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Evidence for Multi-stage Melt Transport in the Lower Ocean Crust: the Atlantis Bank Gabbroic Massif (IODP Hole U1473A, SW Indian Ridge)
Journal of Petrology ( IF 3.5 ) Pub Date : 2020-12-11 , DOI: 10.1093/petrology/egaa082
Wei-Qi Zhang 1, 2, 3 , Chuan-Zhou Liu 1, 2, 4, 5 , Henry J B Dick 3, 6
Affiliation  

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-km2 gabbroic 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.

中文翻译:

低层洋壳中多阶段熔体运输的证据:亚特兰蒂斯河滩杂物质断层(IODP孔U1473A,西南印第安岭)

缓脊和超缓脊的低洋壳结构是多种多样的,但是产生这种多样性的机制还没有被很好地理解。尤其是660公里2亚特兰蒂斯银行(西南印第安岭)的辉长岩地块表现出明显的地带划分,代表着高岩浆供应端成员,在缓慢和超慢扩展的山脊上积聚了下部大洋地壳。我们介绍了来自Atlantis Bank辉长岩断块的809米IODP孔U1473A的橄榄石辉长岩的岩石学和地球化学数据。在结构上,U1473A的上部由一个约600米的剪切带组成。在此以下,孔相对不变形,具有几个较小的剪切区域。远离剪切带的橄榄石辉长岩具有矿物微量元素组成,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石学和化学特征,表明与相对低温,氧化物饱和的熔体发生了反应。这些特征表明,在该辉长岩地块中,从原始到中等程度地演化的熔体迁移处于早期阶段,随后是由变形驱动的高度演化的富铁钛富熔体向高水平迁移。剪切带与氧化物饱和熔体的反应之间的密切关系表明,同岩浆剪切带为后期富铁钛富集的熔体通过亚特兰蒂斯河岸下部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,与岩浆供应基本相关的同岩浆变形程度,在发展低速和超低速扩张山脊观测到的低层洋壳多样性方面起着主导作用。然后,在该辉长岩断块中,由变形驱动的高度演化的富铁钛熔体运输到高水平。剪切带与氧化物饱和熔体的反应之间的密切关系表明,同岩浆剪切带为后期富铁钛富集的熔体通过亚特兰蒂斯河岸下部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,与岩浆供应基本相关的同岩浆变形程度,在发展低速和超低速扩张山脊观测到的低层洋壳多样性方面起着主导作用。然后,在该辉长岩断块中,由变形驱动的高度演化的富铁钛熔体运输到高水平。剪切带与氧化物饱和熔体的反应之间的密切关系表明,同岩浆剪切带为后期富铁钛富集的熔体通过亚特兰蒂斯河岸下部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,与岩浆供应基本相关的同岩浆变形程度,在发展低速和超低速扩张山脊观测到的低层洋壳多样性方面起着主导作用。通过亚特兰蒂斯银行下地壳的富铁钛熔体运输。这个过程对于产生观察到的成分分区至关重要。因此,与岩浆供应基本相关的同岩浆变形程度,在发展低速和超低速扩张山脊观测到的低层洋壳多样性方面起着主导作用。通过亚特兰蒂斯银行下地壳的富铁钛熔体运输。这个过程对于产生观察到的成分分区至关重要。因此,与岩浆供应基本相关的同岩浆变形程度,在发展低速和超低速扩张山脊观测到的低层洋壳多样性方面起着主导作用。
更新日期:2021-01-22
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