section, and related geochemical unit, constitutes a single magmatic reservoir, in which the lower unit is formed by stacked primitive sills formed by repeated recharge of primitive melts and melt-present deformation. Recharge led to partial assimilation of the crystallizing primitive cumulates, and hybridization with their interstitial melts. Hybrid melts were progressively collected in the overlying mushy part of the reservoir (upper unit), whereas the sills’ residual hybrid melts differentiated by RPF processes under a predominantly crystallization regime. Similarly, hybrid melts’ evolution in the upper unit was governed by upward RPF, and progressive differentiation and accumulation of evolved melts at the top of the reservoir. Our results provide the community with the first integrated model for magma reservoir formation in the lower slow-spreading oceanic crust that can potentially be applied to other magmatic lower crust sections.

几项ODP-IODP探险队钻探了海洋岩心复合体，这些复合体被解释为靠近山脊轴线的下地壳的发掘部分，并为社区提供了宝贵的采样机会，进一步限制了缓慢扩散的下层地壳的形成所涉及的岩浆过程。ODP孔735B展示了在亚特兰蒂斯银行海洋核心综合体（西南印第安岭）采样的最原始岩性，岩层厚约250 m，以前被解释为单个地壳侵入。我们将详细的结构和岩石学约束与整个岩石和原位该部分的矿物分析，以精确确定下地壳内的沉积，结晶和熔体迁移过程。该单元的下半部分由交替的橄榄石辉长岩和闪长岩组成，具有侵入性的接触，岩浆纤维和晶塑纤维。这种结构和原始岩性在上半部缺乏统一的辉长岩序。整个岩石成分突出了下部和上部单元的累积特征，并且在下部层序中具有很大的组成变异性，而上部层序是均匀的并且区分了上段。原位矿物相的分析记录了岩浆的沉积过程，并为分化过程中普遍存在的反应性多孔渗流（RPF）提供了证据。我们表明，整个断层和相关的地球化学单元构成了一个单一的岩浆储层，其中下部单元是由堆叠的原始基岩形成的，这些基岩是由原始熔体的反复充填和熔体当前变形形成的。补给导致结晶原始堆积物部分同化，并与其间隙熔体杂交。杂物熔体逐渐收集在储层上覆的糊状部分（上部）中，而基岩的残余杂物熔体在主要的结晶状态下通过RPF过程进行区分。同样，上层单元中混合熔体的演化受向上RPF的控制，储层顶部的演化熔体逐渐分化和聚集。我们的结果为社区提供了第一个完整的低速扩展洋壳岩浆储层形成的综合模型，该模型可以潜在地应用于其他岩浆下地壳部分。