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首页> 外文期刊>Geochemistry, geophysics, geosystems >Compositional variation and Ra-226-Th-230 model ages of axial lavas from the southern Mid-Atlantic Ridge, 8 degrees 48S
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Compositional variation and Ra-226-Th-230 model ages of axial lavas from the southern Mid-Atlantic Ridge, 8 degrees 48S

机译：大西洋中脊南部8°48S的轴向熔岩的成分变化和Ra-226-Th-230模型年龄

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摘要

We present geological observations and geochemical data for the youngest volcanic features on the slow spreading Mid-Atlantic Ridge at 8 degrees 48S that shows seismic evidence for a thickened crust and excess magma formation. Young lava flows with high sonar reflectivity cover about 14 km(2) in the axial rift and were probably erupted from two axial volcanic ridges each of about 3 km in length. Three different lava units occur along an about 11 km long portion of the ridge, and lavas from the northern axial volcanic ridge differ from those of the southern axial volcanic ridge and surrounding lava flows. Basalts from the axial rift flanks and from a pillow mound within the young flows are more incompatible element depleted than those from the young volcanic field. Lavas from this volcanic area have Ra-226-Th-230 disequilibria model ages of 1000 and 4000 years whereas the older lavas from the rift flank and the pillow mound, but also some of the lava field, are older than 8000 years. Glasses from the northern and southern ends of the southern lava unit indicate up to 100 degrees C cooler magma temperatures than in the center and increased assimilation of hydrothermally altered material. The compositional heterogeneity on a scale of 3 km suggests small magma batches rising vertically from the mantle to the surface without significant lateral flow and mixing. The observations on the 8 degrees 48S lava field support the model of low-frequency eruptions from single ascending magma batches that has been developed for slow spreading ridges.
机译：我们介绍了在8度48S缓慢蔓延的中大西洋海脊上最年轻的火山岩特征的地质观测和地球化学数据，这些地震证据显示了地壳增厚和岩浆形成过多的地震证据。具有高声纳反射率的年轻熔岩流在轴向裂谷中覆盖约14 km（2），并且可能从两个轴向火山脊喷发，每个火山脊的长度约为3 km。沿着山脊长约11 km的部分出现了三个不同的熔岩单元，而来自北部轴向火山脊的熔岩不同于南部轴向火山脊和周围的熔岩流。与来自年轻火山岩田的玄武岩相比，来自轴向裂谷侧面和年轻洋流中的一个枕墩的玄武岩中的不相容元素被耗尽。来自该火山区的拉瓦斯的Ra-226-Th-230不平衡模型年龄为1000年和4000年，而来自裂谷侧面和枕墩的较旧熔岩以及一些熔岩田的年龄则超过8000年。来自南部熔岩单元北端和南端的玻璃显示出比中部低100摄氏度的岩浆温度，并且增加了热液蚀变物质的同化作用。 3 km范围内的成分异质性表明，少量岩浆从地幔垂直上升到地表，没有明显的横向流动和混合。在8度48S熔岩场上的观测结果支持了单个上升岩浆批次的低频喷发模型，该模型是为缓慢扩散的山脊而开发的。

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