Magma Reservoir Formation and Evolution at a Slow-Spreading Center (Atlantis Bank, Southwest Indian Ridge)
Magma Reservoir Formation and Evolution at a Slow-Spreading Center (Atlantis Bank, Southwest Indian Ridge)
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DOI：
10.3389/feart.2020.554598
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发表时间：
2020-09-15
影响因子：
2.9
通讯作者：
von der Handt, Anette
中科院分区：
文献类型：
--
作者：
Boulanger, Marine;France, Lyderic;von der Handt, Anette
若干大洋钻探计划-大洋钻探计划考察队钻取了被解释为靠近洋脊轴的下地壳挖出部分的大洋核心复合体，并为该社区提供了宝贵的取样机会，以进一步限制缓慢扩张的下大洋地壳形成过程中所涉及的岩浆过程。ODP孔735 B展示了在亚特兰蒂斯岸海洋核心复合体（西南印度洋脊）取样的最原始的岩性，该岩性类似于250米厚的剖面，以前被解释为单一的地壳侵入体。我们结合详细的结构和岩相学的限制与全岩和在situmineral分析这一节，以精确地确定侵位，结晶和熔体迁移的过程中的下地壳。该单元的下半部分由交替的橄榄石辉长岩和橄榄岩组成，显示出侵入接触、岩浆组构和晶塑组构。上半部分缺乏这种构造和原始岩性，相当均匀，辉长岩序列。全岩成分显示出上、下两个单元的累积特征，下部层序成分变化较大，而上部层序成分均匀，并与上部层序有明显的差异。矿物相原位分析记录了岩浆侵位过程，为分异过程中普遍存在的反应性孔隙流（RPF）提供了证据。研究表明，整个剖面及其相关的地球化学单元构成了一个单一的岩浆岩储集层，下部单元由原始熔体的反复充注和熔体现变形形成的原始岩床叠置而成。再充导致结晶原始堆晶的部分同化，并与它们的间隙熔体杂交。混合熔体逐渐收集在上覆糊状部分的水库（上单元），而窗台的残余混合熔体分化的RPF过程下的主要结晶制度。类似地，混合熔体在上部单元的演化受向上的RPF以及演化熔体在储层顶部的渐进分异和聚集的控制。我们的研究结果为社会提供了第一个综合模型岩浆储层形成在较低的缓慢扩张的洋壳，可以潜在地应用到其他岩浆下地壳部分。
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 similar to 250 m thick section that was previously interpreted as a single crustal intrusion. We combined detailed structural and petrographic constraints with whole rock andin situmineral 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 situanalyses 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.