Brown Amphibole as Tracer of Tectono-Magmatic Evolution of the Atlantis Bank Oceanic Core Complex (IODP Hole U1473A)
Brown Amphibole as Tracer of Tectono-Magmatic Evolution of the Atlantis Bank Oceanic Core Complex (IODP Hole U1473A)
复制标题
棕色角闪石作为亚特兰蒂斯浅滩海洋核心复合体构造岩浆演化的示踪剂（IODP 孔 U1473A）
DOI：
10.1093/petrology/egac089
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发表时间：
2022
影响因子：
3.9
通讯作者：
Ferrando C
中科院分区：
文献类型：
--
作者：
Ferrando C
棕色角闪石是下洋地壳中一种次要但常见的矿物成分。它通常被解释为富含 SiO2 和 H2O 的流体或熔体运移的产物，这些流体或熔体可以是大洋中脊玄武岩 (MORB) 高级岩浆分异的残余熔体，也可以是包含海水成分的热液流体。在亚特兰蒂斯浅滩海洋核心复合体挖掘出的下洋地壳内，沿着超慢的西南印度洋中脊，棕色角闪石在从橄榄石到氧化辉长岩和闪长岩的所有岩性中普遍存在，包括未变形和塑性变形品种。我们在这里展示了系统岩石学研究的结果，该研究旨在揭示棕色角闪石中记录的富含 H2O 成分的性质，并记录：（i）岩浆阶段迁移熔体的演化和（ii）整个地壳横断面记录的不同程度的熔体承载变形事件。棕色角闪石中的低 Cl 含量和轻重稀土元素 (LREE/HREE) 比率以及高 Ti 含量表明它们是从具有岩浆含水成分的熔体中结晶出来的。一致地，它们的 δ18O 值与大洋中脊玄武岩 (MORB) 成分保持平衡，但可能记录蚀变矿物局部同化的闪长岩角闪石除外。在未变形的橄榄石辉长岩中，间隙斜长岩在超固相线条件（~1000°C）下从 MORB 分化后期的熔体残余物中结晶出来。我们推测，在橄榄石辉长岩晶体糊达到完全固态之前，一些残余熔体的等分部分被提取并在离散的时间间隔内积累。在那里，玄武岩熔体通过 Fe-Ti 氧化物和单斜辉石作为液相的早期结晶而分化，最终形成氧化物辉长岩。这一过程促进了残余熔体中硅的快速富集和铁、钛、钒的贫化，随后被提取形成横切闪长岩脉。糜棱岩橄榄石辉长岩在超固相线条件下记录了高温塑性变形（~900°C±50°C），涉及辉长岩先前结晶的熔体残留。进一步的固态塑性变形导致晶粒尺寸大幅减小，从而导致孔隙率增加。这为随后的熔体聚焦创造了路径，这可能代表了晚期分化熔体在整个下地壳部分的迁移。研究表明，亚特兰蒂斯浅滩下洋壳中的棕色角闪石是高级岩浆分异残余熔体的结晶产物，也局部参与了地壳增生过程中的塑性变形事件。
Brown amphibole is a minor but common mineral component in lower oceanic crust. It is generally interpreted as products of migrating SiO2and H2O-rich fluids or melts, which can be either residual melts from advanced magmatic differentiation of Mid-Ocean Ridge Basalt (MORB), or hydrothermal fluids including a seawater component. Within the lower oceanic crust exhumed at the Atlantis Bank Oceanic Core Complex, along the ultraslow Southwest Indian Ridge, brown amphibole is ubiquitous in all lithologies from olivine- to oxide-gabbros and diorites, including both undeformed and plastically deformed varieties. We here show the results of a systematic petrological study conceived to unravel the nature of the H2O-rich component recorded in brown amphiboles and document: (i) the evolution of migrating melts during the magmatic stage and (ii) different extents of melt-bearing deformation events recorded throughout the entire crustal transect. The low Cl contents and the light over heavy rare earth elements (LREE/HREE) ratios and high Ti contents in brown amphiboles indicate they crystallized from melts with a magmatic hydrous component. Consistently, their δ18O values are in equilibrium with Mid-Ocean Ridge Basalt (MORB) composition, except for diorite amphiboles that possibly record the local assimilation of altered minerals. In undeformed olivine gabbros, interstitial pargasite crystallized at hypersolidus conditions (~1000°C) from the melt residual after late stages of MORB differentiation. We speculate that before the olivine gabbro crystal mush reached fully solid state, some aliquots of residual melts were extracted and accumulated within discrete intervals. There, ferrobasaltic melts differentiated through the early crystallization of Fe-Ti oxides and clinopyroxene as liquidus phases, ultimately forming the oxide gabbros. This process promoted rapid Si enrichment and depletion in Fe, Ti, V in the residual melt, later extracted to form the crosscutting diorite veins. The mylonitic olivine gabbros record high-temperature plastic deformation (~900°C ± 50°C) under hypersolidus conditions, involving melts residual from previous crystallization of the gabbroic rock. Further solid-state plastic deformation led to substantial grain size reduction and, consequently, to an increase in porosity. This created pathways for subsequent melt focusing, which likely represent late-stage differentiated melts migrating throughout the lower crustal section. This study shows that brown amphibole in the Atlantis Bank lower oceanic crust is the crystallization product of melts residual from advanced magmatic differentiation, which are also locally involved in the plastic deformation events during crustal accretion.