Evidence for Multi-stage Melt Transport in the Lower Ocean Crust: Atlantis Bank Gabbroic Massif (IODP Hole U1473A, SW Indian Ridge)
Evidence for Multi-stage Melt Transport in the Lower Ocean Crust: Atlantis Bank Gabbroic Massif (IODP Hole U1473A, SW Indian Ridge)
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下洋壳中多阶段熔体迁移的证据：亚特兰蒂斯浅滩辉长岩地块（IODP 孔 U1473A，西南印度洋海岭）
DOI：
10.1093/petrology/egaa082
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发表时间：
2020
影响因子：
3.9
通讯作者：
Heny J.B. Dick
中科院分区：
文献类型：
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作者：
Wei-Qi Zhang;Chuan-Zhou Liu;Heny J.B. Dick
慢速和超慢速扩张脊处的下洋地壳的结构是多种多样的，但产生这种多样性的机制尚不清楚。特别是亚特兰蒂斯浅滩（西南印度洋中脊）660平方公里的辉长岩地块表现出显着的成分分带，代表了低速和超慢速扩张洋脊处下洋地壳增生的高岩浆供应端元。我们提供来自亚特兰蒂斯浅滩辉长岩地块 809 米 IODP 孔 U1473A 的橄榄石辉长岩的岩相和地球化学数据。从结构上看，U1473A 的上部由约 600 米的剪切带组成；在此之下，孔相对不变形，有几个较小的剪切带。远离剪切带的橄榄石辉长岩具有矿物微量元素成分，表明与氧化物不饱和熔体发生高温反应。相比之下，剪切区内的橄榄石辉长岩显示出岩相和化学特征，表明与相对低温、氧化物饱和熔体发生反应。这些特征表明原始到中等演化熔体迁移的早期阶段，随后在变形驱动下，高度演化的富铁钛熔体在该辉长岩地块中迁移到高水平。剪切带与氧化物饱和熔体反应之间的密切关系表明，同岩浆剪切带为晚期富铁钛熔体通过亚特兰蒂斯浅滩下地壳的输送提供了通道。这个过程对于产生观察到的成分分区至关重要。因此，与岩浆供应根本相关的同岩浆变形程度在慢速和超慢速扩张脊观察到的下洋地壳多样性的发展中起着主导作用。
The architecture of lower oceanic crust at slow- and ultraslow-spreading ridge is diverse, yet the mechanisms that produce this diversity are not well understood. Particularly, the 660-km2gabbroic massif at Atlantis Bank (Southwest Indian Ridge) exhibits significant compositional zonation, representing a high magma supply end member for accretion of the lower ocean crust at slow and ultraslow-spreading ridges. We present the petrographic and geochemical data of olivine gabbros from the 809-metre IODP Hole U1473A at Atlantis Bank gabbroic massif. Structurally, the upper portion of U1473A consists of a ∼600-metre shear zone; below this, the hole is relatively undeformed, with several minor shear zones. Olivine gabbros away from the shear zones have mineral trace element compositions indicative of high-temperature reaction with an oxide-undersaturated melt. By contrast, olivine gabbros within shear zones display petrographic and chemical features indicative of reaction with a relatively low-temperature, oxide-saturated melt. These features indicate an early stage of primitive to moderately evolved melt migration, followed by deformation-driven transport of highly evolved Fe–Ti-rich melts to high levels in this gabbroic massif. The close relationship between shear zones and the reaction with oxide-saturated melts suggests that syn-magmatic shear zones provide a conduit for late-stage, Fe–Ti-rich melt transport through Atlantis Bank lower crust. This process is critical to generate the compositional zonation observed. Thus, the degree of syn-magmatic deformation, which is fundamentally related to magma supply, plays a dominant role in developing the diversity of lower ocean crust observed at slow- and ultraslow-spreading ridges.