Stability of rift axis magma reservoirs: Spatial and temporal evolution of magma supply in the Dabbahu rift segment (Afar, Ethiopia) over the past 30 kyr
Stability of rift axis magma reservoirs: Spatial and temporal evolution of magma supply in the Dabbahu rift segment (Afar, Ethiopia) over the past 30 kyr
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裂谷轴岩浆库的稳定性：过去30 kyr达巴胡裂谷段（埃塞俄比亚阿法尔）岩浆供应的时空演化
DOI：
10.1016/j.epsl.2014.11.002
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发表时间：
2015
影响因子：
5.3
通讯作者：
Medynski S
中科院分区：
文献类型：
--
作者：
Medynski S
利用宇宙成因（36 Cl和3 He）玄武岩熔岩流地表暴露测年、野外观测、地质制图和地球化学资料，揭示了埃塞俄比亚阿法尔坳陷Dabbahu/Manda Hararo裂谷段的火山历史，表明该裂谷段的岩浆活动在两个不同的岩浆室之间交替进行。Dabbahu裂谷最近的活动（特别是2005-2010年的岩脉危机）是由裂谷轴内一个地震上识别良好的岩浆储层所推动的，我们在这里展示了这个岩浆体在过去30年中一直活跃。然而，除了这个轴向岩浆库之外，我们在本文中强调了第二个，独特的岩浆库的重要性，它位于当前轴以西15公里处，从15 ka到次近纪一直是岩浆聚集的主要焦点。20 ka至今，轴向储层的岩浆供应大幅减少，而侧翼储层自15 ka以来似乎有规律的岩浆供应，导致岩浆分化程度较低。两个储层的岩浆微量元素特征是由单一地幔源不同程度的部分熔融作用产生的，但它们各自的岩浆在不同的地壳管道系统中分别演化。Dabbahu/Manda Hararo裂谷段的岩浆活动并不集中在当前的轴向坳陷内，而是在西侧翼扩散至少15公里。这与大地电磁观测结果一致，大地电磁观测结果表明，在该剖面下方存在两个岩浆体，目前岩浆主要聚集在西翼下方，正是在地表观测到最近（< 15 ka）规模最大的侧翼火山活动的地方。将这些观测结果应用于缓慢扩张的洋中脊表明，岩浆体的寿命可能至少为20 ka，岩浆活动的连续性是由一个独立的接力储层系统维持的，这反过来又可以控制扩张的位置。相对于板块而言，位于大致相同位置的不同地壳储层之间的这种长期交替似乎是在地幔稳定软点上组织和维持活跃扩张中心的关键特征。
Unravelling the volcanic history of the Dabbahu/Manda Hararo rift segment in the Afar depression (Ethiopia) using a combination of cosmogenic (36 Cl and 3 He) surface exposure dating of basaltic lava-flows, field observations, geological mapping and geochemistry, we show in this paper that magmatic activity in this rift segment alternates between two distinct magma chambers. Recent activity in the Dabbahu rift (notably the 2005–2010 dyking crises) has been fed by a seismically well-identified magma reservoir within the rift axis, and we show here that this magma body has been active over the last 30 kyr. However, in addition to this axial magma reservoir, we highlight in this paper the importance of a second, distinct magma reservoir, located 15 km west of the current axis, which has been the principal focus of magma accumulation from 15 ka to the subrecent. Magma supply to the axial reservoir substantially decreased between 20 ka and the present day, while the flank reservoir appears to have been regularly supplied with magma since 15 ka ago, resulting in less variably differentiated lavas. The trace element characteristics of magmas from both reservoirs were generated by variable degrees of partial melting of a single homogeneous mantle source, but their respective magmas evolved separately in distinct crustal plumbing systems. Magmatism in the Dabbahu/Manda Hararo rift segment is not focussed within the current axial depression but instead is spread out over at least 15 km on the western flank. This is consistent with magneto-telluric observations which show that two magma bodies are present below the segment, with the main accumulation of magma currently located below the western flank, precisely where the most voluminous recent (< 15 ka) flank volcanism is observed at the surface. Applying these observations to slow spreading mid-ocean ridges indicates that magma bodies likely have a lifetime of a least 20 ka, and that the continuity of magmatic activity is maintained by a system of separate relaying reservoirs, which could in return control the location of spreading. This long term (> 10 5 yr) alternation between distinct crustal reservoirs located broadly at the same location relative to the segment appears to be a key feature for organizing and maintaining active spreading centres over stable soft points in the mantle.
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